Blake

I used two-inch primaries (started with a Racing Beat 12A header that I chopped up), through two Rotary Performance bullet-type presilencers and collected to three inches approximately 66 linear inches from the exhaust port (not the flange). The collector was custom fabricated by a real pro whom you cannot even talk to without a referral...unlisted number and a shop in the securred part of a small airport complex. Real cloak and dagger stuff, which struck me as funny. Great fabricator, though. I ran the 3-inch pipe under the axle and into a big magnaflow straight-through muffler and then ran an extraordinarily long tail pipe across the rear bumper. One of the secrets to scavenging is having the right critical lengths and -- moreso with a rotary -- as long an exhaust as possible. Rob calls it the "freight train effect". You want that freight train of exhaust pulses moving in the right direction, at a certain pace, working for you rather than against you. If you get it right, you can place a negative wave (less than atmoshperic pressure!) right on the exhaust port as it opens up, creating a suction that will draw in the fresh intake charge. Not only does this scour out all the exhaust but, due to the overlap, will get the intake charge moving faster, sooner...that headstart gives it more inertia as well, so the benefit extends all the way to IC. Scavenging: the gift that keeps on giving!

Anyway, we ran an oily rag over the entire exhaust while it was running and could determine by the dark and light bands where the high and low pressure waves were "standing". This confirmed that we got a negative wave over the exhaust port and positive wave in the presilencers and muffler, plus a positive at the exhaust tip. Here is a picture of the old (long) exhaust tip with a very visible dark band:



That band is high pressure in the exhaust. There is another one beginning right where the actual tip is, but most of that falls just beyond the tip (an area you want to prevent being low pressure). It worked great but was a bit ungainly, so I eventually shortened it to where the visible dark band is.

A properly scavenging exhaust improves Volumetric Efficiency (VE) all over the powerband and VE = Torque (bigger 'bangs'). Also, because the excellent VE continues to stratospheric RPMs, that component of work has the leverage of RPM (more 'bangs') to create more horsepower.

We called the exhaust "the street cleaner", as it would blow all the leaves and dust off the street for several feet behind the car even at idle. The positive pressure in the muffler literrally blew it apart after 7,000 miles. The damn thing exploded in Redding, CA on my way home. I had to stop and pick up the pieces and drive straight to a muffler shop with my ears ringing (even the ear plugs were not enough).

I still have that exhaust at my shop, sitting under a trailer collecting dust. Someday, I'll have to put it back to work...

rotary emotions

The description you make, makes me think of two stroke race exhausts. I remember in my last year of school we had to train at a workshop, and I was in a motorcycle shop. The owner raced bikes in two classes: 400cc 4stroke and 125cc two-stroke. Both were so called prototype classes: almost all was allowed, so a lot of custom work was performed. He did all the testing and designing himself. With an own rolling road/dyno (for motorcycles) a lot was possible. These exhausts he made were all done to create exactly what you mentioned: a sort of suction at the exhaustport to get the exhaustgasses out and the new fuel/air in.

Siraniko

iTrader: (13)

no wonder its quiet. Here in LA, I never seen a street driven P-Port with a 2" exhaust.