Race Engine Forum UK

Hey All

Sorry again for my ignorance :-) but I need a little advice again please. I'm in the proccess of getting my VX up and running and we suspect the wastegate may be past it's prime :-(

So I was wondering if I could by a off the shelf dump valve and replace the existing unit. Personally I love the sound of a dump valve and the are a lot more reasonably priced than an original wastegate replacement.

Now, what pressure valve would I need to use, and I believe some of them even have speed controls for how quick the dump.

Many Thanks
Geoff

If you mean the tube screwed onto the top of inlet manifold it's a backfire protection valve. Just strip and clean and fit new O ring.

Given the low pressure it generates the blower doesn't need a wastegate or dump valve.

GC

There is no wastegate or "dump valve" on a Volumex engine, there is no need for them. These items are intended for a turbocharged engine.

Hi,

Thanks for the update guys, I will try referbish the unit as suggested. But out of interest what sort of boost does the VX put out, as I has thinking bout putting a boost gauge in the engine bay.

And i know it's not required as Steve pointed out, but would it actually be possible to use a dump valve (if this unit is beyond repair)?

No, you would definitely not use a dump valve.

In short - the standard Vx has a suck-thru type carburettor so when the throttle closes the pressure outboard of the blower drops and since the blower is crank driven so does the engine speed, which is what you want. On turbo the rotational inertia of the fast-spinning compressor tends holds the pressure up in the the inlet manifold, and will do so till it runs down or the throttle is closed, but in that case the airstream tends to hold the throttle plate open so without a dump valve you couldn't ever slow the motor down from an on-boost condition.

However fun they may be to listen to (for a driver) I myself happen to find them annoyingly intrusive outside of the vehicle. I speak as a race engine professional you understand. Round here young chaps with turbocharged this_and_that seem to get a real buzz out of winding their cars up to hear the thing go off, knowing full-well that they generally alarm people. Actually dump valves are a prodigious waster of stored energy. But then, I am definitely biased - and in a minority - my feeling on turbos (speaking as a former Chief Engr for a turbo manufacturer) is that they are a device, frankly, ill-suited for the purpose of uprating a reciprocating petrol engine.

If you must have one - dump valves are only for pressurised throttle plate systems, turbo or supercharged. Sorry!

GC

Hi Guy,

Thank you for the quick answer. I must admit that does make more sense than how I've imagined it :-)

And with regards to the dump valve, I do believe you are right. I think after a week of sitting in traffic and having that thing hissing at me every 20 seconds might drive a man insane :-)

And my mechanic just phoned, the engine has just started for the first time and the current valve is working like a charm. So no need for any form of replacement.

Geoff,
As Guy mentioned, the valve on the intake manifold on a Volumex is a “blow off” valve not a “dump valve”. The purpose of this valve is to relieve pressure in the intake manifold due to an engine backfire. Without this valve, to relieve the pressure in the manifold due to a backfire, the supercharger and or the intake manifold would be blown right off the engine. The valve on a Volumex opens at 14PSI. Since the VX system is a “suck-thru” the carb system, the intake manifold contains a very flammable mixture of fuel and air. With a Volumex, you do not want to dump or vent the intake manifold through this valve. That is unless you are trying to create an explosion or start a fire! Modern turbo systems vent only boosted air, not boosted fuel and air. I would clean and lubricate your valve as it is 26 years old these days. If the valve is stuck shut, you run the risk of blowing the supercharger off the engine in the event of a backfire. If the valve opens in a backfire but does not shut, you run the risk of filling the engine bay with an explosive mixture of fuel and air that is easily ignited. BOOM That’s one way to open the engine compartment lid!
tmvolumex

Good post Tom.

GC