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      10-10-2007, 01:17 PM   #1
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Exhaust Gas Scavenging (56k warning)

The question is how much of a role does exhaust gas scavenging play post turbine..

I'm looking for numbers. Not what your uncles friends cousin's grandpa came up with when he licked his finger and put it in the air. No finger licker's!:smile:
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      10-10-2007, 10:40 PM   #2
atr_hugo
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Try this link for a start. I know a guy that's been building turbo systems for Miatas that I can contact - I need to e-mail him. There's another guy hanging around town that builds custom headers (and we aren't talking muffler shop work here - he's done some really interesting things on some VERY intersting cars ; -) - he's hard to get ahold of but maybe one of the ATR guys knows a good way to do it.

Here's a set the guy built for the KU Fromula SAE car a few years back:
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      10-11-2007, 12:54 AM   #3
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WOW those steps in the piping is F1 technology! I'm impressed! But that isn't a turbo manifold...

What kind of coating did he use?

Edit: duH! i just re-read your post.. Im an idiot. I just saw the picture and went 'ooo pretty'post
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      10-11-2007, 01:46 AM   #4
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Quote:
Originally Posted by hugo's link
Improved engine scavenging capability affects quality and quantity of the fresh charge and consequently influences the exhaust gas emissions. If, in addition, valve overlap period is increased the benefit of this system is still more evident.
I am uncertain if tuners are able to control the vanos systems yet but this would help move the n54's powerband to the right. Possibly preventing high rpm fall off.

eye candy


80's porsche F1

80's BMW M13-1 (max 5.5 bar!!:eyebulge

Late 80's Zakspeed F1

(Zakspeed?! Does that mean it looks like mine?!)


No? aww...

Zak
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      10-11-2007, 06:55 AM   #5
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Good gawd! I had totaly forgotten about the Porsche Indy effort - I think I gotta poster of that car hanging in the garage.
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      10-11-2007, 04:33 PM   #6
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Quote:
Originally Posted by atr_hugo View Post
Good gawd! I had totaly forgotten about the Porsche Indy effort - I think I gotta poster of that car hanging in the garage.
Yea.. it's been collecting dust!:tongue:

I found an adequate right-up for the uninformed..
Quote:
A long time ago, back in the caveman days of carburetors, changing anything on the engine meant adjusting the carbs, because they're designed to work for that very particular engine flow rate, ve, blah and blah. That is why when people would install larger exhausts, they might burn valves, because the car was actually running lean, because they wouldn't adjust their carbs to compensate for the exhaust. Not because of the exhaust itself.

To explain the loss of torque in the lower rpm ranges that does happen: exhausts, if well designed, use the principle of scavenging to help pull exhaust out of cylinders. How this works is, imagine you have a manifold with equal length runners, and they all meet up in a collection area. One exhaust pulse races down its runner, into the collection area, and down the exhaust. It has forward momentum, but is eventually slowed, because the valve has closed behind it, so the pulse will create a slight vacuum behind it, then the next valve opens, and it's pulse is helped slightly by that vacuum. This is scavenging. Scavenging only works at one particular frequency/temperature/volume level though. Move out of that frequency, and you have exhaust pulses that actually run into eachother, creating a restriction. The frequency all depends on the size of the exhaust, runners, and many other factors. In general, a smaller exhaust (read stock) is most efficient at a lower frequency/rpm range. Car manufacturers do this, because most people drive around at 1-3k rpms all the time, and making the exhaust efficient there increases gas mileage. Larger, aftermarket exhausts are more efficient at scavenging at a higher rpm. Thus the apparent loss of low rpm torque. The exhaust is like a musical instrument that can only play one note. Try to play another note through it, and it's not going to work as well. However, if you were to remove the exhaust system altogether, you would pretty much see gains all across the board (unless you just removed a very well constructed/designed/tuned exhaust, and then you would only lose torque where the scavenging effect was the greatest). This is because there is _no_ exhaust back pressure anymore at all. There is no scavenging effect, but you wouldn't have to worry about it. However, you would have to worry about your car burning down.

The easier the exhaust can get out, the better, no matter what. Back pressure in the exhaust is always bad, and just causes the engine to have to work harder to push the exhaust out, losing torque. Saying that the engine needs back pressure in the exhaust is like saying that you need vacuum in the intake. The intake and exhaust are very similar, and all the scavenging applies to the intake too, that's why runner length/diameter/shape on both the intake and exhaust are important.
via http://www.dsmtuners.com/forums/show...=back+pressure
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      10-11-2007, 04:49 PM   #7
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more indepth info.. same source.
Quote:
The reason you lose power when you remove your N/T exhaust is because you are actually INCREASING your backpressure in the process. To understand this, you need to consider the split second timing of the pressure pulses in the exhaust. An exhaust system, when operating at the correct speed range will actually LOWER the pressure at the port below atmospheric. This occurs for a split second during the cycle, and, when timed correctly, leads to a momentary below atmospheric pressure condition. This happens not only because of the slug of air moving down the pipe at high velocity, but more because the pressure waves that travel at the local speed of sound in the pipe are timed with the mechanical operation of the engine. If you remove the exhaust system, you inadvertently "tune" you exhaust system to a MUCH higher RPM range than you can safely operate, so you raise the exhaust port pressure to atmospheric, and you lose power.

Further, there is MUCH more advantage to be had than just the fresh charge during scavenging; for instance, during overlap, the piston is moving slow and can do little to move the incoming air charge. By scavenging during overlap, we can get the intake charge moving BEFORE the piston begins to move downward. This initial movement creates a pressure wave that has effects inside the intake manifold as well.
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