Seafoam

[PwrSmoke]

I read somewhere that a fresh idi should push anywhere from 450-500 psi and do any other years have a minimum pressure that I can compare too? Thanks

Well, I think that 450-500 psi was in error.
I have manuals from '84-94 (6.9-7.3 IDI + 7.3 IDIT) and they all say the same thing as I posted above.

The GM 6.2L diesel (also an IDI with a similar Ricardo-type combustion chamber) manuals do list a compression pressure and since they had a nearly identical compression ratio (21.5:1 and the cam profiles are similar), it might be relevant. The GM manuals lists 380-400 psi on a warmed up engine. That's right about where you are, though if the engine wasn't warmed up when you tested, it will read lower. Again, I think you have nothing to worry about.

 

[icanfixall]

My rebuild shows 530 lbs of cranking compression with all the glow plugs out and batteries fully charged. I know what was done to my engine to reach this compression too. It was plenty of machine work plus CCing the head valve recession and the precup volume. I did all I could to blu print my engine to get all the lost hp out of it from the loose tolerances from a factory assembly line build. I wanted each cylinder to make the same hp too.

 

[franklin2]

If you have low compression, there should be other signs to go along with it, like excessive oil burning and stubborn starts.

About the snake oil stuff, I agree I am not normally a fan either. But there are two "snake oils" that I have seen work over and over. One is Rislone. If you have a lifter ticking, pour this stuff down in the oil and run it for a couple of weeks. I have seen over and over the ticking go away.

Another one that just floored me was "Lucas Tranny Fix". I had a problem with my daughters car lurching and dropping out of overdrive(computer recognizing slippage). Over and over people on the web said this stuff got rid of the problem. I said oh well and bought some, it's not cheap, it was $12 for a little bottle. I then go to pour it in the tranny and it's so thick it won't hardly come out of the bottle. That's when I ask myself what in the world and I doing?. I finally got it all in there, and I could not believe it, it did fix the problem. I talked to several other people and they have said it worked for their small slipping and surging problems also. You guys with E4OD torque convertor slippage problems may want to try some of this stuff.

 

[PwrSmoke]

There is an old Formula from the Bosch Engineering Handbook about calculating an approximate cranking compression pressure for any given static compression ratio.

compression (psi)= (compression ratio - 1) x 1.2 x atmospheric pressure (14.7 psi at sea level) so 21.5-1= 20.5, 20.5 x 1.2 x 14.7=361.6

According to that, a 21.5:1 ratio should pump out 362 psi.

I also have an old chart from an engine manual from the 1930s that has a chart showing what the CR should be based on a compression psi reading. It only goes to 8:1 but extrapolation (17.5 psi per ratio or 17.5 x 21.5) yields a close result of 376 psi for a 21.5:1 ratio.

None of these take into account carbon buildup, which will raise the pressure, or the cam timing (inlet opening and overlap) which will lower it. Since diesels do not have aggressive cam timing events vs a gasser, that is not a huge factor (vs a "race" cam in a hot rod gasser that will make an 11:1 CR engine show only 105 psi cranking pressure)

More food for thought.