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Old 02-03-2017, 06:06 PM   #1
motoman354
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Default Boost bottles, Help or Hype for your motor?

There has been some discussion lately in the chat room on boost bottles. I feel the first thing is to explore what a bottle does and how it works. 1st thing is if you think you are going to get some huge horse power or torque boost you are going to be very disappointed. The bottle only affects the 1st quarter of throttle. Above that you will notice nothing. All you will notice is better throttle response just off the bottom.
So how does the bottle work? It is really brilliantly simple. It acts as a surge tank for the intake track. Every time the reeds close (or the piston in a piston port) the flow through the carb stops (in fact it can actually reverse) When the reeds reopen it takes a moment for the flow to regain full speed. By adding the boost bottle you gain a place to store low pressure during intake. When the reeds close this low pressure keeps the flow moving through the carb. The result is smoother carburation at low RPM.
Big bores respond better than tiddlers. This is for 2 reasons. One is the stronger signal due to the volume of the big bore. Second is because the smaller the motor the less you ride at very low RPM. Also the larger ports and longer timing of an MX will be helped more than an Enduro that is tuned for bottom end torque.
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Old 02-03-2017, 06:52 PM   #2
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Nice write up. Thanks for clearing some confusion on the matter.
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Old 02-04-2017, 09:38 AM   #3
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First I have to say that I'm not disagreeing that big bores get more benefit... or that on some bikes the "boost bottle" might only be effective at lower rpm/throttle.

My 1988 DT50's have the factory Yamaha YEIS... aka "boost bottle"... little 10,000 rpm screamers... maybe 7 HP? I tried capping off the boost bottle (I had one that was cracked/leaking) and the performance was noticeably reduced on this engine that doesn't have much HP at all under 5,000 rpm and that I pretty much ride at wide open throttle all the time.

(the DT50 felt "boggy" without the boost bottle even at high rpm.)

My understanding is the volume of the bottle and hose is critical and that small changes can make big differences in how the bottle performs.

The YEIS fell out of favor when Yamaha went to the "power valve" variable exhaust timing (YPVS). My understanding is that the exhaust pulses have quite an effect on the intake pulses on a two stroke and the exhaust power valves rendered the "boost bottle" technology ineffective.
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Old 02-04-2017, 09:58 AM   #4
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Quote:
Originally Posted by MarkT View Post
First I have to say that I'm not disagreeing that big bores get more benefit... or that on some bikes the "boost bottle" might only be effective at lower rpm/throttle.

My 1988 DT50's have the factory Yamaha YEIS... aka "boost bottle"... little 10,000 rpm screamers... maybe 7 HP? I tried capping off the boost bottle (I had one that was cracked/leaking) and the performance was noticeably reduced on this engine that doesn't have much HP at all under 5,000 rpm and that I pretty much ride at wide open throttle all the time.

(the DT50 felt "boggy" without the boost bottle even at high rpm.)

My understanding is the volume of the bottle and hose is critical and that small changes can make big differences in how the bottle performs.

The YEIS fell out of favor when Yamaha went to the "power valve" variable exhaust timing (YPVS). My understanding is that the exhaust pulses have quite an effect on the intake pulses on a two stroke and the exhaust power valves rendered the "boost bottle" technology ineffective.
I would not say you were wrong. May remarks were aimed at people considering adding a boost bottle to a bike that never had one. Bikes that Yamaha designed them into can behave differently. I have read that the Blaster quad can be horrible to jet without its bottle. In my experience I built my 81 YZ175 using an 83 IT175 top end. The IT was designed with a bottle. No matter what I did it was blubbery on the bottom. I added a bottle and it became nice and crisp off the bottom with no jetting change. I believe power valves killed the bottle because they aide power in the same range. The bottle became redundant and so by eliminating it they save on money and complexity of the machine.
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Old 02-05-2017, 10:07 AM   #5
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Here's some info from someone who seems to have spent a lot of effort to understand the Yamaha YEIS patent:

http://www.dragonfly75.com/motorbike/YEIS.html

One inconsistency is that the author says the YEIS is not for high rpm racing engines yet he says Yamaha introduced it on the YZ models in 1980.

A significant "torque trough" happens at pretty high rpm on bikes like the '77 YZ125D... right about 7000 rpm... so the YEIS on a later YZ125 was likely tuned for 7000+ rpm to improve the power from 6000+ to 8000+ rpm. (Like the IT200 mentioned in the article that was tuned for 6400 rpm)

My conclusion is the YEIS will help "boost" power in the "torque trough" area of the power curve... and if the trough occurs at high rpm, the YEIS will work at high rpm.

The documentation shows it works best at 1/4 throttle opening... what's not clear is how big the carb was for the engine used in the testing? If Yamaha wrote the paper based on YZ data they might have been using a pretty large carb for the engine... maybe even a 38mm on a 125? It stands to reason that the same engine with a small carb might experience a positive effect from the YEIS over a larger range of throttle openings?

At least that seems to be the case on my high revving DT50 with a tiny VM16 carb.
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Old 02-05-2017, 11:38 AM   #6
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Very interesting. I only found a couple weird things. One is that he seems to call for larger hoses than Yamaha uses in relation to carb. size. I used 19/32" PCV hose to match to the IT200 manifold I used. The bike uses a 34mm carb. According to him Yamaha should have used 3/4" hose. As far as why it was used on the YZ series. It was only used on the YZ250 in 1981. I believe this is because the 465 didn't really need it and you don't ride the 125 where it would have helped. It worked very well on the IT series and got added to many bikes more for marketing than the improvement it gave. Basically they do work, but they are one of the last mods you would do to refine your power band. Not the first if you are looking for power gains.
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Old 02-05-2017, 04:32 PM   #7
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I think I understand now, why some people run these on vintage trials bikes. 75-90% of trials is between idle and 1/4 throttle; fine tuning low end power and perfecting crisp throttle responses at part throttle is make or break. However after reading that article MarkT posted, makes me wonder how many of these homemade bottles I've seen in use are actually tuned correctly.

Thanks for posting this up here and explaining the finer points guys.
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Old 02-07-2017, 04:08 PM   #8
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Those bottles do look purty trick anyhow. I am guessing they work best when velocity is low through the carb and help clear up some hesitation, especially on overly carbed engines at part throttle openings. Also, the boyesens reeds may work best on such engines, but for all out power bikes may lose horsepower over standard. For DT's the improvement may be moddest to none due to the small carb and high velocities. Just my thoughts.
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