Blow-Off Valves & Waste-Gates - What's the Diff???

Hammer · Postby **Hammer** » Sun Apr 20, 2008 2:35 am

Hey ... I was hoping to tap into some of you that are in the know, and educate me on the difference of Blow-Off Valves (BOV) and Waste-Gates (WG).

Are they just two different solutions to the one problem (Kinda like 4-Stroke & 2-Stroke. Different solutions to an an engine's combustion process, but both address the same process). Is that what BOV & WG are?

I'm familiar with BOVs - my car has one. But from time to time, I read about WGs on other turbo cars (ie. Porsche 911 Turbo/GT2 - both of which were using WG, that is until the current model moved into a Variable Geometry (VG) Turbo setup. Apparently having a VG turbo eliminates the need for a WG).

Are there any turbo engines that have both BOV and WG? Or it is one or the other type arrangement?

Cheers

rhysk · Postby **rhysk** » Sun Apr 20, 2008 3:47 am

No, they have two separate purposes.

- A wastegate is a bypass around the compressor turbine, which is used to control the speed of the turbine (and therefore, the amount of boost). Normally these are built in to the turbo (as would be the case with your car).

- A blow off valve is used to prevent air coming back through the compressor in the wrong direction, thus preventing compressor stall, when the throttle has been closed.

Boags · Postby **Boags** » Sun Apr 20, 2008 9:57 am

What Rhys said; your car, and the vast majority of turbo cars, have BOTH.

Boags

Craig · Postby **Craig** » Sun Apr 20, 2008 5:59 pm

My waste gate blows off all the time...even more so after McDonalds! :lol:

fastfreddygassit · Postby **fastfreddygassit** » Sun Apr 20, 2008 7:08 pm

Craig wrote:My waste gate blows off all the time...even more so after McDonalds!

qft
and don't forget the Indonesian food farty bum :mrgreen:

bruce · Postby **bruce** » Mon Apr 21, 2008 11:30 am

Craig, perhaps you should avoid cheese (or maybe all types of food!)

Hammer · Postby **Hammer** » Mon Apr 21, 2008 12:36 pm

Thanks for the information guys.

Craig, you've been doing the duff-duff cruise at your local MacDonalds again? :mrgreen:

OMY005 · Postby **OMY005** » Mon Apr 21, 2008 12:54 pm

Hammer wrote:(ie. Porsche 911 Turbo/GT2 - both of which were using WG, that is until the current model moved into a Variable Geometry (VG) Turbo setup. Apparently having a VG turbo eliminates the need for a WG).

Sort of makes sense, If angle of the vanes change enough the boost pressure will be decreased and vice-versa. Changes spool up time and allows more boost at low rpm. I could be wrong though.

Andrew.

Casey · Postby **Casey** » Mon Apr 21, 2008 7:31 pm

OMY005 wrote:
Hammer wrote:(ie. Porsche 911 Turbo/GT2 - both of which were using WG, that is until the current model moved into a Variable Geometry (VG) Turbo setup. Apparently having a VG turbo eliminates the need for a WG).

Sort of makes sense, If angle of the vanes change enough the boost pressure will be decreased and vice-versa. Changes spool up time and allows more boost at low rpm. I could be wrong though.

Andrew.

The Porsche system allows the exhaust gases to be directed at an oblique angle into the core of the turbo when the engine is operating at high rpm - effectively bypassing the turbine and reducing backpressure. At its extreme it can bypass the turbine sufficiently that minmal boost is produced by the compressor - same function as the wastegate.

I remember reading an article about BMW's system for actuating valves - not sure if it was their Double VANOS available now or if it was something still being developed. The upshot was that it does away with needing a throttle butterfly as the inlet valves can be precisely controlled to provide the correct "throttle opening" - possibly combined with direct injection.

Clever Germans.

Uncle Arthur · Postby **Uncle Arthur** » Mon Apr 21, 2008 8:40 pm

The diff is what transfers drive from the drive (or tail) shaft through the axles (or half shafts) to the rear wheels.....

It's purpose is to compensate for the fact that the inside and outside tyres actually track different distances when the car is cornering

Mr Starlet · Postby **Mr Starlet** » Mon Apr 21, 2008 9:09 pm

My turbo experience was back in the days of the Starlet GT so could be a little hazy but here goes anyway as it's slightly different to what's already been said.

To me the term Blow-Off Valve, BOV, usually refers to a valve that reliefs air pressure built up by the turbo/supercharger on throttle lift off, the idea is to let the pressure out or else trap pressure will push the butterfly open (throttle creep) even at full lift off, causing throttle instability. In cars with FI, this relieved air is then fed back in between turbo/supercharger and airfilter to help keep turbo spooling. The atmospheric vented BOV makes cool sound but takes way a fraction of response; without that air fed front of the turbo, it's now having to work to suck air back in through the air-filter, hence extra lag.

BOV was more of a street term but is quickly becoming 'the standard', I guess the more technical term is Air-Bypass Valve, doesn't quite have the same ring though...can you picture the young guns with hotted up turbos talking Air-Bypass Valve?

Rhys pretty much sums it up here; the Waste-gate is use to control boost by controlling exhaust gas to either go through the turbine housing or around it. I always remember Waste=exhaust gas, the unwanted by-product of an engine.

Hope this helps, or be corrected if it isn't.

Cheers
Minh