How do i convert 11.55:1 compression into a psi #? I tested 3 of 8 cylinders today and got 165, 180, and 182. The 165 is a little low, but i don't think i turned it over enough to get it up to 180. Probably tomorrow i'll attempt to do all 8.

 

There IS no conversion from compression ratio to cylinder cranking pressure. The compression generated in a cylinder is as much a function of valve (the cam being used) timing as it is of the CR. If there were, what would you hoping to figure out? Is one cylinder in your engine built to a different CR than the others? The actual pressure is not what you need to be looking at. It's the variation that should concern you. You need to go back and redo that and preferably all the cylinders. If you have a low cylinder after making sure you have done it correctly, you should go back and retest with a squirt of oil in each cylinder. If the pressure jumps up, "wet", the rings are leaking. If not, it's more than likely the valves or a head gasket. Two low adjacent cylinders are a classic indicater of a blown head gasket between these two cylinders. Good luck.


[Modified by CFI-EFI, 8:41 PM 6/16/2002]

 

Ok, thanks. A friend with 13.5:1 compression was getting 230psi, and another friend with stock 10.5:1 comperssion was getting around 160psi.... so I figure 180psi is just about right. I didn't check the cylinder adjacent to the one with 164psi (because i couldn't figure out how i'm going to get the compression tester in there). I'm going to try all of them over again and crank it longer this time. Probably 3-4 seconds to get a full reading. Hopefully i don't wear out my starter with this high compression, lol.

 

Crank the same number of times (4 or 5) at each cylinder and note the initial and final pressure values.

 

you recommend hand cranking, or running the starter? hand cranking by what you're saying to do.


[Modified by Glock'94, 10:19 PM 6/16/2002]

 

I know of a 9.5:1 that reads about 200 psi..... :eek: :D

CFI-EFI is correct, there is no formula for calculating cylinder pressure as a function of compression ratio for an internal combustion engine. If your car didn't have valves and a cam then that would be a different story. :)

:seeya

 

Yes, that is correct. there is no real conversion table for the ratio to psi question. Ratios vary with the various parts used. Most ratio claims are only estimated guesses.However when you take compression readings, you do have to be consistent on the amount of cranks to some degree. Throttle position also can affect the reading to some degree. The wet method serves to give some indication of ring condition. However, one test that also could give some indication is a leakdown test. None of these tests however are completely conclusive. But they are a good place to start. If you have good readings consistently throughout the motor, you should be in pretty good hands. I am not sure what you are looking for. I normally do a compression test right after the rebuild and periodically as the motor gains some run time. I use this as a basis for judging motor condition. Any deviation may indicate a problem worthy of a teardown to find the real problem. For a normal "gas burn", you need a minimum of 175 psi. Anything over that should be okay with readings over 225 to usually require a higher octane fuel. This is very general though considering that air density and humidity also play in to the equation. I am not a real expert though. So any questions regarding the normal psi reading for a particular engine should be referred to the person who designed that particular engine.

:cheers:

 

I bet I could model it closely.. but I don't wanna. :lol:

 

:lol: :lol: :jester

 

Oh come on.. you know you want to :jester

 

thanks sharky.

 

In addition to the above:
1) Remove ALL spark plugs first.
2) Make your throttle body stay wide open during testing.
3) Get an angle adapter for your compression guage, or remove/loosen exhaust manifolds as needed.
4) Crank engine same approximate number of times for each cylinder.

If you skip any of the above then you will have wasted your time. Good luck.

Thomas

 

Can you explain #2 and #3?

 

Sure:

Throttle should always be wide open on compression test - this is SOP, has been so since the compression test was first done. *edit* With throttle closed, compression readings will be artificially low from airflow restriction of closed throttle (remember that your starter only spins the engine a fraction of your idle rpm, so ANY flow restriction will seem to be a large flow restriction to your engine.

Angle adapter; if you don't have enough space to thread in straight fitting and hose to compression guage, get an angle adapter that threads in first, and then thread your compression guage straight fitting. I can't tell you where to find such a part, my brother is a machinist and I just tell him whatever it is that I need and he makes it up for me if he can't find one in industrial supply stock.

Thomas


[Modified by The Green Rocket, 10:58 PM 6/16/2002]

 

ok, thanks thomas. So you recommend doing like i did before and letting the starter crank it?

I unplugged the injectors while doing this to keep from getting a bunch of fuel in the exhaust system (i got a backfire the first time i started it up after testing one cylinder).

 

I've been away for a day, so for all I know you have redone the compression test already...but if not:
I'm not sure how else you planned on cranking it over - yes, use the starter to do this.
I like to have between 3 to 5 spins of the engine, just make sure to be as consistent as possible from cylinder to cylinder.
It should only take a couple of seconds to get 3 - 5 spins, four secnds is probably excessive.
I don't like to do a compression test on a stone cold engine, fire it up and get a little heat (it is not neccesary to bring engine up to full operating temperature) in the engine first.

Thomas

 

If you are going to remove all the spark plugs to run your compression test, don't remove them from a hot engine. Aluminum heads are easy to ruin the spark plug threads when hot. When you are cranking the engine during the compression test, you can hear the starter labor on each compression stroke and I always do 5 compressions on each cylinder and then record the number on the compression gauge. If you have someone operating the starter, explain to them to listen for the "labor" on each compression stroke and to stop cranking after five compression strokes. In an ideal theoretical case, you can multiply the atmospheric pressure by the compression ratio to obtain the compression gauge reading, but like everyone else in this thread, this reading is modified by the slight leakage past the rings and valves and by the fact that there is a pressure drop in the intake system during the intake stroke which must be accounted for. Absolute pressure isn't as important as the difference in pressure between cylinders. A rule of thumb is no more than 5 psi on a low miles engine or up to 10 psi on a high miles engine.


[Modified by jfb, 6:33 PM 6/19/2002]

 

Let me repeat, just a little heat in the engine prior to compression test - NOT fully warmed up.

Thomas

 

:yesnod: :yesnod: