How much horsepower can you get out of the peanut port heads? I have a used 454 that the previous owner said he thought was 400-450 horsepower. I tried to figure out what heads it had, and from the casting number, it appears they are the peanut port heads, which surprised me because the engine has a lot of other good upgrades on it like roller rockers, Gil Exhaust, etc.

Finally, until I can verify what is in the engine internally, what should I limit my RPM to? The engine started life as a Mercruiser 7.4 Litre, but I'm not sure if the crank was upgraded to a forged one, what the rods and pistons are, etc.

Thanks,
Steve

You won't make any power after 4800 RPMs with the small port heads. Make sure you're not confusing them with the Vortech heads. They're much better than the small port heads and can make decent power to 5500. The rectangle port heads are the best though but if you have them on a weak big-block (ie small cam, flat dual plane intake) you will actually lose power with the rectangle port heads.

Roby

I'm just wondering if the claim of 400-450 horsepower is believable with peanut port heads. The casting numbers appear to definitely be for the peanut port heads.

If an engine builder knows what he is doing, can you get that much power out of these heads?

-Steve

With a supercharger you can. N/A? No I don't think so; however you will get those as peak torque numbers. Those heads were designed for low-end torque. Trailer towing for trucks and torque for heavy cruisers

Roby

Ten years ago I spent most of the winter season porting a set of peanut port heads that were on a 1992 7.4L carb motor. I had seen an article in Hot Rod Magazine a year or two earlier about how John Lingenfelter's shop had taken a set of these and ported them to the max, installed larger valves and then made 505 horsepower at around 5000 rpm in a pump gas 502 using a very small hydraulic roller cam, edelbrock perfomer intake and 750 Holley. The article (I think it was entitled "Maximum Rat") gave a step by step description of how they did the heads. So, I took a shot at it myself.

I installed my home-ported heads with the larger valves, slightly larger flat tappet hydraulic cam (218/222 @050, 112 LSA), performer intake (re-used the stock Weber AFB carb but re-jetted it richer) and Stainless Marine exhaust on my 7.4L that was in a 24 Cobalt Condurre. It picked up an amazing 10 mph in top speed and performed better throughout its entire rpm range. I propped it to run around 5100 rpm at WOT. I never dyno'd it, but by the increase in mph, it must have picked up close to 100 horsepower. Granted, I am sure part of the overall gain was due to converting the exhaust from through-prop to through-hull and adding better manifolds, but it was a very surprising result.

The peanut port heads are pretty easy to identify by their nearly round intake ports.

I am not suggesting that anyone go through the time and effort that I did to port a set of these heads, especially since there are now so many aftermarket head options. But, it does show that they do respond to porting if someone wants to take the time.

Rich

Lingenfelter sure could make those heads sing. I remember the very same article that you are refering too. Of couse, the company doens't offer the service any more, because the value just wasn't there. You'll be better off with some aftermarket heads.

35/25: Funny, I did the same sort of thing. Had the heads pocket ported, then I destroyed my electric drill opening the intakes up to match a Performer oval port intake. Richer rods in the Quadrajet and the 454 Magnum camshaft. I picked up 5 mph going through the prop. I sold the boat before installing through hull exhaust. I figure I added 65 HP. I propped the boat to run 5000 RPM with a 23 Mirage.

sgr: With more cam and valvetrain upgrades to match, plus Gil exhaust I don't find 425 HP hard to believe, but it will only happen at higher RPM and the rest of the 7.4L engine was not built for that. Find out if the crank and rods are the good ones.

I think that JimV can really do those heads well.

Thanks Marty

Yes 500 hp is obtainable, but with a lot of port work. By the time the smoke clears your better off with the vortech heads. The vortech chambers are smaller and will produce more compression ratio, and flow more air.

Hi Guys,
Thanks for the input. I'm not looking to do anything to these heads. I was just beginning to doubt this engine put out much more than 350 horsepower because I couldn't get any more than about 63 mph out of it and only 4600 RPM.

Last weekend I tried a different prop and tried trimming the drive out more than I thought it wanted and unleashed a whole new world of speed. The speedometer ended up pegging out above 75 and the boat ran at 71-72 mph GPS at 5100 RPM.

With those kinds of numbers I am pretty convinced that it is in the 425-450 hp range.

Thanks again.
-Steve