Rb26dett Compression

[Chris_S15]

Im going to import an R33 GTR soon and it will be compression tested over in Japan, what sort of compression across all cylinders indicates a healthy motor? Also are there any other tests I can get done to indicate engine health? Cheers.

[Topaz]

150psi?

Testing of injectors/spark plugs/coil packs

Maybe ask to see a dyno sheet to look for boost pikes, AFR.....

[andylaurel]

there are a few variables when testing compression, such as;

closed throttle

open throttle

number of cranks

hot test

cold test

so rather than testing for a number, look for even figures across the board.

eg: 140, 139, 141, 143, 140, 139

[Chris_S15]

I've also heard of the "leakdown" test. Can anyone explain this test and what it is for?

[frankxinyu]

yeah, there are several factors involved....

mine was done, and i got 165 in every cylinder....i was told that was a good sign

[MerlinTheHapyPig]

manual spec is above 127psi and no more than 15psi variance between cylinders

[rev210]

The compression on my old R32 GTR was 160psi just about dead on with all cylinders except one at 100psi. To check out a piston with lowish compression pour a few cc's (mls) of oil into the cylinder via a small funnel (you only need a little oil, too much is very very bad and may damage things). Let it sit for a few minutes. If the compression on that cylinder shows marked improvement once re-tested to the order of 25psi or greater you probably have worn rings.

That motor was 250,000kms old or so. And the bang on compression between cylinders is perhaps testimony to the fact that the aussie engines were blueprinted from the factory, unlike the jap domestic ones.

[rob82]

Well my GTR returned 150Psi across all cylinders. 6 months later I spun a big end. To read the bearings and to see the pitting on the cylinder head/pistons I say it had been absolutly hammered and seen lots of big detonation events and I can garuntee that it never rattled(detonated) or was run lean in my hands.

Imports are usually an unknown item.

[MerlinTheHapyPig]

A good result from a compression or leakdown test will verify the condition of:

-Piston Rings

-Head Gasket

-valve seating

-cracked head

-general condition of bores and pistons

it's a good overall test of motor as you would assume these components are in similar condition to the rest of the engine.

It MAY be able to identify damage to:

-pistons

-bores

-valve springs, guides or lifters - or rocker arms if it's an SR20 (maybe... if they are causing a valve to stay open)

(depending on if this damage is causing a drop in compression)

It will NOT verify the condition of:

-Bearings

-Crank

-Conrods

-Oil & oil pump

-other head problems such as stem seals.

You can have an engine with a spun big end bearing, and it could be using mud instead of oil and it will probably still give you a good compression reading! What i'm saying is it isn't a complete indication of the health of your engine.

A leakdown test can pinpoint the percentage of compression leakage and where it's leaking from, but it tests the same things that the comp test tests.

----------

A good example of a BAD compression test was the one I did on my daily driver before i rebuilt the engine.

Dry: 95psi 95psi 165psi 165psi

Wet: 165psi 165psi 165psi 165psi

A large increase in compression indicates piston ring problems...

Pistons 1 and 2 had cracked ring lands.....

http://i2.photobucket.com/albums/y19/merli...dia/piston2.jpg

^ if you look at the piston in the pic this was the damage....

Edited October 31, 2006 by MerlinTheHapyPig

[Chris_S15]

Is there any way to test the bearing, rods, crank, oil pump?

[rob82]

Is there any way to test the bearing, rods, crank, oil pump?

The bearings where scoured from detonation.

Pull the oil filter off and open it with a heavy duty can opener and look for copper material and or anything suss. If you find nothing then its still not an indication that the bearing's aren't damaged but it may be a heads up for damage allready done.

[MerlinTheHapyPig]

how is it possible to kill bearings from detonation?

don't you mean pistons?

bearings usually die from forign particles, lack of oil or just general wear / driving it too hard.

[rob82]

how is it possible to kill bearings from detonation?

don't you mean pistons?

bearings usually die from forign particles, lack of oil or just general wear / driving it too hard.

You sometimes get little black oval scours along the verticle parts of the bearings. They also loose their tension and depending upon how big the detonation you may actually crush the bearing.

Remember when detonation occurs the bearings are transferring the force to the crank.