Back pressure and torque
10 May 2002, 11:22 PM
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hi...
something i have yet to understand...
technically, what and how is back pressure good for low down torque?
why does a more restrictive DP give better low down torque... whereas a less restrictive DP give better torque high up?
is it related to the way the turbo works?
something i have yet to understand...
technically, what and how is back pressure good for low down torque?
why does a more restrictive DP give better low down torque... whereas a less restrictive DP give better torque high up?
is it related to the way the turbo works?
10 May 2002, 11:49 PM
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I think some people apply normally aspirated tuning ideals to this, I'm not so sure they apply to the same degree to turbo'd engines ? It all starts to get a bit complex when you look at pulse wave tuning !
FWIW I found that when I fitted a bigger bore exhaust, I reduced backpressure and lost bottom end torque
As the new exhaust had increased torque further up the range, the initial loss seemed ever greater !
However I later 're-found' the torque by adjusting the air/fuel ratio
The engine had been running weak due to the reduced backpressure causing less exhaust gas contamination of the combustion.
My conclusion is that big exhausts on a Scoob need not mean loss of bottom end torque, my car is producing over 330lb torque from 3500rpm on a 3" bore straight through system
Just need to find someone local now to re-map them
FWIW I found that when I fitted a bigger bore exhaust, I reduced backpressure and lost bottom end torque
As the new exhaust had increased torque further up the range, the initial loss seemed ever greater !However I later 're-found' the torque by adjusting the air/fuel ratio
The engine had been running weak due to the reduced backpressure causing less exhaust gas contamination of the combustion.My conclusion is that big exhausts on a Scoob need not mean loss of bottom end torque, my car is producing over 330lb torque from 3500rpm on a 3" bore straight through system
Just need to find someone local now to re-map them
11 May 2002, 12:06 AM
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very nice CC!
my car is a my99 PPP...
seems alot of folk have fitted a DP to that type of car...
including JB...
thing is, as i understand it... its quite likely ill get overboost... and so need to tweek something on the car..
dont wanna go through all that ... when all i wanna fit the DP for is more low down torque... (2500 pulling, instead of 3500)...
my car is a my99 PPP...
seems alot of folk have fitted a DP to that type of car...
including JB...
thing is, as i understand it... its quite likely ill get overboost... and so need to tweek something on the car..
dont wanna go through all that ... when all i wanna fit the DP for is more low down torque... (2500 pulling, instead of 3500)...
11 May 2002, 12:12 AM
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Overboost is a sign of good things to come 
It normally means that something is working more efficiently as a result of whatever you changed. It happens because the turbo spools quicker, it is normally pretty easy to get things back under control again with a tweek to the Dawes (assuming you have one ?) or restrictors.
Your TD04 turbo will produce better torque low down than my TD05, it should be getting on song by 2500 rpm I would think.
Andy
It normally means that something is working more efficiently as a result of whatever you changed. It happens because the turbo spools quicker, it is normally pretty easy to get things back under control again with a tweek to the Dawes (assuming you have one ?) or restrictors.Your TD04 turbo will produce better torque low down than my TD05, it should be getting on song by 2500 rpm I would think.
Andy
11 May 2002, 12:26 AM
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not got a dawes...
seems like a bit of a sledgehammer... doesnt it fool the prodrive chip?
ive been told drilling the restrictor to 1.3mm would do the trick...
half the time i wanna go for it... like now...
then i wake up all sensible
seems like a bit of a sledgehammer... doesnt it fool the prodrive chip?
ive been told drilling the restrictor to 1.3mm would do the trick...
half the time i wanna go for it... like now...
then i wake up all sensible
11 May 2002, 08:31 AM
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I will sort the restrictor for you in quicksticks m8. You are not running a massive amount of boost at the moment and you might just be able to put it on and get away with it, otherwise 1.3mm will probably do the trick nicely.
The Dawes is a good method of finishing the job as it gives you a much wider torque curve.
The Dawes is a good method of finishing the job as it gives you a much wider torque curve.
11 May 2002, 09:39 PM
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Hi All
Yesterday I fitted a P.E. exhaust manifold to my MY00 Impreza PPP.
Interestingly, I lost 1-2 psi boost pressure all over the range (!!!), turbo reacts later than before (it seems turbo lag but IMHO it isn't) and the car pulls away to the high rpms linear as not turbo cars do. In side-by-side tests with another PPP Impreza I've lost low down torque (Stephen Done where are you?) but now it is better from 4000 rpm to the red line (by the way the service who fitted it told me that this isn't Maxim Racing manifold but cheap copy).
Normally I had a - high for a standard PPP car - 16,4-17,3 psi turbo pressure, now I have maximum 14,5 psi, the mixture is more or less the same rich as before but my gains are minimal presumably due to lost turbo pressure!
As you probably remember I had a maximum -3 knock correction between 4200-5000 rpm, now I have zero or instant -1 and only the first green lights up from my knocklink sensor.
From what I understand when I'll fit my Magnex downpipe I'll gain low down torque (and turbo pressure) or am I wrong?
I haven't yet too much tests with Delta Dash but I'll post the new log files later here just for comparisons.
JIM
Yesterday I fitted a P.E. exhaust manifold to my MY00 Impreza PPP.
Interestingly, I lost 1-2 psi boost pressure all over the range (!!!), turbo reacts later than before (it seems turbo lag but IMHO it isn't) and the car pulls away to the high rpms linear as not turbo cars do. In side-by-side tests with another PPP Impreza I've lost low down torque (Stephen Done where are you?) but now it is better from 4000 rpm to the red line (by the way the service who fitted it told me that this isn't Maxim Racing manifold but cheap copy).
Normally I had a - high for a standard PPP car - 16,4-17,3 psi turbo pressure, now I have maximum 14,5 psi, the mixture is more or less the same rich as before but my gains are minimal presumably due to lost turbo pressure!
As you probably remember I had a maximum -3 knock correction between 4200-5000 rpm, now I have zero or instant -1 and only the first green lights up from my knocklink sensor.
From what I understand when I'll fit my Magnex downpipe I'll gain low down torque (and turbo pressure) or am I wrong?
I haven't yet too much tests with Delta Dash but I'll post the new log files later here just for comparisons.
JIM
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