Thread: Compression test results?

I had to replace the pistons in my 3100 and now each cylinder reads almost exactly 120 psi. Is that pressure good or bad?

Thank you for looking! The new pistons were nothing special. They are the same stock size but hypernutectic and i got them from napa along with the same brand rings( i think speed pro was the brand) I just went in there and asked for replacement pistons and rings put together at a machine shop and had at it. Any reason you can think of why it would be so low? Im planning on a 600 mile trip and just wondering if it is safe to drive. It it helps any i cant tell a big difference in power, no misfiring, it does idle a little higher ( 900 rpm), and ive noticed low vacuum as well. Thanks for the help!

Before i put in the new pistons, i only cleaned the cylinders with brake cleaner really well. No honing or boring. I did use a break in oil that the napa guy said to use and changed it after about 150 miles to valvoline synthetic? I remember i was overjoyed that It ran just like it did before haha. Later doing some porting and tinkering i picked up a test gauge and came up with almost exactly 120 each piston. So if the rings never broke in that would explain why the low pressure is even?

Also from what i found low compression also means low vacuum at high speeds under a load, is that right? Because it would explain alot!

Cool! Hey thanks a bunch

Have you tried a cylinder leakage test too?

Nope but i will definitely look into that before getting remand cylinder heads and new piston rings, and trying this again. But leaking cylinders make a lot of sense. Any special test method on this 3100 that you prefer?

JC When did you do the Compression test? When the engine was HOT or when it was COLD? I remember there being something about the 3100 engine being designed with a Piston that was Smaller than the bore when it was Cold so when the Piston Expanded due to the Heat of Running it was the correct size. This may be something you want to look into, I think it is called "Piston Slap".

I remember handling really hot spark plugs with my bare hands..... Haha. But that reminds me, the hyper pistons dont expand very much when they get hot due to the silicone in them, compared to the cast aluminum which are the same size ( 3.55 ish ). The pistons did have a little movement after installed and rings were tight with about a 1/16 gap ish. But they wouldn't sell bunk pistons right?

I think that they sold you Pistons which are the same size as the Originals, but, if they don't Expand Slightly (like the originals are designed to do) they will be too small when the engine is Running and up to temp. The real question is that going to matter?? I think NOT, but then again I have Originals so it does not affect me.

I think your right, and i don't have piston slap like I've seen other 3100 on a cold morning. So either i didn't prepare the cylinder walls right or my valves are leaking the exact same amount. What else do you think i could have messed up? Also when i read up on compression tests, it mentioned to have it at WOT. Does that matter with the 3100?

So did a new compression test at wot and no fuel pressure. All cylinders at 140 psi except for cylinder 1 which was at 130. Does that seem better? Vacuum is still low..?

I have Nothing to go by for the "real" Numbers it is in a Book somewhere I don't put too much into actual Numbers. Does it sound right and run right? Or do you have Issues?

Unless you have a brand new block and the correct (OEM) Pistons I would say you can't go by the numbers anway, as that is what they were supposed to be when they came out of the Factory, not as it sits NOW. Yes they should be close, but I like the Fact that they are all relatively even that says a lot to me. Besides what are your alternatives?

4 weeks late...Sorry i didnt write back guys, thank you for your help. I wasnt testing compression right when i said 120. I didnt stop the fuel pump and the TB was closed. The psi is now 140 and everything seems healthy except for low vac. Thank you for your help

I'm piggy back off this thread. My dad's GS L67 is rusted out, before I think about swapping it out into my car, I want to do compression test.
1. What is considered good compression test range?
2. Hot or cold? Does it matter?
3. WOT? Should I?

I did front bank, cold, and closed throttle, got 130,145,125...