bago Baja

What limits RPM in an engine. I have an '88 7.4 liter, 330 crankshaft HP. Book says max rpm is 4600. What determins this limit? Is it valve springs? Desktop dyno shows max power at ~ 4600. Is that the reason? Another way of asking the question. Desktop dyno shows a 40 - 60 HP increase by changing from small oval heads to aftermarket large oval but the power peak moves up to ~ 5000 rpm. If I put new heads on with the same cam and springs will the engine run up to 5000 rpm if properly propped or will the valves float at 4600? Thanks for any input.
Tom

SeaRay Jim

You can modify or swap the heads, add a cam, better fuel delivery etc. and turn much more RPM. But the questions is, what are the internals (cast or forged etc.) and how long will it last turning more RPM before melt down.

I've got a 7.4MPI with the same 4600 max RPM and with the work being done on it right now, the limiter has been moved from the stock 4750 (I think) up to about 5100. The only reason is so that I can get those 1-2 minute 4900-5000 RPM blasts for peak MPH simply for bragging rights. I surely wouldn't expect to keep it together running 5000 RPM for 2 hours! Some may, but I'm not taking chances.

Now let the experts answer your question.

bago Baja

I understand the point searay. I don't spend much time at WOT either. I'm interested in the technical aspects of this question, from a theoretical standpoint as much as anything. There must be a reason for a manufacturer to suggest an RPM limit. Maybe reliability is part of the equation. I'm sure there are also some physical restraints that limit RPM. Just want to understand this.
Tom

Whiteknuckle

I think the 4600 RPM limit is a figure established by Merc for this engine for longivity and warranty issues. The 330HP 454 is a workhorse of an engine, and I think a couple of years ago, Powerboat Magazine said it was the best production marine engine every built. However since it is a mild engine, many of the components, are not built for high substained RPM. Since the cam is very mild, the springs are very conservative, the pushrods are 5/16", rod bolts are only 3/8". The way I see it, the components were chosen for the intended purpose of the engine.

If you go to the large oval heads, without changing anything else, I will guarantee a loss of performance. I don't care what the desk top dyno says. The peanut ports are a good low speed, midrange head and are the best head for that camshaft and exhaust. Now, change the cam, valve train, and exhaust, then put your large ovals on and you might have something. Also, remember you have cast crank and pistons. The cast crank is good for well over 400 HP, and the pistons will hold up better than you think, however the pistons would be a priority upgrade if you get serious. Check this board especially Kaama on how to put this motor on "steroids".

How was that for a rambling answer?

bago Baja

Thanks for the reply Whiteknuckle. The standard duty marine 454 came with large ovals prior to 1984. The large oval head dropped out of existence along with street muscle cars in '84. That left the small oval and rectangular port heads, neither of which are best for a marine engine. Large oval heads returned again in 1996 in the vortec Gen VI engine. The small ovals are definitely a big compromise. By large oval, I'm referring to the 230 - 260 cc stock chevy oval port heads. I've not heard before that converting from small to large ovals could actually hurt performance. I have read the threads about 330's on steroids. Interesting stuff. I'm probably not that ambitious right now. Seems like every engine probably has a bottleneck that restricts airflow, therefore HP. On the stock 7.4 / 330, I think it's the small oval ports. Sure interested in more opinions.
Tom

Ric232

The manufacturer recommends a WOT operating range of 4200 - 4600 rpm because the peak HP occurs at 4400, right in the middle of that range. There is no benefit to running higher in stock form, which is why the internals are not up to the task of running a lot higher for sustained periods.

If you make mods that bring your max HP up to 4800 or 5000, I would think new valvesprings would be enough to allow you to operate reliability in the higher RPM's. Roller rockers certainly wouldn't hurt while you've got things torn apart, but I really don't think they're necessary.

Whiteknuckle

bago,
I understand exactly what you are talking about. I did an upgrade on a 330 myself, and replaced the peanuts with the #049 Chevrolet large ovals, for the very reason you mentioned. I did run the peanuts one year with the large 2.19 intakes, 1.88 exhaust valves, and the heads really did pretty good. But to clarify what I said, or meant to say, is if you don't change anything else, don't expect a power increase by just installing the large ovals. The peanuts compliment the rest of the components on a stock 330 HP engine.
Hey, I got to go and to the Christmas Eve thing. Have a great Holiday.

blue thunder

To me, what limits rpm is how much load is applied to the crankshaft and how much hp the crankshaft has to deliver...ie prop pitch and engine hp. The trick is to find the right prop/hp combination to work with the hull for max whatever. Whatever could be speed, fuel economy or reliability. Each goal will take a different path. My goal is speed, so I am trying to maximize hp. This could happen at any rpm from up to 5100. I say 5100 because I have cast components as well, and I think reliability will become a problem after that rpm. Not really srue, because that all depends on how balanced my enignes are, but you get the idea.

BT

28Eliminator

Ric232, is right on with the RPM limits of that motor. You could reprop and get 5000 RPM, but you wouldn't gain any speed because you are on the down hill side of the prower curve. If you installed a bigger cam and springs, you would see an increase in HP, but possibly at a higher RPM. Now if you go with the large oval port heads and a new cam you can pick up even more power.
So 'bago Baja to answer your question. The heads and cam are the real limiting factor, but Merc put this together this way for reliabilty. If you put on new heads, why not put on new springs anyway. They are not to pricy for this application. My .02 c

bago Baja

Thanks for all the great input. Merry Christmas to all and to all a good night!