Saabscene Saab Forum - Saab Technical Information Resource banner

1 - 7 of 7 Posts

·
Registered
Joined
·
24 Posts
Discussion Starter #1
Ok, my basic goal is to rid myself of this lag induced by the APC. My other car is an MR2t and I use to have a DSM, both of which did/do not rely on electronics and would spool up almost instantly.

So my basic question is, is the only "real" function of the APC to control boost? Because Im thinking about by-passing the APC and using an MBC to control boost, Im not really looking to turn up the boost much right now, I just want better spool up and instant on boost (which I should get with a little t25).

My concern is that if I take the APC out of the game that it will mess with the ECU's controls of fuel/timing, etc, but if the ECU is getting its boost pressure reading from some place else (I assume the MAP sensor which is all the way the throttle body) then removing the APC control should not be a problem, Correct?

Thanks in advance guys
 

·
Registered
Joined
·
2,277 Posts
Yes the APC only controls the boost pressure, increasing it (within the defined limits of the ECU programming) to the maximum possible with the fuel, ambient temp. etc. shown to it at the time. If knocking occours the fueling, timing, and boost can all be altered to eliminate the knocking. By removing the APC from the system the boost cannot be altered by the ECU.

It has been mentioned in a previous thread that the ECU does alter it's parameters when it senses the APC is not functioning. Why? would be self explanatory, for the safety of the engine, but how and what it alters are a different thing.

Nick.
 

·
Registered
Joined
·
24 Posts
Discussion Starter #3
hmm... thats a good point that it does act differently without the APC. Im not too concerned about knock, as you just have to be smart about boost (ie., paying attention to the temp outside, the gas you got in the car, drive it nice if its really hot out... ) Welp, I guess we'll find out! But like I said, Im not planning on going real big with the boost, just trying to get rid of some of the lag
 

·
Registered
Joined
·
1,676 Posts
Er, you've got a '89 900T, right? In that case, APC has no influence whatsoever on ignition or fueling systems. You gotta have DI/APC or Trionic for that.

The lag is more a function of a very crude ignition timing strategy through that vacuum advance/boost retard thingy on the side of your ignition distributor. A simple way to imporve off-boost response is to bend the stop that limits the retard on the actuator rod of the vacuum unit slightly (1 mm = 5 degrees, so I'd be careful not to overdo it!) and increase static advance by 2-3 degrees. The downside is that the vaccuum unit goes to full retard in one go at about 1 psi boost, and by bending the stop it wil dump more of it within the same, narrow, band. Go too far and you'll experiene a 'stumble' as the boost gauge needle goes slightly into the yellow.

The more advanced strategy would be to add something like an MSD BTM (Boost Timing Master) which allows you to set a proprietary boost/advance curve, or -better still- a conversion to DI or a computerized, mapped ignition system. But at that point, you might want to go the whole hog and go aftermarket for both fuel and ignition...

Oh - an oversized exhaust downpipe will greatly help turbo response to, especially as your car has a catalyst, which is quite restrictive. Swapping your ceramic-cored cat for a 'metal (sports/racing) cat' would be the obvious next step on this front.
 

·
Registered
Joined
·
312 Posts
Hi,
You didn't mention what mods, if any, have been made to the car already. The biggest single improvement to spoolup times, other than the APC chip, on my '92 Griffin was the K&N air filter. The turbo will spool faster when the engine can breath better. Also, if you are in CT. you may want to confer with the people at Swedish Dynamics. I can highly recommend their products.
Regards,
John
 

·
Registered
Joined
·
14,305 Posts
APC has NO effect per se on spool up time, which is governed purely by mechanical factors. It is true however that a different mapping may seem like it is providing a quicker spool up time, but in reality all it is doing is allowing a faster rate of rise within the maximum possible.

I make this comment because different turbos will have different characteritics and will have different spool up times e.g. the "standard" LPT/FPT compared to an Aero turbo on a 9000. It will also depend on what rev range you're in for any particular turbo.
 

·
Registered
Joined
·
1,676 Posts
My understanding was, that APC modulates the solenoid valve varying from 92% duty cycle to fully open. So even if all parameters (knock sensor input, pressure transducer input, rpm) are within the set values, APC will still allow a small percantage of airflow to go to the wastegate actuator instead of back into the intake tract.
 
1 - 7 of 7 Posts
Top