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Old 01-17-2008, 09:31 PM
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Power gains on 454 with stock heads?

Hello everyone,


I have the little jet boat with a 454 in it. I yanked the heads tonight and found that I have flat top pistons with valve reliefs in them and I have the peanut port style heads.

My question is, stock vs stock, how much more power will a set of oval port or rectangle port heads make over a set of peanut port ones. Would it even be noticible? Just curious because i can get a set of either ovals or rectangular port heads from a friend for about 500 bux.

Also, did 454 heads come stock with the dual coil valve springs? because the ones that are on there now have the outer spring with another coil in the middle. its a gen IV engine by the way.

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Last edited by maddogg20; 01-17-2008 at 09:37 PM.
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Old 01-17-2008, 11:00 PM
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The peanut port heads don't flow much and in a jet boat RPM is a concern. I would go with the oval port heads, you might even would want to deck them a little for a touch much compression since 454's were very low on comp. to start with.
The rectangular port heads would work properly only if you rebuild the engine with hipo parts to match the high flow of those heads. The inside spring is a dampner coil, not really a 2 valve spring head.
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Old 01-17-2008, 11:08 PM
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The pistons are flat tops with 2 reliefs in them. Im not sure if the stock pistons had reliefs, but even so, Ill try and get a nice set of oval port heads. U think i will notice a difference?
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Old 01-17-2008, 11:19 PM
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I really doubt it, you probably have a stock 8:1 comp. 454, they are usually wheezed out by 4500 rpm and really didn't make a lot of hp. You may see a little more rpm but the low compression and weak cam are limiters.

If you have some EXTRA (???) money laying around you could bore it and go with those oval ports, 9.5:1 flat tops and a good Comp Cams marine cam and THEN you would feel a difference. Good luck
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Old 01-17-2008, 11:40 PM
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If I were to go with some iron heads with a 119cc chamber, what would the compression be like with my stock pistons?
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Old 01-17-2008, 11:53 PM
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I think the peanut port heads were 119 cc's also so you wouldn't gain any compression that way. Without tearing it down and rebuilding the only other option is too mill the heads and use a thin head gasket. A COMPETANT machine shop can tell you how much to take off.

But remember, just a point of compression isn't going to do much if any for you, you need the right combination of parts to make an engine perform.
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Old 01-18-2008, 06:36 AM
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If the heads you have are the peanut port (what part # are they?). And if they are open chamber and the pistons are flat tops you could try a closed chamber pass head (the ports are bigger vs peanut port heads) and that will bump your compression up. You can go here to check on chamber size if you have the casting numbers http://www.mortec.com/
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Old 01-18-2008, 07:50 AM
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stock cr is in the mid 7's.

To get 9:1 cr with a 454 and 120cc heads, you will need a dome piston. The dome should be about 0.125" tall. Will need to run higher octane fuel if the cam is short duration.

The power you gain will depend on the cam. Do you know what cam you have?

You could try a set of 402 heads (820's i think). They have 112cc chambers. came on early 70's engines. They should rise cr to 9:1 and they flow well.

I would use a set of 781 or 049 heads before using the peanut heads.
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Old 01-18-2008, 06:06 PM
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Because your 454 has the flat top pistons, any of the {"049-781" open chamber heads will give you a very, very low compression ratio. So to get a decent compression ratio with flat tops pistons, you want to find a set of the early, closed chamber 396 (325hp, 350hp) oval port heads. these heads have a "closed chamber" design (96 to 101cc) For a 92 octane friendly 454 you want to have a finished head volume of 96cc. The best heads from these 396-325-350hp motor were the 1966 1967 "215" castings. But any of the closed chamber oval port heads from these 65 to 68 396 oval port motors will work well. Give them a home port job, 3 angle competition valve job, cc the chambers and mill them to a finished 96cc chamber volume. They range from 96 to 101cc stock. Bolt them on your 454 using the GM factory 1970 Chevelle 454-450hp "LS-6" head gasket (.022" steel shim) {GM Parts dealer item} to get the right finished assembled compression ratio. Once you've done this , there are two good cams that will work well in your 454 jet boat. Isky #396278 "278 mega cam" and Comp cams CB-280H-10 magnum #11-208-3. They are very similar in lift and duration.
Use the recomended Isky or Comp or equal valve springs.
Use a Edelbrock performer RPM (oval port) manifold and 800-850cfm carb.
Use 92 to 94 octane fuel. Spark timing will be 36-38deg total.

If you follow this budget hop up formula based on your flat top pistoned 454, you'll see a very impressive horsepower/performance gain over what you have now. Go to www.mortec.com to see what the various 396 cylinder heads castings numbers you are looking for (1965 to 1969 396-325-350hp) 96 to 101cc chamber volume.

Your finished cr will be 9.3 to 9.45 depending on your 454 engines bore size spec. Do not try to run this motor on 87regular or 89 "marine" octane fuel.

Last edited by F-BIRD'88; 01-18-2008 at 06:16 PM.
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