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Engine Rebuild Question

Race Bred

By Race Bred,
in Engines & Forced Induction

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Race Bred Rookie

Race Bred

Posted
Posted

sorry for the newb question fellas, but i was wondering; by rebuilding the bottom end, does it affect the compression of the cylinders in any way? ie. would a low compression turbo engine (which is bad) still be 'bad' after a rebuild? Also, what are the safe compression levels on an rb20det?

cheers, miguel

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r33_racer Grand Master

r33_racer

Posted
Posted

Are you referring to the engines CR and it being bad cause it was built bad originally? or do you mean the engine is shagged from old age and abuse and the compression has dropped and needs a rebuild...if the latter then yes an oversized bore with new pistons and rings to suit will ensure compression is good again and tolerances are tight.

Id imagine compression would be around the 160-170 mark..similar to an rb25.

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MerlinTheHapyPig Mentor

MerlinTheHapyPig

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Posted (edited)

from memory, anything between 127psi and 170psi is good compression, and variance of no more than 15psi between cylinders (these are from rb20det manual if i remember them correctly)

low-compression related problems can arise from

-piston rings (or cracked piston / stuffed bores)

-head gasket

-valves / head

bottom-end rebuild will include new piston rings and will fix any bottom-end related compression problems.

Usually to tell if your rings / pistons aren't holding compression you can add 10ml or so of oil to the cylinder when doing the compression test, if the comp increases then your rings are gone.

(not to be confused with 'compression ratio')

when rebuilding an engine you can lower the compression ratio by taking material out of the head, however this is completely different from what i've mentioned above, and low compression ratio's aren't necessarily bad, particularly for turbo's which are more prone to detonation. An aftermarket thicker head gasket can also lower the comp ratio.

it would be impossible to detect if the comp ratio has been altered without taking the motor apart.

hope this answers your question

Edited by MerlinTheHapyPig
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