Been thinking (I know you shouldn't, but I was)

On Imprezas before 97 the dumpvalve is before the intercooler whereas on all cars after 97, it was moved to after the intercooler.

Now as far as I can see it makes sense that if you are dumping air to atmosphere, it's better to dump air that hasn't been through the intercooler, otherwise the intercooler is cooling air that it doesn't need to. That heat that it takes out has to be dissipated, so you are reducing the intercooler's potential efficiency by dumping air to atmosphere that it has already cooled.

There must be some logical reason why Subaru changed the design (and stuck with it), but I'm damned if I can work it out.

Is there anyone out there who can shed some light on this?

Cheers

Rich

[Edited by RICH WILD - 7/31/2003 1:18:18 PM]

AFAIK Subaru changed the design of the dump valves due to leakage problems on the early versions but can't think of the upsides/downsides of positioning before or after the intercooler.

Alan :cool:

I thought it was the MY97/98 dump valves that leaked? Subaru then changed to the version which fits on to the actaul intercooler. I did remember reading the reason why somewhere Rich.... i'll try to find it again but my memory really is ****e so the chances of me finding it are minimal ;) .

Tony.

It is because it is better to have the dump valve as close to the throttle body as possible, this is to keep as much air in the system as possible. The only way of doing this was to fit the DV onto the I/C (or have the oulet pipe on the I/C).
With the older style DV systems the turbo has to refill the whole system again after dumping as it releases the pressure before the I/C.
Rob

Also standard subaru DVs are all recircs, so dumping air back into the turbo inlet from pre intercooler (140 degrees C) versus post intercooler (50 degrees C) may make a significant difference to resulting charge temps.

Simon

Thanks for the replies guys.

Si Hethers makes a good point. I was thinking purely for VTA dumpvalves as I can't remember the days of having a recirc :) :o
so I suppose with a recirc system, the reason is obvious. Just me not thinking before I speak as usual :)

I was thinking from my own perspective having a VTA valve on there. Would it be better to have one of these before or after the intercooler (that's what I should have said first time).

I take Rob's point about having to refill the system, but would the subsequent effect on intercooler efficiency outweigh this or not?

Interesting stuff this. I love it. :)

Cheers

Rich

Lesson for me now :p

How would it effect the ic efficiency??

Rob

Rich, you are thinking that you are dumping the cooled air(new type d/v) when you could be dumping the hot air(old type d/v) means less cold air into engine?.

You are assuming that it would be better to get rid of the hot air than cold, well it makes no difference surely?.

With both setups under the same conditions(assuming identical dumps for old and new) the exact same volume of air
will pass through the intercooler and into the engine. This surely means the same air temps going into inlet.

hope thats what you mean:D

LOL

Maybe I'm confused and my confusion is becoming contagious :)

What I thought was... If the intercooler has taken the heat out of the dumped air already, then that heat will be stored in the intercooler and then dissipated to atmosphere (interccoler acts as a heatsink and all that stuff), so the intercooler is now warmer (slightly) than it would have been had the air been dumped out from a VTA situated before the intercooler. Hence it's efficiency is reduced.

I'm reading a book at the moment you see called "Forced Induction Performance tuning" and it says on p106:

"Vent valve location has a bearing on performance. If placed after the intercooler, intercooler efficiency falls as it has to work overtime taking heat out of the compressed air, some of which is later dumped. Therefore, the place for the vent valves is somewhere between compressor outlet and intercooler inlet. In this way the charge cooler will only be cooling that portion of compressed air that is actually going to be used by the engine"

What do you reckon?

Cheers

Rich

Hang in there Rich, I'm sure all will be made clear :D

Mikey :cool:

PS What's a Dump Valve and Intercooler ;)

Great topic Rich ;)

I'm no expert so anyone who is please feel free to blow me out of the water on this but...

Perhaps there are some financial cost implications to placing the DV before the I/C, eg. it has to be built stronger, the extra piping is more expensive. I'm sure this is why it moved on the the I/C for '99.

I once tried measuring the temperature in the pipe from the IC to the DV on my MY98, the air in that bit of pipe got quite a bit hotter than the air flowing through the IC very quickly(can't remember the figures but around 10-15C), as soon as the dump valve opens the temps equalized.

Andrew...