Compression Test Results... Good Or Bad?

[Beef]

Hey lads, i just have a question about cylinder compression,

depending on what power your making will the cylinder compression be different?

FOR EXAMPLE if a r33 skyline is making 400rwkw will the compression in each cylinder be higher than one making 200rwkw? or should it be the same...

i really dont know anything about this so its probably a stupid question and obvious answer....

I have a 1994 R33 GTS25T making 209.8kw atw @12psi:

the compression test was as follows:

-----Leak Down--------Compression

1.-----15%----------------125psi

2.-----15%----------------125psi

3.-----15%----------------125psi

4.-----15%----------------125psi

5.-----15%----------------125psi

6.-----15%----------------130psi

may car had travvelled 123,500kms when this test was done.... and now only has 126,200kms on it

is this a good result or not so good result, please.... any advice would be much appreciated.

Beef

Edited June 22, 2006 by Beef

[djr81]

The compression ratio is a physical property of the engine. Broadly think of the volume in the cylinder when the piston is at bottom dead centre & compare that to the volume in the cylinder when the piston is at top dead centre. You (for an Rb) will get a number about 8.5 to 1 or slightly higher depending on which Rb variant you have.

[khunjeng]

with compression tests there are a number of things to look for...one is the total variance between highest to lowest. Your results look good in that regard.

However the "stock compression" from memory is 175/180 in a R33GTST...when I had mine tested my results were in the 165 bracket and its done about 100k. I was very happy with that.

I would be suggesting your figures are low...however the total variance is well within range.

Not being an expert though. some1 else might have a better analysis

[Beef]

The compression ratio is a physical property of the engine. Broadly think of the volume in the cylinder when the piston is at bottom dead centre & compare that to the volume in the cylinder when the piston is at top dead centre. You (for an Rb) will get a number about 8.5 to 1 or slightly higher depending on which Rb variant you have.

excellent description of what "compression" actually is, but it fails to answer both my questions:

1. higher output engine = higher compression per cylinder?

2. are my results of my compression test good, average or bad?

Beef

Edited June 22, 2006 by Beef

[Beef]

with compression tests there are a number of things to look for...one is the total variance between highest to lowest. Your results look good in that regard.

However the "stock compression" from memory is 175/180 in a R33GTST...when I had mine tested my results were in the 165 bracket and its done about 100k. I was very happy with that.

I would be suggesting your figures are low...however the total variance is well within range.

Not being an expert though. some1 else might have a better analysis

ok, so what does it mean.... lower compression more worn engine?

is there any way to make the compression higher if it gets a bit low.... for example, is there any way to get the compression back up to 170psi from 125psi like my engine currently is?

Beef

[djr81]

ok, so what does it mean.... lower compression more worn engine?

Yes.

is there any way to make the compression higher if it gets a bit low.... for example, is there any way to get the compression back up to 170psi from 125psi like my engine currently is?

No. You basically have to rebuild the motor.

To answer the question implicit in your original post:

Testing a modded (Stock internal) motor for compression should give the same numbers as an unmodified motor.

BUT

When operating at full output the BMEP of the modded motor will obviously be higher.

[Beef]

Yes.

No. You basically have to rebuild the motor.

To answer the question implicit in your original post:

Testing a modded (Stock internal) motor for compression should give the same numbers as an unmodified motor.

BUT

When operating at full output the BMEP of the modded motor will obviously be higher.

cheers DJR.... makes sense now, but to thebest of my knowledge my engine was rebuilt in 2004 at about 80,000kms.... and im not sure if the compression test was done before or after... maybe i should get another one done, how much do they cahrge approx for it...

Beef

[Beer Baron]

15% leakage coupled with the low comp suggests it's just a reasonably tired motor. but nothing to worry about too much as they are all fairly even, albeit at the lower limit of acceptable compression.

[Beer Baron]

ahh, if the engine was rebuilt that changes things. does it have forged pistons?

[Beef]

ahh, if the engine was rebuilt that changes things. does it have forged pistons?

how does it make any difference... i dont think it has forged pistons.... no..

so what does this mean now?

Edited June 22, 2006 by Beef

[djr81]

It makes a difference on two counts:

1. If the pistons are not standard they may have a different compression ratio and also the new engine may have a thicker head gasket (to give but two examples) In short the number shown in the Nissan manual may no longer apply.

2. If it is only two years old it should still have good compression.

Your first post read like the test was done only recently on this motor.

But then you have said that you are not sure which motor it was done on?

If you are having problems with the motor (Are you?) then it sounds like you need another test done as the numbers are indicative of an old, tired motor. What I cannot explain is why the numbers are so close together. Usually the lower they are the further apart (cylinder to cylinder) they get....

[Beer Baron]

sorry beef i asked for the reasons djr81 posted. also some engine builder may use a larger piston to bore clearance when cold which if the test was done on a cold motor (shouldn't have been) you would get a very low reading like you have there.

[khunjeng]

yeah any1 who has done a compression test knows to warm the car...so i assume that was done properly...125 is very low IMO...and you are right it seems wierd cos usually one or two cyl. are llower than the rest...from my experience anyway

without factoring in the rebuild..mine is a 94, 101,000km and was in the 160s-165s as above...this is the origional engine...mainly stock. Just as a reference.

If one cyl. is a bit low this can be put down to carbon bulid up etc...but not that low...

[SECURITY]

125psi is pretty low.

ive rebuilt my motor with standard internals and im getting 125psi BEFORE the run in.

go get another comp test done.

the figures seem TOO close together. if they were that low i would think there would be small variances of 3-5psi between them at least. thats my opinion rather than expert advice so just get another one done somewhere else.

[Beef]

ok, well it looks like im gona need to get another one from a different person.....

what is the process of a compression test.... like how do they do it?

just by comptuer testing through the console or what....?

and also what does it roughly cost to get one done?

Beef

[Beef]

no need to answer those questions:

1. how do they do it:

first they take off the coil packs and remove the spark plugs and test it that way.... that way i can get my coil packs and plugs looked at at the same time....

2. i can get one done by a genuine nissan dealer for $40

sound good?

Beef

[Beef]

hey all,

just another question, its booked in to have another compression test done this afternoon, but if it turns out the same results...

what does that mean?

is there any real worry.... if i drive it normally is there going to be any detrimental effects?

is it going to have any bad repocussions?

please advise...

Tim

[Beer Baron]

it's no big worry mate. just keep an eye on the oil level to make sure it's not using oil too much (and if it is just keep topping it up). it wont cause you any major problems, it's just symptom of a tired engine with worn rings. it may cost you a little power but if you cant afford the rebuild now dont worry about it, just start saving for when you can rebuild it.

[Beef]

it's no big worry mate. just keep an eye on the oil level to make sure it's not using oil too much (and if it is just keep topping it up). it wont cause you any major problems, it's just symptom of a tired engine with worn rings. it may cost you a little power but if you cant afford the rebuild now dont worry about it, just start saving for when you can rebuild it.

ok well thats good to hear, so what your saying is that there is no huge risk and if i drive it nice and steady adn normal, there will be nothing to worry about?

Beef

[Beef]

it's no big worry mate. just keep an eye on the oil level to make sure it's not using oil too much (and if it is just keep topping it up). it wont cause you any major problems, it's just symptom of a tired engine with worn rings. it may cost you a little power but if you cant afford the rebuild now dont worry about it, just start saving for when you can rebuild it.

as for teh oil, it doesnt use any.....

its last service was on 123,500 and now its 126,700 and it hasnt used a drop.....

so there is nothing to worry about there, and it doesnt blow smoke or anythign.... it goes good.....

so yeah.... maybe its not as bad as i first thought..

Beef