Thread: New type of BOV
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beepbeep beepbeep is offline
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Join Date: Jun 2001
Location: Sweden
Posts: 5,714
Hello Adam!

We are here to "collaborate and discuss different ways of producing power, tuning, different setups" but critical remark on fundamentaly wrong strategy of venting boost pressure into athmo at WOT is "irrogant/sceptic"?

Well I guess I have to talk from Porsche perspective then.

It's simple physics. You are wasting fuel pressurizing the gas by spinning turbocharger. What's even worse, gas that has been metered up by fuel injection which will inject the bookmarked amount into fuel into engine. So instead of using this gas to raise VE, you dump it overboard but still inject the fuel? Of course, air dumped aboard is also warmed up by inefficient compression process and then cooled by intercooler. Turbo is also demanded to compress more air than it should, as part of it is vented overboard.
Results on otherwise stock car (be it CIS or any other non-Speed/density system): Higher turbo RPM, more heat soak into intercooler, lower MPG, higher backpressure (as turbo isn't bypassed), richer mixture.
Frankly, I don't see the benefits except maybe (and this is a strech) the fact that turbo is spinning faster and it's inertia might let it lag less between the shifts. Thus this might work on a one-off drag-racing car with gigantic turbo, with lot's of shaft-inertia and typical working point to the left of efficiency island. Then it needs to be custom-maped to compensate for VE-missmatch due to bleed. It's not a scenario that can be applied to typical P-car. Potential problems are much bigger than eventual gains.

Let's take a look at typical K27 compresor map, should we?

The pressure ratio is vertical, the mass flow is horisontal. What cold side bleed does is effectivly pushing the working point on the map to the right. As turbochargers are most often sized to have most effective working point somewhere in the middle range, and then allow some extra huffing on the high revs (w/o surging or overreving the impeller), pushing the point to the right (while retaining the same pressure ratio) will only make efficiency drop even more. Let the bleed open too much and you will punch trough Vne of compressor blade tips or a stall, with damages to follow.

Call me ingorant but I really fail to see benefit of bleeding the air from pure efficiency point of view (and then there are the fueling issues on the top of it).

You are of course welcome to explain the special ordeal that makes popper better than ordinary BOV/Wastegate combo and/or debunk my analysis of it. We might learn something from it.

Thank you for your time,

Last edited by beepbeep; 01-28-2008 at 07:09 AM..
Old 01-28-2008, 06:50 AM
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