sr20det compression numbers

[HeroofNobody]

question about compression results


I?m about to buy this sr20det that?s still in the car and his car has been sitting there for a few months because it got state reffed i was wondering would 130psi across all 4 cylinders be normal it?s a upgraded bottom end with thicker head gasket, arp studs and new seals we did a compression test after it idled for about 5 minutes and we also had to jump the car because battery was completely dead. i know you want around 150 psi but this head was rebuilt 500 miles ago just been sitting so could that be affecting the compression?

[Future240]

Moved to chat. While technically engine related (good job there) this is a more general question and you are likely to get more traction here.

[Kingtal0n]

Compression test results need to be verified
1. is the tester good? Always use 2x testers because one will say 130 the other will say 150

2. did you do the test properly?
A. The engine needs to spin some minimum RPM to create the compression, so Battery need to be full and powerful
B. All plugs out
C. throttle body wide open
D. other factors I probably forgot?

3. test should be done DRY + COLD, then WET + COLD, then HOT and compare

4. Write down the numbers using a decimal, i.e. 130.5, 132.3, 137.9, etc...
5. repeat the test to be sure it went well
6. How many times did you let the engine spin? It typically takes at least 6x revolutions to 10x revolutions (some even number) to get the real final results. One revolution or two won't cut it.

My recommendation is repeat the test under these conditions and use 2x different testers (everything I posted about)

Finally, if you find that the compression is only 130~psi then it means the engine is high mileage, perhaps 200,000+ miles. Otherwise, the cylinder ring and bore was worn prematurely, which I have seen happen to engines where an atmospheric bypass (blow off) valve was used with a maf sensor. When you shift with the atmospheric blow off the maf briefly washes the cylinders down with fuel, ruining the oil film and causing excess wear.

[HeroofNobody]

Originally Posted by Kingtal0n

Compression test results need to be verified
1. is the tester good? Always use 2x testers because one will say 130 the other will say 150

2. did you do the test properly?
A. The engine needs to spin some minimum RPM to create the compression, so Battery need to be full and powerful
B. All plugs out
C. throttle body wide open
D. other factors I probably forgot?

3. test should be done DRY + COLD, then WET + COLD, then HOT and compare

4. Write down the numbers using a decimal, i.e. 130.5, 132.3, 137.9, etc...
5. repeat the test to be sure it went well
6. How many times did you let the engine spin? It typically takes at least 6x revolutions to 10x revolutions (some even number) to get the real final results. One revolution or two won't cut it.

My recommendation is repeat the test under these conditions and use 2x different testers (everything I posted about)

Finally, if you find that the compression is only 130~psi then it means the engine is high mileage, perhaps 200,000+ miles. Otherwise, the cylinder ring and bore was worn prematurely, which I have seen happen to engines where an atmospheric bypass (blow off) valve was used with a maf sensor. When you shift with the atmospheric blow off the maf briefly washes the cylinders down with fuel, ruining the oil film and causing excess wear.

thank you he?s going to buy a new battery and test it again.

[inopsey]

should mention that a thicker headgasket will lower compression (pressure) due to an increase in volume. numbers dont mean much on their own, its the consistancy between cylinders thats important and how the engine runs. (does it burn oil etc) another question to ask is how recent the rebuild was; has the engine been properly broken in?



combine the weak battery with a lower compression ratio from the thicker headgasket, im willing to bet that the engine compression numbers are good, but compression numbers only tell part of the story when determiniing if an engine is in good condition.

[burnsauto]

it's always a good idea to do a leak down test in conjunction with a compression test. I know it's a pain in the ass and the seller may not like it, but if you wanna spend it, and he wants to make it..