Join Date: Apr 2002
Location: The Woodlands, Texas
Here's something I found a while back that somewhat explains it. I don't remember where I copied this from, unfortunately. In essence, to minimize surge, you must increase the compressor flow rate so that it's farther inside the operating envelope of the compressor(which in effect, gives you more power). You can also create surge(even with high flow rates) if you don't have a blowoff/bypass valve when you close the throttle. The high pressure air column in the inlet tract stalls the compressor wheel and turbine.
<snip>To understand what is happening inside a compressor you should think of thousands of little airplane wings all flying around in a circle.
Airplane wings must make lift, or in other words, increase the pressure on the bottom side of the wing so the airplane will lift off the ground. The wings do that by the angle at which they hit the oncoming wind and the curve of the wing. The compressor blades are the same. Each little blade acts the same as an airplane wing.
A wing needs a certain amount of velocity to start making lift at all. This is the same as the air velocity in the compressor at minimum flow, otherwise called the surge point.
Below this minimum velocity, for which the volume flowrate is given (rather than the airplanes lift-off velocity), a pressure increase on the discharge side of the compressor blades can not be generated very well, if at all.
This velocity is equivalent to an airplane's stall speed. The flow separates around the wing, or the blades in the case of a compressor, and the flow patterns disassociate, losing the capability to generate lift or make a pressure increase on the discharge side of the compressor.
At the same time as the flow patterns disassociate, there is still high pressure air that has already been packed into the discharge areas of the compressor.
When the compressor blades can no longer maintain the high pressure in the discharge area, that high pressure air that has already been packed in there, blows back through the blades into the still lower pressure suction area of the compressor.
This, of course, wants to turn the compressor in the opposite direction and plays heck with the drive shaft and just about everything else, and the turbine starts to slow down. It soon recovers and begins to build lift or increasing pressure again, until it happens all over again. If you have a lot of capacity and its a large high pressure compressor, it can sound like a pretty good thunderstorm is going on.
The opposite to this surge point is the stonewall effect(choke flow in turbo-speak) which can occur at maximum flow. The density of the air in the discharge portion of the compressor becomes so great that no more can be moved through and a pressure build up occurs which can no longer be sustained. The compressor has "stonewalled" or reached its maximum capacity and some of the high pressure air bleeds back through the blades with no further net increase in the flow.
90lx with a 304ci 4v, Turbo400, and 105mm turbo
Thanks to TranzKing, MMR, BS3, Don Bailey, Team Z, and Wolfe Racecraft
Last edited by NoSlix; 01-22-2003 at 08:06 AM.