Compression Test Readings.

[inark]

Dont know if this really counts as performance or maintenance but i will put it in here anyway.

I had a compression test run on the R33 RB26DETT thats in my ceffy. Now most readings from a brand spankin new RB26 are about high 160+ or Low 170s or something.

Now i know when u do cam work and pistons and stuff that it will change the reading u get... BUT... will it change it so drastically that it would read 125psi across all 6 sylinders????

Can u guesstimate from that reading whats been done or not?

[ace_nz]

Cam timing, specifically inlet closing point

[ace_nz]

also slow cranking speed

[fatz]

yea mine had about that

just before i rebuilt it

4k gone!!!!!!

but yea if you ahave aftermarket cams that could make a diffrence

[cameron]

As long as its fairly even across all the cylinders they should be right. Should vairy about 1 - 3 psi max each cylinder.

Jun

[Poakey]

The main thing I'd be looking at is the variation between cylinders. If you've changed cams and cam timing then the reading are going to change, and sometimes drastically. The variation will tell you if one cylinder has done a ring land or has a bad bore etc. The only time this doesn't apply is when you've blown all the ring lands or fried all the rings, but this will be evident in other ways!

[Busky2k]

Yep as the others said, variance is definitely more important because the absolute readings of every gauge are different. I would try disabling the injectors as I found they deflated the readings by upto 10psi. So pull the fuse to the ECU or disconnect each injector if its accessible! Oh and, dont forget to have all 6 sparkies out when you do it, and also have the throttle wide open!

[siksII]

  cameron said:

As long as its fairly even across all the cylinders they should be right. Should vairy about 1 - 3 psi max each cylinder.

 

Jun

1-3psi max?

have you tested 8 year old motors which put out readings that close to each other?

i was thinking more along the lines of 1-13

what guage do u use? guages i have used u cant even see 1psi difference lol.

[cameron]

  siksII said:

1-3psi max?

have you tested 8 year old motors which put out readings that close to each other?

i was thinking more along the lines of 1-13

what guage do u use? guages i have used u cant even see 1psi difference lol.

If your getting 13 psi difference usally indicates you are losing a bit of compression with worn out ring lands and it will only get worse....

eg: testing RB25

Piston 1 - 160psi

Piston 2 - 163psi

Piston 3 - 162psi

Piston 4 - 161psi

Piston 5 - 162psi

Piston 6 - 150psi - Doesn't look to good!

Maby you can get away with a little more slack but on an old engine you carn't have 1 cylinder down a fair bit eg 13psi down and the rest fairly even It can result in time for a rebuild.

Not sure on what guage it was but just compression test.

Had one RB25 with cylinder number 6 on 0psi compression !!! :headbang: - I dont know how the thing started, it ran like a hairy goat :uh-huh:

Jun

[DoughBoy]

Cams + headgasket would be my bet if the engine is in good condition. If you've got those you're laughing

[siksII]

cameron, most readings i have seen are more random than that! ie there not one cyl that is way out compared to the rest, they are all a few psi diferent from each other....and yeah cyl 6 usually lower (first to get damaged)

could be the head though (burnt valves etc) which means a lot less to fix than bottom end and i would even bother because it probably wouldnt be noticeable if there was a diff of say 8-10 psi in one cyl..

in relation to the ceffy, like cam said, if they are even readings and it makes good power dont worry about it at all... if anything do a leakdown test to double check.

guages/cranking speed/battery/auxilaries etc all make sure you will never get a reading like new unless you are just lucky and the guage reads high..

[lwells]

siksII: It could be the method of measuring you (or whoever) is using. To get accurate readings you have to be very careful about your methodology.

I seem to remember SK posting a very good rundown about it somewhere on here.

LW.

[siksII]

I have read what SK said, and ill sum it up(from memory):

it is impossible to get readings like the factories give.

as long as they are pretty even its fine.

[lwells]

No, he was more specific than that, ie. engine temperature can be quiet important, etc, etc.

LW.

[siksII]

yeah thats true as well, but i just summed it up in terms of high/low readings

[ylwgtr2]

  cameron said:

If your getting 13 psi difference usally indicates you are losing a bit of compression with worn out ring lands and it will only get worse....

 

eg: testing RB25

 

Piston 1 - 160psi

Piston 2 - 163psi

Piston 3 - 162psi

Piston 4 - 161psi

Piston 5 - 162psi

Piston 6 - 150psi - Doesn't look to good!

 

Maby you can get away with a little more slack but on an old engine you carn't have 1 cylinder down a fair bit eg 13psi down and the rest fairly even It can result in  time for a rebuild.

 

Not sure on what guage it was but just compression test.

 

Had one RB25 with cylinder number 6 on 0psi compression !!! :headbang: - I dont know how the thing started, it ran like a hairy goat :uh-huh:  

 

 

Jun

your kidding right?It did have 5 other cylinders to push it along you know....

[BBQ]

  ylwgtr2 said:

your kidding right?It did have 5 other cylinders to push it along you know....

Yes it did, but the other 5 cylinders had cracked ring lands all the way through.

So it wasen't pushing along too well. Mainly because the plugs would oil up, preventing it from starting !!