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Old 04-09-2006, 05:26 AM   #1 (permalink)
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Compression for 5.9 6BT engine?

What is the comrpession suppose to be for a good Cummins 5.9 6BT engine? I've found the compression RATIO (17:1) but can't find the target pressure I should read on the gauge. I'm getting 470-480 pounds in 5 cylinders and 425 in #3, dry. A wet compression test showed 510 in all six cylinders.

I'm getting ready to replace the lower end bearings to bring my oil pressure up. Should I do a ring job while I'm in there?
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Old 04-09-2006, 10:12 AM   #2 (permalink)
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Your numbers sound very good.
Bad idea to do a wet compression test on a diesel as the oil can ignite and start the engine.
The pressure can then go so high that the gauge will actually blow up.

Cummins doesn't usually use a compression test to determine engine condition but rather a blowby pressure test.
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Last edited by illflem; 04-09-2006 at 10:14 AM.
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Old 04-09-2006, 02:04 PM   #3 (permalink)
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Thanks for the information Bill. The thought of ignition did cross my mind but I dismissed it because I was using synthetic oil. I never did consider igintion causing the gauge to explode.

Are there any crankcase pressure specifications to help determine a good engine from a bad one? The blow-by method explains why I couldn't find any information about cylinder compression.
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Old 04-09-2006, 02:16 PM   #4 (permalink)
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You need a blowby tool.
The Cummins blowby orifice tool is simply a tee with one .221" (15/64-in) outlet. Connect one end of the tee to the end of the blowby tube. Put a manometer on last tee outlet. That is your blowby tool. They sell them at the Cummins, but I have made my own plenty of times, less than $10. A simple manometer can be made by looping into a 'U' 6 feet of clear tubing with water in it half way. Measure how high the water level rises with a tape measure, multiply it by 2, convert it to LPMs.

Rough conversion is 1"= 27 lpm, add 3 lpm for each one inch (1/2'' of rise in the tube) of water

The reason for multiplying by 2 is that inches of water equals the water rise in the open end of the tube plus the inches the water is pushed down on the engine side of the water tube. For simplicity my numbers below are the measurement of rise only.


Cummins new 5.9 engine numbers are:
63 liters per minute(2.5" water rise) @ 2200rpm,
76 L/Min (3.5" rise) @ 2500rpm
85 L/Min (4.5" rise) @ 2800rpm.

Worn engine that needs rebuilding are roughly double i.e.
126 L/Min(10.5"rise) @ 2200rpm
152 L/Min(14.5"rise) @ 2500rpm
170 L/Min(17"rise) @ 2800 rpm

Beside indicating a compression problem the valves could also be out of adjustment.

Another way (mine), same idea, is to block the blowby tube with a 1/2'' pipe nipple with a cap that has a 15/64 hole drilled in it. Use 3/8'' id looped clear tubing with water in it slipped over the oil dipstick tube. Other tubing end remains open. Use a sharp tipped felt marker to mark the water level with the engine off, have a helper start an already warmed up engine and run the rpms up to 2.2, 2.5 & 2.8k rpms. Mark each water level with the pen, measure the distance from engine off mark then multiply each by 2.

This is all very simple to do, just hard to explain with words.
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Last edited by illflem; 04-09-2006 at 02:32 PM.
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Old 04-10-2006, 01:30 AM   #5 (permalink)
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Capt. Dean,

What is the oil presure reading? How many miles on the engine?

Put a pressure gauge in the extra port at the top of the oil filter housing before replacing the bottom end. The gauge is the quickest just to check the pressure and the reading of the dash unit. If you have good pressure there, try replacing the sending unit.The sending unit might be reading wrong. My pressure dropped to zero then came back up slowly and the problem was the sending unit (thank God!)
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Last edited by Paccool; 04-10-2006 at 01:32 AM.
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