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Acceptable compression on new car?

andrewwr32
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andrewwr32 Community Regular

andrewwr32

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well you mentioned the manual so youve probabaly allready read this but anyway

Standard is 171psi at 300rpm

Limit value is 128psi at 300rpm  

maximum variance limit between cylinders is 14psi  

so 138 is still above that, but not a huge amount

Yeh got that, thanks.

Something I noticed just looking at the manual again is the fact that you're supposed to use an engine speed sensor, and get a reading at 300rpm - they did not use a speed sensor, nor release fuel pressure (as stated in the manual). Is it possible 138psi is all the guys arm could hold down? If it did hit 300rpm, that's 5 firing's per second. I'm wondering, with a rubber "bung" holding the gauge in - just how reliable that is? If there's dirt around the area the rubber is sealing against - air could escape, maybe?. I'm clutching here I know, just want to bounce some ideas around ;)

See, I was thinking at first fuel pressure would if anything RAISE compression figures. But thinking just a bit more - fuel does not compress. You end up with less air in the cyl. But - that could INCREASE pressure - same force on less air. It's confusing!!! I wish I knew what the **** I was talking about ;) lol

Also, my mechanic did not have a screw-in compression gauge (Allen gauge, whatever the hell they're called) - it was just his arm and a bit of body weight holding the thing in (leaning over car, obviously).

I do trust them, they do this shit all the time and had a new gauge (different to the last time I had my Silvia tested).

I'm wondering if perhaps this isn't really a true reading. Well, I'll rephrase that - I'm HOPING it's an incorrect reading :)

When I get the car, it's stitched up now - I'll take it to a tuner for a baseline test - see what they get as a compression reading, get it dynoed, and get some more results. Worst comes to worst I have to rebuild @ say 5k? Maybe 6k?. In a year or so... hopefully longer. That's BIG outta pocket - but at the same time I get exactly what I want - a new RB26 engine.

Thoughts please :)

andrewwr32 Community Regular

andrewwr32

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yeah thats how I looked at it

You had a rebuild done Duncan? If so, can I ask where? And how long ago?

I'll be honest I'm a little afraid of an "Australian" rebuild. My inclination would be to find uber-rep high priced engine builder and get it done there for peace of mind. Is it much more difficult than rebulding your average NA motor?

Reason I'm asking is my best mate is a mechanic, and that makes life very easy - they do rebuild motors and his old man (partner) was in his younger years a race mechanic and can blueprint motors and stuff - loads of experience, but not with motors such as this.

Should I be trusting them with my RB26DETT?

Aaargh, so many questions, not enough fingers ;)

Bring on the keyboard/mouse implants... lol

:crazy:

predator Grand Master

predator

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Sounds a bit odd the way they did the test without a screw in compression gauge. I can see how it may not be fully sealing the cylinder.

just buy it.. if its the car you want, the paint is good, everything else seems pretty good (gearbox, clutch etc).. at worse its an engine strip one day... gtr engines don't last forever, so it may happen with any you buy. At $28k and you are getting a trade in on your old boys, may as well. If its an 1989/90 just be aware that the value may not hold as well as a newer GTR.. but otherwise, enjoy.

yaf28f Rising Star

yaf28f

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I'm about to get my car compression tested. But is is rocket science or anyone can do it without mistake. This makes the difference between getting CRD to do it for $180 or my cousin to do it for free (just finished his apprenticeship).

andrewwr32 Community Regular

andrewwr32

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Just picked my 32 GTR up a 12/89 model with 101500 on the clock, all cylinders at 165 across the board, so either yours really has more than 100K or its seen a harrrrrd life  :uh-huh: mine had been sitting for a little bit too.

Or, yours has been rebuilt. Maybe with inferior parts that will fly apart after 10 thou....

JUST JOKING! But seriously, it's had a rebuild or a reverse ODO transplant. 165 across the board on a 100,000km engine? It doesn't sound very realistic. Maybe you just scored a freak! Who tested it, by the way - not the seller, right?

Good on ya though, but I've done my research now (lots and lots) and I'm not concerned with the compression anymore. I'd like it to be higher of course!

I'll take it to a reputable tuner and get full comp test, leakdown and dyno. Then I'll have some real info. Kinda moot though. If it dies - it dies. And I buy an N1 motor :P

andrewwr32 Community Regular

andrewwr32

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Sounds a bit odd the way they did the test without a screw in compression gauge. I can see how it may not be fully sealing the cylinder.  

just buy it.. if its the car you want, the paint is good, everything else seems pretty good (gearbox, clutch etc).. at worse its an engine strip one day... gtr engines don't last forever, so it may happen with any you buy. At $28k and you are getting a trade in on your old boys, may as well. If its an 1989/90 just be aware that the value may not hold as well as a newer GTR.. but otherwise, enjoy.

Ah I do love the positive posts :P

Yes, with me no motor will last forever - they never have yet that's for sure. Longest motor I've had is 2.5 years with the Silvia SR20DET. That's including old NA cars, fords, all the cars I've had.

GT-Rs are expensive to own, I've known that going into to it. No point crying if I have to spend a few (or fair few) grand at some point in the future.

I'm very emotional about GT-Rs, so I'm not concerned about holding value. This is my "classic car" not my thrash-n-bash. I'd get a race-only non-registerable Silvy with ridiculous mods for that :)

Like I've said (possibly not on this thread) I'll have a whole new attitude with this car, coming from Silvias. You know how you get old farts with mustangs they polish and love and wank over? Kinda like that, but a decent car instead. I don't care if it's not rare or joe bloggs has one too. It's all about me :)

It's really not that much of an expensive car, so chucking a few bucks at was is to me a cheap supercar - don't sound so bad really does it?

:)

And the gearbox is tighter than a nun! ;p

Andrew

:crazy:

andrewwr32 Community Regular

andrewwr32

Posted
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I'm about to get my car compression tested. But is is rocket science or anyone can do it without mistake.  This makes the difference between getting CRD to do it for $180 or my cousin to do it for free (just finished his apprenticeship).

I'd be getting your cousin to do it as long as he has a screw in gauge. I don't trust the "just hold down 140+ psi with one arm leaning over the car" idea. That's a fair bit of force, and hits like a hammer - not a constant push.

No, certainly not rocket science.

EDIT: The hold-in gauge leaves more space in the cylinder. There is more air, and so doesn't compress as much. This IS nit-picking, but a spark plug/screw in gauge will fill a few more CCs. Minor diff, but I know people who have compression tested with a hand-held and then a screw in and read 7 more psi on the screw-in gauge.

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