Recirculating dump valves

[domV8]

Before we start, I am not a complete newbie to FI, owning a Rover Tomcat Turbo for the last 8 years - however it has always had an atmospheric dump valve (fitted before I bought it), so I have no experience of RDV's.

HOWEVER, I simply do not understand the theory behind RDVs...

In short, the system (whatever type of DV you run) dumps when you lift your foot off the throttle (vacuum created behind the butterfly activates the DV) - the point to take away here is that you are lifting off the throttle...

A RDV will dump this pressurised charge from in-between compressor (SC or turbo) and butterfly, back into the intake before the compressor - and this is the part I don't get...

The intake tube is essentially an open-to-atmosphere tube (filter end), so any pressurised air dumped into it is going to go straight back out the tube to atmosphere, as the pressure differential evens out.

Also, having taken my foot off the throttle, the butterfly is now closed, so any notions of an RDV helping it "spool up faster" are moot, because it shouldn't be spooling up at all due to the engine now being on over-run rather than under load, and - the butterfly is closed!

Any recirculated pressure will surely be lost to atmosphere almost instantly, so no matter how quick you lift-off then re-throttle, the pressurised charge will have gone way before your foot was back on it.

Then, when you factor-in the increased temperature of the pressurised charge, which will have displaced the cooler air (atmospheric temp) which was originally in the intake tube, it looks worse - due to the detrimental effect on performance that charge temperature has...

So I'm lost as to how RDV's are supposed to be beneficial? From my point of view, you have a system which delivers no spin-up benefits - but puts hotter air into the engine...????


Can someone please explain why RDVs are supposed to improve performance..?


Thanks,


Dom

[Thom]

Recirculating valve are used to keep the engine quiet when closing the throttle, and on engines using a MAF sensor pre compressor inlet the fueling is calibrated so that the air being recirculated is measured by the MAF sensor.

[domV8]

Thanks Thom,

Should have said that this was a general question about RDVs rather than the specific instance of MAF-equipped vehicles (I have seen something in a another thread about the MAF requirements).

These type of DVs have been put into many vehicles, including engine management systems that use MAP rather than MAF sensors - so I guess quietness of operation is one reason.

But really it was about the perception that an RDV helps spool-up turbo's and so forth, propositions that I regularly see put forward - and I just cannot fathom why this would work, as I can't see that it can..?

Not to say it can't, just wondering if someone can explain to me how this could be so...

Thanks,

Dom

[Thom]

My car had a dump valve when I bought it. I refitted the original recirculating valve shortly after and could not feel any difference in spool, for what it's worth.

A recirculating valve is usually controlled by a vac/boost line plugged straight to the inlet manifold past the throttle body, so as you change up a gear and floor the accelerator, there will be a very short period during which both the throttle and the valve will be open before the compressor builds boost back again in the inlet tract, thus closing the valve. However, this period may be so short that my guess is that the compressor will spin pretty much regardless of the type of valve being used.

If a recirculating valve still makes a difference in turbo spool, then I would guess the turbo is badly oversized to begin with.

[JonRB]

My understanding is that the main advantage of a RDV is that they are simply quieter. For many people, this is a major advantage. Of course, others love the extrovert noise of an atmospheric one although personally I think they sound chavvy and "Fast & Furious" but each to their own and all that.

[domV8]

Thom said:

A recirculating valve is usually controlled by a vac/boost line plugged straight to the inlet manifold past the throttle body, so as you change up a gear and floor the accelerator, there will be a very short period during which both the throttle and the valve will be open before the compressor builds boost back again in the inlet tract, thus closing the valve. However, this period may be so short that my guess is that the compressor will spin pretty much regardless of the type of valve being used.

Surely the pressurised charge would have been "lost" to atmosphere in the first few 10ths of a second after the valve opened (when your foot lifted), so all the benefit would have gone by the time your foot was going back onto the throttle - so the fact that both butterfly & RDV are open simultaneously at that point doesn't matter, as the pressurised charge has already escaped to atmosphere, meaning you are back to building boost from atmospheric pressure only..?

[Thom]

Well, something along those lines, yes.

[BarnFind]

JonRB said:

My understanding is that the main advantage of a RDV is that they are simply quieter. For many people, this is a major advantage. Of course, others love the extrovert noise of an atmospheric one although personally I think they sound chavvy and "Fast & Furious" but each to their own and all that.

That would make sense,the earlier valves(pre Chav actually by many years) fitted by Abbott Racing for me were full woosh wheras the later RDV were as quiet as a Nun in a cloister.

[DamianS3]

BarnFind said:

That would make sense,the earlier valves(pre Chav actually by many years) fitted by Abbott Racing for me were full woosh wheras the later RDV were as quiet as a Nun in a cloister.

This... But my question would be why run a DV at all?

I seriously suggest you have a go without this was how I ran my cosworth in its most powerful guise and IMHO it was less laggy than with the DV,

You get a cool fluttering noise too as the turbo spools down a little.

This was with a t34 running 26psi as recommended by Harvey.

Try it see what you think..

Damian S3

[rigga]

Some plain bearing turbos like the td05 fitted to early impreza cars are quite sturdy and can take the force applied when not running a dv at all, just blanking off the hose and aperture on the top mount intercooler and it would run fine for many thousand's of miles, the later vf series as well as being roller bearing were not as robust and could quite literally fall apart doing the same thing. To me the main difference between a rdv and a atmospheric one is this ...air once measured either by maf or map has the required amount of fuel added by the injectors according to the ecu/map fitted, once the atmospheric valve dumps this air out of the induction pipework, the original measured amount of fuel is still delivered, and now you have a rich mixture scenario, in extreme cases bore wash can happen, best to run a recurculation valve unless you're running serious amounts of boost.

[BarnFind]

DamianS3 said:

This... But my question would be why run a DV at all?

I seriously suggest you have a go without this was how I ran my cosworth in its most powerful guise and IMHO it was less laggy than with the DV,

You get a cool fluttering noise too as the turbo spools down a little.

This was with a t34 running 26psi as recommended by Harvey.

Try it see what you think..

Damian S3

Sorry can't help further

[JonRB]

Dump valves (of any type) are to stop the back pressure stalling the turbo and (potentially) damaging it. Anything else is a secondary benefit.

If modern turbos can withstand the back pressure with no ill effects then perhaps dump valves are superfluous.

If they are not, then it pretty much comes down to whether you want to be noisy / extrovert (atmospheric) or quiet / discreet (recirculating).

[Thom]

rigga said:

air once measured either by maf or map has the required amount of fuel added by the injectors according to the ecu/map fitted, once the atmospheric valve dumps this air out of the induction pipework, the original measured amount of fuel is still delivered, and now you have a rich mixture scenario, in extreme cases bore wash can happen, best to run a recurculation valve unless you're running serious amounts of boost.

By essence, a MAP-driven ECU does not care about air flow, so using a recirculated valve or a dump valve will not affect mixture when closing the throttle. Also true when using a "blow through" MAF, ie when the MAF sensor is placed between the compressor outlet and the throttle body
I suppose most drag cars use MAP-driven ECUs, the compressors are in 99% cases gulping air straight from the open atmosphere with no filtering, etc.
(In fact since a drag engine is supposed to run at full load the short time it's running, I'm curious if they actually don't use some kind of heat sensor instead of MAP to drive the fuel delivery through whatever ECU, but maybe that's for another topic).

Edited by Thom on Friday 13th June 07:03

[dnb]

Surely most drag cars would use throttle angle instead of messing about trying to get a stable MAP signal to an ECU?

Getting back on topic - why would you have a recirc dump valve if you didn't have to? You'd not have anything to attach the duck whistle to. Vroom, QUACK! Vroom QUACK! all the way through the town centre of an evening...