| The 3 Blues Songs of Motorized Bicycle Land Expansion Chamber Blues Pros -An expansion chamber gives more top RPM power throughout a range of around 2000 RPM. -They can look cool with a professional installation. Cons -For about 1500 RPM before the last 2000 RPM the return wave from the baffle causes less power than if no expansion chamber was used. -They are loud as bloody hell even with a silencer. This is bad for the public image of MB riders and puts their legal status at risk. -You have to modify the header length till you have the peak speed you want. If the header is too long then you get less top speed than with the standard pipe. -If they are routed upwards then great care has to be taken in positioning so that you won’t burn your leg. A racer of the Maico 400 (w/piston port intake) motocross bike emailed me this: "Did you know the Maico motor runs really strong without an expansion chamber till about 5000 rpm. Maico even used a similar motor for enduro racing that had basically a straight pipe and muffler. When you add an expansion chamber you get much more torque above 5000 rpm and less below. Of course the expansion chamber robs some of that smooth torque from the low end (because of the pressure waves from the pipe being out of sync with the normal scavenging and trapping)". Before I designed my torque pipe I used a standard pipe with a header extension welded in which gave it more low RPM power. The only recommendation I can give you is to make my expansion chamber designed for these engines along with making my stealth silencer. Cylinder Head with Squish Band Blues Pros -They give more engine power due to increased compression ratio. -It takes longer for them to reach maximum temperature because there is more metal to heat up. -They look nice. Cons -They cause cylinder plating to flake off due to excessive combustion temperatures and increased combustion pressure.* -The presence of the squish band (which normally does not give the necessary .6-.8mm piston to head clearance) actually lowers the power potential. Squish bands only work as intended if that correct clearance is present. -They are much more expensive than buying a piece of glass and some sandpaper and applying some effort to grind down the stock head 1mm (more or less) for higher compression. -Excessive compression causes the early demise of the upper connecting rod bearing which has to bear all the excess combustion pressure. This can be countered by using an aftermarket CDI which delays (retards) the spark more than the stock ignition does. * one of the cylinder replating companies had this on their web site: “The finish in the early years was a simple chrome or hard chrome electro-plating. The plating was generally only a few thousands of an inch thick. If it was more than about ten thousands of an inch thick it was prone to chipping or flaking because of heat expansion and contraction.” US Chrome had on their site that the old style chrome plating can only withstand 400 degrees Celsius (752F). Also the excessive combustion pressure causes the rings to press harder against the cylinder wall which probably also contributes to the flaking. I had been recommending people increase the cranking compression up to 135psi but now I’m thinking that even that much may be too much given the common problem of the plating coming off. For high performance I recommend getting the cylinder replated at US Chrome and then you can use high compression safely. Boost Bottle Blues Boost Bottles were originally designed for 2 stroke engines with a reed valve and expansion chamber to offset the power loss in the RPM range when the pipes baffle wave is returning to the cylinder around BDC. This happens around 3500 RPM for an engine that revs to 9000. The bottles cause the fuel mixture to lean out drastically for almost a 2000 RPM band which can be put to good use because when the baffle wave is around BDC it causes the mixture to be too rich. So if you use a boost bottle with an engine with reed valve and expansion chamber you need first to know when the baffle wave is returning around the time the piston is at BDC. The boost bottle was not designed for an engine with piston port intake because there is no RPM range at which the mixture is too rich (if it's properly jetted). When put on these chinese engines for bicycles the idle jetting usually has to be richened by raising the needle one notch and the carb slide has to have JBWeld put under the entrance side of its bottom to lower that side by 2mm which richens the mixture till 1/4 throttle open. So it can be done but unless you are good at tuning a carburetor and unless the bottle is designed to have its resonant RPM around 2000 then it's not a good idea. I have yet to see a bottle that was large enough to tune to 2000 RPM. Click here to read more about boost bottles and to download my Boost Bottle Calculator. |