Blow off valves - (was GOING FASTER - IN BIG CAPITAL LETTERS

[zorruno]

Woosh noise is due to installed blow off valve. Aftermarket product, but
included with some turbo kits.

Designed to protect the turbo vanes from compressor surge when you take your
foot off the accelerator. Also said to help the turbo spool up quicker on
next boost cycle.

e.g. http://www.miata.net/products/intex/bailey_bov.html
A pic of one here
http://makeashorterlink.com/?H45616C4

The ones that add approx 250hp to your turbos performance are equipped with
a horn, to increase the volume of the escaping gasses, and to scare old
ladies on the street. (i.e. poseur value, not performance based). I don't
have a turbo, but I know you can buy quiet blow off valves & I doubt it
would add much to the performance/life of the turbo anyway, unless you had
very high boost pressures.

As for superchargers, I'd say they would have no value, as boost is
directionally proportional to engine revs & therefore boost would drop off
directly as revs do.

Anyone else know any more?

just MHO
H.

[darryl]

As said, blow off valves are there to prevent the turbo from 'stalling' from
compressor surged caused by boost pressure building back up to the turbo
from the closed throttle butterfly. This helps with removing some of the lag
between gear changes. And I have foundt that they do make a difference.
Factory BOV's are vented back into the air stream to prevent the noise, and
because on Air Flow metered cars, such as GTX 323's, the AFM has already
measured the incoming air and has added fuel to match. When the air from the
BOV has been vented to the atmosphere, the engine has been over fueled for a
second. When slowing for an intersection this can be enough for the car to
stall. This can be a pain in the but some times. The flames in the exhaust
that are visable at night and the accomanying noise can be rather
spectacular though and add to the driving experience on tight winding roads.
Most aftermarket BOV's are vented to atmosphere I beleive due to simplicity.
Trying to run bigger pipe work for the higher flow of air (that required the
use of an AM BOV) would be a pain in the butt, as well as being an
unessasary exspence.
Yes I would say that the majority of car enthusiasts ( or more so the 'boy
racer' as some prefer to call them, us) fit them purely for the noise. On
some cars, such as Skyline GTS-t's, the BOV becomes audable after just
fitting an aftermarket pod filter.
Most of the BOV's are not effected by fitting a trumpet. RacePro BOV's are
one of the ones that are. They have two outlets, one of which is blocked off
with a blank when fitting the trumpet. This causes all the out going air to
flow through one outlet making it louder.
I experimented with RacePro, and Trust BOV's, the Trust BOV's made no
difference when fitted with a trumpet. I had nothing to do one day so tried
them all on our Starlet GT.
Aftermarket BOV's are used when higher than standard boost pressure is used
as factory BOV's don't hold higher pressure too well and can't flow very
much air. After market BOV's are also adjustable for response and stiffness
so that you can set them to handle high boost yet still respond well.
Supercharges and BOV's? not sure if they are required. I know a lot of the
guys in OZ that are using sentrifugal superchargers (on V8's) are using
BOV's on their cars. I am going to fit one on our supercharged 13b. I have
plenty of them sitting here, and well, it shows a product i sell, weither
required or not. And it will act as a safety against backfire.
more to add to the conversation.

Have you seen the article on the Bullet roadster in the latest Motor Mag?
try prices from AU$98K! and from 235kw to 335kw blown.
Cool.