Boost Bottles?

[tz375]

As John A says, go ahead and try it and see the effect on your bike. I just noticed a typo in my last response. What I was trying to explain is that if the thing works as we have rationalized, then all he has done is to change the jetting to match the new situation and when the tube is pinched, the jetting is no longer correct and at idle that's enough to shut it off.

Chuck Quinzler (sp?) has done some experimentation on Yamaha twins and I'm sure his tests concluded that there's some effect under certain circumstances but IIRC there was a net nil-nil effect most of the time.

We tend to think of motors as being a relatively static state so that at a certain RPM there is one intake suction wave and one exhaust pulse, but in reality the wave in the inlet will oscillate 4-6 times during one cycle and after the piston closes the port it continues to pulse like a string waving up and down. It's those second and third and 4th order waves that make 2 strokes work and changing any individual variable will potentially have an effect.

The theory back in the day was that energy was stored in the bottle that could boost the next intake stroke, but that can only happen if that pressure is timed to match the port open timing. Imagine a positive pressure wave traveling up to the bottle as the port closes and traveling back down to fill the port next time it opens. Good news is that a pressure wave reflected off a solid plane returns as the same sign, so positive up is positive on the way back. Let's say we get the length right for idle so that the wave comes back as the intake is open and that it is not so strong that it pushes back out the carb. So far so good.

Now increase the revs to say 2,000. Now that positive wave is arriving later - maybe while the port is fully open. Not too bad. Increase revs to say 4,000. Now the pulse is arriving after the port closes so it surges out through the carb causing the reversion that we were trying to avoid. So if it works at one RPM range it will hurt at most other revs. Kind of like an expansion chamber but with simpler pulse dynamics.

So I'd expect that if you had a motor with very long intake timing that had a lumpy idle, it might be possible to smooth that bottom end out. Let's say it adds 2 hp at the bottom where it counts and loses 2 hp everywhere else. 2 hp loss at 6,000 is probably not going to be missed but the bottom end would feel much better and the bike would be easier to ride.

That leads me to speculate that if a particular motor had a bad flat spot at certain revs with wave interaction - maybe a boost bottle might change the wave dynamics at that spot and smooth things out as long as it was the correct shape and size for that particular motor at those rpms.

And any wave action in that channel will tend to reduce the energy in the main intake wave, so there could be other effects if the change were large enough which it probably isn't. ll good fun to explore and think about. Did the late John Robinson talk about the Yamaha boost bottles in his book on 2 strokes? There's a mention in one of those books. I don't recall which though.

As Ja-Moo mentioned, there are better ways to change intake wave dynamics that that and most people have ceased using them. There was a test of one on a single in M/C Maniacs / Performing Berks some years ago and on that bike there was no measurable change.

[pull the wire]

My only experience with boost bottles were on my stock RZ350. When I upgraded the reed valves and carbs, I had to delete the boost crossover pipe, not a bottle. The concept seems plausible on a twin and the boost bottle shared with both cylinders. I would believe the low end rpm drivability would be greatly improved since the potential air fuel charge is equalized/shared between the 2 cylinders. At high rpm the boost bottle can not store the energy needed since things are moving so fast. I have never seen a boost bottle used on a piston port engine.