Impreza owners when does your turbo kick in?
Discussion
The reason I ask is that a little while back I had my car remapped by a reputable mapper with great results, 4th gear is an absolute beast. Since having it done though i have noticed that the turbo doesn't always come into play at the same revs. I was always under the impression that at a certain rpm it spooled up.
For example if i put my foot down at low rpm in 4th it will make boost from 2000rpm all the way to the redline making .15 bar boost (that what it says on my gauge, is that the same as 15PSI or 1.5 bar??), if you resume cruising at say 4500rpm it doesn't seem to make any boost until you put your foot down again. is this always the case?
Should the car make the same boost in each gear as it seems to make .14 in 2nd and 3rd and is less keen to pick up early like 4th. in fact 4th isn't much slower than 3rd when i tested it with timing gear. Any ideas why this might be?
Forgot to mention this car is an 03 STi with PPP and an additional remap.
For example if i put my foot down at low rpm in 4th it will make boost from 2000rpm all the way to the redline making .15 bar boost (that what it says on my gauge, is that the same as 15PSI or 1.5 bar??), if you resume cruising at say 4500rpm it doesn't seem to make any boost until you put your foot down again. is this always the case?
Should the car make the same boost in each gear as it seems to make .14 in 2nd and 3rd and is less keen to pick up early like 4th. in fact 4th isn't much slower than 3rd when i tested it with timing gear. Any ideas why this might be?
Forgot to mention this car is an 03 STi with PPP and an additional remap.
Edited by alfa daley on Thursday 14th January 20:57
alfa daley said:
if you resume cruising at say 4500rpm it doesn't seem to make any boost until you put your foot down again. is this always the case?
A turbo provides boost when there is sufficient exhaust gas flow to spin the turbine up, when you are cruising the throttle is almost closed and there is very little gas flow so little or no real boost. When you open the throttle more air and fuel flow into the engine and the turbo starts to spin up, as it does the airflow into the engine increases producing more boost and more flow. The time it takes to do this is the famous turbo lag!Boost is controlled by the 3D boost and wastegate control maps on the ECU. The amount of boost you get depends on how open the throttle is and how many RPM's you're doing at the time. You normally get max mapped boost by about 75%-80% throttle up to WOT (Wide Open Throttle). Of course other maps also factor into the equation, timing, fuelling etc...all count towards how much boost you ultimately get.
Equally the weather also affects how much boost you get. You'll often find when it's cold you can get overboost and boost creep due to much cooler intake/charge temps and the fact that the air is denser in cold weather so you make more power, and less dense when it's hot and you make less power.
Here are the standard turbo boost and wastegate duty maps for your model/year STi just so you can see what i mean. I've no doubt the numbers on your recalibrated maps will look very different to these.
HTH
Equally the weather also affects how much boost you get. You'll often find when it's cold you can get overboost and boost creep due to much cooler intake/charge temps and the fact that the air is denser in cold weather so you make more power, and less dense when it's hot and you make less power.
Here are the standard turbo boost and wastegate duty maps for your model/year STi just so you can see what i mean. I've no doubt the numbers on your recalibrated maps will look very different to these.
HTH
I think this has already been covered.
But a turbocharger is load dependent not rpm dependent.
I personally find it all rather complex, but this is my take on it.
Essentially you have a thing called boost threshold. This is the engine rpm range where the turbocharger should make useful boost, e.g. 2000-6500rpm sot of thing. Below this threshold the turbo won't make any significant boost.
But its more complex than that, as exhaust gases power the turbocharger and the amount and flow rate of the exhaust gases will vary depending on how hard you are pushing the throttle pedal.
For example on the motorway you might be able to cruise at 70mph within the boost threshold range but on a very light throttle. This means the exhaust gas flow rate is also low, thus the turbo isn't spinning all that fast or making much boost.
Hit a steep hill and you'll have to accelerate (push the pedal down) to maintain the same speed and same rpms. Under this situation the gas flow rate will increase and produce more boost from the turbo.
Flooring it will evidently increase the exhaust gas flow rate to maximum and you'll produce the most boost (to your set limit) under such situations.
This is also one of the main contributors to turbo lag. Turbo lag can occur at any point in the rpms and it is the lag time for the exhaust gas flow to spin the turbo fast enough to make boost, going from a light or trailing throttle to full throttle well within the turbo boost range is an example of this.
But a turbocharger is load dependent not rpm dependent.
I personally find it all rather complex, but this is my take on it.
Essentially you have a thing called boost threshold. This is the engine rpm range where the turbocharger should make useful boost, e.g. 2000-6500rpm sot of thing. Below this threshold the turbo won't make any significant boost.
But its more complex than that, as exhaust gases power the turbocharger and the amount and flow rate of the exhaust gases will vary depending on how hard you are pushing the throttle pedal.
For example on the motorway you might be able to cruise at 70mph within the boost threshold range but on a very light throttle. This means the exhaust gas flow rate is also low, thus the turbo isn't spinning all that fast or making much boost.
Hit a steep hill and you'll have to accelerate (push the pedal down) to maintain the same speed and same rpms. Under this situation the gas flow rate will increase and produce more boost from the turbo.
Flooring it will evidently increase the exhaust gas flow rate to maximum and you'll produce the most boost (to your set limit) under such situations.
This is also one of the main contributors to turbo lag. Turbo lag can occur at any point in the rpms and it is the lag time for the exhaust gas flow to spin the turbo fast enough to make boost, going from a light or trailing throttle to full throttle well within the turbo boost range is an example of this.
If you ask the OP what engine load really is it's likely he won't know the answer although i could be wrong. 
I'm sure there are many on here including yourself that could go into the relationship between AFR's, Timing, Load/Air Flow, EGT's, IAT's etc...etc..., and how said variables affect boost but he might have asked the question differently for a more in-depth answer. I kept it simple and posted some pics.
Your car will make different boost in different gears and there are lots of reasons for that. To compensate you can recalibrate the per gear boost compensation maps to add boost in the lower gears although they are set to zero as std but your mapper may well have made some changes there. Again that's another bag of balls to go into and something that right now won't make any difference to the OP whatsoever. It sounds like and OP's car is running like it was mapped to.
I think the OP will get the gist.
I'm sure there are many on here including yourself that could go into the relationship between AFR's, Timing, Load/Air Flow, EGT's, IAT's etc...etc..., and how said variables affect boost but he might have asked the question differently for a more in-depth answer. I kept it simple and posted some pics.
Your car will make different boost in different gears and there are lots of reasons for that. To compensate you can recalibrate the per gear boost compensation maps to add boost in the lower gears although they are set to zero as std but your mapper may well have made some changes there. Again that's another bag of balls to go into and something that right now won't make any difference to the OP whatsoever. It sounds like and OP's car is running like it was mapped to.
I think the OP will get the gist.
Edited by ScoobieWRX on Friday 15th January 11:56
Hi OP here, I doubt I could give a technical definition of what engine load is, I assume its the amount of work the engine is having to do, like hoofing it in 4th (the resistance it experiences) up a hill but could be wrong...some great replies and now i understand why I 'feel the force' at different rpms in different gears. I still reckon 3rd and second could be tweaked to be more like 4 in delivery - whooosh.
Syndrome said:
The turbo will spool up at about 2 - 3000 RPM. The ECU will dictate when you "feel the force Luke". 
Lots of people mistake what's happening with the engine map for turbo lag. In reality there isn't much turbo lag with an impreza.
Oh i so love that quote 'feel the force luke' Lots of people mistake what's happening with the engine map for turbo lag. In reality there isn't much turbo lag with an impreza.Edited by Syndrome on Thursday 14th January 21:53
But i can't at the moment and i'm having serious cold turkey as my Scoob is being repaired
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