Does the difference in compression height of a piston have to be calculated in when determining the piston to deck volume of an engine. For example:

With a 0.020 overbore 1.56 comp. (std) height piston with a 3.5cc dome set 0.025in into the block with 76cc heads and a 3.245 compressed volume head gasket gives me 9.94:1

Would I have to add the .135 inches difference to the deck height, using a 1.425 compression height piston, or would it take effect when calculating the displacement of the engine, effectively adding 28cc's per cylinder?. Or is this number irrelevant and only there to further confuse the mathematically illiterate?

 

yeah you'd want to consier it, if you were wanting to be exact otherwise you could use zero. are you just wanting to compare two differant types of pistons? if so just use a zero deck clearance.

 

I'm just curious about the difference in the pistons in relation to compression ratio when used on the same block, with the same crank and rods. I want to get the math correct so I can correctly input the info in to DD. Their compression ratio calculator is terrible if you use anything other than pure flat-top pistons, and of course, it doesn't allow for different compression-height pistons (atleast not that I can find).

In order to use zero, wouldn't I have to get the block machined down that 0.135"?

 

The different compression heights are for 2 different stroke cranks. The compression height is the distance from the centerline if the wrist pin to the top (flat part) of the piston.
If you took 350 rod&piston assemblies and put them on a 3.75" stroke instead of 3.48" they would stick up higher by 1/2 of the stroke difference. 3.75-3.48=.27" divide that by 2 and you get (drumroll) .135 . So you need pistons with a compression height that is .135" shorter to have the flat surface of the piston stop at the same height in the bore as the original 350 pistons.
What you have discovered is the difference in 383 pistons and 350 pistons. Needless to say if you put the 383 pistons (ones with compression-height -.135) on a 350 crank, then it is going to have really low compression. Milling that much off of the block is not practical. More later on the extreme importance in actually measuring deck height during blueprinting....

 

That's all I really needed to know, thanks. I thought it had something to do with the crank, but didn't quite make the connection, so to speak.