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Turbo Lag When Changing Gears


nuffsaid
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Hi all

In my R34 GTT I noticed that when I change gears hard between 1st to 2nd, to 3rd etc etc near redline there is a split second where the revs just hover at around 5000rpm then the car pulls hard to redline again.

I initially thought it was the clutch slipping a little, but Im almost certain its not. Other than a full exhaust there arent power mods, plus I just had an Exedy HD clutch installed. So not much power, and a strong enough clutch.

I'm pretty sure Im changing gears pretty fast, the short shift kit helps also.

Question is, does the car seem to have to build power up again because as I let off the throttle and step back on it after the gear change, the turbo and IC pipes have to spin and fill up with air again.

This may be fundamental turbo car behaviour, maybe its not, dont know, my first turbo car! haha

Anyways, my BOV is standard. Does a plumb back BOV make things better? or is the factory one already plumb back. Secondly, would putting a screw to block up the tiny hole in the BOV help this?

It is kinda annoying coz you want the car to pull hard as soon as the throttle is back on, but it takes a tiny bit of time before the car picks up and pulls.

So

1) is it normal?

2) can you do anything to stop it happening (without getting a new BOV)?

3) will modifying the BOV help?

4) do you need a remapped, piggy back, standalone ECU to prevent it?

Thanks

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Well after reading this

http://www.skylinesaustralia.com/forums/in...=146996&hl=

Sounds like it is a pretty standard thing, a lag before the torque comes tumbling though. Just didnt think it the lag would be as much as it is.

In NA cars I noticed that as soon as you get back on the gas, the car starts pulling. The feeling isnt so apparent with my car. The pull comes in a bit later.

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Doesn't sound normal to me, especially with a standard car. It either has modifications that you are unaware of (bigger turbo, ecu, etc) or there is a fault/leak.

I have a HKS GTRS turbo, making 255rwkw, and once power is made there is no noticable lag between gears. A standard turbo is on full boost by 3000rpm, so stating that there is lag at 5K is surprising.

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I got the same thing with my R34GT-T after a power FC and EBC it feels a little better. A large or split dump pipe should help and make sure your bov is plumb back. You could always go buy a Ball bearing high flow!

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When I say lag I mean as you are off the throttle, the turbo kind of has to spool back up again when you are back on it. And you dont get that nice rush until the spool is 100%

Almost certain there are no other mods, plus the R34 GTT turbos are already ball bearing.

Missfiring? the exhaust does pop and rumble every now and then on idle, however when stepping on the gas, there is no popping/stuttering at all all the way to redline. But you're right Links, there is like a 500rpm hesitation.

However, the car doesnt jerk, pop or splutter at all.

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yeah this doesnt sound normal for a stock skyline.

Ive got a large highflowed turbo on my baby RB20det and it does what you discribe, however only if i change gears at a normal pace, if i change slightly quicker it just stays spooling and pulls hard straight away, So i find it strange that a stock turbo has that moment of lag...

check pipes, all hose clamps and so forth

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Difference is in my cast, it was stuttering like itd jump back then jump forward on boost, what your describing seems normal, it's just all the piping refilling (although if it's standard with standard cooler it's a bit odd, i'd be looking at leaks)

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Block the bov off.

Much better next gear boost response as the turbo doesn't have to fill the ic and ic piping.

Made mine run a little crappy when doodling around.

So... I'm going auto as I'm bored of the manual for now. :thumbsup:

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i kinda had the same thing.. you'd change gears and the boost would go up but nothing seemed to happen for a second or so.

it happened after i crushed my bov like 2cm instead of 2mm :thumbsup:

bought a new bov and she was bak to normal after that.

....new bov as in a second hand one off sau :laugh:

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Yeah its just the stock BOV, its main fuction is to ensure driveablilty. A big rush of power when you get back on throttle is not what nissan wanted for there new production cars at that time so they designed it to bleed off a bit of boost to ensure more linear power. Get Greddy type S BOV this should fix your off throttle problems.

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Why do you need a Blow-off Valve (BOV)?

To eliminate compressor surge when changing gears. Excessive backpressure is created when the throttle is closed during gear changes or deceleration, causing turbo cavitation. This is detrimental to the life of your turbocharger; however fitting a BOV will substantially improve this problem. (not an issue in an auto as the throttle isnt closed when upshifting)

By installing a BOV you will significantly reduce turbo spool up time. You will notice a substantial improvement in response from your turbo between gearshifts. Boost will come on earlier giving an increase in torque at lower RPM.

Sounds contradictory to some of the advice you have been given above.

In the old days Turbo Lag was a real big problem with F1 cars. Drivers had to use the accelerator befor the apex of a corner just to ensure they were on boost as they exited. The introduction of the BOV was considered the answer to these problems and alowed manufacturers to add turbos to mass produced cars.

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4door_sleeper,

Unfortunately that's incorrect. :P

A bov is there for nothing more or less than emissions and driveability.

The reason for turbo failures in the old days was poor turbo design, lack of water cooling and journal bearing.

Most definitely nothing to do with a bov.

It really is only there for emissions and to improve driveability (smoothness when off/on the throttle).

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I have noticed this with my RB20det too, when winding it out and shifting hard there is like a half second delay before power comes back on. Also being my first turbo car I don't know whether there is anything wrong or not. I've got a fmic and some long piping.

My best guess would be that in my case it's the aftermarket atmo bov. Maybe the ecu is expecting to see that little bit of extra air in between gearchanges so it gets a little rich flat spot then once the air is flowing is goes hard.

I'll see if anything changes after I put the standard bov back on. Although I have also noticed a little vaccum whistle.. sitting stationary at idle, give it some revs so it gets a little bit of boost and the bov lets go, as it's winding back down to idle I can hear a little suction noise, we couldn't locate where it was coming from. When I get motivated enough I should probably redo all the vaccum hoses and see if it goes away or fixes the hesitation between gears.

My thoughts.

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