Alright i just did a compression check on my motor and I was wondering what the compression ratio would be converted from PSI like the guage reads to actual compression ratio. Is it possible?? My readings range from 174 psi to 195 psi.

 

It's not possible.

 

With #'s like that, I'd be wondering more about where I'm going to get the money for a rebuild! You've got 21lbs difference between cyl's at 25lbs the idle start's getting a little choppy!
cyl. pressure variance is one of the best indicators of engine condition and yours is starting to decline.

 

What do you mean its not possible? It is possible to derive an empirical dynamic or cranking compression ratio.

 

Basically you can have a low compression engine with high numbers, and a high compression engine with low numbers...it all depends on a lot of factors, ring seal, camshaft overlap, etc...

And I also agree. I've done with 5% maximum variance for a while, but after that I would rebuild. Even at 200psi that's still 10psi from one cyl to another...time for a rebuild...

 

Sure, but what's that good for?

 

Thats what really matters, static compression ratio means nothing. Engines are not a static system.

 

The problem is the engine doesnt run at 250 rpm. A running compression test gives a more accurate dynamic compression ratio, which isnt what supermaxxbasher was looking for.

 

Not sure if I'd be thinking rebuid just yet.

Coulpe factors come into play..
First, you're so close to being within 10%, I think I'd want to know if it's in the topend or not, add a little oil to the cylinders and see if the readings increase, if not, the the loss is in the valves, not the bottom end.

Also, was the test ran on a warmed engine? Throttle at WO? Results can change.

Lastly, are there any issues with it running?

 

Main thing you are looking for in a compression check is difference in pressures between cylinders as stated earlier. If you have differenes of more than 5-10%, then its looking to be time for a rebuild. Do you have to rebuild?? No... You don't have to, but your engine is no longer operating in an optimal state.

 

the car leaks alittle oil but it does use it and anything about 3500 rpms and it blows a blue smoke screen.. The idle is very inconsistent as in it jumps around almost 250 rpms at idle which is about 600rpm but normally idles up to 850 rpm then goes back down and it does this constantly. I know i have a bigger cam but I know the difference between a lope and say a miss which it acts like but everything on the ignition side is brand new and in good working order. As for it being the valves I will be pissed off if I have any heads problems whatsoever cause these are brand new edelbrock rpm heads. but with my luck you never know. Oh yeah the engine was warm but the throttle wasnt touched.

 

Blue smoke...burning oil...ossibly bad rings, and that would show bad compression from one cyl to another.

 

First off, if the engine was warm, your readings will be much higher than they would be if the engine was cold, because the gas went in cold and became warm while it was in there, which raises its pressure; how much higher, is anybody's guess. Totally unpredictable. So right there, your numbers are totally meaningless.

Second, you didn't have the throttle blocked wide open, so there's no way of guessing how completely the cyls got filled. How much less than a complete fill, and consequently a lower reading, is anybody's guess. So there again, your results are useless.

But even if you had performed the test correctly and come up with numbers that are good for something, there are still problems with getting what you want out of them.

Any cam with an intake duration of more than 180° will not close the intake valve before the compression stroke begins; and some amount of the intake charge will be forced back out into the intake. Exactly how much, depends on the engine's rod length/stroke ratio, the amount of intake valve opening, etc. So right there, some unpredictable amount of compression will be lost.

Compression in an engine is a somewhat adiabatic process. That is, when a gas is compressed, its temperature goes up; when its temperature goes up, its pressure goes up; when the pressure goes up, the temp goes up; and so on, up to an equilibrium point of temp and pressure. So the actual cyl pressure in a perfectly sealed and perfectly insulated engine would be FAR greater than merely the CR x atmospheric. If you could take in air at some temp, compress it, and hold it in the cyl without leakage until it cooled back down to exactly the temp it was at before it was compressed (in other words, convert it to an isothermal process - which is one where temp is the same after the process as it was before), and you could totally eliminate the valve opening loss problem, then you would be able to figure out what you want. Or, if you could be certain that no heat escaped (the gas didn't cool) during the compression process, then you could solve the differential equation for adiabatic compression. But engines aren't perfectly sealed and are far from perfectly insulated; so the actual pressure will end up somewhere unknown between what it would be if the process were adiabatic, and what it would be if it were isothermal. Problem is, it's very difficult to characterize the heat loss out of an engine, and therefore how adiabatic or isothermal the process is; and even if you could, the actual cyl pressure would depend on how fast you cranked the engine (and therefore how much time the gas in the cyl had available to cool down as it approached max compression).

As you can see, it's IMPOSSIBLE to make a correlation between compression tester readings, and compression ratio. There's just way too many variables to be able to come up with some sort of calculation that says if my tester says X then my CR is Y.

I can tell you though, from years of taking readings, that a motor that will run on pump gas will usually read between 160 and 190 psi on the gauge when tested correctly (COLD engine, throttle blocked open); and as stated earlier, you'd like to see as little variation among cyls as possible.

So go back and do it again, and do it right this time, and then we can see if there's anything to be learned from what you observe.