Boost bottle testing

[Deleted member 12676]

There has been controversy on this subject and being open minded and wanting to explore every possibility for power gain I decided to research it myself.
Looking for more mid-range power I installed a home-made boost bottle made of PVC. Area was 86cc (3.14 x L x radius squared) which was more than the believed-to-be-ideal of making it the same size of your combustion area (60cc). Reading about it some people said it was good for low end power if size-tuned for the rpm range you want. Others said it didn't work. I suppose they utilized too small an inner hose diameter because the original research from Yamaha said it needed to be the same diameter as the carb. My first one had 4.8mm (3/16") as smallest passageway diameter. It made the idle stronger, no noticable difference in power, made the top speed 2mph less. So then I tripled the passageway area by increasing the inner diameter to 8.6mm (11/32") and I didn't notice any power difference but it seemed to be too lean at idle, taking a long time to settle to a low rpm after blipping the throttle. Theoretically that made sense because the bottle prevents as much back flow thru the carb, thereby not letting it enrichen the mixture as much. So I would need to go up two numbers on the Dellorto jet. From that experience and reading the stories of others I believe it might make a small enough difference when climbing a street at low rpm. Unfortunately I now live where everything is flat and so if I don't feel increased acceleration then I assume it doesn't make a big enough difference. Maybe if I was back in Cuenca where the streets are hilly I could correctly test it.
If you live where there's inclined streets then please throw in your two cents worth if you did a before and after boost bottle test.

 

[Deleted member 12676]

Here is a picture of the setup:

 

[Deleted member 12676]

Since this leans out the low rpm mixture it is not recommended unless you have a carb with a low speed jet.

Tomorrow I will retest the setup with a total volume of 60cc that matches my engine. Without the bottle it is running a bit rich at low speed so this should be a good match. I increased the boost port a bit too much and lost some needed umph before 20mph. I hope to get it back with the boost bottle.

 

[Deleted member 12676]

Final analysis: It helps a little but the way it screws with idle makes it not worth it. Even when jetted just right it still tends to idle too high till it slowly settles down.

 

[Skyliner70cc]

My experience was similar but the leaning of idle was a good thing at my altitude (over 1 mile asl) However, once a high comp head and tuned pipe were installed, low end became too lean. No benefit top speed or power.

 

[Deleted member 12676]

Turns out the extra intake crank pressure caused one of the crank seals to mal-form and cause crank pressure loss. I fixed it and tried it again today and the idling problem was very much minimized. It does give a noticeable boost to low rpm running. You can especially feel the difference when taking off from a stop. So if it weren't for the idling problem I would consider recommending it for street use. I may make a bigger one with the idea that the bigger it is the more it harmonizes with low rpm. If its set to harmonize at idle speed then maybe it won't have the idling instability.

 

[misel]

Turns out the extra intake crank pressure caused one of the crank seals to mal-form and cause crank pressure loss. I fixed it and tried it again today and the idling problem was very much minimized. It does give a noticeable boost to low rpm running. You can especially feel the difference when taking off from a stop. So if it weren't for the idling problem I would consider recommending it for street use. I may make a bigger one with the idea that the bigger it is the more it harmonizes with low rpm. If its set to harmonize at idle speed then maybe it won't have the idling instability.

Thanks for the update, Jag.

 

[trydrew]

Jag, any thoughts on how boost bottles might have different effects on a reed motor as opposed to a piston port?

 

[Deleted member 12676]

Since a reed valve engine has 3 times the intake duration and more intake restriction it sucks in the air/fuel mixture with less velocity and less abruptness. Therefore I suspect the effects of a boost bottle would be less advantageous for a reed engine than a piston port intake engine.