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Normal compression on 40 Desoto flathead 6 motor


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#1 40desoto

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Posted 07 October 2012 - 09:48 PM

Just wondering what would be the normal compression readings on a 1940 flathead6 engine. I'm getting 115-120 on all pistons and wondering if that is too low

#2 Plymouthy Adams

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Posted 07 October 2012 - 09:54 PM

you doing good....keep in mind that in 1940 the CR is only 6.5

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#3 Rusty O'Toole

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Posted 08 October 2012 - 06:08 PM

That is excellent. 120 psi is the spec for a new engine.

#4 Plymouthy Adams

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Posted 08 October 2012 - 09:08 PM

lets crunch some numbers here...now long ago I worked this all out for the average 218..and to get to where the numbers are a dead on match..we assumed the piston below deck at .030 and the gasket thickness compressed at .043 and came out dead on the money for CI and CR 6.5 with a head volume of 91.5 CC

ok..now your engine is a bit larger..but we going to assume the gasket and low deck piston is the same and thus to get to your CR of 6.5 yields a head volume of 117 cc

now all the above really does not matter in the smiple rule of thumb of PSI pressures at TDC as CR times the atmopheric pressure of 1 bar is 14.5 (simple rounding) yields a pressure of 94.25 and if you want to figure as 14.7 as most folks using a calculate for 1 bar...95.55 PSI

the larger the bore and associated stroke the volume in the head is the key in CR...so that is where most CR increases are made..milling the head and some amount of cylinder boring...I listed the above head in CC just as a reference to show different bores/stroke with CR the same in smaller engine the head volume is the big change in squeeze..

so in averaging the across board reading of your engine we will settle for a mean of 117.5 PSI and using the 14.7 as one atomosphere, you have a CR of 7.99...now to arrive at this in a 1940 DeSoto engine you will have had to have either bored the block...or milled the head and/or a combination of both...only a measurement of the bore and CC'ing the head will give you a true reading of your modification... GIVING, that the increase in CR is not due to carbon build up..within the head and piston tops..


look good on paper..that is where all this data comes from in design..just crunching the facts
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#5 Don Coatney

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Posted 09 October 2012 - 08:55 AM

Good job on the math Tim.

One of these days after I post this someone may actually read it. If you look real closley you will see a chart showing compression ratios and expected cylinder pressures. This chart assumes that the engine is healthy as in no cylinder leakage. The second attachment is a head milling chart.

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#6 40desoto

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Posted 20 February 2013 - 09:38 AM

I installed an oilr pressure guage  when the engine is cool it read compression between 30-40 but when the engine warms up it almost goes down to 0.  Is that normal?



#7 Don Coatney

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Posted 20 February 2013 - 09:47 AM

That is normal for an engine in need of a complete rebuild. A healthy engine will maintain good oil pressure at idle speed and once up to operating temperature you should see an increase in the idle oil pressure at road speed.


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#8 Young Ed

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Posted 20 February 2013 - 10:04 AM

My plymouth did that when I first bought it. Didn't take long 5-10 minutes of idle and the oil pressure would barely register on the gauge. That same night it got pulled out to be rebuilt. Now it shows just over 40 at start up and driving. When its really hot after a long drive it goes down to about 35.


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#9 greg g

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Posted 20 February 2013 - 12:49 PM

There is one thing to take into consideration.  You uses the the piston being below the lip of the block deck.  While that fits a 218, it is different when compared to  a 230.  I did not measure mine but the difference was notable when i compared the two engines I have.  I can not comment if the Desoto design was more related to the 218 or the 230, Being it was the long block engine, but that difference would show up in our calculation as the squash zone would be smaller assumig the piston travelled further up the bore at TDC.


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#10 40desoto

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Posted 26 March 2013 - 08:17 PM

I hate to drive this topic to the groud.  I cannot afford to rebuilt the engine at this time but was wondering  why would It have good cylinder compression reading but low oil pressure when car warms up?  Should I try other things before rebuilding it that can be the cause of the low oil pressure?  new oil pump?  just wondering what else I can try to drive temporarily while I gather enough funds to rebuild?


Edited by 40desoto, 26 March 2013 - 08:17 PM.





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