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Ed, looking back at your previous post, you seem to be under the impression that increasing the bore (swept area) is related to the compression ratio.
It's the piston design that changes the compression. It essentially decreases the volume of the combustion chamber (at TDC) in relation to the total volume of the cylinder.
That's how, back in the day, you would have the same engines in cars with different compression ratios. e.g., 327ci base two bbl carb, 8.5:1 compression vs. 327ci Corvette engine 11:1 compression. Different pistons (among other things). But the pistons are what determines the compression ratio.
Agreed. I don't have a 790 & 865 piston to compare so I made an asuumption based on the published specs of the stock engines and assumed the compression chambers and cams were different. Since it was a "bang for the buck" question and since 790 cams in a 865 engine appear to make a good deal more power I chose cams and headwork. You are correct that I could be completely off base and the 865 pistons could be the answer to the equal compression ratio in the stock motors. I also don't know how much it costs after the inital parts investment to do a big bore kit but I was betting over $1K and "bang for the buck" was the premise. It does make me wonder why I haven't seen higher compression pistons for the 865s out there, that would be a most effective solution to utilize that extra swept volume 
Again, it was an answer for an either/or query and has nothing to do with whether or not Big Bore owners are happy with their engines. 
There's a lot more discussion here My Thunderbike Cams
Last edited by oldroadie; 07/24/2007 8:51 AM.
