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Rb25 Det Neo , Just Forged It

avoiee

By avoiee,
in Engines & Forced Induction

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avoiee Enthusiast

avoiee

Posted
Posted

So finished rebuilding the engine , got Arias custom made pistons in it for neo engine ! when it was freshly finished it showed 120, 120, 130, 125, 130, 120 psi in all six cilinders . drove about 700km and now in all 6 cilinders is 120psi, same in all!! i have putted tomei poncams as well ! gasket is standard size standard has 9:1 as far as i know what number i have now? is it too low?

my friend owns RB26 he did same (forged it) and his car shows 145psi in all six?

so am i ok or in trouble? :))))

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r33_racer Grand Master

r33_racer

Posted
Posted

Normally bigger cams will drop your compression when you do a test, in comparison to standard. But not that much, especially with poncams. Did you build it yourself or an engine builder do it for you?

You did have the throttle fully open when you did the test?

aleks Community Regular

aleks

Posted
Posted (edited)

my healthy neo with pon cams reads 100 psi all round lol

This is why I was persistently asking you about this.

110psi across all six here, with a new engine with poncams which I thought was pretty low but it runs great.

Using the same tester on an a standard RB26 which had 165-170psi across all cylinders then a built RB26 with big cams; 160-165psi :mellow:

Edited by aleks
avoiee Enthusiast

avoiee

Posted
  • Author
Posted

Normally bigger cams will drop your compression when you do a test, in comparison to standard. But not that much, especially with poncams. Did you build it yourself or an engine builder do it for you?

You did have the throttle fully open when you did the test?

didn't touch the throttle and didn't cut the fuel :) lol i'll try to do that way :)

Glad that here is people running that low CR with no problems:)

thx

Rolls Grand Master

Rolls

Posted
Posted (edited)

you sure?? cause mine is as high as 170 and 175,

100psi is low, that must be pretty unresponsive on low revs??

I really doubt its compression is actually almost half of yours, if it did it would drive like absolute shit, either completely wrong pistons or blown rings. The reality is it is probably one motor was cold/hot, different tester, different rpm, different cams etc.

My old rb20 measured 90-95 on all 6, made 155kw on 11psi stock turbo and blew no smoke, all that matters is it is close.

Edited by Rolls
AngryRB Grand Master

AngryRB

Posted
Posted

thats just it.. too many variations to be able to compare one to another.. the main thing is all cyns are the same, and comp testers fall into the same bucket as dynos.. just a tool.

well i wouldnt be happy them just all being the same, id want to know its perfect after spending all that money rebuilding an engine,

a compression guage shouldnt be more than 5% different to the next , thats like saying a tyre guage says its 40psi but another is saying 30psi?

must be quite a big gap with forged pistons when there cold and the cam must be quite long duration to justify a 70psi drop after a rebuild, but just my opinion,

but if your happy with it then great..

JonnoHR31 Grand Master

JonnoHR31

Posted
Posted (edited)

thats like saying a tyre guage says its 40psi but another is saying 30psi?

actually happens a lot more than you'd think and more than 10psi a lot of the time, especially at servo gauges

also dont forget, static compression is different to dynamic compression, way too many variables to go comparing readings. as said aslong as they're all within 5-10psi and its not using more oil than fuel its fine.

Edited by JonnoHR31
Rolls Grand Master

Rolls

Posted
Posted

well i wouldnt be happy them just all being the same, id want to know its perfect after spending all that money rebuilding an engine,

a compression guage shouldnt be more than 5% different to the next , thats like saying a tyre guage says its 40psi but another is saying 30psi?

It isn't just the compression gauge though, it is cam timing, actual compression ratio due to pistons, if they are forged, if they rings aren't yet bedded in, if you have a big cam, all of these will change the compression.

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