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When performain a compression test on a 6bt (I'm sure the 4bt would be the same), what figures am I looking to get out of it? I've searched around and can't find a concrete number for "good", "acceptable" and "problems lie beneath" as far as PSI is concerend.
 

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It takes a minimum of 300 to run, it will run with less but will rarely start without an aid. A good engine will normally have around 500-550 but may have less and still be good. I don't beleive cummins actually lists a specific number. They stand buy the blow buy test and say it more acuratley signifies the condition of an engine if I remember correctly. A compresion check can very on condition of the cranking sytstem also. So make sure it is turning the engine over sufficent to get a good reading. Anything under 350 is likley near an overhaul.
 

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When performain a compression test on a 6bt (I'm sure the 4bt would be the same), what figures am I looking to get out of it? I've searched around and can't find a concrete number for "good", "acceptable" and "problems lie beneath" as far as PSI is concerend.
Cummins Shop Manual
B3.9 & B5.9 Engines
Group 14
Engine Testing
Page 14-22

" NOTE; Due to variables such as starter and battery conditions that effect engine cranking speed, it is difficult to establish an absolute value for compression pressure; however the following values can be used as guidelines:

New Engine (cranking speed @ 250 rpm) 2412kPa (350 PSI)

Used Engine (cranking speed @ 250 rpm) 2068 kPa ( 300 psi)

It is recommended that the compression pressure be checked on all cylinders and then compared to specification. ALL cylinders MUST be within 650kPa ( 100 psi) of each other"

Paul
 
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