Boost Bottle's -Do They Work?

[Matt-]

Just been looking on you tube and i have noticed that some people have boost bottles on their 2 stroke's like RS125'S etc?

How exactly do they work & are they worth the £15.00 price tag ?

All the best

Matt
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[The Shaggy D.A.]

I would have thought that if they worked and only cost £15 retail, manufacturers would be putting them on as standard.
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[MarJay]

No.

They have some effect on two stroke twins, as they balance the air fuel charge between cylinders in a slightly more efficient way than a balance tube, but only on certain tuned bikes, and only if they have a balance tube as standard (like an RD350 YPVS).
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[Matt-]

[N cee thirty]

Yamama RXS 100 has the yeis system as standard

https://au.answers.yahoo.com/question/index?qid=20120308175317AAdCQ7B
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[Teflon-Mike]

Piston ported two strokes, work on the wheeja principle.
In theory they ought NOT work at all.
Think about it, piston rises, and opens hole in the cylinder wall, sucking fuel & air through the carb... but no valves, as piston starts to fall again, the hole in the cylinder wall will still be open...
So whatever got sucked in..... OUGHT to be blown back out!
ONLY thing that makes a piston ported two stroke work, is gas inertia; momentum of gas heading 'in' means it doesn't really want to turn around and come back out, and in the 'lag' of it trying to turn around... you manage to trap a bit before it does!
Other thing is 'reverse flow restriction', basically, long tapering inlet tract, and sharp edged port window at the end, flows easily on the way 'in', but once in the pot, to get back out, it gets 'jammed' in the narrow port opening, and has harder time getting back out than it did getting in, and you can trap a bit 'more' charge in the lag period between it flowing the wrong way!

So the piston ported two strokes, could not use big ports, or there would be no reverse flow restriction; and they couldn't use very wild port timing, limiting how much they could be tuned for perfomance.

Idea behind reed or disc valves is that putting a valve in the inlet tract... once you got charge in the crank-case.... valve stops it getting back out.

This little innovation almost doubled the trapping efficiency of the two stroke, and the power they made in one go..... BUT also aliviating the need to 'port' the engine for reverse flow restriction, allowed bigger ports and more 'agressive' port timing for even more power.

HOWEVER.... the reed valve two-stroke still has 'some' of the properties of the piston ported motor.

Reed valve only opens, or shuts when there is significant pressure difference between the inlet manifold and the crank case, and itself has 'lag' in responding to the change.

Now, on the piston ported two-stroke, one of the problems of setting up the carburation was 'tripple charging'.

As air was sucked into the engine, it would draw fuel up the mixture jets... but, as a proportion of that charge was blown BACK out the engine, it could pass back through the carburettor, and suck even more fuel into the air stream.... then if it hadn't escaped to atmosphere from the carburettor mouth, becouse of a long bell-mouth inlet, air-box or filter.... it double 'charged' air would be sucked back through the carb a third time, picking up even more fuel..... Made carburettor setting rather tricky.

With reed-valve lag, and much shorter inlet tracts modern two-strokes can suffer similar issues......

BUT, if you have an 'expansion bottle' on the inlet manifold, increasing manifold volume, without necesserily increasing manifold length......

Reverse flow can be 'cought' in the expansion bottle, and NOT reverse flow and tripple charge through the carb....

Means carburettor metering can be a lot more accurately tuned...

Also means that the reeds can be bigger and slightly less 'responsive' without loosing benefit.

So WAS a common tuning aid on early reed-valve motors, and Yamaha & I think Suzuki both used it on production bikes at some point.

But on more modern machines with more advanced reed-valves, and probably more 'evolved' inlet tracts.... benefits are possibly more marginal, or the 'principle' is incorporated into the reed-block / inlet port architecture without the need of external volume chamber....
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[stevo as b4]

Sounds logical enough TM, but i'm thinking that by the time of todays latest developments in reedvalve tchnology such as the V-force 3 reedblock for example that the negative effects on carburation from the action of the reedvalve could be totally eliminated potentially?

Also the other aspect of what the manufacturer fits as std to a bike, is probabaly nearly always about cost over performance, and while they might have experimented with then dropped a small plastic chamber and a bit of hose, they would have probably had better ways to control the flow of air and fueling by then, so airbox design was probably developed further as an alternative approach? I know none of the Jap manufacturer's have ever fitted the V-force reed system to thier bikes as std, but alot of modern/current European trials and enduro bikes have adopted this type of reedvalve system, so cost aside it much offer some real advantages?

[keggyhander]

I've had two-strokes, and loved them, but I'm buggered if I can get my head round them.

[Matt-]

Thanks Mike

big help and nice explanation!
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RIP jack (wheelie king), you will never be forgotten.
Current Bike's - Road Legal TZR 125 Pitbike, Aprilia SR50R 2010

[Charlie]

[ruck bodgers]

i was told they need to be able to expand and compress so have a balloon inside . ..? johnny on a tube to suffice.

maybe a weak spring loaded cylinder thing you could knock up .

lol check this out .

https://www.youtube.com/watch?v=y80MOk3IpSs

[WannaBeDude]

Tldr version

But on more modern machines with more advanced reed-valves, and probably more 'evolved' inlet tracts.... benefits are possibly more marginal, or the 'principle' is incorporated into the reed-block / inlet port architecture without the need of external volume chamber