20-04-02, 12:15 PM
Just put a smaller restriction in the turbo to solenoid hose and it hit the overboost fuel cut out straight away - wow. I had read about this mod on the Saabnet BB a few times and seeing as it is easy to do and cost's nothing I gave it a try. I will have to try with a slightly larger hole next time. I was thinking about the theory behind it - reduced pressure signal to the actuator - and reasoned that it is better to up the boost this way rather than crank the actuator arm in . With this method the wastegate is still able to open fully and prevent boost creep and overshoot. I guess the proper way is to put in a higher pressure rated actuator - but this gives an indication of what you can get with one?
I've not tried this method but some people say that it simply delays boost pressure signal. In other words, you get a fairly long spike of boost.
I already have plenty of that at the moment so I don't think I'll be going that route.
You are lucky saabs have tough motors.
Detonation is most likely to strike at the point of peak torque. By increasing this torque in a fairly uncontrolled and not accounted for by the engine computer you are increasing your chance of big/ dangerous detonation.
Personally i wouldn't put that much trust in my APC detonation controller.
With this mod you are 'dampening' the actions of the APC by severely restricting the air flow to & from the wastegate actuator.
It is not the same as fitting an uprated actuator as there is also a delay on closing the wastegate...
With the mod boost will rise quickly as the restrictor delays air getting to the actuator, if you keep your foot down eventually the pressure both sides of the restrictor will equalise and the actuator will start to react as the APC intended, however the APC via it's transducer will be seeing a higher pressure than expected for the given rpm (if over 3.1k). It will then try to adjust the boost back down, (possibly too much) the actuator can now do this relatively quickly as the pressures both sides of the restrictor are now equal, so boost is dumped. The trade off is now you have high pressure air the actuator side of the restrictor that takes longer to vent, and so longer to close the wastegate, though since there is a delay built in to the APC on raising boost after knock is detected this might not be so noticeable.
If you have an uprated actuator, it takes a higher pressure to open it, but it also shuts quicker.
With the restrictor mod I can see it causing a huge boost, no boost, huge boost, no boost scenario over 3.1krpm.
<font size=-1>[ This Message was edited by: Keith on Apr 21, 2002 12:48pm ]</font>
22-04-02, 03:37 AM
Thanks for the info Keith - Iv'e already taken it off as it was a bit undrivable - huge power surge though but perhaps not very sensible. It's an uprated actuator then!
26-04-02, 03:55 AM
Did some more tests with this mod and tried a slightly less drastic reduction in the standard restriction size. What I found was that even a very slight reduction in the size of the hole results in a large increase in boost! - also there seems to be a definate improvement in low boost pickup and response. I thought about Keith's remarks about the delay in releasing boost but the restriction is only in the pressure line from the turbo - when the solenoid switches the flow it vents to the unrestricted port on the intake and therefore the reaction time is not changed. It still hits the overboost but not as hard as before and I reckon a bit of tweaking by a couple of turns off the actuator rod and an increase in the cut out pressure limit may yeild interesting if not worthwhile results. I'm also going to try bypassing the R42 on the APC (Red Box modded a while back) - I'm in the tinkering mood!
Iv'e been concidering upping the F pot on the APC, looking at the M box used on the racing 8V Saabs, they had a very high setting on the f pot with r42 & r138 left as standard.
Anybody have any thoughts on this??
Ahh! I thought the restrictor was in the actuator line! That'll teach me to do my homework :oops: