| This is a quote from a respected tuner about
Atmosphere Blow-Off-Valves vs Recirculation Blow-Off-Valves.
"Some people do think that at the absolute upper extremes
of boost levels - about 30-plus pounds - the blow-off valve does,
somewhat, save the compressor wheel and shaft from trying to
rotate backwards. It doesn't actually rotate backwards at all -
all you're hearing is cavitation. What happens is, you've shut the
throttle, the turbocharger is doing 100,000 rpm and now has a
boost spike of 50 psi. Because it's working in a higher region
than what it's designed for, it slips; it basically does a skid
like a car tyre does when you dump the clutch. That's the noise
you hear - the whoof-whoof-whoof is the air doing a skid."
"We've tested one on a manual gearbox performance car run
at Winton Raceway. I think it was running 1 minute 40s back then,
but it would lose 2 seconds a lap putting the gobble-gobble valve
on. When you look at data acquisition, what you find is - as you
change gear - the blow-off valve dumps all the pressure built up
through the intercooler and pipes. It then goes back to zero
manifold vacuum when you get back on the throttle, you have to
build all that boost back up.
"The fact that people think that they keep the turbo
spinning is a problem. The people that suggest this have never had
an engine on the dyno and never had a turbo tacho in their hands.
What people don't realise is, when you shut off the throttle, you
shut off the air supply to the engine - this shuts off the exhaust
gasses coming out of the engine. When there is no exhaust flow,
there is no energy to keep the turbine spinning - the turbo slows
down at an alarming rate.
"If you change gears at quite a good speed, you can
actually get a boost spike on changes; if you're trying to hold a
constant 30 pounds, when you do a racing change you'll get 32-33
pounds when you crack the throttle open again. If everything's
working well, you've got a full head of stream waiting to go into
the throttle as soon as it's opened.
"I've done this on a rally car and it was quicker through
every timed section without a dump valve. You'll never hear a
World Rally Car going pssshhht because they don't use a dump valve
- you get the woof-woof-woof noise instead." |
