Compression Test

[charlie25T]

Im just about to dyno the car and i did a compression test

i got 190psi on all exept for for i got 187psi..

mind you i have only done 77km on the clock

Now some people say omg it should 150 psi

and other say it should be between 170 - 190

what area your thoughts ???????????????????

[gt-ahhh!]

wet or dry test?

i think its sposed to be about 140-150 dry and and 160-170 wet but i could be wrong. im assuming you are referring to a rb25det.

you are lucky, i made a post yesterday about my 10 000km engine having 0psi in one of the cylinders...

[charlie25T]

  gt-ahhh! said:

wet or dry test?

i think its sposed to be about 140-150 dry and and 160-170 wet but i could be wrong. im assuming you are referring to a rb25det.

you are lucky, i made a post yesterday about my 10 000km engine having 0psi in one of the cylinders...

yeah i have R33 GTS25T

Im about to have a heart attack i dont want my motor to blow.....

oooooo sorry to hear about ur cylinder

[gotRICE?]

  charlie25T said:

yeah i have R33 GTS25T

Im about to have a heart attack i dont want my motor to blow.....

oooooo sorry to hear about ur cylinder

From what nearly every thread that questions like this have been asked, it more or less is irrelivent what the final number is (within reason) so long as each cylinder is pretty much the same within 4-5 points i believe.

[Adriano]

agreed, comp testers seem to vary a lot from one to the other, they are really only good as a comparison tool.

[charlie25T]

Does anyone know what is the compression for a brand new rb25t

[charlie25T]

Wellll how about this

Hills motorsoports said that it is should be about 150psi and 190 is far to much

Atm where my car is at Bel garage. they said 190 is high but excellent

[bubba]

No idea on what is high or not but it could have had the head shaved slightly after blown headgasket or something.. that would increase compression slightly.

[The Mafia]

factory compression in a RB25 is 172psi.

So if you have more you have a shaved head or decked block.

Either that or they put different pistons in when it was built.

But I have seen compression testers vary up to 30psi. My last one said I had 200psi in each cylinder. But they were all within 3 psi of each other so I didn't care.

[charlie25T]

  The Mafia said:

factory compression in a RB25 is 172psi.

So if you have more you have a shaved head or decked block.

Either that or they put different pistons in when it was built.

But I have seen compression testers vary up to 30psi. My last one said I had 200psi in each cylinder. But they were all within 3 psi of each other so I didn't care.

Thanks for that mafia..............

The test on mine went 190 on everyone except for the forth wich was about 186-187.

how i just installed a gt35......... am i in trouble

[MANWHORE]

What the guys said above is correct. Worry about variation between the cylinders rather than the actual number.

If you use a different comp tester, the results will be different again.

The actual number means squat (unless if it is reading ridiculous like 50 across all 6, which would most likely mean that the tester is f**ked, the person is an idiot and did it wrong, or the whole motor is f**ked)

[wrxhoon]

Sounds like you have a healthy engine there , stock pressure should be around 170 psi (12.2kg cm2) on RB25 det @ 300rpm, warm engine ( normal operating temp) and WOT. If any of the above vary you will get different results.

If you use a thinner h/gasket , deck the block or cyl head the pressure will be higher , if you have carbon built up the pressure will be higher .

If you want to be sure on the condition why not do a leakdown test ??

[charlie25T]

  wrxhoon said:

Sounds like you have a healthy engine there , stock pressure should be around 170 psi (12.2kg cm2) on RB25 det @ 300rpm, warm engine ( normal operating temp) and WOT. If any of the above vary you will get different results.

If you use a thinner h/gasket , deck the block or cyl head the pressure will be higher , if you have carbon built up the pressure will be higher .

If you want to be sure on the condition why not do a leakdown test ??

Leak down test is used for when you have a low compression..................

what do mean deck the block????/ herd that alot lately but not understanding

[wrxhoon]

  charlie25T said:

Leak down test is used for when you have a low compression..................

what do mean deck the block????/ herd that alot lately but not understanding

You can have a perfect comp dude on all cyl with worn piston rings and or bore because the oil can take up the wear and hold perfect comp. If that was the case when you do a leakdown test you could have say 25% leakdown pressure loss). Now I'm not saying for one minute that this is the case with your donk, as I said before it seams its in good nick.

Deck the block/head= shaving a few thou" . Has the same effect as using a thinner head gasket..

[charlie25T]

  wrxhoon said:

You can have a perfect comp dude on all cyl with worn piston rings and or bore because the oil can take up the wear and hold perfect comp. If that was the case when you do a leakdown test you could have say 25% leakdown pressure loss). Now I'm not saying for one minute that this is the case with your donk, as I said before it seams its in good nick.

Deck the block/head= shaving a few thou" . Has the same effect as using a thinner head gasket..

Well the boys at bel my engine is in good nick and you should be happy it has a high compression

i just spent a crapload on my car dont wanna spend no more

btw..... why would someone shave the head on a Skyline

[r33_racer]

if the head is pitted, warped/not perfectly flat....You shave the head so it has a flat face for a new headgasket to seal. Generally when rebuilding a motor, you shave the head in which they only take off what they need inorder to get it flat, same with the block. Unless your building a wild motor with custom pistons/rods and you need to deck the block to achieve a desired CR and squish area.

A smaller area for the same air/fuel to be squeezed into results in a bigger/powerful bang equals more power...however it can also make it more prone to preignition.

[RodMonsta]

  wrxhoon said:

Sounds like you have a healthy engine there , stock pressure should be around 170 psi (12.2kg cm2) on RB25 det @ 300rpm, warm engine ( normal operating temp) and WOT. If any of the above vary you will get different results.

If you use a thinner h/gasket , deck the block or cyl head the pressure will be higher , if you have carbon built up the pressure will be higher .

If you want to be sure on the condition why not do a leakdown test ??

whats the stock pressure for RB26dett?

also is carbon build up a bad thing?? or is it just the black smoke that comes out when the car is floored.

[wrxhoon]

  RodMonsta said:

whats the stock pressure for RB26dett?

also is carbon build up a bad thing?? or is it just the black smoke that comes out when the car is floored.

A little lower than RB25, 12.2kg cm2,

Carbon built up is not a good thing but it happens on most engines with age .

[gotRICE?]

  wrxhoon said:

A little lower than RB25, 12.2kg cm2,

Carbon built up is not a good thing but it happens on most engines with age .

It usually gets blown out the tailpipe when you really floor that shit.

Carbon buildup is a funny thing, its not good, but half the time when you run an engine cleaner or whatever through the oil and fuel you tend to start burning oil and blowing blue smoke..

So sometimes the carbon buildup is a good thing as its sealing off its wear and tear from being old.

Id be shit scared to run any in the oil in my Hiace, 400,000km (that the speedo reads, its wayyy more) and piston slap when its really cold... dam if i cleaned the carbon out of that id probly burn more oil than i could put in it.

[RodMonsta]

  Quote

A little lower than RB25, 12.2kg cm2,

Carbon built up is not a good thing but it happens on most engines with age .

is carbon build up necessarily a tired engine?? rebuild soon?

this is from 130,000 kms 1997 R33 GTR with 175 compression im looking at

Edited December 2, 2007 by RodMonsta