compression question

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oklahoma
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1973 mustang sportsroof - 351c 2v - fmx trans.
Hello folks, I am currently putting a 302 together for my wifes mustang. The engine block is an 84 casting that had e5te cylinder heads with 69 cc combustion chambers making a whopping 8.4 to 1 compression. The e5's are going in the scrap pile and I am installing GT40P heads with the appropriate longtube headers. The GT40P heads have 58 cc chambers. Does anyone know how much of a jump in compression this will produce? Thanks everyone.

 
it doesn't work that way.

what is your goal?

what octane do you want to use?

if the p's are stock, they are not 58 cc's.

are your pistons flat top with 2 valve reliefs?

how far below the deck surface are the pistons at tdc?

 
Hey Barnett I know you are an authority on Ford engines lol. My book says GT40P heads have a combustion chamber size of 58-61 cc's so sorry if I mislead you. I am going to be using Felpro 8548-PT2 head gaskets. I was just curious to a ballpark figure on a compression ratio so I would have a rough idea. This is a budget build using the stock cam, Gt40P heads, Flowtech longtubes, AOD transmission, a recurved Duraspark distributor (the reason I was needing a rough idea on compression ratio), 3.73 gears, typhoon intake, dual snorkel ram air, etc. I already have all of the parts for this build and looking to build a torque engine not a high rpm screamer. Thanks.

p.s. yes flat top pistons with 2 valve reliefs. 91 octane is not a problem for me.

 
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ok xlnt.

the heads must be physically measured.

mill the heads to 56 cc's or leave them at 58 if thats what they are.

measure how far down below the deck the pistons and use the appropriate thickness cometic head gasket to try and get the distance from the piston to the head between .034" and .043".

 
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Knowing gasket used and assuming the motor is stock and for a 84 block:

Bore = 4"

Stroke = 3"

Piston Deck Clearance = .011 for a stock motor in 84

Compressed Head Gasket Thickness = .049

Piston Head Volume = 0 (with valve relief the CR would be lower)

Head Gasket Bore Diameter = 4.1 (manufacturers spec)

CR by a calculator for 58cc is 9.71:1

Milling the heads to 56cc the CR is then 9.97:1

I also plugged in the 69cc heads that you have now with the above values and the CR came to 8.54:1. If you account for the valve reliefs you would be around 8.4:1 like you mentioned.

These numbers should give you a ball park CR to help you determine what you want to do.

EDIT: Did not know how much volume the valve relief in the piston was that's why I used 0. You would have to actually measure that to know but it can't be more than a couple cc's.

 
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I came up with the exact same numbers using the old paper method.

- have fun with your build, sounds like you have it all lined out.

Paul of MO

 
You guys are great! I appreciate it so much. This engine is going to be for my wifes new daily driver, a 1987 mustang lx hatchback. I figured she has been so supportive of the 73 build that she needs a stang to terrorize the streets in:p

 
It's always interesting to play around with the various variables to see what effect they have on compression ratios. Here's a spreadsheet that lets you easily see cause and effect. You didn't say what cam you'll be using, which determines the dynamic compression ratio and the octane requirements, so you can play around with those numbers, too.

http://www.7173mustangs.com/thread-engine-and-compression-calculations

 
getting the quench/squish clearance between the numbers i posted will reduce the potential for detonation which allows you to run more compression than you otherwise would.

increasing compression will increase power EVERYWHERE.

The numerically higher your gears are, the more compression you can run because it reduces the load on the engine.

5.0 cc's is reasonable to use for the volume of the valve reliefs.

 
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