Modern diesel engine air intake requirements

bob lico · Post by **bob lico** » Dec 20th, '25, 09:35

the temperature of the air going into the turbo is the same as ambient air in the shade BUT accelerated . a typical 70 degree day the temperature of the air going into the turbo is 71 degrees (sensor) engine box temperature about 95degrees and above.phoenix can maintain 34 knots at wot ,315 hp mechanical cummins 6BTA 2006 ,20 years without a break down !!!! (changed raw water pumps to tony Athens specials for impeller longevity. i am sure this falls on deaf ears for some but is reality.best of all 1.8 miles per gallon exactly double the fuel mileage of twin 300 hp yamahas on a 31 Bertram at . manufactures specification 4650 rpm Northeast Yamaha certifications back by Regulator,everglades,

Yannis · Post by **Yannis** » Dec 20th, '25, 10:15

Bob, gallons per mile or miles per gallon? Nautical miles?

Ironworker · Post by **Ironworker** » Dec 20th, '25, 11:01

Tony Meola wrote: ↑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.

Tony,

A couple of points. Getting more air either denser or not dense is moot when you already have more than enough for the full combustion process. Cooler air does help lower the overall temp of the engine which can be a good thing to a point. There are a couple of reasons Diesel engines are not used in airplanes. The primary reason is that its too cold and diesels don't like the cold. However, the reason diesel engines have fuel coolers is that cool fuel is more dense than hot fuel. Yes you can get more power from denser fuel but only marginally which comes back to my point of why would you need it.

Now if the air is too hot at the turbo intake then the exhaust gas will be higher which again the overall heat of the engine is detrimental to lubrication. Once the oil breaks down then the engine wears quicker. Also the way to pump more air is to increase the RPMS. The pistons aka pumps are pumping more air.

Now in a Modern electronic diesel (like I have) the ECM is constantly monitoring all aspects of the combustion process and taking a generally safe operating position. The ECM is making those decisions and I have little control. I can inject more fuel or less fuel otherwise the ECM is in charge of engine operations.

In my airplane, I have an engine monitor that gives me the information at the turbo intake and EGT. I also have an intercooler which allows me to a safer margin of operation i.e. reduced pre-ignition and detonation. I can decide and manipulate the piston engine operation parameters. Some airplane piston engines have FADEC which is basically the same as the ECM in the diesel. The ECM makes the decisions and operates engine is generally a safe margin. I prefer mine because I think I'm smarter than the computer in FADEC

Now the question is can you manipulate the air or fuel in a diesel engine to get a more efficient engine and the answer is maybe yes maybe no.

There are other ways to improve Performance in a boat that is much more effective than pushing more air into the turbo.

bob lico · Post by **bob lico** » Dec 20th, '25, 13:05

the next discussion will be cooling the fuel ,i just want to emphasized it is not just one improvement that makes a performance/fuel economy change in our Bertrams some are obvious and some are not .some adjustments are made for the sole purpose of running in rough seas by keeping attention to the "attack" angle of the hull . you want to maintain bow proud at 23 knots in small craft warnings and most important prevent stuffing the bow into a oncoming wave.after our discussions in engine performance we will get into keeping the boat dry in heavy seas

Yannis · Post by **Yannis** » Dec 20th, '25, 14:33

So, Rick, would you install a direct air feed to the turbo with the aim to lower inhaled air temp, or not?
Mind you, when I say cooler air it's not any polar ambient temperatures in the Med in the summer, I'd say anywhere around 35C give or take a few degrees (35C=95F).

Ironworker · Post by **Ironworker** » Dec 20th, '25, 16:13

Yannis wrote: ↑Dec 20th, '25, 14:33 So, Rick, would you install a direct air feed to the turbo with the aim to lower inhaled air temp, or not?
Mind you, when I say cooler air it's not any polar ambient temperatures in the Med in the summer, I'd say anywhere around 35C give or take a few degrees (35C=95F).

Yannis,

I would not. The juice is not worth the squeeze.

Tony Meola · Post by **Tony Meola** » Dec 20th, '25, 21:33

Ok so now we can take this down a whole new track. I remember discussion on adding water injection into gas engines. This was a big thing during WWII in some of the fighter planes. They used to increase horsepower and therefore speed.

So I went Rick's route and asked about water injection in a diesel engine and AI provided with the following.

"Water injection significantly increases horsepower and torque in diesel engines, especially turbo charged engines.

It cools the intake air for denser oxygen, reduces destructive Exhaust Gas and Temperatures (EGT) and allows a more aggressive Tuning/boost. The mixture used is water and methanol which leads to gains of 50 to 100 HP in the mid RPM range.

Who would have thought.

But again I still wonder how much it reduces the temperature of the air. That is the key question.

Yannis · Post by **Yannis** » Dec 20th, '25, 22:14

Tony,

I'd hate to imagine cruising in very bad weather and the water/methanol nozzle going crazy...

Ironworker · Post by **Ironworker** » Dec 21st, '25, 10:24

Tony,

I'm very familiar with the water injection process used during WW2. They were trying to push MAX HP out of these engines but they were tearing themselves apart to to Pre-ignition and sometimes detonation (engine knock). The military doesn't care how much it costs to kill people and break things as they are immune to cost and efficiency for the most part. Those engines were running very close to EGT peak which is max HP output but also the absolutely the worst place to run an engine for durability. Water slows the flame front as does lead in aviation fuel. We still don't know exactly how lead additive does this but it works and is why we still have leaded aviation fuel much to the chagrin of the tree huggers.

Aviation piston engines are air cooled and each cylinder is it's on little science experiment.

HOWEVER....eventually computers were invented and then electronic sensors, then sensors and computers were applied to engines, then eventually multi-probed engine monitors were invented. With the benefit of actually seeing what is happening in each cylinder the mission was to get every cylinder operating as closely together as physically feasible, i.e. controlling precisely spark, fuel and air in gasoline engines. With diesels its just fuel and air, but the effect is the same. Dump in as much air as possible and machine fuel injectors to as close as tolerances as possible. This allows for modern engine management.

Now the question for us on this forum is what is the outcome we are looking for from our engines? Is it MAX HP (racing), best fuel economy or perhaps engine longevity. I'm not racing so I'm not running my engines at WOT. I do care about best fuel economy and if the engines will last for 15 years then I'm good to go.

The science of engines and the combustion process is very interesting.

Ironworker · Post by **Ironworker** » Dec 21st, '25, 10:30

Tony Meola wrote: ↑Dec 20th, '25, 21:33 Ok so now we can take this down a whole new track. I remember discussion on adding water injection into gas engines. This was a big thing during WWII in some of the fighter planes. They used to increase horsepower and therefore speed.

So I went Rick's route and asked about water injection in a diesel engine and AI provided with the following.

"Water injection significantly increases horsepower and torque in diesel engines, especially turbo charged engines.

It cools the intake air for denser oxygen, reduces destructive Exhaust Gas and Temperatures (EGT) and allows a more aggressive Tuning/boost. The mixture used is water and methanol which leads to gains of 50 to 100 HP in the mid RPM range.

Who would have thought.

But again I still wonder how much it reduces the temperature of the air. That is the key question.

BTW, Did you ask AI what is the lifespan of these particular engines. My guess is they are measured in tens and hundreds of hours and not thousands of hours.

Tony Meola · Post by **Tony Meola** » Dec 21st, '25, 22:01

Rick

Didn't get lifespan of the engines.

What I found interesting in the response was the fact that if cooled the air temperature coming into the turbo and the denser concentration of oxygen was increasing horsepower. We are back to how much cooler is the air going into the turbo sucking the air through an extension on the turbo running to cooler air vs just giving the engines the opportunity to suck cooler air into the engine room cooling the air down.

Given that 90% of our boats use is during the hot humid summer months, and that between friction running through the extension and engine room temperature boosting the temperature in the extension, what is the real temp. difference at the turbo with and without the extension.

Without knowing that I am not sure how anyone can say that by adding the extension you are pushing up the HP.

Like you said, the juice just isn't worth the squeeze.

I agree with you.

They are 31 foot boats, running in an open ocean. I don't use all the HP I currently have. How many days can I really run in the ocean at 24 knots.

Plus, burning 10 gallons less fuel in a given summer does not make enough difference for me to go crazy. Maybe 10 less gallons a trip would but not in a summer.

Yannis · Post by **Yannis** » Dec 21st, '25, 23:31

Tony,

Don't break your head, the calculation of the temperature difference is easy to find. The ambient temperature is what it is, usually anywhere between 30C and 40C, and for the box temperature you can place a thermometer. If you also insulate the air tube that goes to the turbo, then by subtraction you have your figure.

The point which has not been clarified yet, is WHAT effect this temp difference has on engine performance. To simplify things, one can only bother to pin this performance difference at cruising speed - there is no real need to know performance diff for every RPM.
So, where can we find a table that says: At 3000rpm with ambient 35C, the horsepower is X. Same rpm with box temp of, say, 65C the horsepower is Y.
Where is this table to be found?

Ironworker · Post by **Ironworker** » Dec 22nd, '25, 10:23

Tony Meola wrote: ↑Dec 21st, '25, 22:01 Rick

Didn't get lifespan of the engines.

What I found interesting in the response was the fact that if cooled the air temperature coming into the turbo and the denser concentration of oxygen was increasing horsepower. We are back to how much cooler is the air going into the turbo sucking the air through an extension on the turbo running to cooler air vs just giving the engines the opportunity to suck cooler air into the engine room cooling the air down.

I could be wrong but I believe this AI statement is misleading. The denser O2 and methanol/water is not actually increasing HP over anything other than a rounding error. What it is doing is allowing the engine to push more fuel at higher compression running closer to Peak which allows for higher HP. Keep in mind this requires very high ICPs (Internal Cylinder Pressure) and CHTs (Cylinder Heat Temps). If the flame front slows due to design of the piston and timing of the stroke then ICPs are reduced. The two main killers of engines is Temperature and ICPs.

What cooler air (This is a relative term) does is slightly lower the CHT's so that you can push more fuel and still maintain the detonation and Pre-ignition margins. Bad things happen with pre-ignition, for example a runaway diesel engine. Density of O2 requires much colder air not just a few degrees to have any marginal effect.

Yannis · Post by **Yannis** » Dec 22nd, '25, 15:01

I looked up the Yanmar manual, it says air intake temperature if of almost no concern.
I'm saying that even if there is, it is so un-noticeable that at least for the 4lha's one shouldn't attempt to squeeze for any juice...I tend to agree with Rick.
The only reason I would install a dedicated air duct is to relieve my pinky from hitting that damn clamshell !

Tony Meola · Post by **Tony Meola** » Dec 22nd, '25, 22:22

Rick

That's why I keep questioning the cooling difference. It can't be that great of a difference to make that big of a difference.

As you said it just isn't worth the work.

Carl · Post by **Carl** » Dec 23rd, '25, 08:01

I linked to information showing possible power gains due to temp. A 3 degC drop in temp is 1% O2 denser. A 10 deg C drop can yield 1.5% more power.

Dropping intake temp 10 deg pre-turbo does not mean its 10deg less in the cylinder.

Bob is also bringing in laminar air, less turbulent so could be flowing better.

Cool air does make a difference, if it did not why do they put inter-coolers/after-cooler /charge-coolers pick your term for the thing a ma jiggy that goes between the turbo and intake that cools the compressed hot air from the turbos. Biggest difference between my 210's and Tony s 250's he has an inter-cooler to cool that intake air for more power. Step up to the 370's and that inter-cooler goes from coolant cooled to seawater cooled for additional cooling, giving additional power. Point is cooling the air goes hand in hand for getting more power as more fuel can be pumped in. I also believe it can burn the fuel more efficiently, also maybe cause more boat gets out of the water as speed increases.

The water injection is sprayed into the intake piping at high pressure vaporizing and evaporating which cools the air like an intercooler. Methanol is a fuel but also cools...both are supposed to cool the EGT while providing more power.

To the original topic...cooler intake air is a plus, less turbulent air is a plus, I have little doubt about both. However is the work worth the couple percent increase in power and efficiency gain. By itself, probably not for many of us. For someone trying to squeak every drop of juice, one more drop is worth it.

mike ohlstein · Post by **mike ohlstein** » Dec 23rd, '25, 11:20

You're considering injecting salt water into your engines?

Tony Meola · Post by **Tony Meola** » Dec 23rd, '25, 21:52

mike ohlstein wrote: ↑Dec 23rd, '25, 11:20 You're considering injecting salt water into your engines?

LOL

You have to follow the whole thread. We are comparing our 31's to the old P51 Mustangs.

bob lico · Post by **bob lico** » Dec 24th, '25, 10:34

tony that "old P-51 " mustang is were we got the idea for the world champion pro - stock air intake contrary to all opinion it is the shape not the size of the air intake. the government gave me seven figures to give them all my ideas a patterns to protect their secret computer rooms .a true contranian this is the same that led to pro-stock,modified world champions.we beat the open class at Sarasota ,totally humiliating and they protested and paid to take apart the top of the engines not one picked up the obvious HOODWINKING. exact same argument we are having now. HOW can 2+ 2 equal 5 !!!!! just accept it .

Amberjack · Post by **Amberjack** » Dec 27th, '25, 15:36

Answered my question, thanks.

Happy New Year everyone!

Amberjack · Post by **Amberjack** » Mar 18th, '26, 18:28

This topic has been well discussed and I think we all learned something, at least I did. But in the process of unbuttoning Amberjack for spring I uncovered another data point which helps me to better understand air flow to my engines.

Amberjack lives in the water year round and is always in commission although I take steps to protect against freeze by covering the engine compartment ventilators and install a pair of heat rods under the engines.

I run the engines for 45-50 minutes every two-three weeks to keep them sweet. Last winter during a serious cold snap I had the bright idea to stuff rags in the side deck vents for further protection and ran the engines later that week. They smoked like the devil even at idle and I finally had to shut them down after neighbors walked by holding their noses. I had forgotten the vent plugs. Data point number one.

Normally they run during winter at idle with some minimal smoke but I have to pick my days (stiff south breeze to dissipate exhaust fumes) to run them up to 2000 rpm where I keep them for 30 minutes to really warm up and circulate fuel and fluids. At 2000 rpms they produce a smokey amber haze. Data point number two.

Two weeks ago I removed the ventilator covers and ran the engines up to 2000 rpms—no smoke! Data point number three.

What I think I learned: the heavy smoke at idle with the side deck vents plugged indicate there is very little air leakage from the cockpit. At idle the four 4” side deck ventilators provide just enough engine air. They do not provide adequate engine air to run at 2000 rpms. With the two Vetus 150 sq in ventilators uncovered there is enough air to run smoke free at 2000 rpms. The lack of soot on the transom when cruising indicates there is enough air up to 3500 rpms.

I know Captain Patrick maintained there was enough air leakage to supply diesel engines with only the original side deck ventilators. He may have been talking about earlier generation, lower horsepower diesels but I don’t think that holds true for modern higher hoesrpower engines. Also Amberjack’s cockpit deck has been replaced. The deck hatches are lipped and fit tightly. There is little meaningful air leakage through the cockpit and on my boat the opening under the side deck adjacent to the engine box has been closed. The engines live on the air supplied by the Vetus ventilators and I think it is just adequate.

Tony Meola · Post by **Tony Meola** » Mar 18th, '26, 20:57

Doug

That is an interesting observation. I still have the open bilge area and the old vents on the gunnels both midship and towards the stern, as well as the open storage area in the cockpit side panels.

Plenty of places to grab air.

I will be changing the side panels this spring, and will be replacing the opening with the hatches that open and close. That will take away some fresh air.

I may need to look into those vents you have.

Carl · Post by **Carl** » Mar 24th, '26, 12:27

Doug,
I think your right, your deck does restrict some air flow. I know mine with side panel open on top, deck floor in cockpit does not extend to hull sides, just resting on stringers. Stern panel has air access all around. Air can be pulled in from all over.

But something I just took notice of that is not present on other models...the Express has an additional panel past the flybridge. That panel sticks out 3 " or so goes back 2' or so. But whats interesting is the panel that extends outward has a big vent hole going into motor compartment just over the shelf. Its like an induction cowl. Kinda cool now that I just noticed its not on other models. I ran my upper station cables and hydraulic lines into the hole, snaking to bilge. More air...not that my 210's need it.

I do recall Captain Pat saying the stock air setup was enough...yet know he installed gunnel side vents on his builds, also selling them as a kit. I'm not sure where or if their was a recommended motor size when they came into play. More cool, clean, dry fresh air is a good thing I'd think.

AI-
Engine Size: Larger engines will need more air; a rough estimate is about 30-40 cubic feet of air per minute per 100 horsepower.

I'm guessing that running,,,that is a rather large volume of air.

Tony Meola · Post by **Tony Meola** » Mar 24th, '26, 21:33

Carl

IF you remember, back in the day, we thought going above 33o HP was pushing the limits of the hull as far as speed goes. If not balanced right they got a little squirrely.

We have learned, with the newer engines and some balancing of weight we can run the boat with larger engines pushing the hull speed limits higher.

I haven't been on a 31 past 30 knots, and I think the boat doesn't ride well until you hit 24 to 25 knots.

Jimmy G's boat could easily hit 35 knots and he said no issues. So the game has changed.

Carl · Post by **Carl** » Mar 25th, '26, 11:25

Back in the day if you cruised 20 MPH you were among the faster boats. Today a 20 knots is considered kinda slow.

How many boats from our harbor needed 7+ hours to steam 110 miles to the canyon. Kinda crazy to think about doing that before Loran...never mind GPS. Before my time though.
I just wonder who thought to go out that far...then again whale and fishing boats under sail must have seen and known long before.

Tony Meola · Post by **Tony Meola** » Mar 25th, '26, 21:04

Back ibn the day outboards couldn't do what they do today.

But 24 knots is moving over the water is a pretty good clip. Use to take us 4 hours to hit the Hudson out of Barnegat. That was with the old gas engines.

Haven't tried it in a while but I should have cut it down with the diesels by at least 45 minutes.

Carl · Post by **Carl** » Mar 26th, '26, 06:18

How far is Hudson from Barnegat? I had 60 miles in my head, but maybe that is from Manasquan. I had thoughts of taking my boat, but 162 gallon tank said no. I had thought about running south, staying over, fueling and leaving from there...but OPB's were easier and safer (Other Peoples Boats).

Tony Meola · Post by **Tony Meola** » Mar 26th, '26, 21:36

Add 30 to the 60. Itis about 90 miles due east of Barnegat. Believe it or not, it is closer out of Barnegat than our of Manasquan. The angle changes as you head North and South.

The Toms is closer than the Hudson. I use to like hitting the Toms when we went out because it was a shorter run, and when the SE winds kicked up in the afternoon you had it more on the stern for a better ride home.

Carl · Post by **Carl** » Mar 28th, '26, 12:35

At 90 miles I wasn’t making a canyon trip from there…only save me 20 miles…ok 40 getting back. 162 gallon tank was tough with range and I wasn’t willing to bring drums of fuel.

Maybe it was Monster Ledge I was thinking of….stay around Hoffmanns, fuel n go. That was years ago when fish were showing there.

Robert lico · Post by **Robert lico** » Mar 29th, '26, 20:17

yes Tony I did check air temperature at the turbo the air is one degree above the ambient temperature of the air on on the cockpit floor usually around 70 degree. who gives a damm about the engine box temperature I am not the least bit concern and if you do you completely missed the entire scope of my presentation.

Tony Meola · Post by **Tony Meola** » Mar 29th, '26, 22:01

Bob

Just out of curiosity, how do you check the air going into the turbo? That has to be pretty tough to do.

Carl · Post by **Carl** » Apr 1st, '26, 08:15

...going to go out on my limb again. Hate that, I am not fond of heights...

Any-hoo as I am out here I am thinking, I'm going to take a gander the temp was taken with a thermometer, perhaps one with a probe or infrared gun shot at the hose...although the later may be somewhat insulated.
A little hole in piping could fit the stem of a digital oven thermometer or similar. Just care needs to be taken not to allow anything near that could be ingested. That would be bad.
The hole n bung is how my exhaust temp pyrometer probes are fitting the the exhaust side of the turbo piping.

Tony Meola · Post by **Tony Meola** » Apr 1st, '26, 21:11

Carl

that would be the only way. Having some type of short tube coming off the Turbo in place of the filter. Inserting a probe as close as you can to the turbo opening.

You would also need to know the ambient temperature of the outside air. That would impact the air temp at the turbo. A 90 degree day has to have at least 90-degree air coming into the turbo.

Interesting test, but what if we did that without using Bob's design, and we used Dug's. In his case the air comes from the companion way. Check the ambient air temperature then the temp at the turbo.

It has to be at the turbo and not where the air enters the engine box. Believe it or not, depending on the length of the run from the filter to the turbo, the friction of the air running down the tube would also impact the air temperature.

It would be an great test to run. Would I do it just to check how the two systems differ. Probably not.

Carl · Post by **Carl** » Apr 2nd, '26, 07:30

Personally I'd like to see how the air moves within an engine compartment, within the boat. I'd like to see how patterns change as intakes and restriction come into play.

Yannis · Post by **Yannis** » Apr 3rd, '26, 01:28

Carl,

Blow one of those boy/girl firecrackers (without the confetti though) inside the engine bay and watch how it disperses while the engine runs !

On the other hand, I agree with Tony. Too complicated a test to perform, and for what purpose really? If there is no concrete course of action afterwards to influence/modify temperature, what does it serve to only know how much this temperature is?

If I understood correctly what Bob said, the best way to guarantee optimal temps to the turbo intake is to attach a hose/filter combo directly to the turbo intake and lead it outside of the engine room. Temps inside the engine bay become irrelevant...

Carl · Post by **Carl** » Apr 3rd, '26, 07:02

Yannis, that was my take on what Bob said.

I agree a colored smoke bomb would show how the air traveled, although tough to see with hatches closed. I guess a light and cameras would work. As to the reason why...just curiosity.

Pete252 · Post by **Pete252** » Apr 3rd, '26, 11:51

Decided to put my limited knowledge(Marine engineer, PE, etc) together and possibly clarify this question.
I left off the math, but here is what I came up with:

For a marine engine room the targets are:
Vent face velocity: 10–20 fps (7–13 mph) — low enough to avoid turbulence and rain ingestion
Total air volume: engine combustion air + cooling air for the engine exterior + general compartment ventilation (typically 3–5× combustion air volume for an enclosed engine room)
For 3208T (my boat, so why not)
COMBUSTION AIR - 310 cfm (basically 1 cu ft/hp, rule of thumb, not 100% on where it came from, but I have always recalled it like that)
COOLING AIR (3X Combustion) - 930 cfm
Total 1240 cfm
363 sq in/2.5 sq ft per engine at 11.2 mph/16.4 fps
For A 320 HP 6BT
COMBUSTION AIR - 320 cfm (basically 1 cu ft/hp)
COOLING AIR (3X Combustion) – 960 cfm
Total 1280 cfm
390 sq in/2.7 sq ft per engine at 8.9 mph/13.05 fps

So basically I need to make the openings on my boat bigger, possibly go to a remote air intake.

I hope this helps someone.

Pete

Carl · Post by **Carl** » Apr 4th, '26, 08:02

Pete,

Thanks for the information.

But math on the weekend and I didn’t have my coffee yet… ok I had some but not enough lol.

Engine needing a cfm per hp, got that, straight forward. Find power curve n map hp at rpm under load.

Engine cooling air has me scratching my head as to how that is figured out. 3x is an easy calculation but does it take into account circulation and speed of the air. Does it make a difference? I believe when engine coolant flows too fast in engine block the coolant does not have time to absorb heat loosing cooling efficiency. What about areas of the component that do not see air flow. Does it matter.
Last thought I wish not to think about and that is the vent flow. 20 fps from vent at 13mph is 1200 fpm of air flow, but now that has to get divided by vent size and take into consideration turbulence and boats not exactly running at 13 mph. lol

Lots of questions for inquiring minds. All in all, it seems our motors need a lot of air, cooler is better. Ballparking it, standard vents are barely adequate with higher hp motors especially if box n bilge are buttoned up well. Me, I’m fine lots of openings everywhere.

Thanks for info

Pete252 · Post by **Pete252** » Apr 5th, '26, 08:37

Carl,
Not implying that the "cooling air" has anything to do with the operational cooling of the engine, just air flow into the engine space.
while it may help with cooling the engine, and there does need to be some heat transfer out of the space, which is why 3x or 4x is used.
Important of you have your intake in the engine box, cooler air to the turbo, etc., as stated in the posts above.

The opening sizing/intake velocity has nothing to do with boat speed, (assumes an out of the wind type intake), unless you're adding a hood scoop.

The velocity of the air was chosen to limit rain ingestion and turbulence. No need for chip bags, wash down spray, and rags to be sucked into the engine compartments.

We could deep dive in CFD, air flow paths, laminar/turbulent flow, cooling effects of both types of flows, the differences in everyone's boats, but I was attempting a quick and dirty air supply for diesel engines, thought it would be somewhat helpful.

Agree, no math on the weekends unless related to my boat.

Happy Easter.
Pete

Carl · Post by **Carl** » Apr 6th, '26, 06:41

Pete, your right I had in my head forward facing clam-shells. Motor is pulling the air in...

Tony Meola · Post by **Tony Meola** » Apr 6th, '26, 21:49

Pete

3208 cats. That 31 sits low and rides a lot different than ours. Yours goes through the slop not over it. Always heard it was a great riding boat with the 3208's.

Carl

I interpreted all of Pete's numbers to say, at cruise take the engine box cover off.

I am not an engineer, but if you take Bob's extended intakes, I will say if they are 4 inches all the way to the turbo, you probably created a restriction actually reducing air flow to the Turbo. I would think you go with a something like a 6-inch duct to carry the air and then reduce it to 4 inches at the Turbo.

I am not sure how a 4-inch intake that is let's say 3 feet long, allows enough volume of air to reach the turbo vs it just being 4 inches at the turbo but more volume of air moving into the engine compartment through the openings.

Don't even know how you would verify what is happening in that instance.

My head hurts.

Carl · Post by **Carl** » Apr 7th, '26, 08:15

Tony- I think going from 6" piping to 4" before turbo would create turbulence restricting flow. In my head I have this thought rattling around that a straight run of piping creates better air flow as it aligns on its travel. Straight best flow, turns would benefit from large radius over tight and never have corners. I guess in short I'm thinking a straight 4" run to the turbo would flow better then open turbo or with filter.

Going the other way with exhaust piping, I think we go larger in diameter to account for the added water and steam created from dumps.

If I'm wrong...I wouldn't be shocked

Pete252 · Post by **Pete252** » Apr 7th, '26, 09:13

Tony,
Taking the engine boxes off, or even opening them at cruise would require ear plugs and headsets all around. The 3208's do add weight, and the boat rides more like a loaded dump truck. I can still touch 28-29 knots, clean bottom, medium fuel load, etc.

Concerning sizing ducts:
I have actually done the math on sizing the remote air intake, for my engines, I need to use 5" ducting(minimum), long radius elbows, but can get away with reinforced silicon hose for the connection at the turbo. Will likely use 6" because it is easier/more available.

As far as turbulence: you can add laminar flow back by stacking thin wall pipe inside the duct, similar to what they use at high end fountains and water "sculpture" where the water is a solid un-aerated stream. A lot of neat work getting that right, though considering losses and friction. Much easier to pick a slightly larger duct, where lower velocity reduces turbulent losses.

Back to rain, dirt, and mud.
Pete

Tony Meola · Post by **Tony Meola** » Apr 7th, '26, 21:56

Pete

My head still hurts.

I understand the concept. I think you are saying you need to upsize the intake run and can overcome the turbulence Carl talks about. But in reality, how much turbulence would there be in a 3 foot run.

Carl · Post by **Carl** » Apr 8th, '26, 06:36

Straight run, not much, add a few bends or a transition and that changes.

As per much of this conversation we are talking small percentages of performance difference...unless motors are being starved for air. Easy check there...when running, have someone open motor hatch.

Pete252 · Post by **Pete252** » Apr 8th, '26, 08:54

Tony,
Turbulent flow is a function of velocity.

Velocity, in this situation; is a function of mass flow, cubic feet per second.
Increase the cross section, reduce the velocity, therefore reduce turbulence.

We could dive into Froude numbers, friction losses, etc., but I am being considerate
for your headache...

Basically, as Carl said: as large as will fit, long sweeping turns.

I have been on "High End" boats where you cannot open the engine hatches at cruise due to the vacuum from the engine intakes.
Was worse when every boat had Detroits.

Pete

Tony Meola · Post by **Tony Meola** » Apr 8th, '26, 20:15

Pete

Believe it or not this is all very helpful and interesting.

But at the end of the day, unless you can build on all the small pieces of performance gained by doing this, the real question is if it is worth all the trouble.

I guess if I had one of the newer Cummins and was pushing more HP, then I would consider going through all the trouble.

I still think I will add more ventilation using Doug's venting in the companionway.

Robert lico · Post by **Robert lico** » Jun 29th, '26, 10:26

carl do you have a photo of my cummins engine showing the turbo air intakes ?

Carl · Post by **Carl** » Jun 29th, '26, 13:57

As per your request Bob, a picture of your intake.

Robert lico · Post by **Robert lico** » Jun 29th, '26, 18:44

thank you carl, just wanted put it out there