16 replies to this topic

[cyberface]

I've done a search and it appears the consensus is that dump valves / blow-off valves are not recommended on the VX220 Turbo. However it's not clear why not. I've noticed behaviour on mine that suggests a dump valve would be beneficial, so I'd appreciate if the experts here could let me know why it's a bad idea. Basically if I close the throttle after accelerating hard on-boost (for example, aborting an overtake because the car pulls out to block me, or backing off coming up to a corner), then the engine jerks before slowing, like the intake gases are pounding into the butterfly. Equally, if I get immediately back on the boost after lifting off, there's a noticeable lag before full boost regardless of the revs (which would be expected if the compressor wheel has stalled due to the pulse being reflected back to the turbo). My old Noble M12 2.5 had two turbos and two dump valves. Disregarding the boy-racer fun you could have (the noises could scare pedestrians), it was very clear that the valves allowed boost to return *immediately* after lifting off - the turbo did not slow down between gearchanges. This is not the case in my VXT. What concerns me more than the last tenths of a second on track is the jerk and hesitation I get when backing off on full boost, and I've always thought that all that turbocharged intake air ought to be dumped if you close the throttle... It should be clear from the tone of my post, but I'd like to reiterate that I'm not after teenage ppfft-whoooosh type noises for the sake of it, but I am after smoothness and throttle response. The Noble was better in this respect on- and off-boost, and that was an aftermarket engine - the Vauxhall unit should be much better as it's been developed much more... Where am I going wrong with this train of thought???

[urbanmac]

they sound chav - end of story

[cyberface]

they sound chav - end of story

Thanks for that

Presumably you aren't aware that not all dump valves make a noise? Only the atmo valves (that vent to atmosphere) make that puffing sound. Equally there are valves that are specially made to make as much noise as possible (i.e. the ones you call 'chav'), and valves that don't make anywhere as much noise.

[jasvxt]

Yeah but whats the point though, what do they actually achieve ? All the ones Ive heard just make your car sound like its got Tourettes Rs Jas

[cyberface]

Yeah but whats the point though, what do they actually achieve ?
All the ones Ive heard just make your car sound like its got Tourettes

Jas

When you slam the throttle shut (lift-off) on a car with no dump valve, the pressurised air from the turbo (that was due to go into the cylinders) bounces off the throttle butterfly and slams back into the turbo in reverse (simplistically). This tries to force the turbo to spin in the other direction, in practice it just stalls the turbo.

When you open the throttle again, the turbo is not spinning and needs to spool up before it delivers boost (i.e. turbo lag).

With a dump valve, the excess air is vented instead of bouncing back to the turbo. Therefore the turbo can keep spinning, so when you next open the throttle the turbo is already spinning and ready for boost. Voila, less turbo lag. Not only faster response, but less strain on the turbo as it doesn't need to keep stopping and starting.

This clear enough? Dump valves were originally pukka performance parts used by rally cars *before* the Max Power brigade started using them cosmetically. There's a real reason for their use - performance and reliability.

[RWinstanley]

The reason for a dump valve is to empty the pressure from the compressor outlet as when there is a low pressure in the turbine volute a compressor wheel can act as a turbine, albeit a very inefficient one, and reverse the direction of rotation of turbo. This is not good for the engine performance and even worse for the life of the turbo.

AFIK the VXT has an internal recirc valve to perform this function.

As covered here clicky.

[cyberface]

Aha. Cheers for that - so it appears that the VXT engine actually already has a dump valve, but it is controlled by the ECU instead of mechanically by manifold vacuum. Presumably this means that the odd hesitation and jerky off-throttle behaviour of my VXT could be due to the OEM dumpvalve not being operated correctly by the as-yet-unidentified ECU remap on my car? I honestly thought there was no dump valve on my engine, as the turbo very definitely seems to stall on gearchanges and closing the throttle quickly. Given that I thought incorrectly in this case, I'd like to redirect this thread to whether all other VXT engines do this or whether it's just my example. Can other VXT owners chime in please? Does your turbo stop between changes and if you close the throttle?? (i.e. how much lag, how much time after changing gear before the turbo starts whistling again, etc.)

[jules_s]

>voila<

[cyberface]

OK then sounds very much like I'll have to visit Thorney Motorsport for a remap, if it's only around £500 then it will be money well spent. I've already got a VXR exhaust on the car so this would probably be around a stage 2 then?? Only trouble will be resisting temptation - there are a couple of other things I want to do with the VXT (wheels and tyres primarily) and Thorney appear to be a veritable toyshop of kit. I said I wouldn't start modifying the car until next spring!!!!

[jonnyboy]

I think it depends which dump valve you have wether it sounds chav. HKS SSQV ones are cool as they are more of a chirp than a crappy phhhssstt noise. There no problems running them alongside a recirc valve although you may get a wee bit of jerkyness in the throttle at low revs as it will affect the airflow meter slightly. If you cars stalling on changes it might be worth changing the recirc valve too as OEM ones are usually cack anyway (on my old meg 225 you could hear the standard recirc valve passing air) I ran a HKS valve on my 225 for about a year with no probs

[stevenf]

My car has a stage 2 re-map while evrything is great on the throttle or come to that part throttle I must admit to being a little unsettled with jerkiness on lifting off the power. Coming down Craners Donnington and lifting of the power mid way down I suffered this issue and when a car is trying to deal with adverse cambers it's the last thing you need. I thought that it was just one of those trait's but if it can be got rid of I'd like to know how. Cheers Steve

[ronbot]

Sounds like your standard recirc dump valve might be faulty?? Might be worth trying a new one.

[stevenf]

Sounds like your standard recirc dump valve might be faulty??

Might be worth trying a new one.

Is this a job I can do myself ? being pretty usless with the spanners or is it a dealer job?.

cheers

Steve

[ronbot]

Sounds like your standard recirc dump valve might be faulty??

Might be worth trying a new one.

Is this a job I can do myself ? being pretty usless with the spanners or is it a dealer job?.

cheers

Steve

Im not sure where the dump valve is located on the VX. Providing you can get to it, they are normally just a couple of hose clips. As you know it will be somewhere on the compressor outlet pipe but could be anywhere between turbo and intake manifold.

[vectraboyv6]

I thought that you had to have a recirculating valve. the dump vakves can not be use as the ECU measure the amount of air entering and exiting, and thus cant work out where the air had gone causing fueling problems.... or that was the case with the old cavalier turbo engines.

[garyk220]

My car has a stage 2 re-map while evrything is great on the throttle or come to that part throttle I must admit to being a little unsettled with jerkiness on lifting off the power. Coming down Craners Donnington and lifting of the power mid way down I suffered this issue and when a car is trying to deal with adverse cambers it's the last thing you need. I thought that it was just one of those trait's but if it can be got rid of I'd like to know how.

Cheers

Steve

Just posted on your topic in Tuning & Mods before spotting this thread. My car does the same with a Stage 2 remap. Especially if you floor it and then lift your foot a second or so later. I've put it down to the difference between overboost (15psi) and zero boost when you close the throttle, exaggerating the effect of lifting off. If you feather the throttle when lifting off, it doesn't do it. Just one of those things you learn to drive around, rather than a fault with the engine management/turbo unit imo.

The map could possibly be rewritten to reduce the effect, but I'd rather have control of the boost through the throttle when driving on track, rather than let a map control when the boost is cut.

[stevenf]

My car has a stage 2 re-map while evrything is great on the throttle or come to that part throttle I must admit to being a little unsettled with jerkiness on lifting off the power. Coming down Craners Donnington and lifting of the power mid way down I suffered this issue and when a car is trying to deal with adverse cambers it's the last thing you need. I thought that it was just one of those trait's but if it can be got rid of I'd like to know how.

Cheers

Steve

Just posted on your topic in Tuning & Mods before spotting this thread. My car does the same with a Stage 2 remap. Especially if you floor it and then lift your foot a second or so later. I've put it down to the difference between overboost (15psi) and zero boost when you close the throttle, exaggerating the effect of lifting off. If you feather the throttle when lifting off, it doesn't do it. Just one of those things you learn to drive around, rather than a fault with the engine management/turbo unit imo.

The map could possibly be rewritten to reduce the effect, but I'd rather have control of the boost through the throttle when driving on track, rather than let a map control when the boost is cut.

Yea, thanks for that you could well be right. I need to meet up with someone to compare cars , suppose you dont fancy popping down from Edinburgh for an hour or two .

Cheers

Steve