Can the compression ratio (CR) of a YB,be lowered by changing the 4x4 head with a 2wd one? I found a add for pistons,sayng that if they are used with a 4x4 head shuold have 7.5:1 CR,and if used with a 2wd head should have 6.9:1 CR!!! Is this true??

 

yes it's true.
considering the ad. was placed by cosworth they should know

one of my engines uses a 4wd head yet 2wd pistons so much higher than normal compression

 

That surprises me that it is quoted as that much, as when you fit standard 4x4 pistons with standard 2wd head, the compression drops from 8:1 to 7.8:1, so I am puzzled as to how it could drop so much just because the compression has already been lowered?

 

the effect (i.e. compression ratio change) will be much larger when used with a 4wd head (smaller combustion chamber) than 2wd head. mathematics. as the piston bowl volume will be more influential in the equation than with a more significant (head) combustion chamber volume...

 

I know that, but how come the massive decrease in compression they are quoting, when I understood that swapping heads was just worth a drop of 2 points?

 

Mike do you reckon it is worth putting the 4x4 pistons on with 2wd head for a sub 400bhp track car ?

 

well ihave just been sitting here with my sad calculator and seems mike is right again much as i hate to admit
according to my calculations, the lower the c.r. of the pistons, the less difference between using a 2wd and 4wd head
i reckon cosworth have it wrong (well it was only the salemen not the engineers) any EXPERTS??

 

David,
YES! 7.8 at least for anything over 24psi...

 

Cheers Mike

Will get my hands on some 4x4 pistons then