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Discussion Starter · #1 · (Edited)
I have recently acquired a 1991 900 (high pressure)Turbo. It has the TE05-12B.

I have a lot of experience with TDs, but little with turbo petrol engines.

When I bought it the turbo boost would go to 1/2 yellow, max.
I disconnected the plug from the APC valve and found it would only go to ~1/10 the way up the yellow. I disconnected the wasegate actuation line, at the APC valve, and the boost would go to almost all the way up the yellow.

Is all of the above normal? I have tried ULP and PULP, and there seems to be little difference.

The turbo produces little/no boost <2500 rpm. Is that also normal? I am thinking a VNT turbo might work well for all round driveability, with a reasonable top end???

There seems to be a large bolt in the manifold in a good spot to iinstall an EGT. Has anyone done the same? What thread is the bolt?

Lastly, has anyone calibrated the white/yellow/red boost guage against a real boost guage???
 

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The end of the dash boost gauge should be around 12-13psi, however it is worth checking this as on some cars I have seen it it lies like a politician.

With the APC disconnected it should boost around half way up the gauge, and with the pressure feed removed from the wastegate completely it should head off the scale very quickly.

This would either suggest your base boost is incorrectly set and very low or that the turbo is knackered.

A good starting point for the base boost is to disconnect the actuator rod from the wastegate and shorten it until you have to extend it by half the diameter of the hole.

As to changing to a VNT a properly setup TE-05 will start making boost from below 2000rpm and make 230-240 at the top end with the addition of a free flowing exhaust (including elbow) and a FMIC. A hybrid version of this turbo has the same spool point but flows enough to make 300-320 if desired.
 

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Discussion Starter · #4 ·
Thanks guys, great info.

I assume that moving the actuator pressure line from the turbo to the intake manifold will also increase base boost?
 

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All adjustments to base boost are done via manipulation of the wastegate and wastegate actuator.

Base boost is the starting point. The lowest boost level. After this Saab's use something called an APC system which is Saab's version of electronic boost control. The APC box senses manifold pressure using the pressure sensor under the dashboard. The pressure sensor is connected to a vac line on the inlet manifold. As the revs rise and the boost pressure starts to build the APC then begins to control the position of the wastegate via the actuator. If the APC says you need more boost then it blocks the line running from the compressor to the wastegate and bleeds off the excess pressure which would've forced the wastegate open. That has the effect of holding the wastegate shut by a pre-determined ammount. The APC also has a knock sensor. If knock is detected there is a program within the APC box to start reducing boost until the knock goes away. The knock control is the main incentive for retaining the APC device as it protects your engine from potentially destructive running conditions.

In theory, by tuning the APC box you can bleed off as much of the pressure intended for the wastegate as you want. You could setup the APC to allow the turbo to provide all of it's boost pressure all of the time with just the ability to step in on detection of knock and reduce boost.

At higher pressures the wastegate can creep open as the pressure inside the turbine housing overides the pressure in the wastegate spring. That is why aftermarket wastegate actuators are available as they have interchangable springs or stiffer springs which allow you to keep the wategate shut at higher pressures.

With the TE05 you wouldn't really want that to be provding maximum boost all the time. The spool up is too fierce, shreds tyres, breaks gearboxes, causes wheelspin, etc, etc... and as you get further up the RPM range the turbo gets out of it's efficiency zone and starts to pump useless heat causing more heat and possibly knock without any additional performance. So the TE05 needs good boost control if you want a sensible performance car.

Like Alex says though, if you have disconnected the hose leading to the wastegate then you should've seen the boost needle shoot off the end of the red zone, if you don't hit the boost pressure fuel cut switch first. If your turbo isn't doing that with the wastegate hose disconnected then either the wastegate isn't staying shut (dodgy spring, lose arm, poor adjustment, broken diaphram) or the turbo itself is knackered.

As for the VNT turbo. It's just a garrett VNT designed for a petrol engine manufactured by Turbo Technics specifically at my request.
 

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Before playing with the position of the pressure signal to the wastegate you need to sort the turbo out. Only just reaching standard boost when there is no pressure on the diaphragm isn't right.

But you are correct that moving the pressure off take to after the throttle will increase boost as it will compensate for the pressure drops across the intercooler and the throttle body. However you will need a one way valve to prevent the vacuum produced during low throttle operation from reaching the actuator. A better place is just before the throttle but after the IC.
 

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Discussion Starter · #8 ·
Before playing with the position of the pressure signal to the wastegate you need to sort the turbo out. Only just reaching standard boost when there is no pressure on the diaphragm isn't right.

But you are correct that moving the pressure off take to after the throttle will increase boost as it will compensate for the pressure drops across the intercooler and the throttle body. However you will need a one way valve to prevent the vacuum produced during low throttle operation from reaching the actuator. A better place is just before the throttle but after the IC.
Thanks for that. I agree on fixing the turbo first! When the diaphragm hose was disconnected the car absolutely flew, but I could not detect any detonation.

I will be sticking a manometer on just to make sure the guage isn't lying. When you say 12-13 psi, I assume you mean the end of the red part of the guage? Any idea what sort of pressure the top of the yellow should be?

Thanks ejenner - I have already done a fair bit of reading on how the APC system works. I meant which model VNT though? (e.g. 2256 etc???).
 
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