How have people modded their APCs or increased boost? [Archive] - SaabCentral Forums

: How have people modded their APCs or increased boost?

28th April 2002, 05:48 AM
It seems that people have taken lots of different routes to increasing boost and I though it would be helpful for others if some of us could list what we have done and what the boost characteristics are.

My car is modded as follows:
APC: R42 has 2,3Kohm resistor, R41 and R101 connected together from right side (i.e. boost pressure sensing 'F' pot has been bypassed). This is Tweaker's mod.

I also have a Mitsu turbo, RRFPR, intercooler waterspray and an MSD BTM. Advance = 8 degrees at 15psi and 21 degrees at idle.

Characteristics: quick spool up to 26psi and then quick drop to 15-16psi to red line.

I'd like to get rid of the boost spike if possible.


'87 T16s, not washed in five months, bonnet not opened in five months

<font size=-1>[ This Message was edited by: Alanb on Apr 28, 2002 10:49am ]</font>

Johann F
28th April 2002, 10:21 AM
I was running the 'Red Box' box mod but have just started to tinker with P and F pots. I drilled some holes in the box so I can put a small screwdriver in and fine tune it. I'm running a Trent RRFPR set at 3.0 bar and 20deg at idle/11deg on boost via vacuum capsule arm gap increase. Also straight pipe to pack box, cone Filter on AMM. I have also removed the overboost cutout as with my boost experiments I was hitting it - I reckon it's more dangerous to suddenly cut fuel under high boost/load than to overboost slightly!! I tried putting a restriction in the line from the turbo to solenoid but it over shoots /spikes and I can get the same level of boost with the P and F increase. I'm now looking at getting quicker spoolup via a second solenoid in cojunction with the redundant pressure switch - basicaly only activating the APC over a higher threshhold of about .7 or so bar. This may speed spool up by preventing the APC from bleeding boost in the early stages - ?? Anyway it' s fun to experiment.
You are pulling a lot of retard on boost at 8 deg - I tried less than 10 and although I got more boost I didn't get anymore power.With this much retard the combustion gets very hot - could burn valves but also raising temps and brings on knock - from what I've heard from the Saab Network performance BB anything under 10 is counter productive. My experience overall of increasing boost on Saabs is that it's easy to get breif periods of high boost but it always comes down to sensible levels quite soon. My understanding is it's not do do with intake temps either - you can put all the IC on you want as this only increases the density of the charge and hence the power but this in turn leads to more heat in the combustion. The only way to control this is water injection. I'm also thinking that the stock wastegate actuator is no good for higher boost and can contribute to spikes - they are quite expensive to replace but it may be worth getting the higher rated pre APC version to try - I think they are rated at .7bar?

28th April 2002, 12:07 PM
You are right about the retard. I meant to say 8 deg at 20 psi. Since the MSD BTM dials in retard in a linear way I need 13 deg total retard at 20 psi so that I also have adequate retard at 15 psi (i.e. 11 deg retard = 10 BTDC).

What I want to do is limit retard to a max of about 10 or 11 deg advance from 15psi onwards because, as you say, anything over 10 deg is a waste of time. Thanks to Alex I now have a pressure regulator to fit in line with the MSD BTM so that the BTM never sees more then 15psi and therefore never adds timing retard even if 'real' boost goes well beyond 15psi.

Hope that is sort of clear!


BTW, will you be going to the steam and spares meeting in May?

Johann F
28th April 2002, 01:52 PM
I was going to suggest looking at Juoko's site as he has the same limiting device on his timing BTM setup - I guess you knew that though?
May is all booked up - no weekends spare for spares I'm affraid.

28th April 2002, 02:57 PM
I've tried several APC mods so far.

First the 'passive' mod as detailed by Alan, didn't get the same spikes though as the slower spool up of the hybrid T3 fitted to my car damps things out a bit, but still a fair amount of taper at the top end.

Next tried the same mod with a 'ball &amp; spring' MBC in line with APC solenoid. Removed the restriction from the solenoid as well. Managed to get the boost to peak at about 1.3bar then taper to about 1.1 with no silly peaks.

Am now using the mod as currently detailed on Tweakers site. Boost builds and holds to about 1.2bar ish and holds (its running closed loop as my secondary boost gauge is flickering slightly) until 4500rpm is reached. At this point some pinking occurs as the boost drops down to about 0.9bar then builds back up again towards 5500rpm. I think I need to get some water injection sorted now!!! Car feels happiest with this setting and doesn't feel strained as it revs towards the red line.

Johann F
29th April 2002, 01:58 AM
Tweakers site has changed and just now gives a rather vague setup with quite a few of variables - and did you bypass R42 and if so what did you notice - faster or slower spool up?

<font size=-1>[ This Message was edited by: Johann F on Apr 29, 2002 7:06am ]</font>

29th April 2002, 03:46 PM

Most of the variables mentioned relate to the same thing. Changing the F pot to 1Kohm, bridging the pressure transducer or changing R40 to 1.3K ohm all change the peak boost that the box will allow.

R138 is part of a potential divider, whilst R42 controls the sensitivity to knock. Increasing its value decreases the knock sensitivity.

Bypassing R42 makes the APC very sensitive to knock, currently the setting on my car.

Spoolup speed is the same as when I was using a ball and spring MBC

29th April 2002, 11:16 PM
Has anyone tried using ceramic ball bearings in their turbos? Those are supposed to decrease spoolup time...

30th April 2002, 07:49 AM
My APC has had R42 bypassed, R40=1.3kohm, and R138=40kohm.
Seems to give fast spool up to around 15psi, and holds through to 4500, where it tapers to 12-13psi.

Car is otherwise stock (Mitsu turbo) and run on Optimax.

Hopefully fitting a JR open filter this weekend, along with an intercooler fan. And then an RRFPR soon after...

30th April 2002, 07:58 AM
I'd suggest RRFPR as your next mod. This will probably have the biggest effect.


30th April 2002, 08:51 AM
i'd agree about the RRFPR - made a big difference for me...

30th April 2002, 09:00 AM
Well, the RRFPR well definitly be the next thing i pay for. But the Filter is in the post (or they keep telling me!) and i have a fan that should fit behind the intercooler sat in my garage - So i may as well fit those first...