[Author Prev][Author Next][Thread Prev][Thread Next][Author Index][Thread Index]
Re: Dump/blow-by valve installation procedure
James Fawcett wrote:
> Pardon my ignorance, but what exactly is "lift-off"? Also, what exactly is
> this dump valve supposed to do and can I use one for my 86 5ktq? This
> sounds like a cool project. I hope that all this has not already been
> covered, but I had network problems for quite a while, and I wasn't getting
> any of my mail. Thanks guys.
James, suppose you are running 125mph by the end of the front straight
under 17psi of boost in the 5th with the wide open throttle.
You are approaching the turn No.1 (called "the 90" for a reason). If you
are brave enough, you start braking by the second pylon (braking by the
first would be suicidal). At this point you sharply drop the gas pedal
("lift-off") and throw your 90kg of weight on the brakes (the right
foot) and the clutch (the left foot), 'coz you need to drop down to the
3rd befor you enter that corner (it REALLY helps to double clutch here,
Meantime the air charge is rushing to the throttle @17psi 'coz the
turbine inertia keeps the impeller spinning.
But, since you've just lifted off, you have closed the throttle shut.
The air charge, all upset by the obstacle, redirects it's pressure
partially towards the "accordeon" hose (hence it's shape, it has to be
able to expand), and partially back, towards the turbine impeller. It
will effectively slow it down or stop the turbine completely.
Buy that time you are in the 3rd at the apex and throw your 90kg of
weight on the gas pedal, 'coz you want to power yourself through that
corner. Ooops, the turbine has stopped spinning! You nail it, but... can
you say turbo lag? Net result - that Jetta, that you've just blown away
on the strait is somehow on back your heels in the corner.
There are two ways to deal with this:
1. Heel-and-toe. Since you really need a third foot to be on the gas
pedal (to keep the rpms above 3000 and, therefore, the turbine spinning)
while braking and shifting, that role can be delegated to your own righ
foot's heel. Make sure you install a good set of pedal covers (mine are
MOMOs, really made for heel-and-toeing).
2. Install a dump valve, that would release the positive mfld pressure
on the lift-off and prevent the turbine from stalling.