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The T25 is too small for the 2L turbo Saab engine when you start to do performance mods.
With the boost lag induced by the 'bleeder valve' like BPC valve, the T25, which is undersized, will spin up ok for a OEM sports car. The T25 compensates for the BPC and other designed in features that make the vehicle sluggish enough to sell to kids and grand mothers too.
As you reduce exhaust and intake mods and increase boost, then the CFMs' go up and the T25 is undersized at the higher RPMs and the compressor map points being utilized are rather inefficient. This means the turbo produces pressure, for less than max RPMs but the temperatures are too high. So the charge air density does down and so does the torque. I have long suspected that once certain intake air temps are reached, that things get seriously bad because the ECU is programmed to chicken out. A Viggen IC really helps get rid of the heat from the T25 when its CFM's get high.
When the turbo is running in inefficient parts of its map, then the wastegate will be creating more and more restriction to get the turbo spinnings faster. This exhaust back pressure will act directly on the piston tops and reduce shaft HP! Note, that as a rule of thumb, with a wastegate type turbo, the exaust backpressure will be at around 2X;s the boost pressure. So at 15 psi of boost, there is 30 psi of pressure in the exhaust manifold! So the amount of STP exhaust gases in the combustion chamber when the exhaust valve closes is about 2X;s that of the combustion chamber volume. So you can see how this would expand and limit the amount of air/fuel charge that can get into the cylinder. So the volumetric efficiency is going down hill. So the undersized turbo is robbing shaft HP, reducing volumentic efficiency and the AF charge that gets in there is hot and not dense. So very quickly you get 3 things compounding against you. (turbos are like that, on the flip side, do things right and get three compounding things in your favour)
What do you do? Get a $600 aquamist system to get the density up and get the intake air temperature sensor less freaked out. Install a ball bearing T28 turbo to get the efficiency. To utilize the capacity of the T28, get a modified ECU to get the pressure up, and use a MBC on top of that to get fast turbo spool up. Get a fat bank account first.
Now with variable turbine geometry, such as the VTN turbo in the 2003 models, it is possible to get turbine back pressures that are less than the boost pressure! You read that right. If the setup with the 2003's is that sweet has yet to be known. But when things like that occur, serious performance is possible. But the stock ECU will not let you find out about such things. And you will be getting quickly into territory where the mechanical strength of the engine cannot be taken for granted. There still will be that bleedin BPC valve!
Sometimes a nice night in front of the fire, a glass of wine (or three with a book about turbo chargers, is very pleasant and will not cost more than some firewood and a bottle of decent wine. But I you get serious about doing such things, you are getting into some serious expense. As for me, I will stick to reading! You can understand these things in your head or with you cheque book :)
Now, with intake and exhaust mods, the T25 can still be fun to live with. So you don't have to replace it, just work around its limitations and don't expect to get really high boost without lots of hot air. You can have a real nice street machine. Yes it will be less than stellar at high RPMs, so just avoid drag races and time slips and you will be happy.
95SET, MBC pressurized at the TB, Viggen IC, pop off valve, piston type BPV of course, mod air box + K&N, no TCD silencer, eggshelled TBTC casting, Saab Sports exhaust, VRK and SAS rear sway bar. And Delvac 5W40 true synthetic.
Whats next? perhaps a minor MAP sensor resistor hack, no real expense there. More in the head mods, they're cheap too.
posted by 65.68.10...
, Sun, 15 Sep 2002 21:20:13, Sun, 15 Sep 2002 21:26:58
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