New B28 Owner - Gas Performance Question - Thanks

[MarkD]

I just put my new to me 1988 B28 in the water. She has the original 260 HP Mercruisers. They run great and I experimented with some cruise RPM's. 2800 gave me 15knots, 3000 was 18 and 3100 was around 20. My questions are: 1) What do you suggest I run her at? Is 3100 too high? 2) Are the secondary's kicking in and burning too much fuel at 3100 rpms? 3) What would you guys suggest for engine longevity? Thanks in advance for the help...... very much appreciated!

[Rawleigh]

I run my 330hp 440 Chryslers in my B31 at 2800-2900RPM and they will last forever (or until something other than wear gets them.) 21 knots. Have someone look down the carbs and run them just off the secondaries. What blocks are they?

[MarkD]

Thanks Rawleigh. They are 5.7 small blocks with about 1900 hours.

[Carl]

Rawleigh 21 knots at 2800-2900, what model do you have, what tranny/wheel setup do yo have and do you crank up to 4400 @ WOT ?

At 2900-2950 I am just seeing 20 MPH and am shy of the correct WOT rpm by about 200rpm.


Mark, I'd be comfy running the small blocks at 3100, but would stay off the secondaries unless you own a gas dock or have money to burn.

[Harv]

I run my 330hp 440 Chryslers in my B31 at 2800-2900RPM and they will last forever (or until something other than wear gets them.) 21 knots. Have someone look down the carbs and run them just off the secondaries. What blocks are they?

I had similar performance numbers with my 454 Mercs(330hp). They were from 1988. Always ran good and strong and if it weren't for ethanol, would still be working today.

[Leigh]

Mark: I have an '85 FBC with Merc 260's, 950 orig hours. My boat has a new,barrier-coated VC17 bottom (fresh water), no tower, bimini w/o enclosure. I am probably lighter than average. Here are my numbers with approx 1/2 fuel and water, 3-4 people, moderate seas( all per GPS):

2800 18.5 kts
2900 19.5
3000 20.5

I had my 17x17 props repitched to 17x16 to get the WOT up to 4200-4300 RPM, which is the spec. for the engines. I have not done any scientific fuel economy checks, but 2800-3000 RPM cruise seems to be at about 1 MPG. Also, I don't think the secondaries open up until about 3300. Enjoy your new boat. I really like mine. Leigh

[Rawleigh]

sim: With a clean bottom, not overloaded, yes. I think my props are 17 - 18 with cup. I can't swear to that though. Trans are 1.52 velvet drives. 1 3/8 shaft. I also can't remember if I am reaching all the way up to 4400 or not. I'm reinstalling my port engine today. I burned a hole in a piston trying to run too hard to a burning boat Memorial Day. I still had last years gas in the tank and that is the culprit. I knew better, but had to try! Over 500 hours on the engine and it still had the factory crosshatch in it. I like these Chryslers! The only other problem we ever had with them was the starboard engine tuliped some of the intake valves about 15 years ago. Again, I think it was old gas at the beginning of the season. I hear from my machinist the Chevy 350's are known for this. I removed and disassembled the engine myself, and I found it to be easy to work on and well engineered. Parts are not as easy to come by as chevy, but they are still available if you are resourceful. $2,100 for the rebuild: $1000 parts, $1,100 machining an reassembly. I had to go 0.0030 over as the cylinder was scuffed, had the heads worked with new valves, had the crank polished, all new bearings, timing chain and gears, etc. Of course I found lot of other things to do as well: repainted the engine compartment, re-terminated all of the wiring with new heat shrink connectors, etc. All expensive but worthwhile!

[scot]

V8's are over head valve automotive based engines. I have been running the 5.7 in my (currently for sale) boat @ 3200 for years. A friend of mine owns the best performance head shop in our area and he told me that 3500 should be the absolute max I should ever consider for a long term cruise.

IMHO you should be fine up to 3300, but as stated the lower you go....the longer it will last.

Do you have the "need for speed?"

[DaveB]

Mark,

I have a 1978 B28 with Chevy 305's - 17X16 props, Bimini full enclosure, Radar arch and way too much gear - I run 3100-3200 rpm at 20-21 kn @ 1 nm per gal. - Mine seems to run easier slightly over 3000 rpm - boat stays up on her strakes and engines sound smother. I've cruised at these rpm's for 5 seasons and the engines are running better than ever.

[Bruce]

Rawleigh,
Running "too" hard won't burn a hole in a piston. If you were told that, they're wrong.

You burn a hole in a piston from wrong static timing, bad advance in the distributor or the wrong spark plugs. This will cause either excessive heat or knocking both of which will kill a piston.

Very poor low octane gas will also contirbute to knocking.

Gas marine engines are built to run continuous. Most all the old carb engines were 4400 rpm wot. If they were proped corectly to achieve that then running all day at 4000 won't harm the engine.

Unfortunatly most gas owners don't have a clue as to how important timing, plugs and distributor advance are.

[Rawleigh]

Bruce: The gas in the tank is shot! I was attributing it to detonation caused by low octane, but as I hadn't tuned up the engines yet this year timing could have contributed to it too. The machinist said there was a little slack in the timing chain as well. Its all new now though!! Thanks for the advice though. I spent today aligning the engine. I think we got it within 3 thou, but I'll find out when I seatrial it. If I didn't get it right I will be back for more advice.

[dougl33]

Mark,

I had a 1981 28 with the 228hp 305 Mercs. I would cruise at 3150rpms, good for 21-22 knots. WOT was 4200rpms and top speed was 28-30 knots.

[Carl]

4000 all day long, hmm that would take a bit less then 5 hrs to go thru my 220 gallons of fuel.

Not that it really matters but I thought the raw water pump was only rated to about 3300rpm after that it does not adaquated supply the increased capacity needed to cool the engines at rpm. Then again I guess you would go with a higher volume pump, if you intended to run hard.


Bruce you would stay at the recommended 6deg BTDC for 440's and not vary for any reason. I pretty much keep mine there ( close as possible anyway ) but often hear guys recommending advancing till you hear pinging then back off. Sounds like advice from a back street car tuner but wonder from time to time.

[Bruce]

Never said 4000 rpm was fuel efficient.

A stock marine engine should stick with the recommended timing and also check the advance under load with a good light.

One thing that is over looked on marine engines is the dist. shaft bushings. This can vary timing at higher rpm or make timing irratic.

Your description of a timing procedure is a sure way to detonate an engine.

A competent engine builder or tuner will never do anything remotely like that.

You ever see anyone timing an engine without a light hit em with a 4x4 cause they ain't got a clue.

Never had a stock water pump fail to deliver adequate cooling at high rpm's with a CLEAN cooling system.

[Carl]

Thanks, I need to hear from a good sourse not to screw with the timing every once in awhile. I just hear so much crap info so often I get to thinking... and thinking ain't always good ( and don't mean isn't).

Check advance under load? Not familiar with that, I lube distributer as specified and do see timing advance fairly smooth as rpms increase but thats about all I know to do.

I don't think anyone says 4000 is fuel efficent, I watched carbs dump fuel in at that speed once and now when I crank her up to check WOT every now and then,,,I have flashbacks to that vision.

[Bruce]

Checking the advance under load takes two people.

Most dampers have the tdc and before and after degree marks.

If it doesn't have them, check flywheel if you have a removable housing cover. If not you will need to get a degree wheel and mark up to 40 degrees on the damper or flywheel, where ever you time to.

Hook your timing light up and run the boat out under load watching the light and the marks it hits at wot.
Compare the readings with the manufacturers wot timing advance numbers.

The other thing you look for is a steady light reading. Erratic or jumping around could be the sign of worn bushings, weak plug wires, mis firing plugs, mis firing cap or rotor.

[Carl]

Thanks for that information. Makes sence to check timing at speed.

I may have to look into the worn disrtibutor bushing, last time I remember light being on the erratic side, then again it was in the midst of my ethanol problem. I will check again now with new ignition parts.