Exhast Cross Over and secondary sizing?

[911st]

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:

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.

[911st]

Here is the old thread and post that got me thinking about this.

Originally Posted by Steve@Rennsport

We've done a lot of dyno work on these engines since 1976 so its not really subjective,... That device doesn't lie but one needs to understand the data and how it applies to each set of circumstances and objectives.

First, Bursch headers have no merge collector; its not even close to one.

Second, 911 engines are not as picky about collector configuration until you are operating at very high RPM and looking for that very last 3-4 HP.

Third, Header lengths ARE very important as are diameters and both are chosen based on engine size, camshaft profile, compression ratio, and head configuration. Equal length primaries become increasingly important as camshafts become more aggressive. With OEM smog cams that have little or no overlap, its not very critical.

Lastly, 911 engines ARE very sensitive about secondary lengths and the effects of a crossover, whether its additional plumbing or a common, dual-inlet muffler. One measures distinct torque increases right where you need them with such designs. Open megaphones w/o a crossover makes a narrow power & torque range requiring rather close-ratio gears to be practical.

Exhaust designs make a noticable differences in the quantity and spread of both HP and torque. One needs to be very careful when choosing such systems to prevent compromising a useful torque range that doesn't match the car's gearing.

Hope this helps,

George's headers vs. Bursch