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Rb25-30 Compression. Na Pistons, Head Skimmed Couple O Times

joe blo

By joe blo,
in Engines & Forced Induction

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joe blo Experienced

joe blo

Posted
Posted (edited)

hi mates. just got a question about my setup. its na pistons and the head has been skimmed i rekn atleast 3 times. i had it done twice (after i got the head rebuilt then had to do it again after welded the bit for conversion), and i rekn the guy i got the head off may have skimmed it coz he had used it on a conversion also.

how much would 2 or 3 head skims raise the compression?

and i got the bottomend rebuilt. has na replacement pistons. shud they sit level with the top of the block on the outer rim of the piston? i believe mine do.

reason i ask about this is my last setup broke 4 out of 6 ringlands and doesnt show signs of leaning out. guy says its too much compression. but he also says the rb25 head on 30 bottomend RAISES the compression. but it doesnt hey? rekns id be about 11:1 with his idea that it raises the compression and something like 16:1 on boost

so just wanna see if the head skims and decking the block ( i assumed it only got skimmed to make it perfectly flat again) would cause me compression issues.

ive already bolted the head on again and am about to get it tuned again by different place. havent checked compression. was just going off the guide thinking it wud be 8.2:1 plus abit extra from the head skimming

thanks mates

Edited by joe blo
Gerg_R31 Mentor

Gerg_R31

Posted
Posted

http://www.skylinesaustralia.com/forums/topic/378823-engine-internals-comp-ratios-general-info/page__fromsearch__1

that is a link to some info i wrote up a while ago, it has the correct formular to work out your static compression ratio

as the rb heads have a tappered chamber, and how much was taken off each time it was ground will vary or "how long is a piece of string?"

Do a compression test and post the results, some forum users on here may be able to tell you a rough area on where your comp ratio is with that

for my motor 10.75:1 i get 225-230psi dry comp 240 wet comp test

The Mafia Grand Master

The Mafia

Posted
Posted

8.2:1 is a standard NA RB30E block and RB25DET head.

With 20 thou taken off mine was estimated to be 8.35:1

So with 60thou taken off I'm estimating around the 8.65:1 mark. But this is a very avergage estimation. I'm assuming with tapered chambers etc the more you take off the greater it goes up.

But, I am sure that you compression will be well under 9:1.

The reason your have blown your ring lands off 4 pistons, is because of the tune, and driver abuse. I'd even put money on that.

KiwiRS4T Grand Master

KiwiRS4T

Posted
Posted

http://www.skylinesa...__fromsearch__1

Do a compression test and post the results, some forum users on here may be able to tell you a rough area on where your comp ratio is with that

for my motor 10.75:1 i get 225-230psi dry comp 240 wet comp test

OK well the compressions for my RB30/25 are all around 132psi so has anyone got a similar engine with a known CR to compare ( I would guess about 8:1 but its not that important now)? For reasons too boring to relate my budget engine had dodgy pistons replaced with some good cond but evidently less than ideal pistons that were available at the time. Consequently its only making 230awkw (with a GT3540) at 15 psi instead of nearer 300. I have just done a leakdown test - all between 5 and 8% so I will wind the boost up as much as possible with a new tune and live with whatever power it makes until I can afford a rebore with 9:1 pistons.

KiwiRS4T Grand Master

KiwiRS4T

Posted
Posted

At 8.2:1 I got 155psi per cylinder.

What sort of gauge are you using?

At 8.2:1 I got 155psi per cylinder.

What sort of gauge are you using?

At 8.2:1 I got 155psi per cylinder.

Just the handheld compression gauge I bought many years ago - the tuner got the same figure with whatever he was using

What sort of gauge are you using?

just a quick Q, wouldn't a comp test give you dynamic comp ratio and not static?

Comp test doesn't really give you a compression ratio just wanted to compare with someone who did actually know his compression ratio and also the psi.

The two tests gave me the answers I was seeking - the leak down test is ok so the motor is basically sound and the low compressions are the product of the choice of pistons, head gasket and head with the main culprit being the pistons so as above I iwll just make the best of what I have with the best tune possible until I can afford new pistons, balance etc ...probably a year or two away!

  • 1 month later...
MRHD66 Rising Star

MRHD66

Posted
Posted (edited)

I would think everyones different camshafts would mess with your comp readings from engine to engine.

^id have to agree with this comment ^ Edited by MRHD66
  • 4 weeks later...
  • 2 years later...
AngryRB Grand Master

AngryRB

Posted
Posted

What would be the highest compression you can get with RB30 stock bottom end and a neo head? has anyone achieved 10:1 or higher with say a 0.8mm head gasket and shaving the head, how much can it be shaved?

I wanna get the maximum out of the combo, as in area under curve for the street...

KiwiRS4T Grand Master

KiwiRS4T

Posted
Posted

What would be the highest compression you can get with RB30 stock bottom end and a neo head? has anyone achieved 10:1 or higher with say a 0.8mm head gasket and shaving the head, how much can it be shaved?

I wanna get the maximum out of the combo, as in area under curve for the street...

Have a search - someone built a very high compression N/A RB30 a while ago.

Found it already - its back in this thread^^

Gerg_31 got 10.75:1

AngryRB Grand Master

AngryRB

Posted
Posted (edited)

maybe I should be more specific, I want to get 10:1 compression ( 200plus psi on a comp test) or very close, what gasket and shaving is required ?

pretty sure the 10.75:1 was from a 3.2 stroker 26/30..

Edited by AngryRB
AngryRB Grand Master

AngryRB

Posted
Posted

That was just to give an idea, it will get the drop in tomei cams down the track but when you have a good RB30 stock block and a neo head, what do you suggest to get every drop of torque under the curve from the block and head combo, so hence comp and quench area, im sure a lot of guys will be interested in this...

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