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Blow Off Valve Vs Non Bov


Karma Supra
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this was posted on smartgroups supra thingy...

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."

what ya reckon????? I must admit I noticed a perfornamce loss when I fitted mine.... but I got virgin turbos.. want to trear them right!!!!!!!

wile ya at it.. atmos vs recirc? your opinions please!!!!!! (I vote atmos!)

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hmmm i will forward this to a techinican at TDI

I think if it loses power these would not sell... but i am not so up on this so i'll shut it :shutit: :shutit: :shutit:

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you got to admit tho... none of the rally cars runn DV's....

that BIG bhp toyota tacoma (900 enuf for ya?)at the good wood festival didn't run one!!!!

sayign they wouldn't sell so well is like saying the spicegirls were musicaly talented, and sun strips add 12.75 bhp.

I think the rason they sell so well is so well is so peopel kno you are driving a REAL RS turbo as opposed to an XR3i rep... and it kinda caught on... I blame the kevs and the ricers personally :P

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We are looking at one persons opinion here... I have been to many rallys myself and most of the cars are natuarally aspirated!

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"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."

Wrong. I have fitted / replaced several on Subaru WRC cars. You'll never hear a World Rally Car going pssshhht because they don't run piston valves on the whole. They run a valve system that flutters.

Let's not forget that many rally cars also run antilag to keep the turbo spinning. Antilag does a great job but beats the living daylights out of turbos and manifolds.

Some may well not use BOVs but I say some not all. These people have budgets that far out weigh the average road going person.

If they have no good then why were they ever produced? Why did they run on cars at Le Mans this year? Performance + Longevity. Rally cars can can change bits every few stages if needs be.

Road going cars are not subjected to the same extremes and BOVs add to turbine shaft life by reducing stress.

The real arguement is whether you can fit a VTA or recirculating BOV. This is largely dependant on whether your ECU uses a Lamda probe or AirFlow Meter to monitor fuel points and quantities.

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If there is no recirculation valve or dump valve fitted, the turbocharger

surges evertime the throttle is closed when on boost. This slows the turbocharger

down too fast, and the noise that is heard through the intake is the

buffeting caused by the air that's trapped between the turbo and the closed throttle.

This trapped air can either by vented to atmosphere with a dump valve, or

recirculated (which is currently what you have as standard). Both systems ensure

that the turbocharger does not loose speed when the throttle is closed. The

recirculation valve does introduce hot air back into the inlet (which prefers to

see cold air).

Most people choose their preference to suit their priorities. If you want the

noise of the dump valve on deceleration then choose the dump to atmoshere

type. If you don't want the noise, leave it standard or use an aftermarket

recirculation type.

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but not everyone has a BOV as std :P I don't! not even a recirc!!!

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I just think you need to get enough views on it before judging what one person thinks. :thumbsup:

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Well I have 2 HKS BOV's on my '2, with the plan to rig then on via a solenoid, and have one re-circ.

Then I can flick a switch in the dash and have a silent running (and AFM friendly) blat.

And then flick it the other way for a Max Muppet ptschhh.

Best of both worlds?!

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Well I have 2 HKS BOV's on my '2, with the plan to rig then on via a solenoid, and have one re-circ.

Then I can flick a switch in the dash and have a silent running (and AFM friendly) blat.

And then flick it the other way for a Max Muppet ptschhh.

Best of both worlds?!

thats my idea!!!!!!

have you looked much into this???

solenoid has to be between the charge carrier and DV ;)

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