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52mph WOT! What gives 25' Others get 70+

Old 11-25-2012, 08:56 AM
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Originally Posted by project tt
8 atdc?
You don't want to do that.


try 8-10 Btdc
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Old 11-25-2012, 09:04 AM
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Sounds like you need a 3 blade prop - BUT the prop is not the problem.
You are not producing the HP you think you are.
With 500 HP you should easily turn that prop to over 5000 RPM.
Either you have the wrong cam for a boat engine or something is really wrong inside the engine.
With 500 HP you should be running at least a 25 mirage + to 5100 RPM.
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Old 11-25-2012, 09:17 AM
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Originally Posted by project tt
The guy has been building big blocks for 30 years, he builds race car motors mostly. He said mine has roughly 515 to 530hp. It could be the timing, what should it be set at? I have no clue. 8 atdc?
Race car guys are great at car motors. There are a lot that can not build a good boat motor.

Get us the motor specs on the motor. There are a lot on here that will be able to tell if the specs add up to a good boat motor.

My guess is wrong cam selection.
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Old 11-25-2012, 10:07 AM
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When you upgraded your engine did you upgrade your exhaust.
What do you have for exhaust???
With 500 HP you need aftermarket through transom exhaust.
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Old 11-25-2012, 10:47 AM
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Originally Posted by Tinkerer
When you upgraded your engine did you upgrade your exhaust.
What do you have for exhaust???
With 500 HP you need aftermarket through transom exhaust.
good thinking it might be 515 with car headers but not with crap boat manifolds that are just an open chamber. my guess is that its basically a stock 454 with a cam/ intake manifold
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Old 11-25-2012, 11:58 AM
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The timing should be set around 34* at full advance. Using base timing to set it without knowing the amount of advance in the ignition system will get you no where.
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Old 11-25-2012, 12:07 PM
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Originally Posted by project tt
The guy has been building big blocks for 30 years, he builds race car motors mostly. He said mine has roughly 515 to 530hp. It could be the timing, what should it be set at? I have no clue. 8 atdc?
Do you have anything resembling a build sheet?

Was it dynod? (pretty sure I know the answer to this)

Who tuned it?

If you cross connected the spark plug wires you wont be able to pull any RPM.

The guys so far are all correct-car engines typically get great exhausts and run way more duration and valve overlap than a boat engine will.

Uncle Dave
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Old 11-25-2012, 12:09 PM
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A standard merc ingnition rev limter is 4350 , put in a msd dist and ingntion and see what happens
if you have the hp you say you are on the limiter and thats why you are stuck at 4400 and you should then go with a bigger prop after you fix rpm problem . seen this problem before on a friends build , gave him my extra crane box ( fire wire merc ) and it took care of the problem , You can but a dist from msd wiith a rev limiter in it have fun
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Old 11-25-2012, 05:34 PM
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Did the boat pull more than 4400rpm before the new motor? If so then rev-limiter is not the problem.

If that's not the problem that much HP should pull a 23-24 Bravo past 5000rpm. If you have stock exhaust than the HP number is down under 500 more likely 450-475 in the boat.

My mild 468 on the dyno pulled 407hp but is probably around 365hp in the boat since I still got stock exhaust. My boat is 24ft and I can pull a 22 Bravo to 4900rpm and a 26 Bravo to 4300rpm. I'm still looking for a 24 Bravo to try out.
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Old 11-25-2012, 06:13 PM
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Originally Posted by fastscarab22
A standard merc ingnition rev limter is 4350 , put in a msd dist and ingntion and see what happens
if you have the hp you say you are on the limiter and thats why you are stuck at 4400 and you should then go with a bigger prop after you fix rpm problem . seen this problem before on a friends build , gave him my extra crane box ( fire wire merc ) and it took care of the problem , You can but a dist from msd wiith a rev limiter in it have fun
No merc BBC engines have a 4350 rev limiter. The lowest one is 4750 rpms.
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