Rebuilt Engine Help Needed
 

In the spring, I had my HP500 carb motors rebuilt. Had the heads ported and polished, carb modified to flow 980, cometic head gaskets, new rings bearings etc, and cam with the following:

Intake 620 lift, 238 duration @.050 lift 1.70
Exhaust 629 lift, 248 duration @.050lift 1.70

I now take longer getting on plane with the same prop 31 P5s Hydromotive I used last year and can only spin it 100 more rpms with no speed difference. I can't even get the boat on plane with the 32P5s I could run last year, but did not like the speed at cruise range, it was right in the middle of the porpoise speed range on a 36 Spectre.

Is my problem the wrong cam, or wrong prop? I tested a few Bravo1 4 blades, 30" at 5600 rpm ran 102(same as the 31 P5) and a 34" which was not user friendly and could not run top end due to conditions.

The engine builder said to prop it to turn 5600, but that did not work with the Bravo1s, there must be too much slip.

Any help is appreciated. The engines dynoed at 580 @5600 and 610 at 6,000.

thanks
Shawn

Re: Rebuilt Engine Help Needed
 

to much cam and to much prop. either change the cam to run in the sweet spot at 5500 or change the props to buzz the motor harder to make the power seeya charlie if you want call me on my cell 513-477-2780 and we can discuss the problem

Re: Rebuilt Engine Help Needed
 

Yup, cam to big.

Re: Rebuilt Engine Help Needed
 

CR is 9.25 and heads were stock HP500s

Re: Rebuilt Engine Help Needed
 

Give us a few more particulars on the cam, who ground the cam, LSA, ICL installed degrees.
Are you running the stock Dart intake? What exhaust are you running?
I do feel that you have way to much carb! Between the cam that may be a little on the large side and carb that is too big, you are cutting down on the intake velocity! That will kill your torque which will not allow the motor to turn the larger prop! I wouldn't think anything larger than an 850 & preferably an 800cfm. What were the torque #s when you dynoed the motor?
What are you running for ignition?
That cam may be a little on the large side but you could have seen better results!

Re: Rebuilt Engine Help Needed
 

The cam is a custom grind from the engine builder. Lobe seperation is .110 on the intake side and .119 on the exhaust. Lobe lift is .373 on the intake and .370 on the exhaust. Degree #1 intake lobe is .110 ATDC.
I reused the Dart intake manifold.

RPM HP TQ
4500 460 535
5000 528 554
5200 552 558
5300 568 562
5400 569 552
5500 575 550
5600 581 545
5700 588 541
5800 586 531

Primary jets 79 secondary 88. Running Merc TB4 with V6 set up at 34 degrees total. Exhaust is stock Gil/Merc.

Re: Rebuilt Engine Help Needed
 

Have you read your plugs?? I would guess you are running lean with those jets.

Re: Rebuilt Engine Help Needed
 

Plugs are actually black as if its running too rich. Dyno was done at 70 degrees and 70 percent humidity. A far cry from running the Chesapeake in the summer, where its closer to 85 and 80

Re: Rebuilt Engine Help Needed
 

Something isn't right!! When a carb is too big, you usually have to compensate by richening up the mixture because with the decrease in velocity thru the carb you are getting less of a signal for fuel, hence you need bigger jets.
When you do the math, a 502 running at 100% efficiency (I doubt your motor is much better than 90%) 502x5600rpm / 2 / 1728 = 813cfm. At 90% efficency that would be down to 732cfm. Thats all your cylinders can handle at that rpm! Unless you were planning on running higher revs, your carb flows too much!
Yes sometimes a very efficient race motor can exceed 100%, it's unlikely that your motor does this.
I would re-visit the carb setup! If you have access to a couple of 800 cfm (#9022) Holly like yours was, just bolt them on and try it.

Re: Rebuilt Engine Help Needed
 

You might consider advancing the cam 4 degrees and possibly looking at 1.5 BBC rocker arms. It seems like alot of cam for 9.25 to 1 comp. You might also try a 4 hole spacer under the carb if you can get away with it space wise.