VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1933-1936 compression

asking for someone else what compression should a 207 have thanks

around 110 pounds depending on year.

year 1935 i thought it might be lower than that

Gene: I'm like tony. I didn't expect it to be that high.

Dick

Gene’s number is realistic for an engine in good condition. It might be a little high for one with average wear.

We do not know the year of that engine so these are just my estimate. The static compression ratio is around 5 1/2 to 1.

That means that if you start with the piston at bottom dead center and slowly crank the engine the pressure in the cylinder will be 5 1/2 times higher. 5 1/2 times 14.7 psi (standard atmospheric pressure) is 81 psi.

So what creates the remaining 29 psi? First is that the incoming air and fuel mixture will expand as it picks up heat from the cylinder walls and piston during the compression stroke. You will get noticeably different numbers if you run the compression test on a cold engine compared to one that is at operating temperature. Second is that this is a dynamic fluid flow situation, not static. The incoming air and fuel mixture has velocity and inertia that tend to “overfill” the cylinder compared to a static or low flow situation.

As an example, on my ”˜37 the compression ratio is 6.25:1. That calculates to 92 psi statically. The 1937 specs state that the compression should be 112 psi at cranking speed (65 rpm). I got between 120 and 125 on all cylinders the last compression check I did. I attribute the higher number to carbon build up in the head and on the tops of the pistons. That increases the static compression slightly.

No matter what year and make of car my experience is that the specified compression pressure will be above the static calculated pressure. Likewise if the engine is in decent condition the test results will be at or above that number primarily due to carbon build-up.

ompression should be a minimum of 90. 100 is common and I have seen 110 with carbon deposits, etc.depending on cranking speed.
More important to have all cylinders no more than 10 pounds difference, one from another.

from 1940 Motors manual

1934 Standard 97 pounds\
1934 Master and early 1935 Standards with 1934 Master head 105
1935 Master and late Standard 109
1936 119
1937 125
1938 121
1939 and 1940 135

The valve timing also has an effect as well as comprsssion ratio.

thanks for your help info passed on

you are welcome

Rusty,

"Static compression?" How is that determined? I thought something had to be moving to get a compression reading. "course I'm dumber than dirt.

Thanks and best,

Charlie

BTW: I'm just messing with you.

Static compression is what happens when you walk across a carpeted floor and then touch the door knob!

Your body statically compresses the carpet when you fall backwards from the shock.

Charlie: Great to have you keeping us technically correct!