F75 Turbo P.C.S.

[tabrocky]

Hey,
Just wondering if anyone knows the idea behind the P.C.S. (Power Control System) on the turbo intercooled F75 DL52 motors.
Thanks to the insightful post from RockyF70 back on one of the threads, I was lead to the WARFS page and downloaded the F75 manuals.
When I first purchased the vehicle I wondered what the little do-hickey was and this cleared it up.... BUT I still don't understand its purpose.

For those that don't know, it is a small vacuum activated bypass valve between the inlet manifold and turbo inlet. It hangs off a little bracket on the inlet manifold. Apparently it is activated when in 1st gear only and serves to reduce the boost pressure by bypassing it back to the turbo inlet.

Now the question is why?. Can't the drive train handle the torque? This doesn't stack up because reverse is a lower ratio but the PCS isnt activated then.
Is it to reduce the "jerkyness" of gear changes?

Any ideas greatly apreciated.

I'm thinking I may install a bypass switch or connect it into the 4wd switch so it is deactivated when off-road.
Lord knows on sand, etc you need all the ponies you can muster, so this system would be a bit of a pain.

Cheers.

[Big-Stu]

I too have always wondered what this little valve does.

Maybe in First gear the motor spins up too quickly with the turbo causing the mixture to lean out. But like you say why not in Reverse as well?

We need a turbo expert to give their opinion.

Stu

[tabrocky]

Yeah I was thinking that as soon as the turbo came on boost, particullarly in 1st gear, it would causes a noticable surge in power which may make it not so smooth to drive. The chances of hitting boost rpm in reverse is probably not as likely.

Don't think it would lean out as you said thou'.
Diesels dont run rich or lean. There either overfueled or underfueled. You can feed them as much fuel as you want and they'll just put out more and more power until bang.... They also blow a s^%t load of black smoke when overfueled too. When underfueled they lack power and run super clean.

I had another thought that it might be some sort of emissions control. Not sure exactly what/how thou'.

If I don't get a satisfactory reason for it's existance, it might just get removed. I'll let you know how if I do.

Cheers.

[HotFourOk]

I thought I read this somewhere, and I have been asking everyone about it!

No-one seemed to know what I was talking about.

It may be to control wheelspin in 1st gear hahah.. maybe just a safety feature.

I want full boost in 1st gear dammit hahah

I think mine does spool to 11psi in first.. but I'll have to check. It might just allow it to spool up slower, so there is not that surge of power as you said

[tabrocky]

As mine isn't running at the moment while I do a few things to it I can't check boost pressures. The bypass is only fairly small so it probably wouldnt allow a heap of air to be recirculated, hence still giving reasonable boost as you stated HotFourOK.

Not sure about the whole wheelspin thing though...
As ive only just bought this new one with the turb's, I havent had a chance to really test it out, apart from the test drive. It has required a few important things repaired first.... (See "F75 Timming Belt").

I can only hope it has enough grunt to spin the rubber...

Slower pressure build up is possible I spose... Never thought f it that way.

Any other guesses...

Cheers.