3.8 compression test

XK-Engine
#1

Hi,
I have a 3.8 mk2 and did a compression test this week and wondered if anyone had any comments regarding the results. I thought they were pretty good, but don’t know what they should be. I think it is an 8 to 1 compression engine, standard head and cams. The inlet valve clearances are all a bit tight.
Both tests were done with a good battery, all plugs out and throttles wide open. Engine cold and compressions were in the range 140 to 145. When hot compression ranged from 150 to 155.
All within 5% so I am pleased, but are they good, bad or indifferent?
Also would the tight inlet valve clearances (all about 1 thou) make much difference?

Thanks
Dave

#2

Sounds about right for a 3.8

#3

Cannot see why there should be much difference between engines so I will offer that my 4.2 with new rings at 65,000m, valves at 75,000m and now 78,000 miles has compression of between 155 -160 warm.
As for valve clearance, my feeler gauges only go down to .002 and I think I might find it difficult to push the paper thin gauge under the cam. I would deal with those valves promptly if I were you, the car may soon get hard to start.

#4

If it’s 8:1 then those are fine numbers. Frankly even if it was 9:1 it would pull hard with that compression. Mine was 155 before it had to come apart for a broken chain guide and it was still running fantastic.

#5

Dave,
Are you saying that your inlet valve clearances are all only 0.001 inches? If they are, I recommend that you not run your engine until you adjust them to spec. If you are saying that they are 0.001 inches less than spec then that should not be an issue but it would be best not to ignore them.

Paul

#6

Purty sure he’s saying about 1 thou tighter than spec…

#7

That’s how I have read it and in that case it sounds fine. My 9:1 4.2 had higher numbers but consistency is important. Pressure depends on many factors, starter, battery condition,…

#8

Hi,
Thanks for all the responses. I don’t think I need to worry about the compression then.

The inlet valve clearances, I think are a problem. None of them were at the correct clearance of 4 thou. Most were 2 thou, one or two were too tight to get my 2 thou feeler gauge in. I think this is my next job.

All the exhausts were very nearly spot on 6 thou.

Dave

#9

Oh yes…personally, I would set them all at between 4 and 5 thou on intakes, 6 to 7 on exhaust. You may well see slightly higher compression figures once the intakes are at the bigger clearance.