Dump Valve Questions

[TurboJ]

Hi, Guy

This is my first post.
I have a turbocharged engine and I run it without a dump valve, some tuners say “that it will give extra lag and damage the turbocharger” other tuners say “pay no attention that is and old myth and doesn’t happen in reality.”

I would like to know you thoughts on Dump Valves, Re-Circulating Dump Valves and running with no Dump valve at all?

What would happen if one is fitted to a supercharged engine, I know it will 'dump' but are there any bad/good points'?

Regards Jason

[Guy Croft]

Well the guys who told you it can increase lag and damage the turbocharger are right. Why vent off? Because the whole idea of closing the throttle is to make the engine slow down, so keeping high pressure air in the line from compressor to manifold doesn't make any sense.

The dump valve gets rid of surplus pressure when the throttle is closed thus preventing the compressor wheel from stalling. Repeated stall (or near stall) can fatigue the blades on the compressor wheel and a severe stall can burst it. Allowing the air to vent off keeps the compressor wheel spinning and thus enhances the turbocharger response when the throttle is re-opened.

I don't really recommend running a turbocharged unit without one, especially with boost pressures over 10psi or so.

GC

[TurboJ]

Thanks for the speedy response, but the things I don’t understand are:

1) Dumping metered air out of a blow off valve will cause it to run rich as the air that was accounted for by the air flow meter has suddenly disappeared (excluding a re-circulating dv) surely this cannot be good either? I was also under the impression that the dumping of this air can lose low down throttle response.

2) Modern turbochargers e.g GT series run a 6mm shaft and the blades are not wafer thin like they use to be, thus making them extremely tolerant to what some say is only mild surge.

3) Many “top tuners” with high powered turbocharged engines and a very good working knowledge often choose to run without dump valves.

4) Many high powered (10psi+) diesel cars cannot run a DV because they have no throttle plate/vacuum how do they last?

5) Testing my car on the rolling we noticed no difference in “turbo lag” when running with or without a DV.

This has been a constant argument between tuners for a while and I would really like to know your views on the above,

Regards Jason

[Guy Croft]

1) Dumping metered air out of a blow off valve will cause it to run rich as the air that was accounted for by the air flow meter has suddenly disappeared (excluding a re-circulating dv) surely this cannot be good either? I was also under the impression that the dumping of this air can lose low down throttle response.

True, venting to atmosphere can cause the engine to go rich, that's not good news for the rings and the turbine esp if it causes flame in the header. As far as hitting the low down throttle response you don't have to dump it all, the valve can be set to release only some of the trapped air. Reverse flow thru the system without any kind of dumping does 'put the brakes' on the turbocharger, releasing that air allows it to keep spinning. Of course there are no doubt clever people in WRC et alia (and there are a few on this site) who have devised very nifty ways around this problem - ie: how to avoid wasting that energy. Be aware that the more boost you operate, the worse the problem is. Even Turbo Technics say you can get away without a dump valve system under 14psi boost and some cars, yes, never had them (Lotus Esprit Turbo eg)

2) Modern turbochargers e.g GT series run a 6mm shaft and the blades are not wafer thin like they use to be, thus making them extremely tolerant to what some say is only mild surge.

Tolerant is a rather generalised way of putting it. Fatigue failure is a product of the stress and the number of times the blades are subjected to it. Suffice to say that the pressure waves generated by throttle closure without any air venting (which gives rise to an effect similar to water hammer and can generate waves of higher magnitude than the boost pressure itself) are a bad thing for the blades and the bearings.

3) Many “top tuners” with high powered turbocharged engines and a very good working knowledge often choose to run without dump valves.

I don't have a problem with that. But then professional tuners would take a lot more interest in wear and survivability of the turbo than most folks would (or do..)

4) Many high powered (10psi+) diesel cars cannot run a DV because they have no throttle plate/vacuum how do they last?

I don't know any diesels fitted with a dump valve (although I know some people fit them just to get the hissing noise). Since (as far as I know) none of them are speed-controlled by a throttle plate between the turbo and the engine, all that happens is the compressor carries on feeding more air thru the engine. But since the diesel is controlled only by the fuel supply and since all diesels run 'excess air' (ie: ingest more than they burn anyway) that extra air going thru has no effect on the speed of the engine. Of course as the fuel supply continues to be reduced the turbo slows down anyway because it's not getting so much energy. So there isn't a stress issue in that regard.

5) Testing my car on the rolling we noticed no difference in “turbo lag” when running with or without a DV.

That may well be the case, but I can guarantee that if you did an industry standard test on the turbocharger condition over an extended period you would see the effects on the machinery.

FWIW there are as many arguments over what is 'good' and what is 'bad' when it comes to tuning as the day is long. I can only speak from my own exp.

GC

[TurboJ]

Thanks for the informative reply. Since I’m going on the dyno sometime next week I will adjust my DV to release a bit more that it currently is.

One more question, what happens when fitting a DV to a supercharged engine is there any good/bad points?

[kpsig]

Thanks for the informative reply. Since I’m going on the dyno sometime next week I will adjust my DV to release a bit more that it currently is.

One more question, what happens when fitting a DV to a supercharged engine is there any good/bad points?

I have installed a Rotrex C30-94 to my Alfa 166 2.0 Twin Spark, 2004 model (facelift). The -94 Rotrex model is a little big for this capacity but I bought it second hand at a very good price. Rotrex suggests a recirculating valve and MAF before the supercharger. At the moment, I have the MAF after the Rotrex, 10cms away from (in front of) the output tract for the BOV while the valve acts as a BOV with no recirculation. I have not noticed anything strange, but I will go for the recirculation of the excess air and positioning of the MAF in front of the Rotrex. I do not expect anything strange or different to throttle response etc, however Rotrex installations with no BOV or recirculation have developed leak points to supercharger's body.

[ASF72]

Hi Jason,

Can i ask which Model car and engine you have here???

Regards

Nino