# Compression questions



## nissandrew (Aug 23, 2005)

Alright, sorry in advance for the long post, but I would like some help on this matter. Here it goes:
I bought my '85 n/a Z31 from a guy that said the motor has been swapped out for an '87 VG motor. I wasn't sure if he was telling the truth or not...me and my neighbor checked the compression before we pulled out the engine expecting it to be lower than it was supposed to be (I think it's supposed to be around 170-175psi??). The car was running a little funny and wasn't pulling as hard as it was when I first bought it. 
We checked the compression and it ended up being about 200psi on all six cylinders. With the spark plugs out, we were able to look at the tops of the pistons. It appeared that there was a good amount of carbon on the tops of the pistons, the question was whether or not it was enough to significantly raise the compression.
As we took the engine out and tore it down, we realized that the bellhousing bolts had been replaced along with some of the bolts on the upper and lower intake manifolds. Most of the gaskets were in fairly good shape as well. This pretty much confirmed our doubts that the engine had been replaced or at least had some work done to it.
Well, now all that's left is a block on an engine stand. The pistons and rods are still in there, but the carbon build-up on the pistons is not nearly as bad as we thought it would be. At least, not bad enough to raise the compression by 30-35psi.
The car is a Canadian model, but I don't think the Canadian VG motors had higher compression than the USDM models...? Did the JDM motors have higher compression? It's just funny because the compression was the same in all 6 cylinders. It wasn't like it was one bad cylinder or something. All 6 were right in between 199 and 202 psi.
What are your guys' thoughts on this? Any helpful input would be greatly appreciated.


----------



## nissandrew (Aug 23, 2005)

Not really, no.


----------



## Zen31ZR (Mar 24, 2004)

My lower compression turbo engine was pushing 150 psi out of all 6 cylinders at 5500 feet of altitude. What you were getting sounds about right for your engine. And yeah, carbon buildup always looks worse, when you are trying to look at it through the plug holes, until you pull the heads off. Mine had hardly any either at 167,000 miles.


----------



## nissandrew (Aug 23, 2005)

So do you think that it's normal? I thought that I read it was supposed to be around 175psi...


----------



## Zen31ZR (Mar 24, 2004)

nissandrew said:


> So do you think that it's normal? I thought that I read it was supposed to be around 175psi...


Depends on your altitude, time of day (hot or cool), barimetric pressure etc etc what you'll read for compression. I wouldn't have messed with it either. Certianly not tearing it down. If there were differences of more than 10 psi per cylinder on the low side, then I might have been a bit worried about a blown head gasket, but carbon build up is easily removed without doing that.


----------



## nissandrew (Aug 23, 2005)

I'm not going to mess with it if you guys think it's fine. I do want to get everything cleaned up, though, from the inside out. This was just the easiest way to do it.


----------



## nissandrew (Aug 23, 2005)

Yeah, we're already planning on doing all the gaskets and seals, water pump, timing belt, oil pump, clutch, flywheel, and probably headers and exhaust. I'll let you guys know how it goes! Thanks for the help.


----------

