k j s

when i'm changing gears at high revs i sometimes get a momentary lapse in power, as if the turbo stops spinning... is this what is known as turbo lag. (if there such a thing).
Whats the best way to stop this happening ?
thanks in advance

MY99 turbo, STi exhaust system

WREXY

If it's happening just before the gear change, it means you are over reving it and hitting the rev limiter. Change 200 revs lower. Hitting the rev limiter too often is no good.

Turbo lag happens at low revs under 2700. When you put the foot down at those low revs the turbo takes ages to start spinning and the car feels laggy, (low power), then all of a sudden at above 2800 revs the turbo kicks in and there is a sudden boost of power. Not a very technical explanation but that's what it basically is.

Wrexy.

T-uk

have you had a service recently or added any oil to your engine?,as turbos need to be kept just below the max or the boost solenoid/pipes get oiled up.

john

Sith

K J S

I used to get that sort of thing all the time on hard charges to 3rd gear. Added an SS BB and an ITG Panel Filter, now it will go through the red line, change up and there will be no pause in power delivery. IMO the ITG sorted it as I had the same on my old Cav GSI, put a K&N Panel in that which solved the same prob.
A limitation of the standard paper filters to flow the correct air for high and fast gear changes IMO.

P.

PS. Personally I can't see the prob with hitting the rev limiter often. As the limiter is there to stop you from going to far and over revving it. The limiter is at a safe point. Agreed if you sat next to the limmiter it wouldn't help. Prepared to be correct by Steve Lawson or Moray M etc.

[This message has been edited by Sith (edited 01 September 2001).]

WREXY

Sith,

Regarding the rev limiter have a read of this,

kevin stanton

i'm due the 30month service this month, i'll let you know if the oil change cures the problem.

Sith

WREXY,

Fair 'nuff. Can't argue with that. Just as well I normally change up just before the red line! Have only changed up on the rev limiter once before and the exhaust gave an almighty crack an pop. Very Touring car like. Won't be doing it again. Luckily I have only bounced of the rev limiter a few times and that was early in my ownership. First turbo car. Oh and after I got my SS DP. Wasn't quite ready for the differance.

Cheers for pointing me to the thread,

P.

WREXY

I had also hit the rev limiter a few times. I try keeping away from it after I read that post by Pat. I still sometimes, very few times, hit it by mistake coz the car revs out so fast, especially in 1st gear.

Cheers,

Wrexy.

teknopete

KJS,

Good explanation of lag from Wrexy (simple but effective, jus the way it should be)
Something that no 1 has mentioned here is "turbo backspin" and IMHO that may well b what u have a problem with as it`s not so noticable at lower revs.
Basically what happens is when the throttle
is lifted the air in the turbo trys to exit through the path of least resistance, which in effect is to spin the turbo in a reverse direction (hence backspin) from approx 140,000 rpm. This has the effect of removing any pressure built up on the turbo and as such when the throttle is re-applied the turbo has lost most of its rotational speed and has to build up pressure almost from scratch.
There are different trains of thought on how to overcome this, IMHO the most effective is to fit a dump to atmos dump valve as this vents the pressure in the turbo therefore avioding backspin. It also has the advantage that when the throttle is re-applied only a minimum decrease in rpm of the turbo has occured which in effect enables the turbo to be back on boost a lot faster

Hope this helps / is of use to u

Pete

Floyd

iTrader: (9)

Pete, when I had my 1st scooby (MY99) I tried to change up at high revs as I have done in other non turbo cars. I kept my foot on the throttle (slight lift), dipped the clutch, banged the gear through and dumped the clutch. In all the other non turbo cars this worked quite well giving a fast, no lag change but with the scooby it was horrid! There was a pause in boost and when the clutch was dumped the transmission wound up and banged!! Needless to say I only tried it a few times before having lot be a lot more sympathetic and slipping the clutch as well as lifting the throttle a bit more.

I don't know what the banging was (it may have been the rear diff)but the gear change got a lot slower after that experience.

F

dowser

Sorry Pete - but the atmo dump valve stuff is complete rubbish...imho

The whole idea of any dump valve is to stop the turbo slowing down (more likely than backspin or stalling...unless you're really slow with the change ).

The standard recirculating DV puts the air back into the system before the turbo - increasing the pressure on the input side of it (marginally, b4 it gets blown out the air filter).

Dump to atmosphere removes this small benefit (as well as stuffing the air/fuel ratio).

It's *possible* that KJS has a sticking dump valve (which isn't opening), but I'd expect this to actually improve pick-up after changing gear (unless he's hitting boost-cut because of it). The dump-valve is a performance limiting, turbo long-life, safety device.

If it's a standard UK turbo, I'd say any change around 6k has lost you time anyway - I find the OE turbo runs out of puff early on.

Regards
Richard

kevin stanton

thanks for all the replies.
my car is in for a service this month, i'll get them to check out the dump valve.
I've only had the car about a month, so i think a turbo upgrade is out of the question.

Floyd,
your explanation is exactly what i have been getting. i think i just need to get adjusted to the new car and the way it behaves.

kjs

Floyd

iTrader: (9)

Kevin

After a while you will get used to slipping the clutch more as you change. If you get it right it can be almost as quick as in a non turbo car and very smooth.

The only thing that bothers me is how much stick the clutch is getting when it goes wrong.

F