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jluetjen jluetjen is offline
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Join Date: Oct 2001
Location: Westford, MA USA
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One conclusion that I've drawn from exhaust design is that there are two distinct zones that need to be considered for exhaust design. Specifically they are what I would call "low speed" performance which is from idle up to a maybe 1000 RPM above the torque peak, and "high speed" performance which goes from there up to the red-line. My understanding is that the "low speed" exhaust performance is generally dominated by a combination of inertia and accoustical tuning (think of a slide trombone), while the "high speed" performance is dominated by flow rate (think of a fire hose).

Then you need to layer on top the engine configuration (as Bill Verburg described) and the cam design and intake configuration. The neat thing about 911's is that they all have the a solid engine configuration (120 degree boxer flat 6 with even exhaust pulses) which simplifies that particular variable. But that still leaves you with 2 problems and at least 4 variables to manage (including the collector and muffler designs).

Unfortunately most of us have to resort to swapping parts to see what seems to work out best, but this approach will never tell you why or provide any predictive information. Never the less, I'll wade in with my $0.02.

1) We can most likely start to build a database of examples to understand what's happening.
A) Cam design. Does the cam have much (if any) overlap?
B) Primary lengths. How long are the primaries in the heat exchangers?
C) Primary ID. What's the internal diameter of the primaries? Is there a step?
D) Secondary length? How long is the pipe after the collectors. If it continues into the muffler, you need to include that length too as Bill shows.
E) Secondary ID. What's the secondary internal diameter. Does it change?
F) Muffler configuration. Baffles? Glass? Steel-wool type stuff?
G) Cross-over pipe. Is there a crossover? Is it in the muffler?

2) The next thing is to describe the results fully so that we can understand the change in the torque curve.

A) Just looking at the change in HP isn't meaningful since that just describes the limit of the exhaust's or the engine's "high-speed" performance.

B) Just knowing the peak torque number describes how well the system is in tune at that one RPM.

C) In this case I really think that at torque curve comparison tells the most

Some hypothetical cases.
1) The early 911 race system (a la a 906) was tightly tuned for maximum performance from the torque peak ( with a comparitively high peak torque number at around 5000 RPM) up to the red-line (around 8000 RPM). By virtue of being so "in tune" at 5000 RPM (with a strong low pressure pulse at the exhaust valve and a pressure pulse at the intake valve during overlap), the system was almost completely "not intune" at 4500 RPM when the reverse was true of the pulses. (For those of you with band experience, think of tuning a trumpet by tiny changes of the small tuning slide). The result was a big hole in the torque curve at 4500 RPM.

2) A more "out of tune" system would have slightly different primary and secondary lengths resulting in the low pressure exhaust pulse arriving at the exhaust valves at a different time from when the high pressure intake pulse arrives at the intake. The result would be less torque at 5000 RPM, but no hole at 4500 RPM either. So this design might have a wider torque curve and a more driveable motor.

3) Playing around with the cross-over designs can augment or reduce the affects described in cases 1 and 2. In many cases the muffler designs on most street 911's tend to be in this realm.

4) Changing the pipe diameters can (in combination with the valve size, port diameter and cam lift and duration) affect the amount of flow at peak RPMS -- which means HP. But they also affect inertia which impacts the magnitude of the pulses described in cases 1 and 2.

So, would a Monty work "better"? Well, it depends...
- What changes are there in the secondaries?
- What changes are there in the cross-over design?
- What changes are there in the muffling material or design?

While the number of outlets is the most visible change, it also in many cases may be the least influential.
'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
"Any suspension -- no matter how poorly designed -- can be made to work reasonably well if you just stop it from moving." -- Colin Chapman

Last edited by jluetjen; 12-16-2005 at 06:06 AM..
Old 12-16-2005, 05:58 AM
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