Someone told me because I have big valves 2.02/1.60 that my compression is not the "stock ratio" 10.5:1

I figure my compression must be less cause of bigger valves but how much less?

 

Somebody is blowing smoke in your ear. The valve size has absolutely nothing to do with the compression ratio. Compression ratio is determined by the size of the combustion chambers.

 

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by HITMANSS:
<strong> Somebody is blowing smoke in your ear. The valve size has absolutely nothing to do with the compression ratio. Compression ratio is determined by the size of the combustion chambers. </strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Well...not directly. When putting larger valves in, the size of the combustion chamber usually changes slightly. Therefore, assuming everything else is identical, you might have increased your compression slightly. There are many other factors to look into though. Did they choose a thinner head gasket? Were you heads or block machined (decked)? These can also affect CR. Ask the shop that did the work. They should be able to tell you.

 

Unshrouding the valve will take up some of the combustion chamber space.

Typically, a head with a stage II port job, and unshrouded valves will have about 63 CC of space.

Stock head CC is around 67.

Your compression is probably somewhere around 10.8 to 11.0

Good Luck,
Kevin

 

Heads were not milled or decked. we used stock head gaskets. To my understanding compression ratio shouldn't have been increased at all.

I'm trying to explain my car under-dynoing (BIG TIME) and thought maybe after some guy told me my compression ratio is different that maybe I've lost compression.

there is a guy with same cam and bottom end but stock heads that is dynoing almost as much as me with half the mods.

 

stock compression is not 10.5:1 it is 10.1:1

 

Things like a cam will change your -cranking-
measured compression ratio, whether or not
the mechanical compression ratio has been
modified. Bleed-down during the relatively
slow cranking is increased with valve timing
overlap.

I wouldn't expect valves to have more than
a teensy impact here, but big valve heads
and big cams kind of go together.