# Aluminum head comp ratio



## maroba69 (Feb 2, 2010)

I'm looking to change from stock iron head (62 castings) on my '69 to KRE d-port 72cc. It'll be in conjunction with more aggressive cam and alum intake. I'm currently somewhere b/n 9-9.1 to 1 compression with sealed power cast pistons. How do I determine where I'll end up compression wise with aluminum heads? Would 72cc put me too high in compression zone? The 87cc seems too low if stock was 72. Thanks!


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## geeteeohguy (Feb 2, 2008)

You need to run about a full compression point higher with alumimum heads to make the same power as iron heads, all else being equal. So you're right on at the 72cc spec. The cast pistons are an issue, though. You'd be better off with forged dished pistons and iron heads, IMO for a street engine. You want about 84-87cc chambers with an iron headed engine. There are others out there who have vast experience in this area who will hopefully chime in to help.


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## BearGFR (Aug 25, 2008)

maroba69 said:


> I'm looking to change from stock iron head (62 castings) on my '69 to KRE d-port 72cc. It'll be in conjunction with more aggressive cam and alum intake. I'm currently somewhere b/n 9-9.1 to 1 compression with sealed power cast pistons. How do I determine where I'll end up compression wise with aluminum heads? Would 72cc put me too high in compression zone? The 87cc seems too low if stock was 72. Thanks!


First things first --- metal composition alone doesn't change compression ratio. If you've got a compression ratio of X with iron heads, and then you change them out for aluminum heads that have exactly the same chamber volume using the same gaskets and pistons, your compression ratio is still going to be X.

Static compression ratio is a mathematical expression: It's the ratio of how much "air space" is in the cylinder when the piston is at the bottom of the stroke to how much "air space" is in the cylinder when the piston is at the top of its stroke.

Sometimes you'll find it expressed as (swept volume + clearance volume) divided by (clearance volume).

Swept volume: Stroke * ( (bore/2)^2 * 3.141592 )
Clearance volume: Chamber volume + head gasket volume + "deck height" volume + piston dish/valve relief volume.

(Everything has to be in the same units, so that means you have to either convert cc's to cubic inches, or vice versa)

I built myself an Excel Spreadsheet that I use for all this. I just plug in the numbers. It makes it a lot easier for planning because you can change things like piston dish volume, deck height, chamber volume, etc. and instantly see what effect it will have on static compression ratio and quench distance.

I've attached a copy of the spreadsheet to this post, so feel free to grab it and use it. (The file is actually a Word document with the spreadsheet embedded. I had to do that because the forum won't allow me to attach a file with type .xls, but it will accept .doc. There are additional instructions inside the document).

Continuing on...

The difference with aluminum heads is that you can usually "get away with" more compression with them than you can with iron. The reason being that aluminum doesn't hold as much heat so they resist creating hot spots better. The down side of that is that combustion heat is actually a good thing for making power, as long as it doesn't drive you into detonation - that's the big no-no.

There's a "ton" that goes into selecting compression for any givne engine build. I wish I could tell you something simple like "always run a ratio of X on no less than 93 octane and it will always be best" --- but it just ain't so. You have to consider cam timing, lobe separation angle, quench pad area, deck height, quench distance -- because all these play a part in affecting how much actual pressure (dynamic pressure) is built in the cylinder. You also have to consider the effects of fuel mixture and cooling system efficiency. "Hot spots" and combustion heat have a big influence on detonation, and both those are hevily influenced by fuel, cooling, even the shape of the chamber and upper cylinder. 

Generally, it's not worth the risk of pushing a street engine to get every last ounce of compression. On my 461, a full 0.5 worth of compression would only make a difference of 5-8 horsepower, for example.

Bear


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