Modern diesel engine air intake requirements

Amberjack · Post by **Amberjack** » Dec 10th, '25, 19:01

Raining like the dickens out here so I’m not doing any yard work today and the boat is all buttoned up. The Forum has been pretty quiet but nothing gets a good discussion going better than Mediterranean food (I’ll leave that to Yannis) and diesel engine air intake requirements, everybody’s topic!

This has been simmering in the back of my mind for years and when Steve’s Dutchess rebuild touched on it I did some research. It turns out either nobody really knows or they’re not telling. I’m not talking about CFM or air/fuel mix. That information is in the engine manual. I’m asking how many square inches of opening to fresh air is required into the engine compartment for my Yanmar 315’s to be happy at 3800 rpm/WOT.

It’s subjective because air moving around structures or going around corners restrict flow. An opening direct to the engine air intake is more efficient than an opening on the other side of the engine box. For example an air intake piped in from the side deck on one side will deliver freer air flow than intake on the other side due to length and fewer turns of the duct run.

I called my mechanic, a 30 year diesel mechanic and a smart guy and he said he had no idea. Mechanics work backward. If there is soot on the transom they look at propeller sizing first, then they look at engine air. No soot, no problems.

When Amberjack was repowered we went with the existing side deck clamshells and I was scrubbing the transom 2-3 times a season. So we repropped twice and reduced the exhaust stain but never completely eliminated it. Then a mechanic mentioned in passing that modern diesels require a precise air/fuel mix and restricting either one reduces engine performance. He also told me my new Yanmars required a lot more air than the old Crusaders.

As I said, I’ve done a lot of research lately and found a couple things:

1. Cold air is more dense and therefore better than warm air. Bob Lico touched on that.
2. Clean, dry air from inside the cockpit is much healthier for the engines than salty air. That’s common sense. Bob touched on that also.
3. The engine generates a lot of heat so there needs to be a way to dissipate it. The engine has its own cooling system but the surrounding compartment needs help also. Manufacturers say it shouldn’t be more than 30F above ambient temperature and recommend bringing in fresh air in down low in order to circulate air in the compartment as it moves toward the engine air intake.

There’s an informative article in the July 10, 2025 Power and Motor Yacht that discusses this stuff. I installed a 150 cu/in ventilator on each engine box opening to the cockpit and fairly low. I give up some coolness to the air by the time it moves around the engines but it keeps the compartment cooler. My batteries and wiring are happy. I left the old clamshells (12 sq/inch each) up on the side deck and still use the engine room blowers after a run but removed the ducts reaching down into the bilge so instead they scavage hot air near the top of the compartment. Yannis, to answer your question from a couple years ago, the engines may be happy at 185F but the rest of the engine compartment is not. So I run the blowers until the engines cool down a bit. When under way the engine air intake configuration cools the compartment.

Phew, all told I think I did it pretty much right for my needs. Others needs and priorities will vary. I’m going to put this up and look forward to being corrected.

Still raining out there.

Yannis · Post by **Yannis** » Dec 10th, '25, 20:38

Doug,

I was thinking of the same issue, albeit without that same rigor like you.

I tried to understand Bob's description of his configuration, and to be honest, I think I managed to grasp that the best way to funnel air into the engine is directly to the turbo's air intake itself, rather than try to aerate the whole engine compartment. In fact, after Bob's description, it appears to me that it is of no use to strive for the latter as it really is not all that important.

Come to think of it: Your turbo intake has a given diameter. If you attach a tube, or duct, DIRECTLY and as airtight as possible (Bob explains how) to the turbo, and if you also use the shortest route to fresh dry air, then you should be doing what's best.

Why do you still think that the temperature within the engine box is of importance? How much do you think engine box temperature can influence the engine's performance, when this surrounding air NO LONGER influences your engine's needs of combustion, as it's sealed out? And if the existing clamshell intakes, which are installed on the side deck, already create an inbound current that aids hot air escape from elsewhere in the box, then, what else is there really to do except perhaps contemplate whether those clams should be attached to a duct so as to direct air lower in the box, or not?

I'm waiting for the final consensus on this topic to take corrective action...perhaps!

Amberjack · Post by **Amberjack** » Dec 10th, '25, 21:41

Yannis-Not worried about about the effect of engine compartment temperature on engine performance. If anything Amberjack is a touch overpowered for my uses. I’m thinking about engine compartment temperature on the rest of the engine compartment components: batteries, wiring, steering lines, in my case spare parts stored there etc.

For absolute top performance a short straight cool engine air intake and a separate engine compartment ventilation/cooling system are ideal. I’m looking for simpler, less parts to maintain and optimum engine and associated equipment life. Pretty much everything I’ve learned says to control engine room temperature to within 30F of ambient temperature (Caterpiller says 20F). Likely there is something I don’t understand yet but controlling engine compartment temperature is important to diesel engine professionals.

Tony Meola · Post by **Tony Meola** » Dec 10th, '25, 22:20

Doug

When you think about the design of the 31, there is a lot of places for the engines to grab air. Remember it has the whole bilge area to draw from, and that area is pretty open to receiving fresh air.

I still have the side panels along with the open storage area. The engines can grab a lot of air from that area.

Capt. Pat use to point out that the engines will also draw air from the bridge area through the Wire chase that runs to the bridge.

Given all of that you would be hard pressed to starve the engine for air on the 31, unless you closed up a lot of those openings.

Yannis · Post by **Yannis** » Dec 10th, '25, 23:14

Doug,

The way I understand it is that these F limits apply when ambient (engine box) temperatures directly affect the air used for combustion. When all ambient air is sealed off of the air intake whats the point to bother with ambient air temp?

Also I sincerely doubt that cables and other components would suffer from normal engine box temperatures, even at 20 or 30 or more degrees above outside temperature. Place a thermometer inside the box near the items you worry about being heated and this will give you a good indication of how to tackle the issue, if there is any issue that is.

Carl · Post by **Carl** » Dec 11th, '25, 13:07

I did a real easy check to see if more air, cooler air was better. When running I open a hatch and see if there is a difference. Aside from louder there was nothing. Inside engine compartment it was warm, but not hot like when we stop running. Also pulled Air Filter off...nope, not a change. That's one nice thing about the express...aside from the whole motor box coming off, the tops have access hatches, lift and I'm looking at the turbo.

Now what I'd like to do is put my blowers back in boat for when I do slow down, I'd like to get that heat out quicker along with the fumes from the whatever its called that vents the crankcase. I have that hose now going to a separator but still some stank after a good run.

What I do notice is spring n fall the motors have extra pep...especially comparing to those super hot 100 degree July / August days when she feels a bit sluggish. Cold air good.

Ironworker · Post by **Ironworker** » Dec 11th, '25, 15:07

The weather is absolutely beautiful here in S. Florida today.

There are a number of online calculators to properly size air intakes for marine engine rooms available with a simple search.

When I refitted my boat, I was originally going to put my LiFePo batteries in my engine boxes. My boat before I renovated it had some of Capt Pats exterior air intakes. This was where I was going to put the batteries which did not make sense to me. I closed up the exterior air intakes to eliminate potential water intrusion and installed inboard air intakes. Because of the BMSs in those batteries are temperature sensitive and will shutoff at generally between 145F and 150F, depending upon the brand, I was very concerned about engine box temps and airflow. I say all this becuase I went down a long and winding Rabbit Hole on this subject.

After spending countless hours researching resulting in conflicting information, I looked at the engine intakes (measured surface area) and said as long as my air intake is greater than the surface are of the engines intakes then I was good for the engine's needs! But that left me with the issue of the engine box temps. To solve this I put an exhaust fan to draw hot air out of the boxes and pull in cooler air.

To sum this up. My exhaust fans do not make any significant difference in the operating temps of the engines. Generally, my engines run between 176F and 178F during the summer time in the Bahamas and S. Florida. For other reasons, I moved the batteries out of the engine boxes so box temperature wasn't an issue but I am still planning on installing a box temp sensor to tie into my NEMA 2000 system.

Amberjack · Post by **Amberjack** » Dec 11th, '25, 16:19

Yannis wrote: ↑Dec 10th, '25, 23:14 The way I understand it is that these F limits apply when ambient (engine box) temperatures directly affect the air used for combustion. When all ambient air is sealed off of the air intake whats the point to bother with ambient air

Give me time Yannis, I’ll think of something! While I’m thinking I’ll point out that if the air intake is hermetically sealed it’s not moving and cooling the air inside the engine box. That leaves us still with the question of whether it matters or not.

Tony, I remember Captain Patrick saying that. But my chase to the bridge is jammed tight and my new cockpit sole is pretty water and air tight so I think I still need the increased ventilation. I still miss Capt Pat.

Interesting observation Carl. Maybe Amberjack does have more pep in spring and fall but in the summer I’m carrying probably 100 gallons more fuel, 45 gallons water, food supplies plus all that wine…what is that, maybe an extra 1,000 lbs? So it’s hard to tell.

Carl · Post by **Carl** » Dec 11th, '25, 18:35

I’d have to have a thermometer in engine compartment to prove my point. My thought when running those diesels are huge air pumps sucking all the air and heat out of the engine compartment… or at least a lot of it.

I’d think Bobs Licos compartments will be running a lot hotter as he is pulling cool outside air externally. Add to that sound insulation which only works best when fully sealing leaves cooling air entering and exiting from??? Clamshells, blower…. Interesting

So what is effected by high heat in compartment… electrical, circuitry, fuel in lines, pump, filter. batteries.

Then again aren’t some generators in sealed compartments shoved within engine compartments ? They seem ok…

Tony Meola · Post by **Tony Meola** » Dec 11th, '25, 21:56

Carl

When I was rewiring the boat, I calculated voltage drop and all of that good stuff. The one thing I couldn't account for was voltage drop due to high temperatures, which according to everything I read exists.

Does it make a difference for the length of wire we use, probably not. At least I can't tell, just like you don't see a power increase with the cooler air. (Lets leave that argument for another thread)

As far as you venting the crankcase, When Forked River Diesel set up my engines, they routed that hose back along the side of the engine and back in. I thought I had a picture of it but I don't.

Remind me in the spring and I will take a picture. No burp bottle, no stink.

Yannis · Post by **Yannis** » Dec 12th, '25, 23:50

Carl,

Now that you mentioned it, I also at times - when I can hand the wheel over to someone who I trust - step down, and open the middle hatch of the engine room just to see what's going on in there, while the engines run. And, as you say, nothing except for extra noise doesn't seem to be any different than with the hatch closed. So, more air doesn't seem to affect revs, speed, or anything else.

Second point, I think that as we reach our destination after a long run, we all reduce our revs and speed to eliminate wake and allow for a safer approach. Except for safety reasons therefore, this process of reducing speed during the last, say, 10 minutes helps into drastically reducing the engines' temperature and this is clearly seen in the temp meters on the dash. So, if the engine temps drop so drastically, in my case I think from 75C to 65C, which is around 145F, then I assume there should be no issue with any sensitive battery components. Now, if someone is so sensitive about engine room temps after the engines are shut, cracking a hatch for a few minutes should solve any issue, if there was any issue that is.

Carl · Post by **Carl** » Dec 14th, '25, 13:09

I’d think cracking the hatch would have to be more than a few minutes to sustainably drop temperatures in the compartment. Motors are massive amounts of cast iron, and fluids that hold heat for a good while. True the initial drop from running to idle is a big drop like going from full boil to simmer in a huge pot water, shutting motors is like turning off the flame… still not putting my hand in that water for awhile…even if the lid is opened for a few minutes.

Yannis · Post by **Yannis** » Dec 14th, '25, 13:58

The temp is already below battery components’ upper limit. It only drops from now on. Slowly, but it only drops.

Tony Meola · Post by **Tony Meola** » Dec 14th, '25, 21:47

Carl & Yannis

I think if you could cool down the engine compartment enough, you would probably see and I doubt you could even measure it, a slight increase in fuel efficiency since cold air holds more oxygen and that would mean better combustion.

If you were to normally consume a 1000 gallons in a year, maybe, and it's a real maybe, if you cooled down the engine room you might use 999 gallons. Not enough to go crazy.

The real need for the engines to breath right is so they run correctly and I really believe given the design of our boats, they grab enough air from other areas of the boat to give them the air they need to run properly.

Giving it more air is just insurance and can't hurt. I am thinking of doing what Doug did, put the venting in the companionway.

Carl · Post by **Carl** » Dec 15th, '25, 12:05

Tony, I completely agree and disagree with you....I should become a politician.

Think hot air balloon. The balloon air is heated, expands, becoming less dense and lighter then the air outside the balloon. The lighter air rises, cannot get thru the balloon wall causing it to lift balloon, basket n contents. Now lets go the other way, chill the air, the volume shrinks becoming denser and heavier. Now think BBQ tank...empty tank is lighter as it has less propane in it...a full tank is heavier as its more densely filled with propane. The same stuff is inside the tank empty or full, just more or less of it.
Same goes with air, 21% oxygen (20.9% to be technical) hot or cold. What changes with cold air is it's denser, so more of it by volume. More air means more boom per detonation which equates to more power. Its like a natural turbo packing more air in the cylinder by just being cooler.
So yes, if intake is pulling air from the engine compartment, cooler air would give more power...or efficiency for those that are not looking to go faster, but a little further per tankful of motion lotion. Bob Lico went further to pull air from outside the hot engine compartment.

Which brings us to the next part of the discussion...are the motors and engine compartment items and doo dads hampered being warmer or might they benefit being cooler.

Now to how much does cool air, better breathing really give a motor. I can answer part of that in regards to my Jeep. My customer makes performance equipment...or I make, he develops markets and sells. Anyway, he just came out with a K&N style air filters for my Jeep. As he didn't have a new Jeep on hand to check performance he asked me to bring mine by to test. He hooked Jeep up on Dyno and put it through its paces to get HP, torque etc. Then installed Filter and redid Dyno test. A tad bit more torque and 7hp more by changing the filter. Would anybody ever feel the difference between 285hp and 292hp...I highly doubt it, but it is a measurable difference.

Amberjack · Post by **Amberjack** » Dec 15th, '25, 17:10

This is an interesting topic and the discussion has helped clarify my thinking on several things. It also pushed me to do more research about my engines. When it comes to the engines in our boats people have strong opinions.

Cooler Air = More Oxygen: Carl is correct I think. Cooler air doesn’t have greater O2 density it has more O2 because the molecules are packed closer together. I can see that would increase power slightly. But if diesel engines require a precise O2/fuel mix does the engine computer simply increase the fuel to maintain a precise mix? Wouldn’t this be the same as simply advancing the throttle meaning no particular fuel economy just greater power at that throttle setting? This would take hours of testing with two different air intake configurations to determine unless someone smarter than me already has the answer.

Engine Compartment Temperature: Yanmar engine belts and I suspect Cummins also are EPDM material rated up to 550F (288C) so I concede that point to Yannis. My reading tells me that lead acid batteries need to stay below 140F (60C) for best battery life. Mine are in the engine compartments and I’ve been getting a comfortable 10 years from them so I guess I’m keeping my temperatures below that.

Engine Temperature: We are talking apples to oranges with pineapples thrown in. My Yanmar operating temperature of 185F is the cooling system temperature. Combustion temperature is around 3600F (2000C) and exhaust temperatures run 1100F (600C). The cooling system keeps the rest of the engine at manageable temperatures. The core components of the engine are incredibly hot. When I shut down the engine it also shuts down the cooling system but there is still considerable residual heat in the engine block. It has to go somewhere and I assume it migrates out into the engine room which must create a temporary heat rise after shutting down. Yannis indicates he doesn’t see that. I’d like to hear the from someone who has engine compartment temperature sensors.

Anyhow, still raining out here. I was on the boat this morning, it’s getting dirty wants a wash and even more wants to go for a run. BTW for those that viewed the clips of salmon swimming across the road I have a clip this week of elk swimming across a flooded road but won’t post it-Bruce would think that is way too far off the Bertram topic.

Carl · Post by **Carl** » Dec 15th, '25, 21:42

This is where my diesel knowledge gets real fuzzy…love to hear the real answer.

That said I believe diesel, unlike gasoline motors does not require an exact fuel to air ratio. Gas needs 14.7:1 to combust from idle to WOT. Diesel will detonate whatever is sprayed into the highly pressurized cylinder. Intake air is not controlled by throttle plates it just gets sucked in with naturals or pumped in by turbos. Throttle position tell motor to hit a certain rpm, if the motor is not making that rpm more fuel is sprayed into to cylinders to attain that rpm. Bring throttle back tells motor to make less rpm and cuts fuel back.
While a diesel will run at different fuel to air ratios there is a sweet spot with best burn, I think that ratio is pretty close to gasoline, but motors will run richer without full combustion…black smoke and very lean at idle like 50:1 or more. Which may be the reason idling diesel just don’t get to operating temps

The way fuel is metered in diesel was explained to me was the motor has a governor. Centrifugal spin should attain a certain force, not spinning fast enough, not enough force and more fuel gets dumped in.

I’d love to hear the real story as this is my layman’s understanding.

That and more air in cylinder, more boom. More fuel and air even more boom

Tony Meola · Post by **Tony Meola** » Dec 15th, '25, 22:27

Carl
7 HP increase might be measurable on a dyno, but it will make little difference in fuel consumption on our boats.

Bob did cool the air down by drawing it from the cockpit, but he also did the big change that made all the difference and that is shaft angle. He reduced it. Adding 7HP without changing something else will mean little in terms of fuel consumption and speed.

Even in a car, you might be able to notice the difference in fuel burn on a long trip where you do not have stop and go traffic, and if you were to get into changing rear end gear ratios you could use that extra 7HP more effectively.

There are give and takes and I will stand by, just shoving cooler air into the engine room will not make the enough of difference to go crazy over it. I think you might be surprised at the engine room temperature before and after you make the changes to get cooler air into the engine room. I bet the difference is not that big.

Here is the other thing, I have been running mine since 2010 without any additional air over the stock design. Wires are still good, hoses run the usual life span, belts the same, batteries at lease 6 years or more out of them.

Long story short, unless you want to go Bob's route and make multiple changes, so they add up to something, don't drive yourself crazy.

Tony Meola · Post by **Tony Meola** » Dec 15th, '25, 22:29

Carl & Doug

The more oxygen thing is a matter of semantics. Depends on how you want to describe it. More dense, more oxygen what the heck tomato/ Tomaato.

Yannis · Post by **Yannis** » Dec 15th, '25, 23:06

So, if we completely ignore the air temperature in the box (as it doesn't seem to negatively affect cables, belts, batteries and all other components), while we seal off the engine air intake (via a sealed tube or duct) that sucks air directly from the outside, we achieve cooler air intake and therefore a better combustion.
The notion of "more air" in our boats, where air is coming from all over the place anyway, is not an issue to address, as even when you run with the hatches wide open it doesn't seem to affect performance.

Therefore, how can we measure performance change when only outside cooler air is funneled directly to the turbo?
Is it worth modifying the engine box with the necessary fiberglass cut outs, inventing the ideal straight tube that runs the shortest distance, incorporate the best possible air filter etc, if we cannot sense or measure any performance improvement? Because, like you're saying, if at the end of the summer we burned 10 or 20 liters less...well, I have other ideas for my free time to spend it on.

Carl · Post by **Carl** » Dec 16th, '25, 08:46

Needing more air I think comes into play when going up in hp with snugger, better sealed compartments. Clamshells, open seams spaces around bulkheads in bilge will allow lots of air but may not be enough. I never considered worrying about it with my 210's and wide open lower section in companionway in conversion. However, I did think about it when Exhaust Gas Temp was high on one motor. That's how I got to opening hatch, pulling filter and checking boost pressure. Nothing brought temp down and made no noticeable performance difference.

Ugh...I was gonna tell this one but cannot recall how it's in the memory bank. Not a friend of mine for sure, maybe one of dads, or a past client of his or maybe I just read somewhere or perhaps dreamed it up.

Anyway, boat was running great before winter layup. Following year on first run its blowing black from both motors. Mechanic checks everything out, trial run is good. Owner takes out and blowing black, mechanics called back they check out a few things, no problem found and go for a ride. They blow black, mechanic goes down to check out, gets to cockpit and struggles a bit to open door to engine room, when he does no more smoke. On prior check runs they left engine room door open. During layup boat interior was re-freshened along with new carpet and heavy underpayment. Thought was the motors were pulling air from saloon motor hatches, padding and carpet kinda sealed that access off...end of my recollection.

Doug, After that story we should be able to see the ELK crossing the road video.

Tony, 7hp gain was changing the AIr Filter only. Bob has two modifications...well maybe 3 if including K&N Filter. So high flow filter, bringing in cooler, fresh outside Air and the piping. If I understood correctly the air being directional through the piping flows better, less turbulence then air being scavenged from engine compartment. While it would not be starving a little more air is a little more air.

Again, going back to Bob Lico performance and economy, it has been a few percent here, a couple there. Yes, big improvement we believe is less shaft angle...extensive prop work, dialed in injection pump, straightened out the running strakes, keeping fuel filters outside motor compartment for less heat absorption, cooling fuel (???) Cold clean air intake piped direct to turbo. Lightened up boat while attaining the perfect running weight distribution and I'm sure I missed a few things.
So is 7hp more for changing air filter going to do anything, what about another 7hp for bringing in cooler air from outside motor compartment? Now up to 14hp...that is more then nothing. I look at trim tabs, running 20mph at 2000rpm. Give slight tab and I'm running 21-22mph at 2000rpm. Free speed or I bring down throttle a touch for a few extra miles per tank.

From my observation 50hp jumps per motor give about 4knots additional cruise speed. Cummins 210/220's about 20knot cruise, 250/270, 24 knot cruise, I'll leave Bob out as his 315 setup is an apples to steak comparison. But the 370's seem to be cruising 29-30 knots. Cruising as in cruise speed motors are comfy running, not what the owner thinks is appropriate. Bumping up our HP does add to better numbers, just how much time and effort are we willing to go 1 knot or 2 faster or burning a few gallons less a season.

Yeah, Doug, its time to see how the Elk swam to the other side of the road.

JeremyD · Post by **JeremyD** » Dec 16th, '25, 09:21

I have Cummins 315's and my cruise is around 27-28 knots at 2200 rpm's.

After seeing the elite boats with their vents along the side of the cabin "sail" I have been thinking about modifying mine to push more fresh air into the engine compartment(s) my starboard engine (where my generator is located - lack in there much more) than my port engine seems to run out of umph over 2300 rpms. The engine (recon long block from Cummins) rebuilt fuel pump, new turbos etc runs great - no issues with the port engine. I really only have an on deck exhaust (original clam shell)

I'm looking to experiment a little, see if it makes a difference.

Picture for clarity

Carl · Post by **Carl** » Dec 16th, '25, 09:49

Jeremy, I'm sure you could crunch the numbers and verify with others about how much air needs to be available for motors to run at their peak.

Easiest thing I can think of, take for a ride with motor box closed.

Have someone open the box and take the same ride, or open it midway and see if you get the little extra performance you feel is being missed. If a wide open motor box doesn't provide enough air, I can't see any venting, air deflecting channeling systems doing better.
All testing done for the cost of a ride and maybe an adult beverage to the person running boat or opening motor box underway.

Ironworker · Post by **Ironworker** » Dec 16th, '25, 10:50

Most of what I know about turbos and air is from airplane engines. Throwing an intercooler in the mix adds a lots of benefits to airplane engines especially when you're running Lean of Peak.

While I have only the most basic understanding of turbos on marine diesels is that other than the cylinders (big pumps) sucking in air AKA Normally Aspirated, you'd have to pressurize the air AKA Turbocharged. The Electronic version of our engines is metering the fuel for the ambient environmental conditions. The Mechanical version is set up to produce fuel based on an average ambient environmental standard.

My Ambient Environmental conditions in S. Florida and the Bahamas is raw Water temps ranging from 68F to 84F and Air temps ranging from 60F to 100F.

Lastly, I'll add this specifically about "Modern" electronic Cummins diesels. The engines are all the same in the various horsepower ratings. A 350 HP engine is the same as the 380 and 425hp. The only difference is the ECM. I can buy a new ECM for my 380hp engines and turn them into 425 engines without changing any components other than the ECM and making a few changes on the Engine controls. That tells me that the engines are already plumbed on the air and water side for at least 425HP and the ECM is just metering fuel.

BTW, there is a Boat Talk podcast on Youtube put out by the guys from Seaboard Marine. On their last episode they talked extensively about the pros and cons of the Fuel Coolers on Cummins engines. The cons far out way the benefits which would maybe be 1% increase in power output for a mechanical diesel on an extremely hot day when running at 100% power.

Carl · Post by **Carl** » Dec 16th, '25, 11:31

Pics up! Wow, pretty boat Jeremy. Never saw hot lime before, I think I like it a lot. Definitely stands out from the pack!

JeremyD · Post by **JeremyD** » Dec 16th, '25, 11:58

These examples are from Elite Marine. I've seen the lime/white one for sale in the past year or so - very unique.... Mine is blue/white.

Amberjack · Post by **Amberjack** » Dec 16th, '25, 18:58

Tony Meola wrote: ↑Dec 15th, '25, 22:29 Carl & Doug

The more oxygen thing is a matter of semantics. Depends on how you want to describe it. More dense, more oxygen what the heck tomato/ Tomaato.

Tony what can I say, I come from a family of engineers. This stuff matters to us.

OK, Carl here it is.

https://www.instagram.com/reel/DSI7R0HE ... hare_sheet

I’m not sure they’re swimming across the road although there could be a farm road underneath all that. About 30 miles from here.

Carl · Post by **Carl** » Dec 16th, '25, 21:04

JeremyD wrote: ↑Dec 16th, '25, 11:58 These examples are from Elite Marine. I've seen the lime/white one for sale in the past year or so - very unique.... Mine is blue/white.

Yes, that’s right, on the Tide Slides. Now I remember. Another great looking boat!

Carl · Post by **Carl** » Dec 16th, '25, 21:08

Amberjack wrote: ↑Dec 16th, '25, 18:58 I’m not sure they’re swimming across the road although there could be a farm road underneath all that. About 30 miles from here.

Huh, looks like they were going for a touchdown across football field. Thanks, not something I normally see.

Tony Meola · Post by **Tony Meola** » Dec 16th, '25, 22:46

Looks to me like Doug's Elk are craving more Oxygen. LOL

Tony Meola · Post by **Tony Meola** » Dec 16th, '25, 22:48

That boat does stand out with the lime green. I can handle it at the waterline. Actually I think the water line is pretty slick. Not sure on the topside. I think I would get tired of them after a while.

Are those air vents by the windows on the side pilaster?

Tony Meola · Post by **Tony Meola** » Dec 16th, '25, 22:50

Actually as I think about the whole air to the engines issue, I thought the installer who certifies the engines are running correctly, is supposed to do some type of air flow/air pressure test.

Does anyone know?

Carl · Post by **Carl** » Dec 17th, '25, 07:34

I was the installer, I certified they are engines and my pressure test was opening the hatch.

Also, Waterline height to turbo, Oil pressure, Temps, EGT, Back Pressure, Boost, WOT, strobe to verify tachs, no load WOT, Motor angle, and that may have been it. OK, may be a section on air requirements but I glanced over, My motor box has no bottom section so plenty of air.
I have installation manual/pamphlet, that kinda specifies most stuff tech should go over to certify install for warranty...mine was long up so dragged my diesel mechanic friend along on 1st ride...actually he told me he was going before I asked.

Yannis · Post by **Yannis** » Dec 17th, '25, 08:04

So, we're beating around the bush on the cool air issue.

We're discussing if air is enough, what happens when motor boxes are open or ajar, if air comes from below or from the crevices, or from elsewhere, we're discussing if shaft angle boosts horsepower or speed, if engine box internal temperature ruins cables, batteries or belts, this filter, no that filter maybe...the fact that cool air affects performance and by how much, however, not much is being said except that "cool air improves performance"! Fine.
Shall we all go install a direct tube to the turbo to improve performance and/or consumption? Fine, too...

Has anyone except Bob done it and came up with tangible results? At least any reading that suggests what this improvement could look like, speed wise? Consumption wise? My understanding is that no one has done it and therefore no one has an answer. Even Bob doesn't. Because Bob has made so many improvements, that he probably cannot separate the benefit of cooler air in his overall improved performance results.

Carl · Post by **Carl** » Dec 17th, '25, 15:06

Here's a few things I found on the web about temp and power gains or losses. Interesting read on CAT site about funneling air from outside, their biggest concern is air being too cold for combustion in winter requiring heaters to warm the air. That and funneling having air cleaner at the turbo in case plumbing un-plumbs and lets something in...also it needs to withstand vacuum. I'll share link if I find again...
Also good information found when looking at intercoolers ( Air Charge Cooler)...if wondering if cool air makes a difference just look at what the intercoolers add in power. Of course that is real hot air as its been compressed by turbo...but it is still that reason to add an intercooler, they add power by cooling the intake air.

**********************************************************************************************

For every 3 degree Celsius (5.4f) that you reduce your intake temps you increase oxygen density by 1%.

In your engine our goal is to burn fuel, for fuel to burn it needs oxygen. The more oxygen we introduce into the engine the more fuel we can burn.
The more fuel we can burn, the more power we can create.

Engineering Tool box has a great calculator showing this exact example here: https://www.engineeringtoolbox.com/oxyg ... _2082.html

As an example, if your peak intake air temperature (under boost at the end of a 1/4 mile) is hitting 80c (176f) and via the use of our chiller we get this down to say 30c (86f), the result is as follows:

Full article here:
https://fiinterchillers.com/tech_articl ... gen-power/

***************************************************************************************************

Here is an AI answer which kinda put all I have read in a nutshell. Basically cool, not cold and not hot is best.

Colder air boosts diesel power because it's denser, packing more oxygen for better combustion
, while hot air reduces power due to lower density, though diesel engines handle heat better than gas engines. A ~10°C drop in intake air temperature can yield ~1.5% more power, but factors like turbocharging, intercooling, and fuel quality also play big roles in performance.
Cold Air: More Power

Denser Air: Colder air has more oxygen molecules per volume.
Better Combustion: More oxygen allows the engine to burn more fuel efficiently, increasing horsepower and torque.
Ideal Range: While colder is generally better, some suggest an ideal intake temperature range of 70°F to 130°F (21°C to 54°C) for optimal diesel performance.

Amberjack · Post by **Amberjack** » Dec 17th, '25, 16:18

Interesting information Carl. It tells me that I need a PhD in combustion physics in order to decide whether or not to install a dedicated air intake. But I am comforted by the fact that Amberjack usually runs in about 60F ambient temperatures. Given a roughly estimated 20-30F temperature rise by the time my air flows around a hot engine I think I’m well within the optimum intake air temperature parameters you show.

But still the question nags at me-does cooler, more efficient intake air produce better fuel efficiency? It kind of sounds like it does but none of the diesel “experts” explicitly say that it does. “The more oxygen we introduce…..the more fuel we can burn” seems to contradict it.

Ironworker · Post by **Ironworker** » Dec 17th, '25, 16:39

I'm not disagreeing with Bob, but I don't think that I would recommend a dedicated outside air intake for our boats. You'd have to weigh the air loss due to the plumbing and dirt/water ingestion. I worked real hard to keep moisture out of my air intakes. Say you do this and get 2% to 4% more POTENTIAL power. How often would you use it.

Not venting and actively moving hot air out of the engine boxes is something that I did. I will add a Nema 2000 temp gauges to my boxes and will report back here if the fans make any significant difference. As I said in an earlier post they don't make any difference in the cooling water temps.

Tony Meola · Post by **Tony Meola** » Dec 17th, '25, 22:00

Actually, with the suction of air coming from the turbo's, I would think on our 31's at least that if you don't have the side panels you expose the engines to fresh air through the opening that is right under the gunnel where the side panel meets the engine box.

Without an extension to place the air intake at that opening, the suction from the engines running at speed should suck in enough cool air to drop the engine box temperature. How much who knows without actually testing it.

Same as with Bob's solution, you would have to have the temperature of the air entering the turbo through his extension of the intake, vs the temperature at the turbo, without the extension but with the opening under the gunnel exposed.

That would be the only way of knowing exactly what we are working with. I would bet it is not a big enough difference to even warrant the extension of the intake.

Now with all this being said, in the spring I will be making new side panels. I already have the material. What I am debating is do I put a vent in the side panel near the engine box so it sucks air from the cockpit or do I do what Doug did and put it in the companionway.

I would say, it is probably best in both spots. My reasoning would be, heat rises and at shut down it would help cool down the engine compartment faster. Cool Air coming in from the companionway and hot air being pushed out higher openings.

I have to agree with Rick, and if you can avoid it, don't draw the air from the sides allowing damp air and possibly water into the engine room. Even though a Dorado box is supposed to separate water from the air, you would still have moist air coming in.

Tony Meola · Post by **Tony Meola** » Dec 17th, '25, 22:01

Carl

You realize that in the old days this discussion would have been at 20 pages by know. Could have riveled the Rocker Stopper Debate.

DanielM · Post by **DanielM** » Dec 17th, '25, 23:43

Tony,

I’ll do my part to make it 20 pages. You guys need to discuss the relative benefits of laminar or turbulent flow inside the direct to the turbo air feed…flow straighteners and long sweep elbows perhaps?

Yannis,

I can’t talk for Bob, but keep in mind he raced speedboats a big part of his life.

I’ve worked with engineers that loved to race. If they knew there was an extra 1-5 hp lurking somewhere under the hood, they had to go find it. It was baked into their DNA as racers. Didn’t matter if it was on the race car or the grocery getter, they just enjoyed optimizing. Bob certainly has Phoenix optimized.

It’s been a fun discussion for an aspiring marine diesel boat owner. Interesting content.

Yannis · Post by **Yannis** » Dec 18th, '25, 00:37

Doug,

I was puzzled by this sentence also. But you can, perhaps, look at it in a different way: The more oxygen we can introduce, the more fuel we can burn, can be also read as: We need less fuel for the same performance as more oxygen makes a better combustion. And for as long as we don't touch the gas lever to provide more fuel, then, for a given stable fuel level we achieve a better performance as there is better combustion.

Rick,

Perhaps this additional 2-4% potential increase is for every given RPM, not just an increase at WOT.

Danny,

I didn't question Bob's credentials, to the contrary, I said since Bob did it, it must be worthwhile to put some numbers to it so that we can all understand better by how much we can benefit from a dedicated airfeed.

Carl,

I agree with Doug, so I applied for a PhD in applied combustion Physics, LOL ! I'll be able to explain all this to you in 4 years!
I guess what we all need here is someone with a solid engineering background come and tell us, guys...by constructing a dedicated airfeed you can expect a decrease in fuel consumption by that much, or, a performance increase by that much, so we can each decide what to do. Until then, we can continue this discussion and hope to eventually surpass the Rocker Stopper debate !!

Carl · Post by **Carl** » Dec 18th, '25, 08:39

Daniel- I had to lookup laminar flow...
Better flow as it has all the gas molecules marching in the same direction, opposed to yelling fire in a movie theater with everyone making a panicked mad dash to the door. Bob addresses that with the piping from filter under gunnel to the turbo inlet, I believe same theory for the Up and over exhaust run, except the term he goes with it "scavenges" the exhaust. Side note Bob also insulated his intake run so as not to pickup heat in motor box.
I did see on this topic of Cold Air Kits and better piping adding power over stock by using round piping which takes up more space then the squared / flattened stock runs, but flows better as it is a "laminar flow" hey, go to use it in a sentence.

Tony- I agree with hot motor off, lower vent in companionway with higher in cockpit would have hot air going out the top, pulling cool air in from the bottom. Although a breeze with clamshell vents kinda does that too. Running of course the air is getting sucked out by the motor. And another down side if it matters...noise. When trying to insulate my gas motors I found out the smallest openings would keep that sound level high. At idle the gas motors were quiet, running not so much. Kinda the other way around now...idle those diesel clank. Still really like it though....for now.

Rick- Bobs intake vents are pretty much protected being tucked under the gunnel in cockpit. Aside from a hosing from someone that doesn't know... I'm worried about the someone who doesn't know...or the raw water hose on and we get a strike. Drop everything and go into action.

Doug, when temps are in the 50's and 60's I find boat has a little more. More what??? It's a definite something for sure, I may see a tad more on the tach, just like really hot days I think its a bit less...but still pretty much 2600 on both motors, hot n cold...maybe 2625 and 2575, but that could be the way I am looking at gauges. Its a seat of the pants thing...hit a wave in the heat the recovery is slower. When cold 50-60 there is nothing to recover from. Almost like going from salt water to brackish...aside from the sound of the water, brackish the boat feels less agile, has less spunk.

the 50-60 may be the number as it looks like 100F is the optimum temp. I assume that means in the cylinder as 50-60 gets warmed to maybe 70-80 in engine compartment then compressed in turbo adding lots of heat...some get that air inter-cooled or like me right into cylinder.

Anyway, I don't think it's potential power, although it can be I guess...but looking for the better burn. Looking to utilize the full potential of the fuel when cruising. When I built...okay, my sisters boy friend built my car motor...His dad built drag n race boat motors...anyway he let me help. Back to the thing I was saying...we ( yeah really he) doubled the hp and my mpg went from 17-18 to 21mpg on the highway. Efficient use of the fuel...unless of course I stepped on the pedal and could pretty much see the gas gauge move. Reading about diesel they can run ridiculously lean at idle...Last night I saw 60-70:1, to overly rich with most of the fuel going out in the exhaust as black smoke n soot...but the ideal range is the 14.5:1 - 15.1:1. And how to accomplish that...Doug, I'll wait for you to get your PHD and explain to me. But till then I have think it is more efficient with more Air/Oxygen compressed in the cylinder when the fuel is introduced, has to give more bang for the buck.

Rocker Stoppers...good topic.

bob lico · Post by **bob lico** » Dec 19th, '25, 12:06

OOPS ------Daniel m hit the bell ,bravo, bravo !!!! the highlight of my life was to be part of the most hated crew on the track.show up to the 1985 NHRA nationals with no sponsors on the camaro just plain white paint with Lindenhurst, NY on the trunk.all the others PRO-STOCK entries including factory sponsors looking (who the hell are these assholes ) no spare engines and fancy tractor trailers with big pit crew. LOOK at this idiot he will never last a screaming big block turning 10,000 rpm. after "ROUND ROBIN " there was RICHIE ZUL against the point champion and GM sponsor on the line final round for PRO-STOCK champ .we beat him by a half a fender and the place went wild like a modern day ROCKY .cannot tell you all but look at the one off air intake on the hood!!!!! yes people 2+ 2 does not equal 4. yes we did it again in both pro stock and modified in APBA offshore boat racing (ARMED AND DANGEROUS ) never lost a race AIR INTAKES had something to do with it .turbulant thru a OVAL

DanielM · Post by **DanielM** » Dec 19th, '25, 15:40

Yannis,

I understood, you weren’t questioning Bob’s methodology, just exploring the benefit.

Bob,

I’ve worked on old cars for years, never was a ‘go fast’ guy, but yes you do remind me of my racing buddies. Even though I don’t have a dog in the race, these threads are always interesting.

Carl,

LOL, the laminar/turbulent flow comment was a BS ‘red herring’ comment since Tony mentioned the 20 pages. But you nailed it. Sorry for the side track on the thread. Now....about those rocker stoppers…

Ironworker · Post by **Ironworker** » Dec 19th, '25, 15:45

For increased airflow to make a difference one the engine has to be starved of air. As I mentioned in an earlier post Cummins uses the same blocks for various HP engines and just changes the ECM. Here is what ChatGPT had to say when I queried whether QSB 5.9 engines run ROP or LOP.....

Turbocharged Cummins QSB 5.9 marine diesel engines do not operate in a “lean-of-peak” or “rich-of-peak” regime in the way gasoline (spark-ignition) engines do. Those terms are specific to spark-ignition engines referenced to peak exhaust gas temperature (EGT) at a chemically near-stoichiometric air-fuel ratio.

That said, the most accurate way to characterize QSB 5.9 operation is:

They run extremely lean overall, at all normal operating conditions.

Below is the correct technical framing.

Why “Lean/Rich of Peak” Does Not Apply to Diesels
1. No Throttle / No Stoichiometric Target

Diesel engines do not regulate power with air; they regulate power with fuel quantity.

Airflow is essentially unrestricted and always in excess.

There is no stoichiometric operating point used for control.

2. Compression-Ignition Combustion

Fuel is injected into hot, compressed air.

Combustion occurs locally where fuel droplets mix with air.

The cylinder contains zones of varying mixture strength, not a uniform mixture.

3. EGT Behavior Is Load-Driven, Not Mixture-Driven

In a diesel, EGT increases with:

Fuel quantity

Engine load

Injection timing

Boost pressure and intercooling effectiveness

There is no “peak EGT then richer = cooler” behavior as in gasoline engines.

How Lean Is a Cummins QSB 5.9?

Typical air-fuel ratios (AFR) for a turbocharged marine diesel like the QSB 5.9:

Operating Condition Approximate AFR
Idle 80–100:1
Cruise 40–60:1
Full Rated Load 18–25:1

For reference:

Gasoline stoichiometric AFR ≈ 14.7:1

Anything above that is “lean” in gasoline terms

Even at full load, the QSB 5.9 is still lean compared to stoichiometric combustion.

What Limits Fueling on a QSB 5.9?

Fueling is constrained by:

Maximum allowable EGT

Cylinder pressure limits

Smoke (particulate) limits

Turbocharger airflow capability

ECM fuel maps and marine rating

When you add fuel beyond available oxygen:

Soot increases

EGT rises rapidly

Power gains diminish

Engine durability is compromised

This is why a diesel is often described as being “air-limited”, not fuel-limited.

Practical Summary

Lean-of-peak / rich-of-peak terminology does not apply to diesel engines

Cummins QSB 5.9 engines:

Always operate with excess air I bolded this statement

Are globally lean at all loads

Increase power by adding fuel, not air restriction

Engine health and tuning are managed by:

EGT

Boost pressure

Fuel rate

Smoke output

If you are asking this question in the context of prop selection, continuous load limits, EGT monitoring, or repower tuning, I can address those specifics directly for the QSB 5.9 marine platform. End ChatGPT response

What page are we on?

Yannis · Post by **Yannis** » Dec 19th, '25, 16:52

Στοιχιομετρικό, for stoichiometric! What is this? I haven't got my PhD yet!! LOL

Rocker stopper alright!!

bob lico · Post by **bob lico** » Dec 19th, '25, 17:39

funny thing in life and that is people will not change their mind no matter what you show them or explain. my side hustle was to test 1000 boats of a twenty year period ,some were new boats and i would drive while the salesperson would do their thing all boats coming for repair had to be tested and after repair due to a tremendous law suit a person died in a accident on a trial outing MUST be a license captain to take a client for test run. anyways i just had free dockage and storage and use of all facilities. and earlier incident i witness convince me if your going to reinvent the wheel most likely it has been tried before .case in point normally the owner of a APBA race boat has a open check book with that we took the point leader in "S" class and align all strakes perfect within 1/32" . no different at 100 mph so next step we block sand the bottom down to 1000 grit wet sand THE BOAT WENT TWO TO THREE MPH SLOWER !!!!! ouch, Just so happen a expert on hydrodynamics came in with a hundred mathematical equations to explain so we took it back to stock gel coat specifications . learn a big lesson in life. rocker stopper put the damn batteries in the CENTER of the boat and balance the boat SO THE LAST FOUR FEET OF THE CHINES ARE IN THE WATER AT ALL TIMES ------dry boat and no rock

Ironworker · Post by **Ironworker** » Dec 19th, '25, 19:22

Here is the short version of what I posted.

Modern diesel engines already operate LEAN. Which means fuel is the controlling factor. The only way during the combustion process to increase power is adding more fuel. If they operated RICH then adding extra air/O2 would increase power. Since these engines are already operating LEAN then only adding more fuel will increase HP. Yes there are situations where a diesel engine can be inefficient in the combustion process but that means they are broken or not working as efficient as possible.

Now if we are talking about gas engines then we can easily get to a couple of hundred pages. BTW, I only fly Lean of Peak.

Tony Meola · Post by **Tony Meola** » Dec 19th, '25, 23:33

Rick

I understand what Chat GPT is saying and I understand what Bob says.

I think the additional question is, if the engine is drawing air at 100 degrees vs 150 degrees (just estimating here) what effect does it have. I think Bob's point is cooler air, not more air.

What I am thinking is that you can also get cooler air by introducing more air if drawn from outside the engine box into the engine box thereby reducing the engine box temperature from 150 degrees to 100 degrees without adding special extensions onto the Turbo.

Plus the real question is, what is the real temperature of the air hitting Bob's Turbo. Tough to tell, unless a gauge is inserted at the entrance to the turbo in the extension tube. Then you also need a temperature gauge in the engine box so you can judge the difference.

Yannis · Post by **Yannis** » Dec 20th, '25, 00:42

Correct, Tony, but this is only half of the story, in other words, by how much is this air cooler than the box's air. I assume, as you're saying, that a thermometer in the turbo's entrance will give us some indication.

The other half of the story is to find out what the effect of this cooler air is. Does it improve something and by how much? So far we do not seem to provide an answer here...