...Read about the temp diesel needs to ignite and you will find it to be 942 degrees...
What?! I looked up numerous sources and found the auto ignition point of No.2 Diesel fuel to be around 210*C (410*F). On a cold start that is the temperature that must be achieved in the precombusion chamber for the engine to start. In my experience, the enthalpy of compression is not enough unless the ambient temperature is around 100*F or higher. Below that, heat from the glow plugs is needed.
One thing I would like to know is how much decompression affects the temperature in the precombusion chamber. The compression ratio to cylinder pressure is linear (Boyle's law), multiplied by the atmospheric constant of 14.7 psi. But the relationship between cylinder pressure and specific heat capacity is not linear. (Hess' gas law). Therefore a fairly small change in pressure can make a big difference in specific heat capacity... one of the reasons lowered compression helps EGT's no doubt. I know this is simplistic, since static compression is only a specification and dynamic compression and boosted compression are more relevant to engine performance.
You must be registered for see images attach
Does anyone have any first hand experience with decompression and cold starting on an IDI? What I have so far is the wav file posted here of a 17:1 truck that supposedly would not start with GP's AND ether at 60* temps. Wheeler's truck had the pistons shaved .080" giving him 16.5:1 and he said he had no problems starting it. And then there was Anderson's 6.9 that had .060" shaved pistons minus .010" decked block, putting him in the 18:1 range. He said it started fine also, but not in a particularly cold climate. Something's not adding up. Has anyone on this forum decompressed an engine? I don't mean with rebuild pistons intended to offset block decking, I mean /lowered/ to really shift the CR? Looking for cold start ability as well as running characteristics.