Compression Session

[01sik]

Just wanna know what everyones comp tests where the last time you did one? Mine come back as 159.5 psi on three but no.1 was down 12 psi to 147? Bigport 4age question mainly, but throw results up anyway :)

[altezzaclub]

That's not too good- did you put a shot of oil in that low cyl and try it again. The oil will seal the bad rings momentarily and the compression will go up.

 

If it does, hook an air compressor up to the sparkplug hole an stick 100psi in at TDC. You will hear the air leaking out past the leaking valve.

[01sik]

Cheers ill give it a crack, I didn't really know what it should be for a 20 something year old engine though haha, need to do rings and valve stem seals anyway I think

[altezzaclub]

155-170 would be fine. The Girl's KE70 had 150, 150, 155, 160 when we bought it. I've done the valves but haven't touched the rings at all.

 

The well-thrashed gearbox donor car had 130, 130, 135, 130.

[01sik]

On bigport? Probly gunna do a ringset and stemseals soonish though to be sure, seems down on power too, doesn't seem to mak power all the way through the revrange, do the butterflies on the tvis stay open or shut when it is disconnected?

[iwontarolla]

Does compression ratio and cams change the compression test out come or not?

[orangeLJ]

short answer, yes.

 

Every engine will have a different compression ratio (depends on a heap of things)

 

generally, bar a few random situations, all cylinders should show similar compression figures, abnormal readings for one or more is the best indicator for anything being amiss.

[kickn5k]

Does compression ratio and cams change the compression test out come or not?

 

Yeah it will. These measurements are an operating cylinder pressure. More static compression will give more cylinder/combustion pressure. A bigger camshaft/s will bleed off the compression.

 

But back to OP. That reading is fine, usual rule of thumb(for a mild road engine) is if the operating pressure is less than 10% difference, then the engine is still within its service limit. But if it were me and I already had the engine out then I'd replace the rings and give it a "dunny brush" just so you know it's right.

[you_nes]

Yes on both.

 

I had a small heart attack when I comp tested a Small Block and it only had 100psi till I found out that was right on the money for an engine with 8.5 to 1 compression.

Hot/cold motor, thick thin oil, battery and starter condition and weather you hold the throttle open or leave it closed also all effect results.

[oh what a nissan feeling!]

if you leave throttle closed results will be null and void.

 

On sr20det 8.5, stock cr and stock cams i have generally seen about 155.

[iwontarolla]

Okay makes me feel little better I have shaved head but huge cam and have 130psi over all 4.

[01sik]

So I should do it again with throttle full open? And engine will probly stay in when I do the rings and some other bits, a mate has my crane :( so as lone as within 10 percent comparing highest comp to lowest should be still good, I need to get exhaust done too running factory one from the 4k :D