Anyone ditched the APC? [Archive] - SaabCentral Forums

: Anyone ditched the APC?

Janne Selinummi
16-07-02, 09:27 AM
There's all kinds of problems with my car's APC, and rectifying it all just seems like a lot of work for what you get.
Is there someone in these forums who has ditched the APC and uses a manual boost controller instead?

I have a big FMIC with 60mm pipework, 3-inch exhaust with 3" downpipe and a cold-air induction system with a JR cone filter; these upgrades should mean that I can use much higher than standard base boost safely.
(I always run with 98 octane premium unleaded gasoline)

So, anyone, if you have switched the APC to a manual control device of any kind, please tell me about it!

16-07-02, 09:46 AM
As you know my cars been giving me a hard time lately!
Previously i had been running a MBC controller for boost which i had set at 1 bar (boost tailed off at high rpm's to around 0.7 bar as the turbo tailed off), i recently had my engine changed to what i thought was rod-knock but Tom Taylor thought it could be a broken piston!!
So i guess you take your choice!!
Ive known people running MBC's for ages but most have some form of engine manangement system that picks up knock and retards the timing to compensate for the boost or cuts fuel altogether if the boost is to high for the fueling system.


16-07-02, 09:48 AM
P.s im now running my tweaked APC box( thanks Alan!)


Johann F
16-07-02, 11:03 AM
What problems are you having? - let me guess - boost taper at high revs and boost cutting back after a short time? If so it's just the correct function of the APC detecting knock as the head heats up and combustion temperatures rise. All the mods you have described will do next to nothing for reducing the onset of knock - they get more mixture in and out which makes power but also more heat in the combustion = knock. These engines run at the very borderline of detonation as they are high compression (for a turbo engine)- especialy at higher than stock boost. I would look at water injection as this is the only method of controling in-cylinder temperatures.
You would be ill advised to run anything but stock or lower boost without APC (an LPT upgrade) An MBC will in this case reduce some anhoying tendencies of the APC - delayed boost, unsteady boost and boost taper (it will hold at the red/yellow border all day with the right fuel and tune)

16-07-02, 11:19 AM
I'm running with a Dawes Device and the APC solenoid removed, but I still have knock detection via an LED on the dash. I have water injection fitted as a safeguard against high combustion tempertures and I must say my car pulls like a train all the way into the red, by then i'm into the 110mph+ and I haven't really pushed it any harder than that. I want to add an air/fuel gauge to my set up but need to mount an oxygen sensor first, as my car did not have a cat fitted as standard!! I would have kept the APC but I had so many problems with it, intermittantly running base boost, that in the end I gave up & went for the MBC

16-07-02, 01:32 PM
Running with a larger intercooler will help some, in preventing knock as an increasing inlet temp itself is one of the causes of knock.

I ran with an MBC for a while (similar to the Dawes Device) but have now gone back to running tweaked APC with the MBC in line with it to aid spoolup of the turbo.

16vs run lean as standard, especially over 4500rpm at WOT so holding more boost than the engine really wants isn't sensible. I've also had a AMM failure causing the engine to run very lean and a couple of tanks of super that weren't.

Also worth considering that setting the MBC up on a very cold day could lead to problems on a hot one, especially if accelerating away from traffic lights after the cars been standing, and if the MBC is set up on a hot day you're wasting potential when it gets cooler.

16-07-02, 02:07 PM
I use a MBC only because the actuator rod and nut were so seized together they were gonna snap if i tried to seperate them. The reason i wanted to undo them was to set the boost to a standard level, not because i wanted to run high boost. When i got the car it was hardly giving me any boost so i tried the MBC to see if it would work. I don`t really like the idea of doing away with the APC, but have set my boost so that it just goes into the red. I also have the advantage of turning the boost down during the summer (hot air) in a matter of seconds.

Janne Selinummi
16-07-02, 02:30 PM
Well, there are people in Finland who get constant 1.1 - 1.2 bar of boost with their modified engines, and equipped with APC. My problem is that the car is clearly lacking in power; even with the new intercooler it coudln't hit the standard turbo's top speed.
I now get 190-195 km/h, but it should do 200 km/h standard, and I do have a lot more efficient intercooler.
The APC gives its set maximum boost every now and then, but it's rare. Usually it only raises to about 80 percent of the max level. And if the thing would be normal knock-related stuff, the boost should first peak at the max boost for a very short time and then lower a bit. In my case, the max boost only shows up rarely, and irregular boost seems not to be knock-related.

I haven't tried the car with the other mods yet, but if after the first one, the car was still slower than standard, something has to be wrong. I'm fitting an air/fuel ratio meter now along with a high pressure fuel pump, and see if the problem is in the fuelling system.

16-07-02, 03:18 PM
Janne, have you had the base boost set using an external calibrated boost gauge. There have been a couple of occasions on this site where the dash gauge has been showing half orange and the car has barely been making 2psi.

Also, for investigation purposes only, disconnect the pressure line from the wastegate, plug it and check that the turbo will run easily into the red.

16-07-02, 03:34 PM
Janne, try my setup:
R23 removed (so no rpm signal)
R138=5.15k (maybe I will change this value this Weekend)
Very stabile. Max 1.2bar then go down to 1bar until the red line expect in 5th.
In 5th, until 4500rpm 1bar then go down slowly to 0.7bar at 5500rpm (227kph).

17-07-02, 10:57 PM
ok i have heard alot about R23 but no rpm signal wheres that at in perfromance where and what do you gain by removing it;its like
saying oh yeah that borla sticker gives your car 5 horse pls explain...

Janne Selinummi
17-07-02, 11:59 PM
Well, the APC unit is programmed to start lowering boost pressure in the higher rev range. Thus, by removing the rpm signal, you get full boost from 2500-3000 rpm all the way up to the redline.

I'll do the Type-M spec mod to my APC if I can get it to work, that's the unit they use in Finnish 900 Turbo Cup. It gives 1.0 bar max boost, doesn't drop pressure in the high revs and can maintain high boost continuously given that the engine is properly set up. If only the APC system would work already... >:(

18-07-02, 05:28 PM
Good luck !!!

Janne Selinummi
19-07-02, 05:04 AM
Thanks for your help everybody!
I'm off to spend some time with my car again - and, unfortunately, that won't be driving the car! :smile:

Johann F
19-07-02, 05:07 AM
The APC is programed to reduce boost at high RPM's for a good reason - as engine speed rises the time between knock being detected and the system being able to react and drop boost to prevent damage is reduced i.e at 5000rpm it's 1/2 the time of what it is at 2500rpm. IMO you can't rely on the APC to protect your engine at high rpm boost - mess with this at your peril!

19-07-02, 07:49 AM
I to have had problems with the APC.Clean and "full" boost occurs only now and then,otherwise it goes only 1/3 into the orange.Such a pain in the ***.Hoping that a new APC unit with a chip for the 9000 motor will do the trick.
None of you tried aftermarket systems like Electromotive,Haltech or Hestech??

Roger G.
1985 Saab 900 T8 - soon to be T16

<font size=-1>[ This Message was edited by: smael on Jul 19, 2002 12:51pm ]</font>