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Discussion Starter · #1 ·
Went from a 454 built to 420 specs. to a stroker 496, callies crank, h-beam rods, je pistons, 9-1 cr; same heads but went with 2.25/1.88 ferra 11/32 stems valves, was 2.19/1.88; went with a custom flat tappet 4-7 swap in firing order .551in lift/.555ex; running a 850 sea demon; DUI ign, revolution marine exhaust no mufflers, and a rpm air-gap intake, I have seperate oil cooler and pwr steering cooler with -12 lines and a K&N 5001 filter.
The boat runs really well, it has a ton of low and mid range power when the prop is hooked up (the prop is the same as last year) When running a high trim I can cavitate the prop easily. Normal running and hooked up she pulls hard, real hard it will pull you back into the seat. On top end, I can only get up to 51-5200 she feels like it falls flat, any suggestions. I have no rev limiter, the only thing left stock is the block, starter and altenator.
 

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fuel system limitations? tank pick-up tubing and fittings (I just had to deal with this)? Fuel pump or filter? fuel filter too small / restrictive? I'm assuming that fuel mixture / plugs look good that you've got main jets dialed in (what do plugs look like after wot run?). Just some thoughts...
 

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N'Mocean motorsport said:

The boat runs really well, it has a ton of low and mid range power when the prop is hooked up (the prop is the same as last year) On top end, I can only get up to 51-5200 she feels like it falls flat, any suggestions.
1. Did you add all of the upgrades at once?
2. WHat were the rpm's before with the same props (last year?) Did you have the same carb/intake last year with the 454?
3. It sounds like you expected higher rpm - did you have higher before? Or are these expectations of the new build?

I agree the fuel supply could be suspect and you can get an inexpensive mechanical fuel gauge at Summit that could answer that suspicion real quick.

More likely I'd lean toward the prop given your comment about the cavitation. Good luck :vcol:
 

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Discussion Starter · #4 ·
Plugs look good, fuel psi good, lines from tank are 5/8 from pump -6; everything has been changed from last year except the intake I am running a 1" spacer between carb and intake. I would expect with all the changes I would be able to rev higher and I can't but I do have lots more power.
 

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It may be your prop. I went from a standard 330HP 7.4L Bravo setup to a 512HP 489 stroker. We put a borrowed 25P Mirage on the boat and ended up with about 16% slip and could pretty much use the throttle to cavitate at will. My HP peak was a bit higher RPM wise, so after consulting with Julie at Throttle-UP, we installed a 23P Mirage Plus and ended up with about 9-10% slip. The extra cup in the Mirage Plus made the difference.
 

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I was only getting 4900rpm at 68mph with 489 stroker in my nova 23.I was running a 25p mirage.The low end and midrange was awesome and I expected to go faster,but it just felt like it would go flat around 4900.Thought it was a fuel problem, but sold the boat before I got a chance to look into it.
 

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Discussion Starter · #10 ·
90nova said:
I was only getting 4900rpm at 68mph with 489 stroker in my nova 23.I was running a 25p mirage.The low end and midrange was awesome and I expected to go faster,but it just felt like it would go flat around 4900.Thought it was a fuel problem, but sold the boat before I got a chance to look into it.
That is the way mine acts so I switched carb from 750 eldelbrock to the sea demon made no difference
 

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I was running a 850 demon as well.Are you running a 25p prop also? You had some impressive numbers with your last 420hp set up.I had a 1990 and it was heavier than the newer spyders, I only got 61 out of my 420 combo.
 

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where would peak horsepower be for your engine with the current cam, etc...?


in a couple of cases I've been involved with lately (including my own repower), the tank screen check valve and / or some other portion of the fuel tubing provied to be the limiting factor at higher rpms in stepping up power. Assuming you're not running out of cam, do you have fuel pressure guage right next to the carb and that you can read at speed? just a thought...

if upstream of the fuel pump is too restrictive, once you get to a certain point in flow, the pressure coming into the fuel pump can get low enough to cavitate in the pump... and the fuel supply drops off a cliff at that point. It can feel almost like the engine hits a rev limiter... and on a carbed engine will be largely independant of the carb...
 

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Discussion Starter · #13 ·
I got 68-69 with the 420 set up it is the same with the 496 same prop, same drive, I have more power but not more speed. I did remove the check valve on the fuel system made no difference. You would think if you have if you had more power you would have more speed be able to spin the same prop faster. If I go down in pitch the boat would be even quicker and on top the engine rpm's would come up Hell, I don't know what to think.
 

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what rpm is the approximate / projected / designed max horsepower for your engine build?

if for example you're max hp is at like 4600... and fals off a cliff after that, then going to a higher pitch prop will net you a lot...

On the fuel pressure / flow, the problem with finding the issue is knowing where the bottleneck is. On my current little boat I think it was the pickup nozzle / screen and tubing that were in the tank. When I replaced everything from the nozzle to the fuel pump with larger, it was like a switch was turned on in the engine. That being said, my engine build should put peak horsepower at 5300-5500 rpm and I hit a wall around 4600 rpms with two different pitch props. The difference was dramatic in my case... same prop and everything else, different tubing, 700+ rpm increase

Best way to know if this is any kind of a potential problem is to be able to check fuel pressure at the point where your having a problem.

Just food for thought.
 

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Discussion Starter · #15 ·
I told them I didn't want to turn it over 5500rpm's, I know what you mean though. If I could run the boat side by side each other with the other engine the 496 will eat the 420 for lunch hands down but on top speed they are the same. The 496 has the 420 beat everywhere except on top that is where they are the same
 

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Discussion Starter · #17 ·
I am going to try a friends 25 mirage plus. if it ever quits raining. I do like the air idea my dad has said that all along I need to get more air into the engine. I will just keep testing and see what happens
 

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Discussion Starter · #18 ·
I found out some interesting information; you should not turn a 4.25 stroke motor over 5600rpm because the piston speed would be to great. Also, when you stroke an engine it moves the torque into a lower band and to compenstate and move the hp you can increase valve size and/or cam lift to move the hp around. When stroking a motor the torque will increase and be more flat at the same time, the hp could be less than a stock stroke lenght motor. This is how my motor is acting, so I think I can run a higher pitch prop which will lower my rpm somewhat and increase my top end speed.
 
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