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Shocked With My Compression Test !

andy95'R33GTR
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aerofocker Enthusiast

aerofocker

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If the comp doesn't change with the oil test then you could just remove the head with the motor still in, just be sure you do the oil test properly, with enough oil to ensure it get's around the whole piston, and not to much or this will give you false higher psi reading's, the other way to check is to presurize the cylinder with a fitting that screw's into the spark plug hole, with both intake and exhaust valve's closed on the power stroke, then listen for leak's through the intake manifold and exhaust manifold to verify a faulty valve, test each cylinder and see if their is more hissing on the cylinder's with the low compression, just remember to rotate the crack each time to get the cylinder your testing on the power stroke to ensure the cam's arn't holding the valve's open slightly,

could also be worn valve seat's causeing low compression because the gtr's run solid lifter's they need shimming to change the valve clearance, but with rb20- rb25's use hydrolic lifter's so they never need touching until one shit's it's self, or you get sick of the 10 seconds of tapping every now on start up and then and replace the f-ing thing,

If it was me i would pull the motor and check the piston's and bearing's and fit new one's anyway while you doing open heart surgery on your RB, then you know it's going to be a healthy rb, for a couple of hundred extra it's worth doing now rather than later,

You need to know how to gauge clearances for your bearing's, if your not confident doing this then leave the motor in the car and just do the head, if you get the clearance wrong it could throw a big end bearing and end the fun,

I found the ACL bigend bearing's to have a enough varience to swap and change until i had all 6 with the spec's i wanted, to the point where the thickest bearing would lock the crank up when i fitted it to one cylinder, if i had just put the bearings in any which way, the motor wouldn't have lasted 5 minute's, It's still going strong 2 year's 25,000km later with abuse all the time, shit that means another service is due on my rb,

Hope this helps out a bit,

Glad i started on an rb20, will not be so worried if i can pick up a cheap gtr with compression problem's

you found enough varience to lock up the crank in one position? you got the wrong bearings.

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skyline4lifer32 Rookie

skyline4lifer32

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you found enough varience to lock up the crank in one position? you got the wrong bearings.

Not completly lock up just tight tolerences sorry miss worded it,

RB cranks have slight variences when measured with micrometer's and ACL bearings also have slight thickness variences

If i fitted the wrong bearing's would the RB still be going strong after 25,000km, No it would not

how many RB's have you rebuilt?

skyline4lifer32 Rookie

skyline4lifer32

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with new rebuilt pistons, rings and bores, what is the top compression rb26 have???

Hi Andy

From the factory a brand new RB26DETT will have this compression Compression Pressure: 171 psi / 300 rpm

Did anyone know an R34 GTR V spec is 44 pounds heavier than the standard R34 GTR, That doesn't seem right you'd think they would have shaved weight of not put it on, Probably all the V-spec badges they add, LOL

Edited by skyline4lifer32
skyline4lifer32 Rookie

skyline4lifer32

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1 rb25det plenty other engines,i dont talk crap.

I don't either, I built the RB20det with tight tolerances because of the thinner synthetic oil that it was built to run with, Stock clearances didn't suit the oil, If i was just going for stock clearances then i wouldn't give a sh!t about the small differences

skyline4lifer32 Rookie

skyline4lifer32

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does building aircraft engines count?

Yes it does, probably more so because a f*k up in the air means your in a plane that glides like a brick thrown through the front window of a popo car,

What aircraft do you work on?

andy95'R33GTR Rising Star

andy95'R33GTR

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And I forgot to mention that a wet test won't help with a holed piston or missing ring segment either.

hmmm i can see from the plug holes...there is no hole in the pistons, if the piston is holed, if won't give any compression...

andy95'R33GTR Rising Star

andy95'R33GTR

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ok... sorry... i was wrong, my pressure tester, is a mistake, now... i can hear the leaking air but i cant find it... its not on the oil dip stick... its somewhere in the cylinder head... sometimes when i tried to turn slowly the cams, the air will out on cylinder 1

Edited by andy95'R33GTR

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