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Acceptable Head Compression - Rb25det Neo.

Pulseless

By Pulseless,
in Engines & Forced Induction

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Pulseless Rookie

Pulseless

Posted
Posted

Long story short, buying a rb25 s15 - had a full mechanical inspection done and got the following results from the comp test:

#1 – 180 psi
#2 – 170 psi
#3 – 135 psi
#4 – 145 psi
#5 – 120 psi (145 psi wet)
#6 – 180 psi

Those numbers to me feel off, i didn't even think they got compression levels so high. Naturally, the owners then freaked out and taken
it to another place where the comp test came back 140 psi across all 6 cylinders..

So now I'm chasing a 3rd compression test and will also be forking out for a leak down test as well.

So my question is 140 across the 6 an acceptable number to work with (apparently a 90k motor out of an auto stagea)? especially when I'm spending upwards of 20k on this car.

Thanks for your opinion.

GTSBoy Grand Master

GTSBoy

Posted
Posted

140 even is fine. Remember that it is not a particularly accurate test, so good luck trying to tell the difference between a "high comp" Neo's test results and a plain R33's test results unless done on the same day by the same person with the same comp tester.

The original results are junk. You should not be able to get 170-180 on a normal 25 or Neo.

Pulseless Rookie

Pulseless

Posted
  • Author
Posted (edited)

Haha had another guy saying 175-190 psi is stock on these engines.

Do you think a leak down test will give a better idea on health of the motor?

edit*

factory compression is 173.5248 psi exact apparently. I should also mention the 140 across the 6 was a dry test.

Edited by Pulseless
KiwiRS4T Grand Master

KiwiRS4T

Posted
Posted

Walk away. Too much variation. A compression test is not rocket science so no point in doing lots of them.You will find that it drives ok with dodgy compressions but for how long?

KiwiRS4T Grand Master

KiwiRS4T

Posted
Posted

The number is not that important - its the variation between them that is a worry.

Yes a leak down test if done by a competent operator should tell you if the valves or rings are leaking. Again a variation below 5% is desirable.

iruvyouskyrine Grand Master

iruvyouskyrine

Posted
Posted

Yep I got 170psi exactly on all 6 on a genuine low k RB25 neo. 10-15psi variation depending how anal you are.

Personally I'd be happy even with 20psi variation and just turn up the boost and put things in place to aid the inevitable blow by issue.

  • 3 months later...
V28VX37 Veteran

V28VX37

Posted
Posted (edited)

I got a comp test done recently on my RB25DET Neo ('98, about 150k on it) just out of curiosity, here's the results:

#1 - 165

#2 - 150

#3 - 160

#4 - 150

#5 - 155

#6 - 155

It's within the 10% variation, just. Anything to be concerned about, should I get a leak down done?

Car is running fine, just with an intermittent miss, same as it's always had...

(EDIT: Not sure if above figures are hot/cold/dry/wet, will check)

Edited by V28VX37
admS15 Grand Master

admS15

Posted
Posted

I got a comp test done recently on my RB25DET Neo ('98, about 150k on it) just out of curiosity, here's the results:

#1 - 165

#2 - 150

#3 - 160

#4 - 150

#5 - 155

#6 - 155

It's within the 10% variation, just. Anything to be concerned about, should I get a leak down done?

Car is running fine, just with an intermittent miss, same as it's always had...

(EDIT: Not sure if above figures are hot/cold/dry/wet, will check)

I'd be pretty happy with that for an 18yr old engine.

Dose Pipe Sutututu Grand Master

Dose Pipe Sutututu

Posted
Posted

Quote for truth. Plz take this advice.

I bought my shit box, didn't bother to do a comp test because I was too lazy and it was cold lol (had the gauge and socket set on me too) and still haven't done one 3 years later.

Also not to mention I screw in more boost than 90% of owners and thrash the shit out of it on the track.

Once it does grenade, I'll just throw in a 2nd hand motor and do the same thing.

Moral of the story, thrash it till it blows up. Leak down test or compression tests are pointless once you've already bought the car IMHO.

V28VX37 Veteran

V28VX37

Posted
Posted

I'd be pretty happy with that for an 18yr old engine.

Car is 20 years old man, if it runs then be happy with that, then pop in new ARP studs and screw in tons of boost, bake tyres and smile

Quote for truth. Plz take this advice.

My neo has has 100 to 120 flat.for 3 years with 300 plus kw no blow bye or issues

Thanks guys, great response! And yes it's been running 275rwkw @ 20psi on 98 so almost double the stock power for quite a while now, gotta be happy with that ;)

Dose Pipe Sutututu Grand Master

Dose Pipe Sutututu

Posted
Posted

how I see it (feel free to ignore)...

These cars are shit boxes that are fast, handle well and cost more than they should... they're no Ferrari or no 2015 GT-R, so worrying about a few psi across a 20 year old motor is pointless.

It's like a Commodore VN owner doing a compression test on their 25 year old lump of shit and whinging about a 20psi difference between the boat anchor.

  • Like 1
GTSBoy Grand Master

GTSBoy

Posted
Posted

Those Buicks can be epic motors!

They can be, viz the Formula Holden stuff. But in original VN series 1 form they are a horrible, nasty, shuddering cheese grater of an engine, saved from being a completely useless turd by having stacks of torque at the bottom end.

  • Like 1
Dose Pipe Sutututu Grand Master

Dose Pipe Sutututu

Posted
Posted

They can be, viz the Formula Holden stuff. But in original VN series 1 form they are a horrible, nasty, shuddering cheese grater of an engine, saved from being a completely useless turd by having stacks of torque at the bottom end.

I had one, shameless admission

in 2 years I had it, it went through like 4 power steering pumps, 2 alternators, 1 crank sensor

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