compression testing

[Black dawg]

what kind of numbers are you guys getting?

how many "puffs"

 

[hesutton]

400-450.

Cycle the cylinder until the gauge stops climbing. Once the pressure peaks out, that's your compression.

Heath

 

[Black dawg]

she is blowin 300ish


up from 250ish 8 months ago

 

[flareside_thun]

Was she cold or hot each time you tested? 300 is still pretty decent. The 6.9 I rebuilt at school had 470 lbs. consistently....

 

[Dieselcrawler]

gooses fresh rebuilt motor had 400-425 between all 8 cyls. but his only has 40 some miles on it when we did that, so rings havent even seated yet,

 

[icanfixall]

My rebuild has 530 lbs no matter how many times I have tested it. the 300 sounds like it getting down to the rebuild so watch this closely. Are you using a quality compression gauge. If you are not at least your getting a steady reading across the 8 cylinders. It could be much better than what the gauge is telling you... Or worse. Do you have a direct drive or a gear reduction starter... And was the batteries fully charged up. Generally speaking the faster the engine spins the higher the compression ratio will be..

 

[damac]

When I got the truck in my sig 2 years ago it had 380-440.

Seems to run ok to me although I don't know these trucks well. Can start it right up with one glowplug cycle down into the teens without issue.

 

[Black dawg]

My rebuild has 530 lbs no matter how many times I have tested it. the 300 sounds like it getting down to the rebuild so watch this closely. Are you using a quality compression gauge. If you are not at least your getting a steady reading across the 8 cylinders. It could be much better than what the gauge is telling you... Or worse. Do you have a direct drive or a gear reduction starter... And was the batteries fully charged up. Generally speaking the faster the engine spins the higher the compression ratio will be..

is the 530 from very good ring seal or did you up the compression ratio?

how does it start with no plugs?

 

[icanfixall]

The 530 lbs of compression is from ring seal and line boring the cylinders and crank main webs. First you need all the crank main webs bored true.If you look at a main bearing when you rebuild and see that one edge is worn thru then you know that web is not machined true. Once the mains are done then the cylinders get bored true right angled to the true crank. Looking at the rod bearing wear will tell you if a cylinder is off square to the crank. It also will show more wear on an edge. If I increased my compression ratio by milling down the pistons it was not something I planned on doing. My block was decked to square it with the crank but only about 4 thousands was removed. Maybe 1 or 2 thous more or less. The crappy shop doing my machine work was suspect in everything they machined. To actually find what they cut off the block I need to tear it down. Use a depth gauge that will reach the crank tool I have not made and then add in half of the crank main bearing diameter to find the center line depth from the top of the block to the center of the crank... So I just run it till I tear it down to measure... Thats plenty of work to find out something is right or wrong. Runs fine so I'm just enjoying the ride.. As for starting without plugs.. Yes but it cranks for longer than I like. Maybe 5 seconds. I can see white smoke out the tail pipe. Then as it comes to life a puff of black and I'm off to the races without missing a beat...