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People getting into motorized biking usually don’t know enough about 2 strokes to make informed choices concerning what to buy to get more power and speed out of their ride. The sellers should know and should make responsible decisions about what to promote and sell instead of taking everyone as fools and laughing all the way to the bank. This post is to inform readers and dealers about what is needed. I’ll start off with the two biggest headaches, cylinder heads and expansion chambers.
Expansion Chamber blues
These are the pros and cons of putting an aftermarket expansion chamber on your motored bike. (This list excludes my design of torque pipe which eliminates the majority of the cons)
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.
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.
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I’m not against the aftermarket parts market, I just want them to stop selling things that are inappropriate for motorized bicycles and start making and selling that which are appropriate. Here’s a list of things they should be selling:
1. an offset woodruff key that retards the stock ignition timing by 5 degrees for a boost in power throughout the RPM range.
2. A stock cylinder head lathed down a bit for increased compression.
3. Other designs of cylinder heads that don’t increase the compression more than 120psi.
4. Adaptors so people can easily put really good carburetors on such as the Dellorto PHBG 21mm and Mikuni VM18.
5. Good foam air filters for the Dellorto and Mikuni carbs.
6. Substitute motorcycle pistons/rings because the stock rings are too thick and the ring gap is too much which allows compression/power loss.
7. Stock or replated cylinders that have appropriate moderate porting increases for more power and RPM without losing low RPM power (which is needed for in town riding).
8. Stock crank assemblies that have two 3/8” holes drilled at the correct locations of the crank wheels for a correction of the vibration problem that all these engines experience.
9. Stock wrist pins that are drilled with larger holes for lighter weight to counteract the imbalance that causes vibration.
10. A stock exhaust pipe that has its baffle modified for better flow at high RPM.
If the existing dealers won’t offer these items then any of you readers are welcome to go into business. I can promote your products on my site (click my signature link). On my site are plans for making the torque pipe, how to drill the crank wheels, what dimensions the carb adaptors should have, how to modify the woodruff key, how to modify your head for more compression, substitute piston/rings, and how to port the cylinders.
Expansion Chamber blues
These are the pros and cons of putting an aftermarket expansion chamber on your motored bike. (This list excludes my design of torque pipe which eliminates the majority of the cons)
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.
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.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
I’m not against the aftermarket parts market, I just want them to stop selling things that are inappropriate for motorized bicycles and start making and selling that which are appropriate. Here’s a list of things they should be selling:
1. an offset woodruff key that retards the stock ignition timing by 5 degrees for a boost in power throughout the RPM range.
2. A stock cylinder head lathed down a bit for increased compression.
3. Other designs of cylinder heads that don’t increase the compression more than 120psi.
4. Adaptors so people can easily put really good carburetors on such as the Dellorto PHBG 21mm and Mikuni VM18.
5. Good foam air filters for the Dellorto and Mikuni carbs.
6. Substitute motorcycle pistons/rings because the stock rings are too thick and the ring gap is too much which allows compression/power loss.
7. Stock or replated cylinders that have appropriate moderate porting increases for more power and RPM without losing low RPM power (which is needed for in town riding).
8. Stock crank assemblies that have two 3/8” holes drilled at the correct locations of the crank wheels for a correction of the vibration problem that all these engines experience.
9. Stock wrist pins that are drilled with larger holes for lighter weight to counteract the imbalance that causes vibration.
10. A stock exhaust pipe that has its baffle modified for better flow at high RPM.
If the existing dealers won’t offer these items then any of you readers are welcome to go into business. I can promote your products on my site (click my signature link). On my site are plans for making the torque pipe, how to drill the crank wheels, what dimensions the carb adaptors should have, how to modify the woodruff key, how to modify your head for more compression, substitute piston/rings, and how to port the cylinders.
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