Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Compression Calculator Help
View Single Post
  #16 (permalink)  
Old 11-15-2012, 12:40 PM
bentwings's Avatar
bentwings bentwings is offline
Last photo:
Join Date: May 2002
Location: St.Paul, Minn
Age: 75
Posts: 2,142
Wiki Edits: 0

Thanks: 3
Thanked 86 Times in 75 Posts
As you can see there are many variables especially heads and gaskets.

Years ago when we were checking the CR of our TF motor the discussion got heated over who had the right numbers......several rod lengths, compression height pistons, ported heads with polished chambers etc. Well a long story short some money got laid on the table.

We assembled a block, crank, rod, piston on the motor stand. We found top dead center and set the pointer. Then we dialed the assembly to TDC. Since we ran pistons down the hole we cc'd the "deck clearance." This automatically took into account the small valve reliefs and the radius of the top of the piston. We then turned the motor over to BDC and again cc'd the cylinder. This gave us swept volumn and TDC volumn. From this we could calculate the CR using established vol of the gasket and head. However there is still room for error here so we added the gasket and cyl head. Torqued the head down to spec and using water for a fluid, we checked the vol at TDC and Again at BDC. Dividing BDC vol by TDC vol gives an exact CR as accurate as your equipment can give.

The result was the calculated value (mine) was within .01 of the measured value.

That steak was sure tasty haha.

Bottom line is that when in doubt check your CR as it is's more work but very accurate.

Last edited by bentwings; 11-15-2012 at 12:46 PM.
Reply With Quote