Changing my head gasket from a .040 thick to .015 thic will add how much compression? The book says it'll reduce the compression volume by 5.3cc over the .040 one. Motor is .040 over, small pop up 350 with 70 cc heads. I am looking at Fel-Pro gaskets with either .040 or .015 thickness. Had .040 before, compression was high but streetable. Thanks a LOT.

 

Here is the formula to figure out compression ratio.


V1+V2+V3+V4
-------------------
V2+V3+V4

V1 = Cylinder Volume
V2 = Deck Clearance
V3 = Gasket thickness
V4 = combustion chamber minus dome volume

To find V1 square it and multiply by the stroke then multiply by .7854. That is V1.

To find V2 square the bore and multiply by .7854 again.and then multiply that by your deck clearance.That is V2.

To find V3 square the bore and multiply by .7854 and then multiply by your gasket thickness.That is V3.

To find V4 it is the chamber volume of the head minus the dome volume or add the dish volume if the piston is dished.Get the total number of CC's from the chamber and divide that by 16.387.That is V4 then use the formula above.

Heres a stock 350 for example.
4x4 =16 x 3.480 stroke=55.68x.7854=43.731 That is V1
4x4=16x.7854=12.5664 x .035 (deck Clearance)=.439 That is V2
4x4=16 x .7854=12.5664 x .040 (Gasket Thickness)=.502 That is V3.
76 cc heads with a 3cc dome= 73 cc's total.73/ 16.387=4.454


43.731+.439+.502+4.454 = 49.126
-------------------------------------------------- = 9.105
.439 + .502 + 4.454 = 5.395

The compression would be 9.1 to 1.I hope this helps you.I would have crunched the numbers for you but I did'nt have all of the info needed.I just learned all of this in school and it is fresh in my mind.Sorry it is so long,but I hope it helped ya.

James

 

Turns out that my current compression ratio is about 10.28:1, and going to a thinner gasket will improve that to 10.97:1. I think a bit too much squeez.

 

I'm running 10.8:1 but if you think that is too much, you could clean out the bowl and lower it a tad.

 

Most gaskets that are that thin are basically a copy of a facory "steel shim" head gasket. Tough to seal up especially if your head and decks aren't DEAD flat. Even then, you will usually have to do something special to seal up around the coolant passages.

For a not-too-thick composition head gasket you might check out a set of GMPP head gaskets (I bought mine from Scoggin Dickey) with a compressed thickness of .028". Easy to seal up like a composition gasket but without the really tall compressed thickness of many aftermarket gaskets.