Unintended Acceleration - now left foot braking for boost

[John Cody Forbes]

Doug Johnson wrote:
> The gas dynamics got me stumped, Cody.  <trying my darndest to
> understand how a car that's slowing because you're applying the
> brakes with your left foot continues to allow enormous exhaust gas
> flow that keeps the turbo spinning to produce tons of boost>
> 
> ~ Doug
> 

I'm sending this to the list to help others learn about this topic, not to
insult ;-).

Vehicle speed has nothing at all to do with exaust gas velocity. What does
have to do with exhaust velocity in this case is engine speed and throttle
position (alot more goes into it, but thats more info then this discussion
requires). As long as the engine speed and throttle position remain above
the threshold needed to maintain boost pressure then you will maintain boost
pressure. This has nothing to do with dirt vs asphalt, only to do with
keeping the airflow going through the engine.

Lets say I'm cranking along at 7800rpm in 4th and comming up to a corner
that I can take at say 5000rpm in the same gear. If I lift off of the
throttle the exaust flow stops almost entirely and turbine velocity is lost.
If you keep the throttle wide open and use the brake to slow to the required
5000rpm then you have lost little exhaust velocity and you maintain boost
pressure.

Relatively speaking, very little exhaust velocity is needed to spool a turbo
as you may be un-aware. Most lag is more time based because of inertia of
the turbine and compressor of the turbo, not becase of insufficient flow. My
car in 1st gear doesn't generate boost until near 5000rpm, but starting from
1000rpm in 4th gear I get full boost by about 2700rpm. Once the turbo is
spooled only a small ammount of exhaust velocity is needed to keep it there,
inertia takes care of the rest. This is why we have a wastegate... to vent
the extra airflow arround the turbo as to not over-spin the turbo and/or run
too much boost.

Now the left foot braking technique is used very differently in a situation
where downshifting is required. In a downshift situation you must lift off
of the throttle to downshift safely, but once you are finished downshifting
immediately go WOT and left foot brake to get the spooling process going
without accelerating too fast for the turn or breaking traction. Again as
long as the revs are up high enough (which is what downshifting
accomplished) the turbo will at very least begin to spool up so you have
some power ready to go when it's time to go. There are other methods, but
that is how I do it. My father learned this technique from Bob Akin when he
was doing some races in the #5 Coca-Cola Porsche 962, and subsequently my
father taught it to me. YMMV.

-Cody "Another Long Post" Forbes
http://www.5000tq.com
'86 5k noT noQ
'86 5k noT noQ - Parting Out
'87 5ktq
'87 5ktq - Fast. Really Fast.