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Re: Minimum amount of compression to fire a diesel
I know this is an old post but I have been a diesel mechanic for many years and there is always something that people forget.
That would be battery and starter condition. These directly affect cranking speed. Cranking speed is much more critical on a diesel than a gas engine for reasons mentioned above related to "heat sinking". The stirling engine uses that principle to run. Basically the head, piston, and cylinder wall will sap the heat right out of the air at an incredible rate. If the engine cranks too slow the required heat is gone before the injector fires. Many GM diesel cars and trucks came with 2 batteries from the factory. I actually added a third to help the two last longer and facilitate faster cranking, thus faster starts on my diesel van.
If you are into getting the most out of tired diesels I would highly recommend 3 start batteries or at the very least checking and cleaning all electrical connections and possibly upgrading the starter whichever way you can. I have found some of the newer gear reduction starters (for GM vehicles) seem to be able to crank a bit faster. Just my two cents.