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Discussion Starter · #1 ·
Most riders here have a decently engineered bike already and mostly need to work on their fitness and riding ability. But a few people want to fine tune their ride for optimal performance because they either have good riding skills or just like to drag race others and win. Bikes are put together with manufacturing tolerances that guarantee few engine failures but also guarantee that the power is not optimal.

For instance there is the squish band. Racing teams adjust the head design for different squish velocities till they find one that gives the best power. Too much though can cause detonation. Using a squish velocity calculator they know how much to lathe off the head to acheive a certain increase in squish velocity. Without the program they would just be guessing.

If you are changing a race bike into a trail bike, or a trail bike into a race bike then you need a carb size calculator to give you an idea of what size to use. The size changes the air velocity, and that changes the fuel atomization which affects the burn speed. So a large carb delays the finest atomization till high RPM, and a small carb achieves the same atomization at a lower RPM which is better for street or trail riding.

With that same conversion you also need to consider the engine porting which determines at what RPM range is the max power emphasis. The higher the ports, the higher the RPM of power emphasis. Changing the porting only has a minor effect on the peak RPM which is mostly set by the pipe. So a trail bike needs lower ports than a racing bike. To change a race bike to be better on the trails you can either lower all the ports by lathing metal off the cylinder base or carefully use JBWeld at the transfer roofs. (Don't give me grief over this suggestion because Eric Gorr has been doing this for decades and I too have done it many times.) The trouble with lathing the base is that it might mess with your power valve actuator. It also requires machining your head to keep the same head/piston clearance. The advantage to using a porting calculator is that it also helps you determine the best height difference between the exhaust and transfers for the optimal blowdown at your top RPM.

If you want to raise the bikes top RPM then you won't be guessing at how much to cut off of the pipe header if you use a pipe calculator. Same goes for if you want to lower the top RPM and need to lengthen the pipe header. If you want to cut the exhaust noise in half then you can use the calculator to determine the correct larger diameter of the stinger when you elongate it enough to reach all the way into the pipe to start at the belly area. Jennings wrote about this decades ago and he said it didn't reduce power any.

And if you want to eliminate that annoying drop in power just before getting "on the pipe" you can use a boost bottle (Yamaha YEIS) calculator to design your own one that has a center resonance 500-700 RPM before getting on the pipe. Or you can reduce the baffle wave strength by drilling dozens of holes in it and enclosing that with more sheet metal. Blair discussed this also in one of his papers. I've tried it and it works. On the karting engine he did it on the over-rev was greatly extended which is perfect for karts. It does reduce the peak power a bit though.

If you change your piston to a heavier or lighter one then you may throw the crank balance off enough that you'll need a crank balance calculator to know how much bigger to drill the counter balance holes or how much to fill them in. No one likes too much handlebar vibrations or how a vibrating engine tends to loosen up nuts and bolts.

Some calculators are complex and some are super easy. The website should convey how easy or complex it is to use. If it's a spreadsheet then you'll need Excel on your computer. Computer programs remove most of the guesswork and will prevent you from making costly mistakes when you modify your engine.
 

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Discussion Starter · #2 ·
I forgot to mention that there's a really good jetting calculator on the market that can help you have electric-like power response from bottom to top. It helped me figure out that the stock needle in my AX100 was too short and so I modified the slide to let the needle sit lower which solved the problem.
 
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