- Location
- NSW
- First Name
- Steve
- Drive
- 2010 nt did pajero tow car / 2000 6spd gc8 wrx tarmac rally car / 2000 Manual Subaru Outback 2.5 just a car
Hi all, a bit new to the leggie scene but mine is getting nice results so far and I am trying to mod it in a measured and ultimately much slower step by step way with a dyno run on the same dyno at each major change (although there were a few mods already done so....). Friday is the next day on the dyno purely to play with boost mapping (Boost levels vs RPM) hence me reading this. My aim is to go as far as I can retaining the stock turbos (mainly because I'll never stop if I go past that).
As some have said before there is a FLOW limit to a turbo as well as an increase in pressure limit, talking about pressure alone is not really valid and part of this is due to the varying losses on the intake system due to a change in airflow as mentioned above by Kenneth or Bernard (can't remember which).
In simplistic terms lets say the world is ideal (which it isn't) and that at 3000RPM and 12psi boost we have half the airflow of 6000RPM and 12psi. In this ideal world the turbo is therefore flowing half the air at 3000RPM and the pipework is also flowing half the air. Pressure vs flow is a square relationship so at half the airflow there is a quarter the pressure drop. So if there is a 4psi pressure drop before the inlet to the turbo at 6000RPm the will be about a 1psi pressure drop at 3000RPM. So there are two (ignoring all the heat and intercooler efficiency benefits, start counting them and we'll be here forever) advantages at lower revs. The first is that the inlet pressure to the turbo is higher so it doesn't have to increase pressure so far and the second is that the turbo is only flowing half the air, therefore doing half the work.
My point with all of this is that while 13psi may well be the safe limit of our turbos with a certain level of other mods at 5500RPM plus, 17 or 18 psi may well not be a problem at 3000 - 4500.... so mapping the boost against RPM can give you either a bigger gain or a safer gain with a similar overall result.
On friday I'll know where my turbo's drop off efficiency and I have to wind timing back so much that I lose all the gains but that will be for my car with it's current mods and I guarentee the pressure level will be different depending on revs.
I also run multiple boost maps so you aren't runnning the turbos up there all the time and just because your maximum boost is set to 15psi or whatever it is doesn't mean you are necessarily getting to that point all the time so your driving style makes a difference too....
As some have said before there is a FLOW limit to a turbo as well as an increase in pressure limit, talking about pressure alone is not really valid and part of this is due to the varying losses on the intake system due to a change in airflow as mentioned above by Kenneth or Bernard (can't remember which).
In simplistic terms lets say the world is ideal (which it isn't) and that at 3000RPM and 12psi boost we have half the airflow of 6000RPM and 12psi. In this ideal world the turbo is therefore flowing half the air at 3000RPM and the pipework is also flowing half the air. Pressure vs flow is a square relationship so at half the airflow there is a quarter the pressure drop. So if there is a 4psi pressure drop before the inlet to the turbo at 6000RPm the will be about a 1psi pressure drop at 3000RPM. So there are two (ignoring all the heat and intercooler efficiency benefits, start counting them and we'll be here forever) advantages at lower revs. The first is that the inlet pressure to the turbo is higher so it doesn't have to increase pressure so far and the second is that the turbo is only flowing half the air, therefore doing half the work.
My point with all of this is that while 13psi may well be the safe limit of our turbos with a certain level of other mods at 5500RPM plus, 17 or 18 psi may well not be a problem at 3000 - 4500.... so mapping the boost against RPM can give you either a bigger gain or a safer gain with a similar overall result.
On friday I'll know where my turbo's drop off efficiency and I have to wind timing back so much that I lose all the gains but that will be for my car with it's current mods and I guarentee the pressure level will be different depending on revs.
I also run multiple boost maps so you aren't runnning the turbos up there all the time and just because your maximum boost is set to 15psi or whatever it is doesn't mean you are necessarily getting to that point all the time so your driving style makes a difference too....
