Compression And Leakdown Tests

[Freeman]

Well im weeks away from having the cash to buy an R33, and just wanted to know some information about Leakdown and Compression Tests.

Firstly, how are they conducted?

And secondly, what results are considered acceptable and what should i avoid, or whats a safe cutoff point?

Thanks

[Freeman]

Anyone? Just want to know whats ok and whats a bad result...

[Abo Bob]

A compression test should show reasonably equal pressure across all the cylinders. It's not really important what the readings are (within reason) just that they are fairly even.

The leakdown gaugue that I've seen has a a green area that I think goes up to 30% meaning that anything up to 30% is fine. 20% is a good healthy engine and anything less is probably a new engine.

It's worth getting a leakdown test done before buying the car but you need the gauge and air compressor to be able to do it.

Both tests are done on a warm engine with the spark plugs out. For compression testing the gaugue goes in the spark plug hole and the engine gets cranked over. For leakdown the gauge goes in the same and the engine turned until the valves are closed for that cylinder.

If you are watching Jerry - I was paying attention .

[R31Nismoid]

Use the search button mate.

All these types of things get answered regularly, hence why people arent dropping answers (casue they have 10 times before )

[Freeman]

Yeah i did try the search button, but i got the sh*ts after looking thru 15 thrreads with no luck, including about 10 pages of the RB30DET guide before i found out how damn long it is. Thanks for the information Bob, helped me heaps

[r33_racer]

yes i get the same sorta shits with the search here also....you look through 10 pages of results and it gets you nowhere closer to what your looking for. so you start a thread and then they tell you use the search lol...catch 22. not fun.

[wrxhoon]

A compression test should show reasonably equal pressure across all the cylinders.  It's not really important what the readings are (within reason) just that they are fairly even.

The leakdown gaugue that I've seen has a a green area that I think goes up to 30% meaning that anything up to 30% is fine.  20% is a good healthy engine and anything less is probably a new engine.

It's worth getting a leakdown test done before buying the car but you need the gauge and air compressor to be able to do it.

Both tests are done on a warm engine with the spark plugs out.  For compression testing the gaugue goes in the spark plug hole and the engine gets cranked over.  For leakdown the gauge goes in the same and the engine turned until the valves are closed for that cylinder.

If you are watching Jerry - I was paying attention .

<{POST_SNAPBACK}>

Yea Adam you summed it up fairly well mate .

Compression should the same in all cilinders in a good engine but it depends how its done to what the reading will be and its acceptable to have some varience up to say 5-8 % . The upper readings in your engine 11.5 kg/cm ( 161psi ) ( if i remember right ) is a little on the low side for a good engine but still good for an old engine .

A stock as new engine ( rb25det ) should have 12.2 (173 psi ) , a comp test will not show low if the bottom end is slightly worn ( not broken ) , like worn rings or pistons for intance because the oil takes up the clearance and you still have good ( almost perfect ) compression . It will show if you have worn top end ( valves not sealing 100% ) .

The leakdown test will tell you a lot more as you shaw in your case on that cil where you had good comp but heaps of leakdown .

On a new , as new engine it should be no more than 5% but anything up to 30-40 is acceptable for an old engine , again the lower the figure the better , my car has less than 5% on all cilinders .

Have you desided what you are going to do yet ?

[Abo Bob]

I think I'm going to pull the engine out myself and get someone to rebuild it with forged pistons and about 8.6:1 compression ratio.

[wrxhoon]

That way you will save the R+R cost and i'm pretty sure you can do it yourself , all you need is an engine crane or block + tackle and somewhere to hang it from + a mate to give you a hand when you are ready to pull it out ( i'm sure someone will help you out ) , i would if i was a little closer . If you deside to do it and you get stuck give me a buz .

Good luck with whatever you deside and look at it this way you are going to have a better+ stronger engine .

[BOZ22N]

just be carfull when you do the leak down test that the valves are seated correctly, it can and will leak past them if you are not carfull. if you are getting lots of blow by (ie leak down) put some engine oil down the spark plug hole and see if that seals it, if so it is your rings, if not then it is your valves. take off the intake and listen for air rushing out if none listen up you exhaust. it has to go some where.

cheers happy hunting,

Boz

[Freeman]

So up to 30-40% is acceptable for old engines? Would 95 rb25det be classed as old?

Also, has anybody had experience with asking importers for these tests? Will they mind or get annoyed at you?

Boz22n, as i dont know how to conduct these tests, i would probably ask the importer to test them for me. Im not real sure if they would let me tip oil down their spark plug holes

: S lol.

[Abo Bob]

The engine needs to be warm to do a leak down test so if it's not installed on a car you can't really do one.

I would call 10 years and old engine. Dunno about everyone else though.

As for 30-40% - I don't have enough experience to answer that one. Everything i've posted here I learnt on sunday from wrxhoon .

[wrxhoon]

So up to 30-40% is acceptable for old engines? Would 95 rb25det be classed as old?

Also, has anybody had experience with asking importers for these tests? Will they mind or get annoyed at you?

Boz22n, as i dont know how to conduct these tests, i would probably ask the importer to test them for me. Im not real sure if they would let me tip oil down their spark plug holes

: S lol.

<{POST_SNAPBACK}>

Of cousre a 95 engine is old , its 10 years old ... an engine with 30-40% leakdown will still perform well enough .

You dont need to put oil down the spark plug holes in to the cil to do a leak-down test .

When you doing a compression test if the comp is down in one or more cil , then you put some oil in the cil and repeat the test if comp rises by a big margin that tells you you are loosing comp in the bottom end like rings worn or cil scored . If it doesn't rise with the oil then its top end , like valves .

[MerlinTheHapyPig]

for skylines, good compression is around 145-175psi

anything lower than 125psi is considered bad. (from memory reading it in rb20 engine manual)

and variance of + / - 5psi is bad

to do comp test.

-----------

car should be warm. start engine, remove fuel pump fuse (or disconnect fuel pump from boot) car will stall, remove all spark plugs, insert comp tester into spark plug hole, with throttle floored, crank engine a few times, until comp reading on gauge is steady.

not sure how to do leakdown test, as i've never done one

my 1989 rb20det tested 151 150 150 150 150 149 from memory, which is fine.

Most 4-cyl sr20det's and ca18dets i've tested have come out more like 165 psi but it all depends on how worn the engine is.

[Freeman]

Thanks for the info guys, especially Merlin. exactly the info im looking for. Im going to the importer on Thursday to see what he has got!