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Much controversy swirls throughout the internet on boost bottles. So I decided to do some research on these so called performance parts.

First I must state that you can believe what you want in regards to this article. This is written from data gained from legitimate sources and not lame ebay dealers trying to hawk something for a penny and charging $30 for shipping.

What exactly is a boost bottle anyways? Simply put it is a bottle with a hose that connects to the intake.

What are they for? Sellers will tell you they will help you make a ton of horsepower just by screwing one of their premade bottles onto your bike and drilling a hole into your intake. How much is a ton of horsepower? I've seen claims up to 5HP gains.

What do they do? If you read my article on how a two stroke works then you will understand that in a two stroke (reed type), there are no valves to open or close. Essentially at some point in the pistons motion there is a completely open system. Straight through from carb to pipe with no resistance.

Of course since you read my article on two stroke pipes you would also know that a pipe on a two stoke is designed to push the air/fuel charge (charge from now on) back into the head to be burnt.

A boost bottle essentially works on the other end. When the piston comes down it creates a force which pushes the chage outwards in all directions. On one end is the pipe, on the other end the carb. This creates essentially a pulse against the back of the reeds.

A boost bottle works by simply sitting there. When the cylinder goes up and air is pulled in, there is a vacuum at the bottom of the hose for the boost bottle. This causes the bottle to basically empty itself. When the cylinder goes down (without a bottle) the charge pushes against the back of the reeds and cannot go anywhere else. With the boost bottle this charge is able to enter into the bottle as it is essentially empty. Then it empties itself on the next motion of the piston.

Now you basically understand the theory of how it works. So who came up with the idea of a boost bottle and what were they smoking? Well, believe it or not it was Yamaha. You can take a look at Modification of Two-Stroke Engine Intake System for Improvements of Fuel Consumption and Performance through the Yamaha Energy Induction System' by Noriyuki Hata, Takeo Fujita and Noritaki Matsuo, SAE Paper No. 810923 (Sept. 1981).

The system is called the Yamaha Energy Induction System or YEIS. So now you're thinking this isn't just junk science? You'd be right.

So where do the issues come in and why don't they work? Because no one that makes them knows how to make them apparently. The boost bottle has to be at least as big as the cylinder itself, and the hose has to be at least as big as the throttle opening at the intended RPM you want the bottle to work at.

Take a look at some of the boost bottles out there and you'll see hoses so small an ant would suffocate if it tried to breathe through it.

So should we break out some math? I know you guys just love the math I throw at you.

WOW! If you're lost now, I can't blame you. But as you can see the math shows that any slight change will affect how your boost bottle works. So any old boost bottle off the shelf probably won't be right for your bike, bike setup or application.

I must also note that these boost bottles only really make a difference at the lower end of the RPM range. They could be designed to help at top RPM ranges but they would have to be HUGE.

Now that that's all settled, do boost bottles really do anything? Of course they do, otherwise Yamaha wouldn't put them on their production bikes. But the difference here is that while a boost bottle may give you a slight increase in performance, are you really going to notice that on your 2.5HP cag? Not really, but you might experience some improvement in throttle response at the tuned RPM range (as with the math above).

And to make life a little easier, I've attached an Excel spreadsheet you can use to do the calculations on your own boost bottle.

This article was written by Bruce Peresky for pocketbikeforum.com. Copying is permitted provided you give credit to http://www.pocketbikeforum.com

Attached Files

Boost Bottle Calculator.xls (13.5 KB, 33 views)

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