2 into 1 ‘Street’ Exhaust for BMW R100 Airhead

Images: With the kind permission of Jon-Lars Sorenson. More pics at: http://s44.photobucket.com/user/R90S/library/

When designing exhaust systems for race engines, I pit a couple simulation programs against eachother. This has proved to yield pretty good base designs from which dyno tuning takes over: a complex world of interactions including the tricky relationship between exhaust design and inlet tract design. The whole system can be  likened to a double ended pipe organ with the inlet and exhaust valves as keys – it is literally akin to tuning a musical instrument.

All well and good to put this sort of effort into shaving half a second off a racing lap time, but for your average rider on a street bike with its wide operating parameters,  moderate departures from design optimals will be lost in the noise. The calculated base design should be good enough. Besides, packaging contraints will often have you deviating from design ideals, in which case the exercise may instead become a matter of avoiding lengths and diameters that actively rob horsepower where you want it. This is particularly true of the ‘tail-pipe’ length which includes the collector length (downstream of the merge) and the muffler length.

For the calculated tail-pipe lengths to work, the muffler should be a straight-through absorber type of the same internal diameter as the collector. Unfortunately, the optimal overall tail-pipe length is influenced by the individual muffler characteristics and its length. So the tail-pipe lengths below are a rough guide only.  Anyway, let’s be honest, half the time we choose mufflers for their looks and sound right? Call that ‘ego contraints’, which leads to the following fromula:

Wide operating/operator parameters + packaging constraints + ego constraints = big fat compromise!

This is the world most of us inhabit. Unless you confuse the street for a racetrack and go to considerable extra effort in dialing out contraints and other unknowns for a narrower set of riding parameters, then quit obsessing over mm’s! (from an ex mm-obsessive).

With the above firmly in mind, here’s a set of street calcululations for a stock R100. Caveat: I haven’t built and tested this street system. But the following guidlines should yield better performance than the stock exhaust system and probably better than some of the dodgier after-market 2into1 offerings – particularly the ones that have one pipe butting into the other without a well designed merge.

Main Inputs (among others):
Compression ratio: 8.7
Volumetric efficiency: 0.95 (assumed)
Stock 308 cam duration @ 0.05″ lift: 243 deg
RPM at peak power: 6800
Application: street peformance – tuning for mid range torque and higher RPM HP

Resulting 2into1 specs:

1) Header diameter – stick to stock 38mm* pipe size even for warmer 336 or 324 cams with longer durations.
2) Header pipe length: 770 – 835mm (Third harmonic: the magic 32″!) err on longer side for our air cooled machines and approx 50mm longer if have warmer cam. **
3) Into a collector via a well designed merge.
4) Collector pipe size: 44 – 48mm  (1-3/4″  or 1-7/8″)
5) Best tail-pipe lengths including collector and  ‘straight through’ absorber style muffler: 435, 870, 1740 mm*** (4th, 3rd and 2nd harmonic repectively – 4th is the best)
6) Worst (power robbing) tail-pipe lengths: 650, 1300 mm

* All pipe sizes based on OD and 1.6mm (1/16″) wall thickness
** Ideally, both headers to be same length
*** Tail pipe lengths vary depending on type and length of muffler.

The header, or primary pipe lengths should idealy be the same. Though you are not going to notice moderate deviations (which may actually help spread the torque curve a little). This is most easily achieved by symetrically dropping both pipes back under the sump into a merge towards the back of the engine. Though some clearance is lost, this configuration is considered efficient because it minimises the number of bends.

Alternatively, it’s off to the same side for both pipes with chichanes in the near side pipe to even the lengths. Jon Lars Sorenson’s bike sports a fine example – see pic below.

Then there is the merge (or y-piece). Yep, there are good and bad designs. Try the search: ‘merge collector design’

And some advice to you cafe dudes (sh*t that’s me): Presumably, if you are reading this then you care about performance? If in tandem with this exhaust system you are also thinking of replacing the stock air intake system with nothing but a pod filter… Dont!

Happy fabbing.

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