[peter messenger]

i have a 2.0lpt 1997 9000
it is fitted with maptun ecu. jr filter.jt exhaust system with downpipe and sport cat.
i am very happy with the performance of the car.
but i have never checked the turbo boost i have read on bills site that it will be lowered due to lower pressure from jt exhaust. i do not fully understand how the ecu and exhaust control the boost is there anyone who give me advice on this matter.

 

[billj]

The boost pressure on a simple turbocharged car (and on your 2.0LPT until you upgraded it) is governed purely mechanically by the wastegate actuator. This consists of a diaphragm with one side connected to the manifold so it has boost pressure acting on it and the other side at atmospheric pressure. Under boost, the diaphragm moves the wastegate in the turbo, which opens and lets exhaust gas bypass the turbine, limiting the amount of energy available to turn the turbine, thus reducing the boost pressure. This is designed to reach an equilibrium point at a desired boost pressure, thus limiting boost to that pressure.

On the Saab engines, this particular pressure is called "basic boost pressure" or "base boost" for short.

On engines with APC (like yours has since it was upgraded), there is an electrically-operated valve (the APC or BPC valve - different names for the slightly different designs used over the years, but essentially the same thing). The ECU can use this to bleed pressure away from the wastegate actuator. This means the actuator "sees" less pressure and doesn't open the wastegate so far. It now takes more pressure to open the wastegate (because only some reaches the wastegate actuator) so the effect is an increase in boost pressure.

APC can increase boost pressure beyond base boost, but can't reduce it below that level. If there is enough exhaust gas flow to achieve base boost, you'll always get at least base boost pressure.

Now, what I found when fitting the JT exhaust (and a few others, but not everyone, have also found is that the base boost pressure drops a bit after fitting the new exhaust. I'm not 100% sure of the theory why, but that's what happened on my Aero. On Trionic cars liek yours, this will have no effect on maximum boost pressure, but it will affect how it behaves at lower boost pressures. In particular, my Aero felt a bit gutless in the mid-range - the turbo lag was increased.

It's worth checking your base boost with a cheap boost gauge, especially since your car probably doesn't have a boost gauge built in.

 

[peter messenger]

cheers for the advice bill
if the part i fitted with the ecu that maptun refer to as a solenoid valve and you refer to as apc valve then i begin to understand. i have ordered a gauge to check boost. although i am not sure what i should be looking at.

 

[mark e]

Originally posted by BillJ:
[qb]Now, what I found when fitting the JT exhaust (and a few others, but not everyone, have also found is that the base boost pressure drops a bit after fitting the new exhaust. I'm not 100% sure of the theory why, but that's what happened on my Aero.
[/qb][/b]

The drop in base boost pressure will be due to the drop in pressure (from fitting an exhaust system with lower resistance) on the exhaust side of the wastegate. As the wastegate is mechanically linked to the actuator diaphragm, any change in pressure in the exhaust system will affect how much extra force has to be applied to the actuator diaphragm in order to move it.

That's not a very clear explanation I'm afraid but it's the best I can come up with at the mo. I know what I'm trying to say, I just had one glass of wine too many last night

 

[billj]

Yes, Peter. The BPC valve is also known as a solenoid valve (from the fact that it contains an electrical solenoid that actuates the valve).

The base boost figure for your car should be the same regardless of whether it has been upgraded. Unplug the connector to the solenoid, drive the car hard up a steep hill in around 3rd gear and note the maximum reading achieved on the gauge. This is your current base boost. Compare with the figure for your car on my web site - about 0.4 bar or 6psi.

Mark, I think I see what you're saying. The greater pressure differential across the wastegate (due to the lower backpressure in the exhaust) increases the tendency for the wastegate to "blow open" and requires a greater spring pressure to keep it closed enough to maintain the specified base boost pressure?

(One can too many of "XXXX" last night

)

 

[mark e]

Bill, yes I think we understand each other through the haze of post inebriation recovery...

 

[wrighar]

Excellent.

So the drop in base boost could be equal to, or a direct correlation to the reduction in backpressure in the downpipe of the exhaust.

i.e 2lb drop in base boost = 2lb drop in back pressure....

Andrew
Who was always afraid of reducing pack pressure too much on my old 99T as that used the BP to keep the oils seals working in the turbo...to much boost and too little BP = leaky turbo seals and a tendancy to blow across the turbo.

 

[peter messenger]

thanks alot chaps it makes sense to me now