Bad Compression test results

[Tommo560]

Cant find the exact numbers but the tuning shop just did a test and got numbers like 110-120 with one even as low as 84 in a dry test, and about 120-140 wet.

just had the motor rebuilt 5000 miles ago, and just had the head off for cam change and valve grind 100 miles ago. It is a high performance motor l28, p90 worked head, triple 45 webers, flat top pistons, medium cam (unsure of overlap), and the car makes about 180 bhp at the wheels. i really dont know whats happening, i suspect that it's rings because i just did a valve grind and its blowing a little blue smoke at idle, but then i had problems with the tensioner coming out and read that timing and a stretched chain could mean that the compression is low (chain did look a little stretched according to the datsun repair manual and from advice from a guy who did some work for the p.o (the cam designer of the new cam) the cam/chain is on position 3) but then again this car is making as much power as it used to when the previous owner had it (before rebuild) and before the cam change (switched to milder cam for the rev range we work in, but still has same top end power as expected by the cam designer) and has the same power as another z that we race with that has all of the same mods. these results are really confusing and i just wanted to know if anyone could make any sense of them.

Thanks,

Tom.

[Arne]

Those numbers seem quite low to me. My L28 has flat top pistons and a stock P79 head (stock cam). I get numbers around 165-170 dry, have never done a wet test. The fact that yours increases wet does sound like compression rings or scuffed bores to me.

[PrOxLaMuS©]

keep in mind EVERY compression guage is different..

i've had readings 30psi off between 2 guages.. so dont be too worried..

also keep in mind altitude, and density altitude play a HUGE part in compression tests..

was the test done with the engine at operating temp or cold?

was the test done with the throttle WIDE open?

was the motor cranked over the same amount every time to measure the readings?

if you live at an altitude of 2,000 MSL .. and it is 25* C outside.. the actual air density is similar to a 3,500 MSL altitude.. (not exact but i dont have my calculator next to me)

this makes a difference on a hot day

[Arne]

You're right - I should clarify. My numbers are with my own gauge, engine warm, throttles WFO, and at 200 meters above sea level (600 ft).

[PrOxLaMuS©]

ask a buddy to borrow his guage or ask a shop...

compare the results...

[Cethern]

I just checked mine this week and I had 135 for 5 and one at 140. this was done cold and dry with the throttle wide open. With as far as I know a Stock motor.

I'm off to do a search and see what stock should be...

Eric

[MikeW]

For the L24 my FSM lists 171-185 PSI.

[Tommo560]

Normal compression figures for a new or newly rebuilt l28 ( i think with p90/90a/79 and flat tops) motor should be about 170 psi, and i know this is supposed to be a high comp motor so this is where i think the numbers should be, and 130-150 for an older worn engine.

I dont know how the test was conducted, only that it was done at operating temps. the shop did it so i hope they know what they are doing but it would have been about 20-25 C's outside and about 20 metres above see level.

What i'm confused about is the varying numbers, the large diff between wet and dry, and why the car still makes so much power if it's only making 2/3 compression? i'm not sure of the overlap on the cam but i know it doesn't make any power (i mean very little) until about 3,000 rpm (mild/comparitively mild competition cam) and that it has a flat spot that we can't get rid of (due to bad comp?) at about 2500 - 3000 rpm that even the previous owner had problems with.

Tom.