What is good compression?

[Steve Jelf]

I think I know, but I've been asked the following questions and would like to get some confirmations or corrections of what I think:

What is the correct compression range for a good running engine?
What is the compression range for an engine that will need a rebuild?

[Joe Bell]

50-55 is stock T, with high compression pistons or heads I have seen 75-90

[Kerry]

I reckon that the engine would be getting pretty sad in the mid to low 30's psi.

[Charlie B in N.J.]

Agree with Ken. Low 30,s high 20's will cause cold starting problems and poor performance.. I consider 45/50 real good/OK. The highest I've ever seen was 65 and I was quite shocked at that.

[Russ T Fender]

Just wondering if it's really possible to reach those numbers cranking over by hand. I have several non starter T's that run and start well but I have never been able to get readings over 35-40 and that was with considerable effort.

[Norman Kling]

The numbers also will vary by altitude. Higher at sea level and lower at high altitudes such as Colorado. They should be over 35 and all 4 should read approximately the same.
Norm

[TXGOAT2]

Check compression with the throttle wide open. It's probably best to check compression after running the motor for a couple of minutes, then letting it set for 5 minutes or so. It's normal for pressure readings among cylinders to vary some, but readings should not vary more than 10% or so from lowest to highest.

[Rich Bingham]

Just wondering if it's really possible to reach those numbers cranking over by hand. I have several non starter T's that run and start well but I have never been able to get readings over 35-40 and that was with considerable effort.

I have hand-cranked compression read-outs at 50-55#. Altitude here is 4500'.
Norm, please explain the physics of how altitude affects compression. I'm confused by your statement.

[TXGOAT2]

If you crank your Model T on the Moon, it will have no compression at all.

[Norman Kling]

The air is thinner at higher altitude. That is why it is hard to breath when you climb a high mountain. When you take less air into the cylinder with intake, you will have less to compress. That is also why you need to adjust the fuel mixture at higher altitude. You have a very good set of rings and valves and perhaps you have a low head on the car, the compression will be higher than one with the later high head. The compression ratio was lowered during World War I because higher octane fuel was harder to get. The compression ratio on cars built after about 1917 was never raised.
Norm

[Scott_Conger]

Rich

air pressure in PSIA at sea level and 75F is 14.7
air pressure in PSIA at 4500 ft and 75F is 12.83

with a known compression ration (4:1 stock for example) it's just math as to what is expected, assuming a perfect seal, which rings/valves are not, but that value is at least the starting point for a diagnosis of engine health. Given your altitude, it sounds like you're running a healthy and happy engine and are right at the limit of a stock head.

[Rich Bingham]

Thanks, Norm and Scott! Yup. My "motor" seems strong and happy indeed. A 1913, she has a low head. I see now my misconception of "air", the difference pressures at altitude make.