1940 engine compression test results

[Garyr1016]

Compression test results on my 1940 straight eight engine

engine cylinder # and--lbs/sq.in

1--40

2--70

3--35

4--15

5--80

6--55

7--40

8--65

It does not look good, I'll try to get the car running (see previous post "No spark") to see what other mechanical issues come up. Is there any way I could just do the #4 cylinder? That's to say if it's not cracked. I will check around for pricing on redoing the head, new valves, seats, guides, springs etc.. And I know it will run rough given the cylinder to cylinder pressure differences. But I also know that with a redone head the old engine will probably start smoking like my great aunt Patsy or worst throw a rod/break a piston. Or, is the factory spec. compression ratio of 6.5 to 1 forgiving enough to tolerate a new head???

Advise, comments on how to proceed.

Thanks

[Gene Brink]

Gary,

Assuming these are "dry" readings. Have you run "wet" yet and if so what are they? I'll go out on a limb and guess your problem is in the head and a valve job will probably be all you need (although I'd want to run a bit to see if things did not improve with a bit of use). I'm guessing that with one cylinder at 80# that the other bores should not be worn greatly either so may be in pretty good shape. A wet reading, if the cylinders/pistons/rings are fairly decent, should show little change (indicating air is still escaping past the valves [could even be a head gasket as 3 & 4 are next to each other]). If pressure jumps a bunch on any given cylinder that cylinder will have excessive wear (or even a cracked piston).

[Charles2]

I think your problem is with the rings. On an engine that has been long in storage, the cylinder walls have "drained" so there isn't enough oil there to effect a seal. Pull your plugs, pour about a teaspoon of oil in each hole, wait a while and then recheck your compression; I'll bet it goes up a lot and hopefully will stay there. If your valves were bad, I think the readings would be a lot more variable and higher on more of the cylinders. But, if the readings don't change after oiling then you most likely have a valve job in your future. Also, you may have both bad rings and valves. If your plugs showed evidence of oil fouling when you removed them, then the rings are suspect. Was the car running when it was put away? If so, then you should be able to get it running again, at least well enough for clear diagnosis, without having to tear it down.

[Garyr1016]

Update 4-27-05

Thanks for the advise, I'll work with it a bit and see where it goes. I was able to get the engine running (happy dance) and as you suggested I'll retest and see what the "dry" and "wet" results are after it is broke in a bit. It runs pretty well considering the variance of the orginial readings.

[Buicksplus]

Gary:

Don't panic just yet. I have seen this sort of behavior many times from engines that have sat a long time. I think you have enough compression to get the engine started, once you get the ignition and carburation working.

If you do manage to get it running, I think the low compression readings may improve with time. Valves do not seat properly, valve seats corrode, and rings stick when engines have sat for a long time. But running them for a while can do wonders to loosen things up.

Get some run time (several hours would be good) and then check the compression again. I think you might find considerable improvement in the numbers. If not, then you can think about what else can or should be done to the engine.

Bill.