Jump to content
SAU Community

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

clip14

By clip14,
in Engines & Forced Induction

 Share

Recommended Posts

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.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share
Go to topic listing


  • Similar Content

  • Latest Posts

    • MBS206
      Duncan's V37.......race car?

      By MBS206 · Posted

      Even more fun, leave all the ADAS stuff plugged in, but in different locations, hopefully avoid any codes!   And honestly, all these new cars with their weird electronics. Pull all the electronics out Duncan, and just shove an aftermarket ECU and if needed a trans controller in, along with a PDM. Make it run basic but race car styled!
    • Duncan
      Duncan's V37.......race car?

      By Duncan · Posted

      To follow up a question from earlier too since I had the front bar off again (fking!) This is what is between the bumper and the drivers side wheel And this is the navigator side, only one thing but its a biggy! So basically....no putting coolers in the wheel arches without a lot of moving other stuff. Assuming I move to properly race prepping this car I'll take that job on and see how the computers respond to removing a whole bunch of ADAS modules
    • Duncan
      Duncan's V37.......race car?

      By Duncan · Posted

      So I prepped the car for another track day on Wednesday (will be interesting to see coolant temps post flushing out and the larger reservoir, with a forecast of 3-14 being 20o cooler than last time I took it out). Couple of things to mention; since I am just driving the car and not taking a support vehicle, I took the rear seats out and just loaded the back up Team Trackday style. Look at all that space! To cover off removing the rear seat....it is weird (note the hybrid is probably different because it wouldn't have folding rear seats) Basically, you remove the lower seat base, very similar to a r series but it is a clip that pulls forward to release the base rather than it being bolted down. Easy Then, you need to remove the side section of the rear seat on each side. There is a 14mm head nut at the bottom of the side piece, the it slides upwards off a hook at the top to release; you also need to unhook the seatbelt from the loop at the top. Then the centre piece is weird. You need to release/fold the seats forward with the tab in the boot on each side From there, there are 2,x12mm headed bolts holding the rear of each seat to the folding bracket, under the trim between the rear seat and the boot (4x christmas tree clips there, they suck). The seat is out but you can see where the bolts attach to the bracket
    • RP1421
      S15 Spec R

      By RP1421 · Posted

      As discussed in the previous post, the bushes in the 110 needed replacing. I took this opportunity to replace the castor bushes, the front lower control arm, lower the car and get the alignment dialled in with new tyres. I took it down to Alignment Motorsports on the GC to get this work done and also get more out of the Shockworks as I felt like I wasn't getting the full use out of them.  To cut a very long story short, it ended up being the case the passenger side castor arm wouldn't accept the brand new bush as the sleeve had worn badly enough to the point you could push the new bush in by hand and completely through. Trying a pair of TRD bushes didn't fix the issue either (I had originally gone with Hardrace bushes). We needed to urgently source another castor arm, and thankfully this was sourced and the guys at the shop worked on my car until 7pm on a Saturday to get everything done. The car rides a lot nicer now with the suspension dialled in properly. Lowered the car a little as well to suit the lower profile front tyres, and just bring the car down generally. Eternally thankful for the guys down at the shop to get the car sorted, we both pulled big favours from our contacts to get it done on the Saturday.  Also plugged in the new Stedi foglights into the S15, and even from a quick test in the garage I'm keen to see how they look out on the road. I had some concerns about the length of the LED body and whether it'd fit in the foglight housing but it's fine.  I've got a small window coming up next month where I'll likely get a little paint work done on the 110 to remove the rear wing, add a boot wing and roof wing, get the side skirt fixed up and colour match the little panel on the tail lights so that I can install some badges that I've kept in storage. I'm also tempted to put in a new pair of headlights on the 110.  Until then, here's some more pictures from Easter this year. 
    • joshuaho96
      R32 GTS (RB20DE) Injectors Replacement

      By joshuaho96 · Posted

      I would put a fuel pressure gauge between the filter and the fuel rail, see if it's maintaining good fuel pressure at idle going up to the point when it stalls. Do you see any strange behavior in commanded fuel leading up to the point when it stalls? You might have to start going through the service manual and doing a long list of sensor tests if it's not the fuel system for whatever reason.
×
×
  • Create New...