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Compression Test

AdamsR33

By AdamsR33,
in Engines & Forced Induction

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AdamsR33 Mentor

AdamsR33

Posted
Posted

Hi, yes i did a search and the answer i'm looking for didn't come up.

So my question is when i did a compression check, my values were from 135 to 145, i know an RB25DET static ratio is 9:1 but this doesn't tell me if my readings are fine, How do i calculate my values to see if they are good or not?

These are my results:

1 - 145

2 - 140

3 - 135

4 - 140

5 - 140

6 - 145

Motor was fairly warm and stupid compression tester was hard to get tight but think they should be about right.

No mods other than turbo back exhuast and pod filter.

Cheers , Adam.

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https://www.sau.com.au/forums/topic/130019-compression-test/
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12DFY Collaborator

12DFY

Posted
Posted
thanks for that, just put my mind at ease, was worried about #3 being a bit low.

Cheers.

Ohh and Dave we need to have beers soon. ;)

Good reading would be near 160psi, youre engine is getting a little tired but will still be good for a while yet.....

nfi Community Regular

nfi

Posted
Posted

Guys, The absolute value is not that important. As long as its not excessively low eg. ~125 then what matters is how close all the cylinders are.

The reason it only matters how close the cylinders are is that every compression tester reads slightly differently. Obviously quality, parts used etc effect the result. Also the temp of the engine and the amount of voltage in the battery effects it as well.

My car showed 10 psi difference between two compression test guages within two weeks. And the second one was higher!

Those results look ok to me. Generally the cylinders you do last start to show lower figures as the batteries looses a tiny bit of its beans! Again I have seen this as well.

I wouldn't worry. Keep taking it to the track!

Leakdown test is by far the most accurate means of checking the condition of the engine. It actually tests the pressure the rings can hold as well as valves and stems.

AdamsR33 Mentor

AdamsR33

Posted
  • Author
Posted

Interesting, i wonder what SK would say about this. SK???

NFI, i will be taking it to the track as often as i can. Its got good power for a stocker, at SAU dyno day it made 175rwkw so not too shabby. When i did the comp test it was in this order 1, 6, 5, 4, 3, 2, why??? i was just go front to back but then tought that number 6 is the one that stuffs up, usually, so...

How do i do a leakdown test??? do i need to take it to a mechanic?

nfi Community Regular

nfi

Posted
Posted

Just noticed wrxhoon is watching this thread. I think he will give you a much better explanation than I can. I learnt all about this from him.

Leakdown test you need an aircompressor and a special air tool. Best taking it too a good mechanic. I wouldn't worry though as if you are making good power all signs point to a pretty good engine. I would start to worry when it starts burning oil or starts to not make power.

Craved Grand Master

Craved

Posted
Posted
Hi, yes i did a search and the answer i'm looking for didn't come up.

So my question is when i did a compression check, my values were from 135 to 145, i know an RB25DET static ratio is 9:1 but this doesn't tell me if my readings are fine, How do i calculate my values to see if they are good or not?

These are my results:

1 - 145

2 - 140

3 - 135

4 - 140

5 - 140

6 - 145

Motor was fairly warm and stupid compression tester was hard to get tight but think they should be about right.

No mods other than turbo back exhuast and pod filter.

Cheers , Adam.

Adam, my motor was getting the same readings last time it was checked at UAS..

still makes good power, and nearly 0 blow by in the catch can ..

still made 294rwkw @ 18psi

FATGTS-R Experienced

FATGTS-R

Posted
Posted

Nfi made a good point on battery voltage. The two you tested first, when battery would of had more charge are the highest readings.

Try it again going in reverse order from lowest reading cylinder to highest and see if theres a difference?

And as mentioned, a leakdown test will give a better indication of engine health.

khunjeng Veteran

khunjeng

Posted
Posted

these tests look fine. Even if one cyl. is a bit low some carbon bulid up etc can cause this...

leakdown for more accurancy but really you only need to do ti if u find a problem with the compression test to pin-point the cause of the low reading.

wrxhoon Grand Master

wrxhoon

Posted
Posted

Number 3 looks a little on the low side in comparison with the others. Assuming you have a good gauge , done the test when the engine was at operating temp ( very hard without geting burnt), the starter, battery were good and had the throttle wide open then all cylinders read low, they should be around 170 psi.@300rpm

khunjeng: carbon built up will increase compression..

AdamsR33 Mentor

AdamsR33

Posted
  • Author
Posted
Number 3 looks a little on the low side in comparison with the others. Assuming you have a good gauge , done the test when the engine was at operating temp ( very hard without geting burnt), the starter, battery were good and had the throttle wide open then all cylinders read low, they should be around 170 psi.@300rpm

khunjeng: carbon built up will increase compression..

Actually test was done when the motor has cooled abit so i wouldn't burn myself, and the throttle was shut, I will do it again and post new readings.

Busky2k Veteran

Busky2k

Posted
Posted

Yeah good point about the battery. Thats why I have the car hooked up to a battery charger whilst doing the tests. I wait 5 mins between each cyl so the battery gets topped up.

Btw also some readings may be artificially high if there is a lot of carbon deposits (raising the comp ratio).

wrxhoon Grand Master

wrxhoon

Posted
Posted
Actually test was done when the motor has cooled abit so i wouldn't burn myself, and the throttle was shut, I will do it again and post new readings.

With WOT the redings will be higher, make sure you keep cranking the engine until compression stops rising.

A fully charged 40+amphour battery should be good enough to complete the test, make sure all the plugs are out before you start the test.

Busky2k, they are not 'artificially' high if you have carbon built up, the combustion chamber is somewhat smaller so compression will increase. Same as if you shave the cyl head or block or even have a thiner h/gasket.

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