5kCSTQ Performance Modifications

Robert Deis rdeis at io.com
Wed Mar 26 10:12:53 EST 2003

On Wed, 26 Mar 2003, Steve Sears wrote:

> > While on the subject of boost, etc. I was considering a blow-off valve
> > install. Any gains to be had here at the stated level of tune? If not, I
> > might get one anyway, just so I can sound serious...  "rrRRRRMMMMM
> <KSHHH!>
> > rrRRRRMMMM <KSHHH!> rrRRRMMMM..." you get the idea :)

The import tuners are very imprecise with their language here- a lot of
folks say BOV when they mean CBV.

The valves are identical in operation, the only real difference being that
a CBV vents back to the intake between the air meter and the turbo and a
BOV vents to atmosphere (bad idea, and Steve said.)

Gains here- very small. A properly routed CBV will help keep the trubo
spooled between shifts, so there's some help there, but what is far more
important is the pressure releif on other parts of the system.

When at full throttle and under full boost, there a compressed column of
air rushing through the system.  When to suddenly close the throttle
(like when you shift), this colmun of air slamms into the throttle plate
causing the predictable train wreck.  A shock wave of air pressure travels
back from the throttle through the intercooler (hard on the end tank
seals) and crashes into the compressor (hard on the thrust bearings, and
slows/stopps the wheel).

The BOV/CBV allows this pressure wave to escape a different way, so you
don't blast the IC or the compressor, and the compressor keeps spinning.
If you're careful, you can even route the CBV in such a way that the
pressure wave goes through the compressor the *right* direction and spins
it up instead of slowing it down.

As to the KSHHHH factor, both provide it. BOVs are generally louder, but
the CBV on my other car still has a satiffying sound to it. (-:

  Rob Deis                  "Let the people know beforehand what the law
  MiB3347                      is and what they are to expect."
  rdeis at io.com                              -- 18th Congress, Rec. 75

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