Accelerates a bit more when I back-off slightly!
17 December 2001, 11:03 AM
#1
Anyone know why this happens? Typical case is cruising along the motorway, accelarate hard to pass a lorry, ease off slightly and the car accelarates even harder! Lift off a bit more and it slows. I presume it means the mixture is leaning off at WOT. Is this a symptom of a contaminated MAF sensor? Only noticed after it's recent 30k mile service.
Cheers
Cheers
17 December 2001, 12:34 PM
#2
Scooby Regular
Join Date: Jan 2001
Posts: 1,116
Likes: 0
Received 0 Likes
on
0 Posts
I am pretty sure that this is actualy a weird trait of the car and something to do with fudging the emmisions regs or something? I am sure someone will correct me, but I know the thread was started by Pete at Scoobysport I think - give him a call - I am sure he wil be happy to explain if you ask nicely.
Mike
Mike
17 December 2001, 12:50 PM
#3
Scooby Regular
Join Date: Jun 2000
Posts: 2,965
Likes: 0
Received 0 Likes
on
0 Posts
It's down to 2 things:
1) If you floor the throttle the ECU goes open-loop, i.e. it's not running the normal feedback mechanism to control mixture, so it will run a bit rich which blunts initial response.
2) At a steady cruise, the turbo is spinning OK but still has intertia. Flooring the throttle kicks it out of this, but if you back off slightly the turbo is still accelerating and pushing boost pressure higher ... and combined with closed-loop running you get the meaty surge.
The latest turbo Saabs have a drive-by-wire throttle which opens the throttle a bit wider than you've dialled in with your foot, to cut lag, then backs it off after half a second or so .... you're doing the same but with your foot.
1) If you floor the throttle the ECU goes open-loop, i.e. it's not running the normal feedback mechanism to control mixture, so it will run a bit rich which blunts initial response.
2) At a steady cruise, the turbo is spinning OK but still has intertia. Flooring the throttle kicks it out of this, but if you back off slightly the turbo is still accelerating and pushing boost pressure higher ... and combined with closed-loop running you get the meaty surge.
The latest turbo Saabs have a drive-by-wire throttle which opens the throttle a bit wider than you've dialled in with your foot, to cut lag, then backs it off after half a second or so .... you're doing the same but with your foot.
17 December 2001, 03:49 PM
#5
Scooby Regular
Join Date: Nov 2001
Location: Leeds - It was 562.4bhp@28psi on Optimax, How much closer to 600 with race fuel and a bigger turbo?
Posts: 15,239
Likes: 0
Received 1 Like
on
1 Post
annoying isnt it? Everbody keeps thinking I cant drive when mine does it and I explain that it isnt me...
David
David
17 December 2001, 03:57 PM
#6
Scooby Regular
Join Date: Nov 2000
Location: 32 cylinders and many cats
Posts: 18,658
Likes: 0
Received 1 Like
on
1 Post
A mildly overboosting car will also behave like this as the boost backs off a little and comes back in. Almost undetectable if very mild to wild oscillations if major.
There is also a 10PSI boost situation under WOT from around 3000rpm most easily felt in 2nd gear for noise regs.
[Edited by john banks - 12/17/2001 3:59:07 PM]
There is also a 10PSI boost situation under WOT from around 3000rpm most easily felt in 2nd gear for noise regs.
[Edited by john banks - 12/17/2001 3:59:07 PM]
Thread
Thread Starter
Forum
Replies
Last Post
bluebullet29
General Technical
9
05 October 2015 02:17 PM