Predator 212 compression, stock and modified............ (2024)

A member posting on another thread, BigWes, advised that the stock Predator in that thread should have 90-100 psi cylinder compression. Don't know where he found that, but I looked for it a long time, never did find it.
You out there Wes? Okay, anybody, please help me answer my compression question. First, is that stock psi of 90-100 correct?
Now for my current modified............Compression has been increased by replacing conn rod with a 3.328" over-length billet rod (.020" over). Further increased by replacing stock head gasket (about .050") with steel performance gasket that's .010" thick. Shrinks the piston/head proximity by about .060" total, coming at it from both directions.
Anybody able to figure out what that cylinder compression psi should be? How about ratio, from 8.1:1 stock to how much now?
I know some of y'all are motor math whizzes, any helpful answer would be appreciated.

I think Wes is forgetting about the compression releases these engines are equipped with.

E-Z start is a decompression release. That is why you can’t do a compression test on these engines.

All I've learned about compression release on anything but a Japanese motorcycle engine has come from my Predator 212's, and what I know about it from them is that.......................
My stock cams came with a sprung bobweight that centrifugally swings out of the way, to clear the release lobe from bumping the exhaust lifter, once some cranked rpm's get on up, and that.................
My Superman series .265 race cam (from diyperformance) I replaced the stock cam on my current motor with has the same compression release lobe and sprung bobweight as the stock cams do.
That's why I must have the resettable max reading needle on an analog compression gauge, or I'd never read it with any accuracy.
Just dying to know how much compression improvement I got for my hotrodding efforts.

Summit compression calculator needs info I don't have, but it looks cool. So specs on the motor..........
Predator stock 212cc hemi 4-stroke OHV's 6.5hp. has flattop piston that in the end lightly sandwiched a piece of welding slag smaller than a .177" BB between itself and the head.
Stock stroke doesn't get increased by .020" overlength ARC billet rod, but does get piston .020" closer to head. Head dropped down to .010" cylinder deck clearance (with stainless steel gasket) from what I've only heard estimated at .050" stock paper gasket, compressed thickness unknown.
No more info than that. My math says 70mm bore has 3,846.5 sq. mm. area, or 38.465 sq. cm. area. That means 55mm stroke makes...............
211,557.5 cu. mm., or 211.5575 cu. cm.

Still confused, karl, but thanks for lining that in for me. Called Mark at EC Carburetors to verify the hemi head cc, he said it's about 23cc stock, and as you alluded to, only milling a head deck down to dangerously valve-smacking vicinity would make an appreciable increase in compression on the hemi head, maxing out around 9:1 with the 32/28 big valves.
Next build is going to be a skeleton wheelie machine ("BareBallz") based on a 212. After talking to Mark it looks like I'm going to start with a non-hemi engine, throw a flattop piston on a overlength rod in it, a .265 cam, and then see if an EC Dominator carb is in my future.
Oh God! Now I'm picturing a turbo. I need "professional" help.