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Anti reversion designs - snake oil?

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#1 Greg Locock

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Posted 13 January 2008 - 00:01

I see various enthusiasts are touting the benefits of various features designed to suppress reversion waves. The only graph I found on-line indicates that putting restrictions into your exhaust pipe will have a fairly predictable effect - www.nrhsperformance.com (excellent site)

However, I suppose I could be convinced that the things he tested weren't really a very good example, I imagine that the 'proper' design would have taper one way, and a step the other.

So, do you think any of these geometries have an effect above and beyond what a good knowledge of conventional exhaust tuning would provide?

what about the other things on this search?




Why are they only shown on a certain style of motorbike - and one for which top end power is rather a secondary consideration?

Here's a fairly robust discussion



#2 Canuck

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Posted 13 January 2008 - 01:45

In my limited experience:

Long-stroke v-twin buyers, apart from being notorious herd-followers are of course notorious noise makers (can't go fast, might as well be loud). "Drag pipes" have a tremendous following and are nothing more than lengths of chrome tube with a flange on the end - no more, no less. The lengths of the pipes are of course determined by visual appeal and packaging 9 times out of 10 which means that while diameter is not generally outrageous (appearance garnered by heat sheilds and falsie-pipes not withstanding), the lengths are not tuned to anything at all. As you might expect the power where 99% of riders spend 99% of their time is utterly demolished (never mind hindered). I've seen as much as a 40% drop in power between 2000 and 4500rpm on an engine limited to 5500rpm. When I built my own bike, I had the fortuitious opportunity to beat the daylights out of it on a chassis dyno for several weeks (over 1600km) trying various combinations and products. While a set of "drag pipes" did indeed produce the highest peak power, they had the lowest average power.

So-called torque-cones shift the pipe's characteristics from 5000rpm+ down to a more rider-useable level when used in straight pipes, but are a really poor band-aid on poor pipe designs.

That said, my favourite pipe manufacturer (the ever bombastic Bob Behn of RB Racing ) does utilize an insert in his pipes that sit right at the head. Unlike the other products, these are venturis as opposed to a cone. The chassis dyno is misleading but the stopwatch wasn't - transient response with his pipes was considerably quicker.

But I've been wrong before...

#3 Greg Locock

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Posted 13 January 2008 - 11:30

Ha, he doesn't take prisoners does he?

OK, this is my picture. if you have a taper downstream, then the flow tumbles off the flat face at the end, and forms a back eddy, but the main stream can expand out into the wider section beyond.

However, the return pulse sees a flat plate and a turbulent cone forms inward from the lip, restricting the general flow through in the reverse direction.

Is that it?

My caution stems from many years of completely failing to predict airflows and so on.

#4 McGuire

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Posted 13 January 2008 - 13:14

That is the theory more or less, but I have dyno tested AR chambers twice and never found anything, even when doing an exhaust valve timing loop. I wouldn't call it snake oil exactly as the proponents seem quite sincere... but I think this is one of those deals that is slowly forgotten as dyno testing gets more precise and repeatable. Some people claim to find nothing on the dyno but that the engine is crisper and more responsive on the track. Which may mean something, or only that the engine sounds a bit different.

Aftermarket exhaust systems are sort of a commodity market (cars or bikes) especially at the bargain end. If you want some product differentiation beyond build quality, fit etc, you need some kind of trick or wrinkle to make people want to pick yours. Better invent something.

All that said, I have always found that a header primary tube just larger than the exhaust port exit works at least as well as going for a precise match. It does something but I have no idea if it is AR.

#5 Fat Boy

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Posted 13 January 2008 - 17:32

I have ran an 'A/R' pipe on a race engine several years ago. It was a completely different tuning idea than the pipe that preceded it, so it's difficult to draw many conclusions. This was my feelings after running it for a while and then moving away from it. This particular design had a bulbous piece about 3 inches off the head.

It did seem to have some effect in terms of reducing the tuning peaks of the engine. I'm assuming that this is the 'A/R' part of it. If you reduce the effect of the wave traveling back up the pipe, it'll have less of an effect good or bad. So in that respect, I suppose that it could be said that it smoothed out the curve so some extent. In the end, though, we were better off with a different design that, while stepped, did not have any particular 'A/R' design. It was 'peakier' than the 'A/R' pipe, mostly due to shorter primary lengths. Ultimately, the peakier pipe was better in terms of lap time. Keep in mind, the biggest difference at any point in the curve was maybe 3-4%.

In this case, the saber beat the broad sword.