V8i cylinder compression results

- D
- Dadio
  
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posted
18 years ago

Sat, Aug 20, 2005 7:40 PM

Ok, so it looks as though I will keep my existing 145,000 mile Discovery and I am now turning into a bit more of a mechanic than expected. Needs must....

1995 V8i 3.9 I have just replaced all the plugs and took the opportunity to compression test each cylinder. I did not do it quite according to the book but took one plug out at a time (leaving the others in) and spun the engine for about 10 seconds with the coil disconected and the accelerator depressed. Results were reasonably consistent between 165 and 170 throughout. Is this good, bad or average? I am a bit confused by my Haynes manual as it suggests two different typical compression results for the 3.9, one for the low compression version and one for the high compression version. How can I tell which engine I have (low comp or high comp).

Thank you

Les

- R
- Richard Brookman
  
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posted
18 years ago

Sat, Aug 20, 2005 7:55 PM

so Dadio was, like...

Good to hear this. :-)

Sounds pretty good to me. You really should have all the plugs out, but your results look healthy. The most important thing is that they are consistent - ideally within 10% of each other. Yours are well within this. FWIW, mine were all around the 130-140 mark (3.5V8) and the engine still goes well.

You're not alone there!

I think it's cast on the top of the exhaust manifold. 9.3 (i.e. 9.3:1) is high comp, not sure what low comp is, 8 point something. It's fairly obvious when you see it.

HTH

- T
- Tony
  
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posted
18 years ago

Sun, Aug 21, 2005 7:37 AM

There are two way's to tell the compression ratio: On the nearside of the block there is a small flat with the ratio stamped on it or by the ignition timing Low comp No CAT 2deg BTDC, High comp No CAT 4Deg BTDC, High comp with Cat 6Deg BTDC.

As a matter of interest my 3.5 with a 9.35 comp has pressures of 165-170 across all cylinders.