'48 Chrysler-8 factory compression pressure???

[Spitfire8]

Members: Anyone know what the original (new) compression PRESSURE ( not ratio) was for the post-War Chrysler straight-8? Thanks! 

[Guest]

14.69 X compression ratio.  If you do a search back at "home" you will find a lot of technical information on how cranking speed, wet or dry valve duration etc will affect this number.  However for the average person atmospheric pressure times the compression ratio is close enough.  This is a chart that will allow you to compensate for your height above sea level.  https://www.avs.org/AVS/files/c7/c7edaedb-95b2-438f-adfb-36de54f87b9e.pdf

[Rusty_OToole]

130PSI per my 1953 Canadian Service Data Book. It gives the compression ratio as 6.70. If you divide it out the compression pressure is 19.4 X the CR. I can't help that. Perhaps the air is heated by being compressed.

Edited August 9, 2017 by Rusty_OToole (see edit history)

[Spinneyhill]

13 hours ago, Tinindian said:

14.69 X compression ratio. 

That is what I thought until last week but it is NOT true! I put the formula in a thread somewhere. Let me find it and come back.

 

MoToR's Manual 1947 says the 1946 8 compression pressure was 125 at cranking speed. MoToR 1950 doesn't give the data!

[Spinneyhill]

OK, found it. Post number 8 and 9 in this thread:

This is what I said:

This paper gives a formula.

Nominal compression pressure.pdf

which is

compression pressure = (pressure at bottom dead centre [usually 1 atmosphere]) * (volumetric efficiency) * (nominal compression ratio)^{(specific heat ratio of working fluid)}

 

where the specific heat ratio of petrol + air, corrected for engine heat, is about 1.3 and

 

the volumetric efficiency depends on valve timing. For your car I expect it to be 75 to 80% - lets say 80%.

 

Your standard compression ratio is 6.70:1, so the compression pressure at TDC = 14.7 p.s.i. x 0.80 x 6.7^1.3 =  139.4 p.s.i.

 

The gauge reading will be one atmosphere less than this because the gauge reads zero under air pressure. So the gauge reading would be c. 125 psi.

 

I note that in the 1950 manual, 1946-48 are given the same data, so your car should be the same as the 1946 data given in the 1947 manual. 125 p.s.i. gauge.

[Spinneyhill]

OK, found it. Post number 8 and 9 in this thread:

This is what I said:

This paper gives a formula.

Nominal compression pressure.pdf

which is

compression pressure = (pressure at bottom dead centre [usually 1 atmosphere]) * (volumetric efficiency) * (nominal compression ratio)^{(specific heat ratio of working fluid)}

 

where the specific heat ratio of petrol + air, corrected for engine heat, is about 1.3 and

 

the volumetric efficiency depends on valve timing. For your car I expect it to be 75 to 80% - lets say 80%.

 

Your standard compression ratio is 6.70:1, so the compression pressure at TDC = 14.7 p.s.i. x 0.80 x 6.7^1.3 =  139.4 p.s.i.

 

The gauge reading will be one atmosphere less than this because the gauge reads zero under air pressure. So the gauge reading would be c. 125 psi.

 

I note that in the 1950 manual, 1946-48 are given the same data, so your car should be the same as the 1946 data given in the 1947 manual. 125 p.s.i. gauge.

[Spitfire8]

Good information. Thanks to all who responded! 

[Rusty_OToole]

If all cylinders are fairly close to each other (within 5 or 10 pounds, + or-) and 100 pounds or so you should be good to go. If the car has been out of commission for a while it should run better, and have a better compression, after you drive it a few hundred miles.