Thread: Compression Test Woes

Had my engine rebuilt using CP 8.5:1 pistons, ACL bearings and VET head gasket. Proper break in procedure followed, plenty of downshifting and light-medium driving.

Did a compression test about 20 minutes after being turned off. Came up with about 120 across all 4 cylinders. Very low. Engine has about 1100 miles on it. Any ideas? I wonder if the decking messed up the timing enough to throw off compression w/the S4 cams? I doubt it, but a theory I had. Another idea was head bolts need re-torquing.

If the engine is fucked, I'm parting it out never to return to the SR20 world again(I'll still visit)

-G

8.5 compression plus s4 cams,Enough said, Did you follow the instructions as far as gapping the rings. I wouldn't worry about it if the motor runs and sounds healthy.

I agree with Gio. Don't let this scare you. How is the oil consumption?

dynamic compression doesnt say much as long as you have no blow by and they are all in range then your fine, i get 150 to 160 on regular des so i dont think this is a horrible number one of my very good running motors had 135 across the board

How warm was the motor and did you follow the factory procedure. Also the gauge might just read low. The numbers are all equal and thats what matters. Do a leak down if you really want to know how the engine is doing. I agree those numbers are low but you also have s4 cams in the motor.

I drove about 5 miles to my friend's garage and then shut off car..waited 20 minutes then did compression test. Car was pretty warm, but not operating temp. I want to try another one right after fans come on. We did it with full throttle, fuel pump fuse pulled etc..We also tried 2 gauges and both showed 120psi(Second time we tested was probably an hour later after getting the 2nd gauge).

Numbers are low, especially for the fact that the engine/head were decked quite a bit and everything is brand new. I actually was expecting the compression to go up and numbers to be 160+

Any ideas what could be causing the low numbers though? I want to figure out of it's something I can fix or if something is wrong. Another thing besides the head studs and possible timing problems from the decking was that the piston to wall clearance is so large that compression is getting by the pistons. I remember the builder telling me how much pistons and cylinder walls balloon under high boost that I think he machined for a high piston to wall clearance as I can still hear some piston slap once fully warmed. I've been having a high idle issue I've yet to figure out. Seems to do it only when car is fully warmed up.

BTW, since the last oil change I've put about 600 miles on it and I haven't noticed much if any oil consumption.

-G

Your not going to get 165+.1.you are 8.5 compression 2. S4 cams do lower the numbers when you do a compression test. 3. After market piston have a somewhat looser tolerance since it is a turbo application and the material expands more than factory.
Your not going to get 165.
Not a gasket issue unless your pushing water like crazy.what was the tolerance between the piston and cylinder? What was the gap given for the rings since they have many options depending on the use.

Nope, i wouldnt worry about it if your numbers are even. The s4's will lower the compression like everyone said. I had my head and block decked before my build and my compression was 145 across the board. Only thing with mine was that during initial startup i had no oil pressure due to a broken oil pump (long story) and it caused the rings to wear improperly. Did 3 startups and revs with no oil pressure running 20-45 seconds each time.

Only issue I had was a bit of blowby under boost but that was to be expected. It didnt consume any oil and it ran like a champ and put over 11k miles of torture on it. Compression every time I checked was always the same. 145 across.

Like they said, if it runs good and sounds healthy then dont let the numbers scare you. It could have been too big of a ring gap or too loose of tolerances on the p2w clearances. Either way I wouldnt worry.

I actually don't know the clearances. I'll have to contact my machinist to find out.



I'm going to check the compression every 500-1000 miles or so and see if it goes down. If it stays around 120 I'd feel a little better about it. I'm just worried something is off or wrong.

A friend of mine suggested since the compression is so low to run 18psi daily..nothing a little more boost can't compensate for

I'm going to do another compression test @ proper operating temps this weekend and see what I get.

-G

My DET has 120-130 PSI compression too. Ran just fine. Make sure engine is completely warm, make sure to pull all of the plugs, also make sure to WOT when cranking.