[200q20v] RE: Turbo bypass and Blow off valves

QSHIPQ at aol.com QSHIPQ at aol.com
Tue Jun 19 07:18:47 EDT 2001

The discourse below is my .02 on bypass valves.  Many vendors use the term 
Blow Off Valve, when they mean bypass valve, some use the terms 
interchangeably because many aftermarket Blow Off Valves can be mounted, 
preload adjusted and used as a Bypass Valve.  Anyone that's been in a car 
with a Blow Off Valve (defined as downstream of the throttle body - mounted 
in the intake manifold), and had it go, wouldn't at all think it's something 
you want to use for any on=off throttle response.  A Blow Off (Pop Off) 
Valve, dumps boost with extreme prejudice, and it takes a long while to 
recover (like count in seconds).

All that said, venting a bypass valve to atmosphere sounds cool, btdt, but it 
sure isn't healthy, legal (EPA would have a fit), nor recommended.  When you 
have a turbo spinning up, then relieving the pressure with a bypass valve, 
the demand for air remains higher than actual.  So you end up with a rich 
fuel spike, which tends to burn out O2 sensors, wash down cylinder walls and 
catalytic converters.  IME with bypass valves, there is NO performance 
advantage to open venting a bypass valve.  Even Audi Sports massive S1 
recircs the bypass valve back into the turbo inlet.  OTOH, a Blow Off Valve 
is usually vented to atmosphere, because you want to dump all boost (who 
cares where, you are saving a motor here) due to a failure/inadequacy in the 
WG control system.  

This subject has come up every couple months for many years on these lists, 
and I try to get in on the clarification when I can.  Two huge problems as I 
see it, terminology and understanding.  You need a Bypass Valve (mounted pre 
Tbody) on any turbo car to allow the turbo to freewheel during shifts, you 
need a Blow Off Valve or Pop Off Valve (mounted post Tbody - intake manifold) 
as a final overboost mechanical safeguard (or a boost limit enforcement - see 
indy car use).  If you use a Blow Off Valve as a Bypass Valve, it's now a 
Bypass Valve by definition.

Hoping Rod will forgive the re-up.

Scott Justusson
bypass valves aplenty, no BOV's

In a message dated 6/1/01 8:48:20 AM Central Daylight Time, s4audinut at aol.com 

>I can agree with this to a point and that point is about the place 
>where most will stop with engine modifications. If however you still 
>have a very high flow turbo that is designed to come on low and have 
>more than you need on top (wasting the excess) then you will still 
>need blow off valves (I contend altho I havent convinced Dave yet, 
>but I think a blow off valve should be mounted in the intake man). 

woah up cowboy.  The "blow off" valve you describe in the intake manifold, is 
an absolute pressure device (in case of WG failure), mucho different than a 
Bypass valve which is installed before the throttle body.  If the throttle 
flap is closed, putting a blowoff valve in the IM won't do a damn thing about 
relieving any boost.  A blow off valve in the IM is for ON BOOST.  IF you 
think that's going to help you with an oversize turbo Rod, it won't.  What 
happens with a Blow Off valve in the IM, is that the WG can't let off enough 
steam ON BOOST, so pressure builds up in the IM, which causes the valves to 
open.  IF you have ever driven a car with a blow off valve, I assure you, you 
don't ever want them to blow, they fall flat on their asses.  Why those Indy 
car boys have the little tether in their ear listening for the impending 
blow.  I claim, in your scenario, IF you need a blow off valve, you have a 
too small WG problem.


>think the twin bearings in my turbo will allow quick spinup and I 
>have requested a turbo that will have strong flow early and more than 
>I need at top to make sure I have plenty at midrange, this will work 
>the wastegate hard, but make boost almost instantly. This is probably 
>more of a racing type setup (modeled on SQ's) but hey this is Rod's 
>car and I dont care if anyone else ever drives it.

Unhunh.  And you need to understand the terminology here.  Ned was refering 
to the bypass valve which dumps boost when you SHUT the throttle body, 
causing the boost spike from a spinning turbo to head back towards the turbo, 
eventually causing turbo stall.  Turbo stall means that the turbo has to 
spool back up to get it's boost, as apposed to a bypass valve install which 
allows the turbo to freewheel, so the turbo is back on boost sooner.  With 
REALLY big IC's, you find that that the bypass valve dump causes the turbo to 
need to fill the IC before it can generate boost in the IM, which at a 
certain point exceeds the time benefits of the bypass valve itself.  


More information about the 200q20v mailing list