Hot vs Cold compression

[imdrax]

Trying to diagnose a newly acquired 99 gsx. The other night when I had the car dropped off I ran a compression test on a completely cold engine, results were 180-30-180-180. Today I started the car and drove it around the block and got it partially warmed up. Ran another compression test and this time got 180-160-180-180. I have NEVER seen such a deviation between a hot and cold compression test. Does this stick out to anyone as a potential result of a certain failure, i.e. HG or valve?

 

[bsbllfit7]

this could be something to do with worn piston rings. A way of testing is testing the bad cylinder when cold and see if you can get the same low reading. If you get the low reading pore some engine oil into the actual cylinder. Then try and do the compression test again, if the numbers are higher this time then you have worn rings.

 

[imdrax]

Originally when i did the cold test I added oil and retested, same result 30 psi. When i ran the hot comp test i got 160, then added oil and got 180. I am going to wait till later and retest cold to see if I come out with 30psi again cold.

 

[bsbllfit7]

i would see if you can recreate the 30 psi test again, if not then it might have been a fluke or faulty connection.

 

[imdrax]

Car has sat for a few hours, still got 160, hmmm. I guess i will let the car sit overnight and try once more. I may have gotten really lucky with this car, bought it expecting to pull head($1300).

 

[farmer890]

Reviving an old thread. My cold compression numbers are 135 psi across the board. This is on a fresh rebuild which was done by a machine shop that typically builds V8's.

I fear that the piston to wall clearance might be too loose, especially since a friend of mine (who had his talon built at the same shop) had too much of a PTW gap. He sent his car down to English Racing and they had to rebore his block and everything.

I haven't run the car to operating temp since the rebuild, only did a cold compression test. Is 135 psi too low? Do I have something to worry about here?

 

[biglady112]

That depends. Those would be good numbers for where we live. We
Live at 6000' though. The fact they are even is a good thing. We have run as high as .007" piston to wall clearance and had good compression. It pushed a little more oil but, our compression and power numbers were still very good.

What about cold versus hot numbers? Throttle open during cranking versus not? Leak down test numbers? A leak down will tell you far more than a compression test will. Especially when you have even compression. I say run it and enjoy.

 

[pauleyman]

If you haven't gotten to operating temperature you haven't even seated the Rings yet

 

[jakk220]

Agreed. You need to break the engine in before you will get accurate compression numbers. There are different opinions with everyone on how to break in an engine. This one seems to be the most popular and has always worked perfect for me.

http://mototuneusa.com/break_in_secrets.htm

 

[miliman13]

How about telling us what you ptw Clearance is?
Ptw c , comes in different ranges based on desired results
135 can be considered low if comparing it to an oem block, built with the same tolerances and plenty of break in time.

However higher power builds tend to come with less compression results since they have more room to allow for expansion of alternative metals which have different heat capacities .

As far as break in , 'There are so many interpretations of what is believed to be accurate' i recommend engine braking ... as in drive to 3k let of throttle allow engine to naturally hit idle without changing gear, or raising gear .. Lowering gear is ok so long as it does not raise past 3k

Rinse was repeat to, 4 5, 6k

 

[farmer890]

Thanks for all the replies guys!

Totally didn't even think about the rings not being seated yet. Haha that's a tad embarrassing. But I'm a newb so it's all good! This is my first rebuild and I just got a little to horny to see some numbers is all.

As for piston to wall clearance, I have absolutely no clue where my paperwork for the build is because I'm a huge stoner and lost it . I called the machine shop that did the work and they told me they have no records or extra copies of my specs. So I'm SOL there I guess.

I've done a lot of reading about how to break the engine in, but I'm not that far yet. I've still got a couple things to finish yet.

Thanks again for the help guys, unfortunately I won't be able to give you any numbers or anything until next spring. She's getting put away for the cold months and I need to save up in the mean time. I was just worried my engine was shit right from the machine shop, so I replied to this thread cuz I didn't wanna be worrying all winter long.

I have dsmlink V3 but I'll likely end up taking it somewhere to get tuned as I'm too far into this thing financially to do it myself and f*** it all up.

 

[donniekak]

Whatever you do, don't put a lot of idle time on the engine before you break it in.