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Dump valve for supercharger


harrypotter
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Great explanation. 

I suppose if it works without the vacuum, why bother piping it in. I’m sure there must be a good reason?

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I always think the confusion on this comes from the 2 main types of supercharger, centrifugal and Rootes.

 

As Dave says, the centrifugal is effectively a mechanically driven turbo and does need a dump or blow off valve as it need protecting from surge. The advantage of this type is small size and good efficiency but they are more delicate. 

 

Rootes are the ones normally factory fitted to early R53 Minis, SLK Mercedes etc. These are bigger, heavier superchargers that create more low down boost but aren't very efficient (they create a lot of heat!). These don't need a dump valve, but tend to use a bypass valve that only allows boost into the intake at higher revs (better fuel consumption for normal driving) and then diverts it past the intake when closing the throttle. You dont need surge protection on a Rootes as its more like an air pump than a compressor and they're almost unbreakable. 

 

There is a third type (twin screw) that is quite rare. It's big ,expensive and works like a Rootes, but its closer in efficiency to the centrifugal. I've only ever seen these fitted to big american V8's 

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So I think I’ve grown out of need for ‘hissing’ over run, so I assume a Recirculation dump valve is the way to go, feeding into the intake side of the supercharger. I’m now faced with a choice of tails of different diameters. Is the common 25mm type adequate for my needs? 

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A recirc type will eliminate the noise, but there are some (relatively) quiter versions of straight to atmosphere valves, that I suspect you'd struggle to hear over the rest of a Westfield.

 

2 minutes ago, harrypotter said:

Is the common 25mm type adequate for my needs? 

 

With no experience of your set up, hard to say without doing the math. Not sure if there are any "rule of thumb" guides about?

 

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Googling will probably get there, but it's not immediately helpful, as most are very US centric. a 53 mm BOV should be great up to around 700 bhp on a big V8, doesn't really help! (Anything for a lower than 400 bhp engine didn't even seem to appear on the radar!)

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The Rotrex handbook doesn't really help with sizing, however, if using a recirculating type BOV, then placement becomes critical - to the point of affecting the Rotrex warranty if wrong!

 

The BOV itself must be placed after the intercooler, so that it's venting cooled intake charge. The recirculation output, is then piped back to the air inlet piping between the air filter and the Rotrex.

 

Quoting from the handbook:

 

Quote

it is very important that the re-circulation valve is positioned after an intercooler, to avoid excessive heat soak of the supercharger. Under cruise and/or urban driving with an incorrectly mounted re-circulation valve, the inlet temperature of the supercharger can easily exceed 200°C. Incorrectly placed re-circulation valve will cause damage to the supercharger and voids warranty.

 

And the accompanying diagram:

 

image.png

 

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After a bit of digging, the BOV supplied by TTS with my Mega S2000S SC kit, was a Malpassi unit with a 25mm bore inlet.

 

There doesn't seem to be any real penalty to going bigger, so yes, I'd be inclined to try the 25mm recirc type BOV.

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You should include a dump valve with the rotrex set up. 

 

You could vent to atmosphere or recirculate back to the air filter. With the engine running and the throttle shut, the dump valve will be open as the charger will continue to deliver air through the system. As you open the throttle the valve will close and the air will be redirected into the engine.

 

Your dump valve will also need a suitable spring rate depending on your set up. For example if you’re running with relatively low boost you may need to run a slightly softer spring in your valve so it opens at the appropriate time. 

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