I have asked this question in another thread but I never had a real answer. I have found a jspec s5 engine and I will do a compression test on it. The engine is out and the transmission and the starter still connected to the engine. The engine is inside. How accurate would be the compression test if the engine is not warmed up ? Will the compression be lower or higher than a warmed engine ?

Thanks for help guys!

 

lower. also you should make sure it is well lubricated or it will be virtually zero.

pat

 

If the combustion chambers still have an oil coating (i.e. OMP is still working with oil still in the oil pan), the compression numbers will actually be *higher* by about 5-10psi.


-Ted

 

the results i have gotten when doing tests with my mazda tester is cold motors give lower #'s 10-20lbs or in that area. if the motor has sat a long tim e and the oil coating has gone away it will give really bad #'s and if you put atf/mmo or oil in the motor its going to give you a faules reading... being higher

 

thanks guys, finally there is no way to get an accurate compression test on cold engine. I will put some oil in the engine to make an oil coating and will consider this in my compression result.

Thanks again!

 

i just did a compression test on my engine. I got 50 in the front and 45 in the back. this is on a cold motor by the way. This is from a jspec motor i got shipped to me, so who knows how long it has been sitting in the junk yard. Do you think that these are ok #. As soon as i get it running i will post back so everyone can see the difference from a cold motor to a warm motor.

thanks

 

Even though the engine was cold, its pretty obvious that engine is worthless and will need a rebuild. However, you could be getting false #'s due to your testing method. How did you test it?

-Joe

 

Ted,
Not to doubt you, but in my experience the opposite is true. Cold = lower. Did you get this info from experience or from someone who told you that? I know you have ALOT more rotary experience than I do, but its strange that we get different results.

-Joe

 

This I have seen with my own eyes, and I've confirmed it with others with extensive rotary engine experience even more than mine.

This can be dependent on oil injection versus pre-mix and even down to oil viscosity...


-Ted

 

i know someone that did a cold compression test and got ~50+ and when he did a worm test after it was running had 110+ on both. mybe his tester is fubared but i used the same tester so i feel good about my #'s.

 

The really only meaningful numbers are done under Mazda specifications, which require a warmed up engine and cranking RPM's within the Mazda spec.

It really doesn't matter what the engine does cold.


-Ted