[DMCForum] Slightly OT - DUMP VALVES

[Martin Gutkowski <webmaster@xxxxxxxxxxxxxx>]

Hi All

I would like some input on something that I want to learn about before 
installing my Z7U turbo PRV in my DeLorean.

I know a hell of a lot about DeLorean engines, but bugger all about 
anything else. I'm trying to learn about my engine before stripping it. 
I do have a lot of secondhand info up my sleeve from a friend with a 
ballistically quick turbo'd car, and I'm basically trying to identify as 
much as I can and wrap my head around its operation before deciding how 
to go about it.

The original Renault turbo engine employs a recirculating dump valve. 
This pulls a pressure feed from downstream of the throttles, and uses it 
to actuate a piston which, when under low pressure (ie vacuum) opens the 
piston and dumps the outlet of the turbo back to the inlet. Atmospheric 
dump valves just let the air involves fart out into the engine bay, 
making the typical "sneeze".

But I think, based on a comment from my friend, that a dump valve is 
about as useful as directional speaker cable. (Please insert your own 
metaphor - eg chocolate coffee pot, inflatable dartboard, fart in a 
wetsuit, etc). Here's why.

A turbo is a fan, not a pump. Ok there are loads of different types of 
pump, but by this I mean that you can't stop it turning by closing off 
the inlet or outlet. Ever taken a CPU fan and covered the back of it 
with your hand? What happens? It goes _FASTER_. The reason is because it 
has no work to do - it just "paddle-wheels" the air around in a circle, 
not shifting it. This means less resitance, and results in accelleration 
in the turbo.

The idea behind a dump valve is that it releases excess pressure in the 
turbo outlet reducing the back pressure which would otherwise "cause the 
turbo to slow down". I reckon this is bollocks, but I can't find 
anywhere on the internet to support this theory, so I must be wrong. Can 
someone tell my why?

Certainly in my friend's skyline, after a gear change, there's an 
enormous bang as the throttles are opened, and this is evidence of the 
turbo having gained in speed while the throttles were closed. He has a 
huge T4 on there with no fear of overspinning, so didn't fit a dump 
valve, and this is the only reason one should be fitted - to prevent 
overspinning of a turbo that's working right on the limit

Also, for what they are, dump valves are extraordinarily expensive - it 
seems there's a similar market out there for gullible fools and 
"performance" parts as there are with HiFi equipment.

Opinions anyone?

Martin

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