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Rb25de Vct Not Active - How Much Power Am I Losing?

clip14

By clip14,
in Engines & Forced Induction

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Sandeep Community Regular

Sandeep

Posted
Posted

vct affects your midrange torque and power more than peak (high rpm) power. not 100% sure on the rb series, but on the vg and vh engines it is only on until around 5000-6000rpm, then it switches off.

from what i understand, the idea is to increase valve overlap at low/medium rpm (not at idle though) to help suck in the in intake air/fuel mix and blow out the remaining exhaust gases. however at high rpm the pressure in the exhaust manifold may be high enough that some of the exhaust gas will actually be pushed back into the intake due to the bottleneck in the exhaust manifold... this would cause a decrease in power and increases your cylinder temps. so to avoid this they switch off vct at higher rpm's.

basically with vct disabled, there will be very little (if any) difference in your peak power compared to if you had working vct. your torque curve however will drop away after the peak torque, whereas with vct it will be abit flatter towards high rpm and your midrange power would be reduced.

Eug Grand Master

Eug

Posted
Posted

i know the RB30 guys had a couple of dyno graphs in the forced induction thread with and without VCT. not a 100% sure exactly, but the difference was something like 10-15kw over a 1000-2000rpm range.

i don't know how applicable that would be over to an NA RB25, but i would say probably about the same taking compression into consideration????

MissR34 Grand Master

MissR34

Posted
Posted
my vct isnt working because I'm running a r32 loom and ecu.

I know the peak power is the same as vct is turned off in the higher rev range. was just wondering if anyone had some dyno plots so i could see the midrange difference.

why are you running a 32 loom and ecu? did u do a engine swap?

LUME Enthusiast

LUME

Posted
Posted (edited)

when did the vct's start getting used? what model onwards with the skylines?im not trying to hack the thread but my 33 is extremly gutless low and mid nothing starts happening until 5500 rpm?

its a s1 1993 r33 btw

Edited by LUME
G0DZLR Mentor

G0DZLR

Posted
Posted

VCT got bought in with the S1 R33s and from thereonwards was standard. easy way to tell whether or not it has VCT is whether it has a buldge on the lefthand side of your timing belt cover.

the only RB25DEs without VCT were the ones from the R32s, VL owners usually use them on RB30DE builds as they dont have to modify the block as much.

Luke, i had the same problem that your having. just launch it at about 4-5k RPM and you should be alright if youv got grippy tires, cos thats where the torque band is. i gained heaps of bottom end go when i turboed mine, cos of the high-comp motor.

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