compression ratio

[ricksaia]

Hello all, my 1983 f54 block is all set. What I want to know is with my P90 head. I had the head milled .050, with the stock block (non-turbo), what would be my compression ratio? Is an easy answer possible? Please let me know. Thank you, Rick.

[Phred]

If you can take your head to a race engine shop and get it CC'd, I can tell you what your Compression will be. I will also need to know if you are using a stock head gasket, and what the deck height is, and if those are flat top pistons.

Phred

[Ed]

Phred, I want to take you up on this offer of figuring out the compression ratio. I just finished cc'ing my head and these are the #'s I came up with:

#1- 38.8, #2- 38.6, #3- 39.2, #4- 39.4, #5- 38.6, #6- 38.2

The block is a '81 F54 with the stock flat top pistons. And I'm using a Felpro head gasket.

Also I have a .020 head gasket shim by Felpro but I'm not sure if I'm going to use it. I guess it all depends on what my compression ratio comes out to.

Thanks

[Phred]

Hi Ed,

You're only missing one critical measurement which I will make an educated guess on. If it varies much from my assumtion, I can easily refigure it into the formula.

First, figure the known actual figures:

Bore/Stroke:

3.39 X 3.11 = 2759.95 cc's

Head Gasket:

3.49 X .047 = 7.37 cc's

Head Gasket Shim:

3.49 X .020 = 3.14 cc's

Combustion Chamber volume, (Ave.) 38.8 cc's

Now, the unknown guess:

Deck height:

-.010 (area above the piston @ TDC) =1.48 cc's

Next we just add them up to find total compressed volume:

7.37 + 3.14 + 38.8 + 1.48 = 50.79 cc's

Now divide the displacement of one cly. by compressed volume, Then add 1 (a math thing to arrive at a ratio) and you have 9.06:1 compression

If you delete the shim vol. of 3.14 cc's you come up with a total vol. of 47.65 and a compression of 10.65:1

Class is over for today, remember to play nice with the other Z kids.

Professor Phred

[ricksaia]

The head was at the machine shop for 2 months, I would rather not take it back. To find out the CC's might take another month. The block is a flat top piston F54 and the head gasket is from felpro. Is it possible to find out a ballpark figure? Let me know, thanks, Rick.

[Ed]

Thanks Professor.

[Phred]

Rick,

As you can see in the figures for Ed, a small change can make a big difference in compression. There are so many variables with you head, a guess here could be downright dangerous. .050 off you head? How much before? sunk valves? new Valves? You can't guess here, as a wrong guess could mean a blown engine.

Phred

[ricksaia]

Thank you. I knew my question was a little vague. With all the reading and research, I am going to assume (I know) a 9.5 CR. I hope I am right. Thanks again. Rick.

[Ed]

Rick, PM your address and I will send you the piece of plexiglass and the syringe that I used to calculate the cc's on my head. It's easy to do and will take you less than an hour.