Re: Engine gurus!!!!!!!!!!
 

Thats helps alot.big difference between 26 bravos and 29 hydros.Bravos are lite on pitch where hydros are true.There is probably a 400 rpm difference between those props.What gear are the drives 1.36 or 1.50.I wish my boat went 80 at 4300.

Re: Engine gurus!!!!!!!!!!
 

Drives are 1.5 I believe

Re: Engine gurus!!!!!!!!!!
 

I built a 548 inch motor and used the heads, carb and intake setup from a previous 502 that had been dialed in very well. The first thing I noticed was that the 548 wanted way more fuel than the 502. I had to go up several jet sizes on the primaries and not as much on the secondaries. Also needed larger fuel pump and lines.
Ignition timing seemed best at 32 or 33 max whereas the 502 would like 34 to 36.

Re: Engine gurus!!!!!!!!!!
 

This is EASY,i'm surprised Mcollistn didn't figure this out for you! Punching your numbers into a prop calculator,THERE IS NO WAY YOU HAVE 1.50 ratio drives!! At 4300 rpm's and 0 slip with a 1.50 drive and your 27 pitch prop,your boat could only reach 73 MPH!! So,you have different gearing than what you think and 99% chance you are OVERGEARED!!! Your 27 pitch and your 29 lab are going to rpm pretty close,if you have 1.36 drives your 27 pitch prop (adding 10.2 percent for difference) makes your 27 prop pull like a 29.5,your 29 lab pull like a 31.95 pitch! Your boat is most likely underpropped, sounds like you put it together with a few unknown variables and your drives were one of them,talk to the prop experts after figuring out what your drives are,you'll probably need something in the 24-25 range to get your rpm's up,then your motor will be closer to peak power,itt'l rpm more and rev even higher giving you your target speed,Smitty

Re: Engine gurus!!!!!!!!!!
 

Punching the #'s,with a true 24 pitch ,at the same 9% slip your 29 calculates at,your boat would go 85 mph at 5400 rpm's,88 w/25 pitch at 5400. I doubt your slip will be that low once you get the rpm's up but backwards calculating your 29 and 80 mph asuming a 1.32 gear that is what i get,Smitty

Re: Engine gurus!!!!!!!!!!
 

Ratio doesn't matter - he's still got an issue.
I agree that he should be propping for 5000+ but he runs the same rpm with both 29 and 27 Hydros. That's a significant tip off.

J, if your WOT fuel pressure (taken from the line feeding the carb, after all filters, pumps, and separators) is 7 psi, then you do not have a restriction. If you did, then you couldn't maintain pressure to the carb.

PLUGS. Get somebody to read the plugs. I want to know what they look like after a hard pull and quick shutdown.

Re: Engine gurus!!!!!!!!!!
 

mcollistn,my idea is he only has 3 mph difference between the 2 props at same rpm,it should be more like 5-6 mph unless there ratio is a little closer than 2 true pitches,his boat would run high 90's w/a 29 pitch at 5000 and he probably doesn't have that much motor,so is it dying off at 4300 or is it going about as fast at that rpm/prop and gearing as it is going to go at the amount of power he has at 4300? He DOESN"T have 1.50 ratio drives (he would be at 73 mph at 0 slip with his 27 pitch at 4300) so if his motors make 625 hp at 5200 but make 500 at 4300 i would think he is trying to pull too much gear and prop for the speed of his hull. I'm not convinced he has anything wrong with his motors.J,if you trim it to the max do your rp'ms hang at 4300 or doe's it rev higher?Let us know what you find,Smitty

Re: Engine gurus!!!!!!!!!!
 

I can get more rpms if I really over trim it. It just wont pull more rpms under load. Im out of town until Thurs., but Im going to take it out out Thurs and have someone look at the plugs. I also found some 25p hydros that I can borrow. Thanks for the suggestions.

Re: Engine gurus!!!!!!!!!!
 

I certainly agree that he is propped/geared way too steep to make appropriate assessments regarding his performance at this point.

BUT

Until he goes down the (pretty straightforward) checklist, there's no good way to make recommendations.

1) Float level(s) set correctly.
2) Properly opening throttle linkage.
3) Check total ignition timing.
4) Lock out knock sensor.
5) Rule out rocker arm misadjustment or collapsed liter or broken springs.
6) Verify cam setting (in degrees).
7) Compression check.
8) Fuel pressure at cruise (taken right before the carb) between 6 and 8 psi.
9) Do a plug chop at mid cruise speed to check primary jetting and check for all cylinders pulling their load.
10) Fuel pressure at WOT (taken right before the carb) between 6 and 8 psi.
11) Do a WOT plug chop for WOT burn conditions.

If the FP is 7 at WOT, and ALL of the plugs look good (some light deposits on the face of the barrel and a light chocolate insulator), then drop pitch for 5100-5200 and recheck the above.

(camshafts generally like to stay within plus 4 to minus 4 from symmetrical on a BBC. Your cam comes ground on plus 5, and yours is installed at plus 7. I still think it may be an issue.)

Re: Engine gurus!!!!!!!!!!
 

Is there a way to figure out how much hp he needs to spin 29 hydros with a 1.36 gear at 5400 rpm,My guess is 750 a motor to spin 29,s at 5400 rpm.If the boat went 80 with the 26,s my guess is a 1.36 gear.