How important is secondary pipe sizing and is there value to a cross over on a 911 6?
As to the secondary pipe, one resource I read seemed to indicate that its length of the secondary tube might be as important or even more so than primary tube length.
One of our noted and respected exhaust experts has found that a cross over can improve the midrange on a 911 motor. I understand that a cross over works on an odd fire motor like most V8's to reduce back pressure during overlaping exhaust cycles but I did not think there was a benifit on an even fire flat 6.
I did find the following referance which seems to make some sense:
Quote:
Some of you ask about a Cross Over on a V6.
In the case of the V6, with their “Even Firing Sequence… Having a crossover or not, is very dependent on the size of the exhaust and distance from the engine of the pipes placement. A cross over is not required if the exhaust is tuned to the engine. Tuned systems are nearly always without a crossover pipe because the length and diameter of the exhaust is specifically designed to work with your engine at a specified RPM Range to avoid reversion and scavenging.
On a V6 there is no need for a crossover due to the even firing engine. However, it has been proven that in some instances, a crossover pipe will decrease backpressure and allow for a higher flow. If the crossover pipe is too close to the engine, it allows the pulse timing of the opposing cylinders exhaust cycles, to crash into each other – that is to say, the pressure from a right cylinder will still be present in the left pipe when the left cylinder opens to vent. Too far a placement can create a “Pulse Vacuum”, causing diminished pressure on the venting opposing cylinder, causing decrease in torque (Called Scavenging). .
Proper design and placement of the cross over will allow a balance of pressures across the system and therein increase torque, especially at lower RPM.
Also, it has been shown that having a cross over pipe mellows out the raspy “ricer” sound that occurs above 3000rpm. So some people install them just for their “sound” value.
So, the bottom line is, you probably should have a cross over pipe on a V6 exhaust, even though it is not needed. There is minor performance gain and sound gain as well.
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