1st post all.

I've read up on this on this matter just wondering what everyone here thinks.

110-120 psi across all 8 cylinders. stock 1998 z28 LS1 862 heads.

is this low? compared to the numbers I've read on here its about 30 psi lower than most.

122k miles on motor.

no driveability issues, im considering a motor build and i'm looking to do a cam/port/polish 243 head swap. but if these numbers are a tad low then maybe ill just get a short block assembly. with forged internals. seems like it would cost about the same as getting the old block rebuilt.

 

Squirt a spoonful or so of oil into s cylinder and see if it improves. If it does, the rings are going. If not, that indicates valves. In which case, doing head work will do wonders for you.

The stock ls1 cam has very low dynamic compression and doesn't build big cranking pressures. Most of the numbers you've seen on here are from aftermarket cams.

 

120 is too low. Are you doing the compression test correctly? Open throttle plate, disabled injectors? You don't want fuel spray into the cylinders when you're doing the test, 10 full revolutions per cylinder. All plugs out when testing.

 

Forgot to make it WOT. Yes fuel pump disabled and coils disabled. Cold motor, and I only pulled 1 plug at a time per each cylinder I was testing.

 

From what I understand, inserting oil into the cylinders will make compression better even if the rings are good or not. However if there is a drastic increase its more of an inclination of rings beginning to fail.

 

if low compression is due to valves, it the added oil won't help much of any. That's why it's a good diagnostic check. Since you're planning heads anyway, if you can validate the rings are ok, you'll save a chunk of change

 

Yea this is true. If I were to do this proper for a diag of a culprit I would really need to do a Leakdown test.. Which is what I may do but I only have my hands on a compression tester atm. Compression was very consistent across the board tho which made me 2nd guess my thoughts on the rings possibly being worn. The numbers just seemed low compared to what most got. Service manual isn't much help lol "100 psi or less being bad." I really think though it's probably a ring issue. She consumes oil a bit, not smoking any but I can smell the oil in the exhaust at times (no cats).

 

make sure you double check with a next compression tester.

One time i went to autozone to rent one. It gave me a poor reading... shall I say too poor to be driving the car there. I figured it was bad, got the next one, and the numbers although higher were still poor. I thought that was it though, second opinion and the motor was toast etc...

Figured I would try one more time. third compression tester and everything was good.

Keep in mind that couple other factors will influence the reading also include:
1. engine starter turning speed
2. plugs are all in or out (turning speed)
3. throttle open or closed (maybe, depends on the engine and IACV path if it exists)
4. Battery health / Voltage (try jumper cables if battery is weakly turning motor)
5. oil in the cylinders

Also some food for thinking, I had an engine I thought for sure was toast, sitting on the ground with the worst compression test result, two or three cylinders were terrible (50-80psi) the rest low, and one high. My friend works at toyota, came over with some toyota carbon remover (not sure what the name was) pulled the head and let it soak for a while, cleaned up the valves and the engine came right back to life with the cleaned up head.

 

Yea this was a vatozone rental haha.

Battery was in good shape, wasn't dying down on me.

I got a few buddies from when I used to work in autoshops around town so maybe I'll borrow one of theirs soon and give it another try. I did this yesterday 3:00in the afternoon in unshaded southern Texas summer driveway so needless to say I was kinda rushin. Especially when I got to the #8 cylinder haha... I was lookin for some reference numbers.

 

It'a got to have fast rotational speed and lot's of air going in to be accurate, IAC valve isn't enough in any case, don't introduce oil in the cylinders until you've done it without.

 

A high quality, brand name compression tester is what you want to use. Those cheap/rental ones tend to be wildly inaccurate. I get this all the time come winter (2 stroke season), when engines get checked out by customers with cheap testers, and show very low readings. They freak out and want to know why it runs so well with 80 psi in each cylinder, I have them bring it down and run it on our tester, and boom, 165 PSI!

 

If it were 110-120 across the board my first thought would be bad gauge. It wouldn't run very good with that low of compression, if at all.

If those numbers are in fact accurate I would look for the cam to be out of time. Dynamic compression includes what the cam is doing and can change based on the cam size and timing.