It's almost turbo time!!!!

[Juffa]

They're an emissions control device plain and simple... though they don't control too many emissions when they are atmo venting, then they just make lots of noises.

How do you reckon they prevent damage to a turbine blade? (it would actually be the compressor blade that's neither here nor there).

Purpose of Relief and Blow Off Valves
Blowoff valves are used to prevent compressor surge. Compressor surge is a phenomenon that occurs when lifting off the throttle of a turbocharged car (with a non-existent or faulty bypass valve). When the throttle plate on a turbocharged engine running boost closes, high pressure in the intake system has nowhere to go. It is forced to travel back to the turbocharger in the form of a pressure wave. This results in the wheel rapidly decreasing speed and stalling. The driver will notice a fluttering air sound. In extreme cases the compressor wheel will stop completely or even go backwards. Compressor surge is very hard on the bearings in the turbocharger and can significantly decrease its lifespan. In addition, the now slower moving compressor wheel takes longer to spool (speed up) when throttle is applied. This is known as turbo lag. With the implementation of either a bypass valve or a blowoff valve the pressurized air escapes, allowing the turbo to continue spinning. This allows the turbocharger to have less turbo lag when power is demanded next.
References
http://features.evolutionm.net/article/machvcorner/29
Allard, Alan. Turbocharging and Supercharging. Cambridge, England: Patrick Stevens Limited, 1982.
Gorla, Rama, and Khan, Aijaz. Turbomachinery Design and Theory. New York, New York: Marcel Dekker, 2003.
Society of Automotive Engineers. Turbochargers and Turbocharged Engines. Warrendale, PA, 1979.
Watson, N, and Janota, N. Turbocharging the Internal Combustion Engine. London, England: Macmillian Press Ltd, 1982.

[Sean]

The only people who can \"prove\" blow off valves are worthwhile are those who sell them.

I don't run one, and the minimum boost target I ever use is 16psi, so there's often a lot of air to move...

I've never broken a turbo.

There is no requirement for a wideband sensor as long as the car is tuned and set-up properly to start with.

[Juffa]

Ok, I willing to learn here and even willing to stand corrected.

If we asssume for the time being that a BOV/Re-circulating type valve does not extend the life of the turbo, what about benefits to reducing turbo lag. Why did the turbo era F1 cars use the valves if there was no benefit of some kind?

J

[timk]

I always assumed they fitted them to make the cars quieter, that's just my personal opinion though.

[Sean]

Ok, I willing to learn here and even willing to stand corrected.

If we asssume for the time being that a BOV/Re-circulating type valve does not extend the life of the turbo, what about benefits to reducing turbo lag. Why did the turbo era F1 cars use the valves if there was no benefit of some kind?

J

I thought they used a "relief valve" rather than a blow off valve, each works in s slightly different manner, a releif valve bleeds off air over a given pressure, which can at times allow the turbo to work in it's peak efficiency range even though some of teh air escapes via the relief valve, from memory teh benfit was that you could flow a lot more air via teh turbo, so when it was needed (to maintain boost levels) it was already there. Thinking about it, it's probably a more effective way to control boost than a conventional wastegate, but gee it's shorted turbo life by having them spinning at ridiculous speeds the majority of teh time, even with light throttle.

I'm happy to stand corrected if anyone actually knows the real data.

[Boags]

I'm with Juffa - he is the only one who has quoted references so far!

If anyone has dirt on those references, let it be known!

Boags

[Sean]

I'm with Juffa - he is the only one who has quoted references so far!

If anyone has dirt on those references, let it be known!

Boags

LOL when I click the link it fails to load...

[sabretooth]

Ahh, the BOV debate.

Arguments for:
a) throttle response between gears (compressor surge stalls the turbo - venting the air will apparently keep the compressor spinning
b) turbo damage
c) increase noise
d) decrease noise

I've never heard of (b) occurring, ever. I can't prove (a) with anything real. I have seriously noisy compressor surge on my car, so (d) is nice if you have the right recirculating BOV.

Mine got me (c). I hate it. When I can find someone who will mod my intake piping, I'm getting a 100% plumb-back BOV - purely for point (d).

I've got my BOV's vac line capped off with sticky tape so it doesn't go off - I'm undecided as to whether noisy compressor surge or a noisy whoosh is better.

[Juffa]

I'm with Juffa - he is the only one who has quoted references so far!

If anyone has dirt on those references, let it be known!

Boags

LOL when I click the link it fails to load...

Interesting. I actually stole the words and the reference from another site. I just went back there and the link is broken....

Anyway. I might start another topic on the BOV rather than high jack this one any further. I have Corky Bell's Maximum Boost at home and I seem to remember reading about the BOV in there. Will have to find that and see what it says and post it in the new topic if relevant.

J

[4sfed5]

you can fit the VF10 turbo into a vj20 turbine housing as per the 1800 tx3 and 323 GTX turbos's. they have a square bolt up fitting which is more standard than the triangle type subie one.

[The Pupat]

Purpose of Relief and Blow Off Valves
Blowoff valves are used to prevent compressor surge. Compressor surge is a phenomenon that occurs when lifting off the throttle of a turbocharged car (with a non-existent or faulty bypass valve). When the throttle plate on a turbocharged engine running boost closes, high pressure in the intake system has nowhere to go. It is forced to travel back to the turbocharger in the form of a pressure wave. This results in the wheel rapidly decreasing speed and stalling. The driver will notice a fluttering air sound. In extreme cases the compressor wheel will stop completely or even go backwards. Compressor surge is very hard on the bearings in the turbocharger and can significantly decrease its lifespan. In addition, the now slower moving compressor wheel takes longer to spool (speed up) when throttle is applied. This is known as turbo lag. With the implementation of either a bypass valve or a blowoff valve the pressurized air escapes, allowing the turbo to continue spinning. This allows the turbocharger to have less turbo lag when power is demanded next.
References
http://features.evolutionm.net/article/machvcorner/29
Allard, Alan. Turbocharging and Supercharging. Cambridge, England: Patrick Stevens Limited, 1982.
Gorla, Rama, and Khan, Aijaz. Turbomachinery Design and Theory. New York, New York: Marcel Dekker, 2003.
Society of Automotive Engineers. Turbochargers and Turbocharged Engines. Warrendale, PA, 1979.
Watson, N, and Janota, N. Turbocharging the Internal Combustion Engine. London, England: Macmillian Press Ltd, 1982.

That's not Compressor Surge. Compressor surge is when the turbo under power slips off the left of compressor graph thingy. I've seen a 1.6L corolla motor surging at around idle, an easy way to tell is that it blows air out the turbo (IE it sucks a big amount of air in but hasn't got the guts to hold that air in underpressure so it slips back past the turbo causing the turbo to slip rather than stall (it actually aerodynamically stalls rather than physically stalls).

Turbos spin at a great rate of knots and as such when you cut off the air supply to keep them spinning they will slow down very rapidly whether or not you have a pressure pushing back on them. Secondly if you wanted less turbo lag why would you vent all the pressurised air that will force the turbo to spin up fast again once the throttle is reopened.

Now I'm in agreeance with Sean, more likely they were used as a pressure relief valve and the reason for that is because of piss poor turbo design. The guys at Nizpro used to do this with the VL's etc because of the turbo design and the wastegates available weren't able to keep up the boost the motors could make/handle. The wastegate wouldn't even open in the top end because the turbo would run out of efficiency but it would boost spike at mid RPM in the high VE range which you give a massive spike in torque and blow rods out the motors basically. So what they did is use a relief valve or blow-off valve to bleed air above a certain pressure so as not to keep blowing up motors (the next advantage was of course that you could control the valve using a pneumatic valve and a motec so you could cut the boost at high VE then dial more back in as the VE drops away to make more in the top end). Now the turbo in this situation won't overspeed because when you cut off the air supply to the turbo it slows down plain and simple.

NOTE: Champ cars actually run a pop-off valve which is a relief valve that is the series supplies to control boost pressure only, this does much the same as what I imagine the F1 boys were doing.

Now another interesting one that Simon did was the took a turbo car. Ran it round a race track with the BOV connected, then disconnected the BOV and sent it back out. The car was 2 seconds a lap faster with the BOV disconnected, so they definitely reduce lag.