Combustion Chamber Q's [Archive]

tsm1mt

12-18-2001, 10:18 AM

Originally posted by Chas76SII
Ok, time to pull out the calculators...

For a 345 w/.030 bore, if the combustion chamber is REDUCED by 5cc in volume, what is the compression gain? I haven't got a clue on how to figure this out and I'm hoping someone here will...:rolleyes:

It's a bit more involved than *THAT*..

You have displacement (stroke * bore) which is easy enough to find..

BUT.. TDC is not "zero deck".. so you have the CC of the head chamber, the CC of the head gasket (.038 or .042 thick for the composites, depending if it's Felpro or Clevite and I can never remember which is which) * gasket bore.

Even more fun is that the factory steel-shim gasket was more like .020" thick, so you're starting off with a .020 deficit to make up just to get back to stock ratio.. plus whatever you lose by "destroked" pistons that don't have the same wrist-pin-to-piston-top distance..

PLUS the CC of the cylinder bore * the distance between the piston at TDC and the deck.

Get all of those figures together and you can start calculating your compression change. :D

This is why it's better to shave the block than the heads - you cut down on the overall combustion volume more since the block is a cylinder,while the head is more like a cone/dome..

I have some of the math on my site..

http://www.tmcom.com/~tsm1/scout/faq/ under the first installment of the motor rebuild..


Pirate4x4.Com > Brand Specific Tech > International Harvester > Combustion Chamber Q's PDA : Combustion Chamber Q's AgaveHound 12-18-2001, 09:59 AM Ok, time to pull out the calculators... For a 345 w/.030 bore, if the combustion chamber is REDUCED by 5cc in volume, what is the compression gain? I haven't got a clue on how to figure this out and I'm hoping someone here will...:rolleyes: tsm1mt 12-18-2001, 10:18 AM Originally posted by Chas76SII Ok, time to pull out the calculators... For a 345 w/.030 bore, if the combustion chamber is REDUCED by 5cc in volume, what is the compression gain? I haven't got a clue on how to figure this out and I'm hoping someone here will...:rolleyes: It's a bit more involved than THAT.. You have displacement (stroke * bore) which is easy enough to find.. BUT.. TDC is not "zero deck".. so you have the CC of the head chamber, the CC of the head gasket (.038 or .042 thick for the composites, depending if it's Felpro or Clevite and I can never remember which is which) * gasket bore. Even more fun is that the factory steel-shim gasket was more like .020" thick, so you're starting off with a .020 deficit to make up just to get back to stock ratio.. plus whatever you lose by "destroked" pistons that don't have the same wrist-pin-to-piston-top distance.. PLUS the CC of the cylinder bore * the distance between the piston at TDC and the deck. Get all of those figures together and you can start calculating your compression change. :D This is why it's better to shave the block than the heads - you cut down on the overall combustion volume more since the block is a cylinder,while the head is more like a cone/dome.. I have some of the math on my site.. http://www.tmcom.com/~tsm1/scout/faq/ under the first installment of the motor rebuild..