1936 Dodge L-Head Compression Pressure

I posted the question below on the Dodge Brothers forum also. I'd appreciate any thoughts from any member of the MoPAR family.

Hi, I was wondering if someone could help me out...

I'm trying to determine a correct compression pressure for my 1936 Dodge 217 cubic inch L-Head. I know the compression ratio is 6.5 but what I'm trying to find out is what that means in pressure when it comes to reading a compression gauge. The last page of "Motor's Auto Repair Manual" states that if your ratio is 6.5, then the pressure should be around 110 lbs., give or take up to 20 lbs based on differences in manufacturer engine designs and possible variances in cylinder and head castings. So based on that, am I correct to assume that the pressure should be anywhere between 90 - 130 lbs.?

Can anyone tell me if I am even close to correct, what they think the compression pressure should be, or what their current compression pressure is on their engine of the same size?

Any information anyone can provide will be more than appreciated!

Thanks!

I just ran a compression check on my '41 De Soto last weekend (motor VERY tired, at 101,000 miles): four cylinders averaged 110# (dry), #1 pulled about #70, #6 achieved 100#.

A MoPar flathead in decent shape should be capable of achieving at least 110#...

A freshly broken-in engine should pull about 150#...

Good luck!

Thanks Frank, I appreciate you taking the time to help me out. Mine run an average of about 70 lbs. across all cylinders. It doesn't burn oil and still manages to pull the family up the Rocky Mountains. I can't imagine what 110 lbs. feels like!

What its all about is compressing atmospheric pressure, so 6.5 times 14.7 psi equals ...........

There are other variables; normal operating temps, throttle wide open etc. but as a rule of thumb just multiply atmospheric pressure by the stated compression ratio and look out for any large ( 15 - 20 psi ) variations between cylinders.

Looks like yours is tired but sound.

Chris H

Thanks Chris, your response is more helpful than you know! I currently live at 8,700 feet. According to what I have read, I need to subtract 1 psi for every 2,343 in altitude. Therefore, my outside atmospheric pressure is 10.99 psi. Following your calculation below of 6.5 times 10.99, I get 71.435 psi. Does this sound right to you? At those calculations, it looks like the motor may be stronger than I thought!

Thanks again!

Well paint me pink and call me a pork-chop ! <img src="http://forums.aaca.org/images/graemlins/blush.gif" alt="" />

The altitude variable never once entered my mind !

Sincerest apologies from a "flat-lander"... <img src="http://forums.aaca.org/images/graemlins/crazy.gif" alt="" />

And so aero engine manufacturers developed the supercharger to maintain sealevel engine performance as altitude increased and atmospheric pressure dropped off.

What a wonderful world !!

Chris H

Probably a good buildup of carbon deposits in the combustion area is helping you to maintain a good compression ratio, nevertheless you are doing well

ChrisH

To calculate compression pressure (not 100% accurate, but close enough for an engine in good condition):

- assume atmospheric pressure is 15 psi;

- multiply by 6.5 (for your 36) = 97.5 psi;

- add 1 atmosphere = 97.5 + 15 = 112.5