I know for to do it with a flat top piston, but how do you do it with a domed piston that pops up above the deck, and how do you factor in head gasket thickness? Thanks. Have a nice day. -Jeff
I've never did it for myself, but my old boss at the Ramchargers shop did it for my 426 I once had. You have to find the piston dome volume, and subtract the amount above the gasket surface from the chamber volume (typical for any pop-up pistons). One thing I was told is that, contrary to the factory 10.25 compression rating, it was actually close to 8.5 to 1 on a stock 426 Hemi! With un-milled heads, Direct Connection 12.5 pistons, and a 0.030 overbore, I had an actual 10.5 to 1 compression ratio. Early motors may have a similar rating issue.
Here is the tech post I just did: http://www.jalopyjournal.com/ubbthreads/showflat.php?Cat=&Number=417991&page=0&view=collapsed&sb=5&o=14&fpart=1 It shows you how to do it with the engine ***embled or dissasembled, you basically do it like you would with flat top pistons, except you subtract the volume of the piston dome from the volume of the combustion chamber.
I know enough guys that can help me figure out this next question, but I'd like to see if someone here can answer it so more guys get the use out of it rather than just me when I ask an engine builder. How do you figure out the volume of the dome on top of the piston? For instance: let's say you've got flat top pistons on a block with no heads. Raise piston to TDC, put plastic plate on bore and fill space with fluid from a marked burrette (sp?). Then you do the same with the chamber in the head, and figure in gasket thickness, etc., do the math and you have cc and compression numbers. But if you've got a piston that has a big ol' hump that comes above the deck of the block, how do you figure out the volume of that hump, to figure it into the equation. A Hemi has this, as do the pistons in my 322 Buick. The only way I can see would be to take a cylinder of some type (piece of PVC pipe?) and lay it on top of the bore, seal it, and fill with fluid to get a measurement (with the piston at TDC, dome protruding into your cylinder extension), and then set the extension on a flat surface and fill it again, getting volume without piston dome, which would give you the volume of the dome.(the difference in fluid on engine and on flat surface) I've never seen this done, and usually the CC kits I've seen are complete with everything you need but I've never seen the piece of pipe or tubing I've just described. -Brad
you put a ring on the piston, some light grease on the cylinder wall, move the piston down exactly 1 inch, put your plexigl*** cover back on and fill like you did with the flat top. you can now caculate how much liquid it should have took to fill your cylinder , using your bore size,1 inch down. then compare that to the actual amount of liquid it took to fill the area, the difference is the dome volume
****! And the reader was blinded by a the flash of light as the bulb went off! Damn, that makes perfect sense, and now that you've told me, I feel kinda stoopid for not rfiguring it out. I sometimes over think things. Thank you very much--that's been bugging me for a very, very long time. -Brad
this of course is static compression being discussed here. how about a refresher on figuring dynamic compression? Paul
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