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ShortCircuit

Rb26 Compression Test Results

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Hi Guys

I know this has been covered a million times before. I've been searching the forums and google but haven't really find the answer I was looking for. (Sorta)

I recently had a pre-purchase inspection done on a GTR I'm interested in and the compression test come back as 120 across all cylinders. Some people have said this is OK as it's consistent and others have said it's too low. Can someone please let me know if these figures are ok or too low as everything else has checked out and I'm close to putting down a deposit.

Cheers

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Really? Because everything else I've been reading has pointed to it being too low!! I'm bloody confused lol.

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Can be a number of things, after market head gasket, worn seals/bores

Low compression across all cylinders generally associate with a tired motor, i would only bite if you were to factor the cost of a rebuild into the price, which you would want to do anyway regardless of condition if long term ownership is your goal

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Cams will drop a compression test reading.

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Both statements "its too low" and "its consistent so its OK" are pretty much right.

Standard compression according to the workshop manual is 171psi, so 120 is way low.

However, you don't know if the test was done correctly, with an accurate guage, with a fully charged battery, with a warm engine etc etc as it should be.

So instead people generally look for consistency in the results because that removes all those factors.

The main things you are trying to avoid with a compression test are some sort of major mechanical issue like a broken ring land, bent valve etc etc which will show up even if the test is not done correctly. The remaining issue is general wear on the rings which can show up as low compression everywhere, but even then it tends not to be consistent.

So....the remaining problem is it may be heigh klm and the rings worn. Does it smoke at idle or light throttle driving?

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Seems like the check was done on a cold engine. (not sure why)

Car was re-checked at running temp and come back at 150-155 across all cylinders and I'm much more comfortable with that.

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So to carry this thread on longer the variation here is 30psi.

Is that average/normal?

What should it be cold? compared to 170psi when warm and new.

Cams, head gaskets can create variation as well as a looser build for higher end builds/power

Worn engines reducing in compression due to wear.

A cold compression test can tell you as much as a warm compression test. At least when it's cold you have a better chance of picking up irregular noises.

Hence the no greater than 10% across all 6 as a general rule.

Edited by Sinista32

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I've been watching this thread and I think we're both happy to report the car made 160-165 per cylinder today at Croydon Racing Developments.

We still have no clue what method was used for the first test to get 120 and to be frank, I don't think I want to know lol.

I'm glad it is now put to bed!

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I've been watching this thread and I think we're both happy to report the car made 160-165 per cylinder today at Croydon Racing Developments.

We still have no clue what method was used for the first test to get 120 and to be frank, I don't think I want to know lol.

I'm glad it is now put to bed!

Official paperwork said 170-165 across all cylinders. Car came back with very good results from CRD and I was really happy with their service. I Would definitely recommend them to anyone is Sydney.

Very happy and glad it's been put to bed.

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digging up an old thread .. had a compression test done as per a pre purchase inspection on a R34 GTR with 77,000km. The car is mainly stock and had cooled down quite abit before the test was done but not cold.. the results were ;

 150 - 150 - 148 - 149 - 139 - 143

Cylinder 5 a worry?
 

Edited by jaysevu

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digging up an old thread again.. hopefully get an answer this time haha. Inspected another r34 GTR and did a comp test.

The engine was rebuilt in 2010.. 

152 - 140 - 150 - 160 - 155 - 155

I read there shouldn't be anymore then 10% discrepancy between cylinders so is 140-160 a bit much? 

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Depends on whether you take 10% discrepancy to be "from lowest to highest" or "from any one to the average/median".  On that basis, your typical value is ~155 and the lowest one is just on 10% lower than that.  There is also measurement error associated with both your lowest and your highest values (and all the others in fact, but we'll ignore those as they are so close together).  So it could be worse or a little better than your numbers show.

If it were me - I'd like to see the worst one not be that bad.  But it is probably fine and could and probably will run just fine for a looooong time.  Or, it could be an indicator that the injector in that cylinder is a bit crappy and has damaged the exhaust valve or seat or any of a bazzillion other possibilities.  Did a leakdown test get done also?  Can be more informative than just a comp test.

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Squirt some oil in the cylinders and then do another test...….see if it comes up OK?

At least you will know if its rings or head.

 

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