Diff, BOV and Transmission Qs

RDM · Postby **RDM** » Tue Feb 13, 2007 6:41 pm

OK this one is for the mechanically minded folks out there... it is a three-for-the-price-of-one thread...

One:
I've always wondered what the issue was with towing a vehicle on the driving wheels... it's not too unlike coasting down a hill in neutral. What is the actual damage you're doing to the diff/gearbox by coasting out of gear? I didn't think there would be any damage unless you were also trying to engage a gear while it was already in motion... but obviously some damage is being done... I just don't know what...

So can anyone explain why it is a mechanical no-no to tow your car on the driving wheels or coast down a hill in neutral (yes I know it's not safe to coast in neutral anyway :shock:

)?

Two:
The second part of the query is this... the BOV is meant to release the build up of boost so that it doesn't damage the turbo bearings/housing with back-pressure.... but I've noticed that at full throttle chasing the redline, when I momentarily release the throttle to shift up a gear, it's almost like the the motor/intake suddenly stops from the back-pressure before I get back on the gas... This clearly can't be all that good for the turbo... but I wasn't sure what kind of damage this might be causing or if it is a problem at all. So, the question is, are the aftermarket BOVs actually better at releasing the pressure than the OEM units? I thought they were just there to make loud whooshing noises :roll:

Three:
Is there any need or has anyone installed a transmission oil cooler to their MX? Is there any benefit to having one if you enjoy going out for a nice fang (or especially if intending to tow a small trailer)?

Many more questions still to come...

RDM
PS looks like the towbar cat-herding worked...

I got mine installed last weekend and the others will be getting their towbars installed this weekend. Looks just like a bought one so eager to go get it certified and try it out for towing a small trailer...

The Pupat · Postby **The Pupat** » Tue Feb 13, 2007 11:12 pm

BOV or Recirculating Bypass Valve is there for emissions, plain and simple.

RDM · Postby **RDM** » Wed Feb 14, 2007 2:10 pm

[quote="The Pupat"]BOV or Recirculating Bypass Valve is there for emissions, plain and simple.[/quote]

OK... so does that BOV/Recirc Bypass Valve thingy reduce the wear the tear on the turbo itself by releasing the pressure that the turbo is still generating (as opposed to just closing the valve and stoppng the turbo from spinning in an instant? How good a job does the OEM BOV do to cushion the impact of quickly closing a full throttle? Would an aftermarket BOV do a better job of cushioning the pressure?

RDM

Garry · Postby **Garry** » Wed Feb 14, 2007 2:44 pm

A turbocharger isn't a positive displacement pump so it's going to be pretty hard to damage it by closing off the outlet, especially when it's pumping air. If it was pumping water then more damage would get done by blocking off the inlet causing the pump to cavitate. I'm not sure if it can cavitate pumping air?

Are you sure what you are feeling isn't the delay waiting for the turbo to repressurise the inlet tract after dumping all the boost out through the bypass valve?

What you describe happening is one of the big tuning problems with the SP. As it was explained to me, as the bypass valve opens a rush of air goes back towards the AFM. Unfortnately the AFM reads in the forward and reverse direction and as some spent air from the bypass valve passes back through the AFM the wrong way it tells the ECU to add more fuel to compensate for the extra air it's seeing so the engine runs too rich for a short period of time which causes it to stumble.

It's very annoying when trying to feather the throttle under boost. I didn't think SE's had the same problem.

The only way I've heard of to solve the problem on the SP is to replace the AFM with a MAP sensor and add a new ECU to utilise the MAP sensor. Does anyone know if moving the outlet from the bypass valve further away from the AFM and closer to the turbo inlet would help? Or is it just impossible for the slowing turbo to injest the extra air form the bypass valve quickly enough and some air would still flow through the AFM the wrong way. Currently the bypass valve outlet is about half way between the turbo and the AFM.