Predator 212 compression, stock and modified............

[madprofessor]

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.

 

[Denny]

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

 

[OPmini]

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

I dont think predators have auto decomps, look at the valve train and cam

 

[karl]

I dont think predators have auto decomps, look at the valve train and cam

Ever have one them cracked open? The weight on the cam gear with the spring has something to do with it.

Even popular aftermarket cams like the MOD2 retain the compression release.

So a test gauge reading does not tell very much.

 

[Denny]

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

 

[madprofessor]

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.

 

[OPmini]

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.

cool, didn't think that the little weight came close enough to the lifter to push it, guess I was wrong. If you need to know the compression you could pull the exhaust rocker out after warming the engine up.

 

[karl]

Just dying to know how much compression improvement I got for my hotrodding efforts.

Well, lets start, what engine is it, hemi or non?

If a remember right the Hemi head is around 22cc volume, and Non is 18?cc.

The hemi has a flat top, Non has dished piston (-3cc? ).

You know the gasket thickness, I assume you can approximate how deep in the hole the piston was,
and the bore and stroke is 70mm x 55mm. So now you can calculate your compression ratio.

https://www.summitracing.com/newsandevents/calcsandtools/compression-calculator

I usually sit the wedge heads on the hemi blocks, shave the head down, and CC it
with plexiglas and a syringe until I'm in the 12:1 zone. Spices things up considerably.

 

[madprofessor]

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.

 

[karl]

Nah , your just confused.

I gave it the "best case senario" , piston flush to deck.

Really does not make much of a diffrence at all, thinner head gaskets make a bigger impact
with low CC heads. The big hemi head, practically nothing.

8.4 to 1 is practically stock

 

[madprofessor]

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.

 

[madprofessor]

cool, didn't think that the little weight came close enough to the lifter to push it, guess I was wrong. If you need to know the compression you could pull the exhaust rocker out after warming the engine up.

OP, only just now realized what you were saying about the rocker. That's a truly great idea.
For my hemi's rocker setup it'd be easier for me to just loosen the exhaust lash enough to prevent that little bump, or even enough to just pop the pushrod out from under the rocker. My stainless steel .010" head gasket can be reused also, so no problem there. Always a good idea to check valve lash now and then, so good opportunity to set it then.
The non-hemi rocker setups are easy to pop off the way you said, very different from hemis.