So I bought a JDM engine and put it in my car.

I had some problems starting the car ( bad gass but didn't know) so I tried the atf trick, didn't start either. So right after, I took a compression test and it showed 120 PSI on both rotors. I switched the gas the same day and it started right up. So the ATF didn't have time to eat any seals...well, not in 2 hours anyway.

Now just for fun, I took the compression this afternoon and it showed 92 PSI on both rotors. Why?

Is it because it had oil in the chamber and it showed higher compression or am I slowly killing my engine and dont know why?

 

Probably just due to a lot of oil sealing things up and taking up space. Besides, you'll only get very accurate numbers with the engine warm and a Mazda comp. tester anyway.

Just to be safe, though, your metering oil pump is oiling things good?

 

Because its JDM yo!

Ok got that out. I think it is probably as you stated, the oil in the housings might have given a temporary higher number. But I am not sure. Is the engine cranking at the proper speed to get an accurate test?

 

Running premix 1/100.

My starter turns just fine. And its the same starter I that I used when I first tested it.

 

I would say it was the added ATF in the housing that gave it a higher initial reading. Hey, 92 is higher than some I've seen!!

 

how does the starter rpm afect a comp test?

 

Just wanna be sure its not going down.

If its at 92, I'm not worried. 92 is pretty acceptable for a TII.

The compression test is supposed to be taken at 350rpm if I'm not mistaking. If the starter turns slower, you'll have lower compression readings.

 

Putting ATF or oil into the engine will cause the compression to rise due to better sealing. But the effect goes away as soon as the stuff burns off.

Compression tests are evil anyway. As long as the engine runs, makes good power and starts hot or cold, the compression is fine.

 

haha, thats the default answer we need to tell others when they have a compression issue. Numbers? who needs numbers?

 

Thats what Aaron is trying to say.. You don't have a compression issue if it runs good. When you DO have a compression issue or other "My car won't start" issue, the numbers are exactly what you DO need.

But if everything is running, go drive it!

 

make sure that you didnt do what my friend did.
Do the first test without having the throttle @ WOT, then do the second test @ WOT. There will be a difference.

 

DOE!

Thats what I forgot...

Thats why I got such low readings. I didn't tell my gf to hold it at WOT.

Well, guess I'll take it again when I get home.

Quote of the day.

 

As long as the engine pulls good vacuum at idle, and runs the way Aaron describes, then your engine is good.

Otherwise... you should take back your statement in the other thread!

 

I wont, it still has +/- 120 PSI I'm sure of it.

I just took the compression for fun to see what the difference between trailing and leading plugs would be...Then I pissed my self immediatly.

But now, since I know I'm a retard, I feel better.

Quote of the day.

 

You are welcome.

 

Thanks man.

 

the seals don't form a perfectly airtight seal, so you get a higher reading if the rotor spins faster because there's less time for the air to leak out.