Compression test results, opinions please.

O

old31

Well-known member
I have a 6.1 Snyders head. About 3,000 miles on the engine. Head was torqued down many times.

So, what do you think. Just drive it like I stole it and not worry about anything, or?

1: 89
2: 82
3: 71 (80 with oil added to cylinder)
4: 71 (80 with oil added to cylinder)
 
Drive it!! Those are good numbers!!
 

Attachments

  • walter dripping nose.jpg
    walter dripping nose.jpg
    19 KB · Views: 10
I would expect the numbers should be closer. You may want to do a leak down test which may give you additional information. I am a little concerned with #3 and #4 cylinders both being low and having the same numbers. It may be that there is a little leak in the head gasket between these 2 cylinders.

My opinion,

Chris W.
 
Last edited:
As said, sounds like you can drive it as is - particularly if you feel that it has power at 45 - 50 mph. Otherwise, put a new head gasket on it and then check the numbers after heating it up a few times and torquing it down each time. I am assuming that the head and block are level.



Check the numbers between each torque. This will give you an objective comparison.
 
It appears you did a wet/dry test on 3&4,with a 10psi rise wet.This indicates the rings are losing the 10psi.Assuming you are using stock pistons and rings I would not worry,stock pistons use wide rings that actually glaze rather quickly.Basically its the nature of the beast,wide rings yield a long lasting bore but a reasonably high leakage rate.Using a piston with a narrower ring seals better...Ross pistons,SBC 283 pistons hold compression and control the oil better...me? I prefer the stock rings,leakage is upper cylinder lubrication in my opinion.
 
I would drive it but do another test after another 500 miles. Your test indicates something not quite right, maybe a head gasket, or perhaps valves not fully seating.
 
The fact that the drop you have is cancelled out after adding oil to the cylinder I agree it is the rings. If after oil in cyl and pressure remained low I would look at valves or head gasket. Since the only thing the oil would impact on is the piston ring to cyl fit It appears as the issue is there
 
Most anyone with even moderate mechanical skills can put rings in a Model A in an afternoon, just do it. Then you'll have a new head gasket and an opportunity to tighten up the bearings a bit too.
 
If I you were able to confirm that it was rings and everything else is OK, I would drive it a while longer before I would take it apart and re-ring it. The engine does not have that many miles and may get better with age and running.

Chris W.
 
I will go with what CW in Pasadena said. It could be that it is not entirely broken in and needs to run a bit longer. But, if you are curious you can take the head off and look at the cylinder walls. There may have some minor scratches on them. While you are at it, check the valves and valve lashes. The oil you added may have worked it way up to the valves to partially seal them.
 
Generally what you find when pulling the head is that the cylinder walls are glazed, normal due to the stock ring width. I would let oil consumption guide this repair, not compression readings. We are all guilty of applying todays expectations to a 90 year old design, model a engines will consume oil, cylinder walls do glaze, valves will leak (though not in this case).Its a human condition, we tolerate, even prize some quirks of it being an early design (manual spark advance comes to mind) but with others we demand efficiency. There used to be an oil slick down the center of every highway, oil was kept on the fuel island at the gas station, a little blue smoke wasn't troubling...till the advent of computerization in design and manufacturing changed it.
 
OK, I am going to drive it this summer and then take another reading at the end of the season, and do a comparison.

Its not broken that bad, I am not looking for work, and I am not getting any younger. But as CW said in post #10 "I may get better with age and running".
 
Why do cast iron cylinders glaze? Cast iron has a lot of carbon between the iron grains. The carbon will cause the grains to smooth over and the cylinder to glaze. Among other reasons, this is what make cast iron a good choice for a cylinder wall.
 
I haven't had anything to add, so, I haven't.

I agree with running the monster for awhile. A few more thousand and another compression check. Maybe the rear rings just haven't set yet.
 
I bet it will be fine after ya drive it a bit, and change the oil. Why did you decide to test it anyways?
 
Gene, I was bored, looking for something to do, getting away from Covid.

Actually just trying to get a baseline as to where the compression was.
 
You must log in or register to reply here.
Back
Top