Combustion Chamber Size

Does anyone know the cubic centimeters for a D7 cylinder head? Or did they vary?


-Chris

Howdy Chris:

A D7 head, assuming it is stock, as cast and no valve job or milling, should have a chamber volume of aproximately 62 ccs. Volumes can vary from head to head, and chamber to chamber on the same head. The only way to know, for sure, is to measure.

Adios, David

Thanks for the information but I think I really goofed on my rebuild.



-Chris

well, speak up 'bout it and let us know what you think you messed up
pretty sure someone here will be able to tell you if you did or not (depending on what you mean by "goofed")

Well, I didn't compensate for the thicker FelPro head gasket and I assumed that the combustion chambers for my '77 head was not larger than 58cc or so (should have checked).

I think I'm in the low 8:1 CR range now.


-Chris

ahh!
perfect for oh, i dunno....
'bout 15 PSI?

you can doo eet!!

i figured that was what the problem was from your other post
look around, see if you can find a cheap turbo, have a real good time with it

Chris,

Using 62 cc combustion chamber size and reasonable guesses on the other parapmeters the stock metal head gasket would have about an 8.1:1 compression ratio, The thicker Fel-Pro gasket would lower this by about 0.5 point to 7.6:1.

Our engine has a calculated compression ratio of 8.2:1, has a stock cam, and has compressions (after break-in) in the 175 range. These are obtained after the engine is at operating temperature, all spark plugs out, carb open full, and spinning the engine unitl the pressure no longer goes up. This is about 10 - 12 "pumps."

Interestingly, engines with higher computed compression ratios can and do have lower compression values. This is primarily due to valve timing differences - the cam. If the cams are the same, and the valves and rings seal well (i.e. the engine is broken in) then a higher compression ratio sould result in higher compression pressures.

Hope this helped.

Regards, Dave

Thanks for all the help guys!

Well, I think I'm going to take the head in and have them shave it .055". Considering everything else is in place (and running...sort of) I thought it best to do it this way. This should bring me up to 8.4~8.6:1.

Or should I take off more? Maybe .070"? I don't want to run premium gas though. Ideas?

And Dave, I reran my numbers and you hit the nail on the head! I came up with 7.8~:1 on the current static compression ratio.

Thanks again.


-Chris

Howdy again chris:

Oh, the heart ache of a few thousandth here and there.

We use the Octane rating as a guide to determine CR in worst case scenieros. For example, if you want to safely run 87 octane regular in the heat of the summer, don't exceed 8.7:1. If you're willing to buy a dose of 93 octane in the heat of the summer then go for a 9.3:1 CR. In your case a CR of 8.7:1 should be reasonable.

Since you have to buy a new head gasket anyway you might want to consider a Victor brand From NAPA #3910VC. They a .010" thinner than the FelPro, but of the same composite type for a good seal. That not only reduces the amount you need to mill, but reduces quench too.

A cut of .055" will reduce CC about 11. We use the ratio of a .010" cut = 2 ccs. But it is best to measure.

Hope that helps.

Adios, David

Thanks David as well as everyone else. I really appreciate the expert advice and support I get from this forum. It makes a BIG difference to me.


-Chris