Motor Question
 

I was running a 454 built to 420 specs boat ran 68 gps with a 24 pitch bravo 1
Now I have a stroker 496 with a larger custom cam 552in/555ex; 11/32 stems ported and flowed; 850 sea demon; DUI ign; rpm-air-gap with 1" spacer headers and no mufflers; the boat has a ton of more power low and midrange it is much much quicker but top speed is the same. Shouldn't I be able to turn the top end rpm's more, and turn a larger prop?

Yes, it should. Something is limiting RPM. What ignition is it specifically?

It is the DUI marine ign calibrated for my engine

That tells us nothing. Are these mercruiser 496's? When you say "stroker 496's" that infers 454's 1/4" stroked to 496. The Merc 486 is a different engine altogether.

If this is mercruiser stuff, you're probably hitting the rev limiter.

The motor is a 454 stroked to a 496 with an eagle balanced rotating assembly The rev limiter is set at 6500rpm I am not even close to it. It runs 24 degrees advance at 3000 rpm for a total advance of 36 degrees. The engine will not turn any higher on top then it did with the 420. Low and midrange is incredible. The only "mercruiser stuff" on the motor is the starter and alt. the block is chevy, everything else is aftermarket

I would think you'd want your total timing in by 2500 rpm. That has always been the general rule. So, you probably want to get your base timing set at around 10 - 12 and total between 32 - 36 by 2500 rpm. That could have an impact on your rpm. Did you dyno the engine? Do you know where your peak horsepower is?

No dyno all I told them I didn't want to spin the motor over 52-5300 it doesn't but I would think I would be able to over rev it some by using the same prop

The cam is almost the same as stock 420 cam. You could try retarding the cam timing to move the torque higher in the rpm range. I don't think the timing is affecting the performance

possibly the cam is in wrong. Does it have a multiple-position keyway?

What heads do you have on it? did you touch the heads? your original power bottle neck could have been the heads. Your cam is not much bigger than the original 420 cam, and adding ignition, carb and intake won't always get you all that much power. but the stroker kit will get you the increased torque you are seeing in the midrange.
JMO

What's your spring pressure? You could be right on the verge of valve float, or have some bad harmonics going on. This will kill your power pretty quick up top. Also probably not your problem but the air gap manifold doesn't always like a spacer. It helps balance the signal to the carb but I've sen it rob 10 or 15 horse from a bunch of different combos. Without dynoing your motor we're all just guessing at what kinda power it should be making. Some combos work better then others.. :grinser010:

Do a leakdown, compression test and put a dial indicator on the valves.

I don't know what kind of distributorless ignition this is you're using but I'd be very suspect. With the same cam & heads, you should have picked up at least 45-50 horsepower and 50-60 on torque. That should give you more, not less. 36 degrees is way too much lead unless you're running high octane and more compression. You may be preigniting and not noticing. If you're running 89 octane, you've probably found your problem. Unless your boat is WAY heavy, bring the max timing in fast- even 2000 if your boat is light.

But we're still doing alot of guessing- you'd get better advice if we had the whole picture. Like cam- it's more than lift- duration and overlap mean alot. Are the valve springs new and what pressures? What is this mystery ignition? What exhaust?

This is his ignition, it is just a marinized GM HEI. I just ran one on the dyno with my 469 and I still pulled power to 6500. I know quite a few people who use them.

http://www.performancedistributors.com/marinedui.htm

Still I ask what heads he is using? If they are GM rectangle port they will support the 496 stroker kit. but if they are peanut port, they won't support the extra HP. IMO

Also like Chris said, just because it has more lift doesn't mean it will be a more aggressive cam.

I took DUI as distributorless.

Where does the RPM limiter come in at? Is it something these people have desugned into the unit? Some sort of external limiter?

After being hosed by this so many times, I never assume that any item I have not seen and verified with my own hands and eyes is the correct item and it functions properly, actually is or does. I've seen way too much mis-labelled and semi-functioning stuff come out of pretty boxes.

The heads are large oval ports ported and flowed. they are running 2.25in and 1.88 exhaust 11/32 stems ferra valves everything is port matched. 118cc the cr 9-1 also the firing order is changed 4-7 swap. according to comp the cam is good from 2000-6200
the motor is internally balanced h-beam rods, callies crank; je pistons; crane gold race rockers, the springs are matched to the cam 10 degree retainers. intake ports have been polished. Full -12 oil lines running a K&N 5001 filter, indvidual oil and pwr steering coolers. All fasteners are ARP.

GM uses the 990 iron rectangle head on their entry-level HP engines- both the 454 and the 502, 425 and 450 hp, respectively. They're seeing less RPM and using a dual-plane manifold. Your heads flow somewhat less. On those 990's, you would probably see 75+ hp with no other change by going to a good aluminum. The ovals... probably alot more. I just don't think they'd support a 500+ horse 500 cid engine.

Without knowing your cam specs and what exhaust, that would be my guess.

are you running the same carb and jetting on the 496? Chances are you are too rich on the jetting in the new carb. I am not familiar with the demons but the jetting for a Holley 800 should be close to 80 pri 87 sec with blocked sec power valve and 6.5 in the primaries. 2 jet sizes can be worth roughly 400+ rpms and 6+ mph if they are off and you did not dyno the new combo. Set the timing at 32 max to start ,the longer stroke with the same chamber heads will need less timing.

Do you have a fuel pressure gauge?
If not, hook one up to verify that fuel pressure is at least 5psi at WOT.

My jets are .82pri; ,88sec; pri 6.5;

sounds like that came just not enough for the that size engine, they say good to 6200, in what cu in ?? a wild cam for a 427 turns into a mild one in a 496...Rob

Bingo! not only pressure but volume how big is the fuel line, foot valve stuck, filters clean,some thing stuck in fuel line?. I know from experience I go into panic mode sometimes and forget the basics, check fuel system before tearing into engine
my .02

well... we've had this discussion before. the drag profile of this boat is very high at 70 mph. you need a LOT more hp to go only a little bit faster and there are a dozen threads here that deal with those ACCURATE mathematics. two things strike me here...

1) there is nothing in the work thats been done to suggest a giant increase in power

2) the original combination that seemed to work well, has been comprehensively changed to some OTHER combination of unknown capability.

you say it runs great down low but won't pull up top... yet thats what you told them to do. if someone said that to me , the cam would go in well advanced and the i would be after big port velocity as opposed to big flow numbers... yet it seems that you went after the flow numbers as well...

i wonder if the combination is just wrong and while you have punched up the middle it falls over and runs out of torque too soon up top.

spend the money. go to the dyno and find out the truth before you piss away the summer and a bunch of money guessing.

I am carrying at the carb 6psi at WOT. The anit siphon valve is that done in the tank. my fuel line from the top of the tank to pump 5/8 from pump to carb -6

Listen to what stevesxm is telling you.

If the cam specs where posted I apologize for asking since I only saw lift figures.

What are the seat, duration at .050", LSA, or better yet Manufacturer and cam part# ?

I know Comp makes some 'snotty' lobes with mid .500's lift so I wonder why we think the cam he has is close to 420 Specs ? As far as I read, we don't know.

Again, if I missed the specs, I apologize for the questions/statements.

Check the float level on the carb. And yes I agree on the dyno first , in the boat second the best money you will ever spend Good luck Jeff :ernaehrung004:

Answer the exhaust and cam duration question.

Pt # 11-000-47; SN# N3994-08; @.050 230in; 236ex 112 lobe separatiion; duration @.006 tappet lift 274in; 286 ex

So I am guess set of rect. port heads and a larger cam

Not an expert, but could the Air-Gap dual plane intake manifold be a contributing factor? If you're claiming "a ton of more power/incredible low and midrange" as you say, then I am highly suspect that the "Air-Gap" dual plane intake manifold may be what is restricting and holding you back. I'M NOT SAYING IT IS THE ANSWER, but I certainly do not think a dual plane intake is helping you any in the upper RPM ranges---especially on a 496cid engine with properly selected/ported heads and exhaust headers!

Could it be some of the other problems that some of the other guys have suggested???---YES! I'm not discounting any of that, but if the engine is telling you all the power is from low to midrange then the torque range needs to move towards the RPM scale from midrange to top end side. Just a suggestion---something you might consider. Hope it goes well for you.

the operative term here is "guess" . you can spend your money guessing
or
you can spend your money knowing.

your choice

I had it out yesterday for a good while finally no rain but anyhow the boat runs pretty darn good I might need to jet the carb but I tunned on her a little she runs 53-5400 rpm the boat is runnig 71mph on gps. That was with full load of fuel 2 cases of beverage and 3 people

so.. what changed from the start of this thread until now ?

timing set at 12 degrees and thottle cable adjustment