Expansion Chamber

eDJ · #61 (**permalink**) 08-12-2008, 03:53 PM

Check out some of the motorcycle dealers. Often they sell aftermarket performance parts and accessories including expansion chambers and silencers for their line of dirt bikes. I've bought bags of silencer packing at those places.
One friend of mine bought a damaged glass pack muffler at a muffler shop and cut it open to get the fiber glass packing out of it. I think he gave $5 bucks for it and he got all kinds of packing out of it.

ran49 · #62 (**permalink**) 08-12-2008, 06:44 PM

Well I finally got around to installing a pocket bike expansion chamber exhaust pipe similar to what Ghost0 suggested on my Dax 70cc and I have to say this is without a doubt the the largest performance boost I've gotten from any of the mods.I've done.Thanks Ghost0.
I just switched from a 44 tooth sprocket to a 40 tooth and was thinking about switching back due to the loss of low end torque(ridiculously heavy pimped bike)
Well now it seems I've got it all with the 40 tooth,at least 1 mph higher top end with expansion chamber,but a large hill that would bog me down to 11mph I now climb at 18mph.I highly recommend this mod,just wish I'd done it sooner.Ron
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Ghost0 · #63 (**permalink**) 08-12-2008, 11:15 PM

ran49,
Nice job on the install but if I can make a suggestion? Shorten your header. It appears that you have enough room to raise your pipe without hitting the frame, pedals etc. I find the optimum distance for that pipe on a 70cc is about 12 inches from the exhaust port to the beginning of the expansion chamber. You won't lose much bottom end but you will gain a ton of top end.

ran49 · #64 (**permalink**) 08-13-2008, 07:58 AM

Ghost0:
All along I was thinking that silicone tubing had the added benefit of being adjustable but I couldn't find any around here.I'm at about 13" now.I'll cut an inch out and give it a try.I silver brazed the pipe together thinking it wouldn't hold up too the heat but so far so good.Anyway it will be easy to pull apart.Thanks again,Ron

Weedylot · #65 (**permalink**) 08-13-2008, 11:39 AM

Good animation on the expansion chamber. I've tried to explain its benefits in the past by describing it as "scavenging exhaust pressure" which is a poor description. I've found some sources (Delottoro and Morini) for pipes and chambers for 50cc motors on some of the moped sites.

jasonh · #66 (**permalink**) 08-14-2008, 12:00 PM

So would an expansion chamber have to be really rounded like most of these commercial ones, or could they be very square and angular? Seems like if they can be angular, it wouldn't be hard to fab one up with access to something to cut and weld metal. (this is in regards to the design program previously posted)

filestyle1 · #67 (**permalink**) 08-14-2008, 03:23 PM

you would want the exhaust to flow smoothly and with sq. or angular you would get disturbance of flow within the pipe.

eDJ · #68 (**permalink**) 08-15-2008, 12:49 AM

Somewhere back in this thread I believe someone was asking about the amount of fuel that was wasted by a 2 cycle engine equipped with a muffler. I found some information in a California business publication of the California EPA Air Resources Board today.

It seems about 30% of each scavenging "cycle" is lost with a muffler equipped 2 cycle engine. I don't have figures yet for how much is saved when using an expansion chamber but certainly it is much better than 30%. Clearly the
expansion chamber compresses much of what would be wasted back into the combustion chamber.

You can see this information used in the report here:

Business Information: Clean Air Two-Stroke for Utility Engine Applications Demonstration, BKM

But this information may be general enough for these Chinese engines in the kits presently available.

MrLarkins · #69 (**permalink**) 08-16-2008, 08:30 AM

where can i get a pocket bike expansion chamber? should i look around at local cycle shops? are they most likely to carry them?

would something like this (47cc 49cc Pocket Bike HP PERFORMANCE Exhaust Pipe part - eBay (item 150160565919 end time Sep-05-08 09:13:10 PDT)) work?
or is that one too small?

Pablo · #70 (**permalink**) 08-16-2008, 08:43 AM

That is essentially the one I have.

You can make it fit. We have a kit in the (slow) works.