I see many people talking about the SP motor, the thing I dont understand is the ports! from what I have seen the SP has a very small exhaust port where the RR has one massive port.
as for my little RR there has been massive work done flowing the ports and I do mean a lot of work!! I spent the best part of a day in total working on those ports. dont get me wrong it was not a "grind it out to the max" it was a technical job with many an hour spent designing the flow.
as for the hp number that people quote, you have to remember that they are running filtration on the carb this in turn gives restriction. My unit is direct unrestricted airflow with the addition of a trumpet that helps to increase the flow. A lot of people run the common pod filter, this again is not good, if you look to the inside of a pod filter they have very sharp 90 deg corners this will mess up airflow for sure!
when looking at the possible power the easy way to gauge things is look at the fuel flow, this can be done by looking at the size of the main jet, as it will not be possible to hit the high numbers if the fuel flow is anywhere close to standard.
To get anything close to the correct size jet I had to increase the diameter of the main jet by 50% this is well outside what people are doing for jetting. the power of an engine cannot exceed the energy of the fuel supplied, in this case I had to supply it stupid amounts of fuel to get somewhere close.
To give you an example
a 100mm pipe has a flow rate of 1,185 liters per min
a 150mm pipe has a flow rate of 2,650 liters per min
50% increase gives over double the flow rate.
Double the fuel is double the energy, I understand you are not going to get double the power as things dont work that way.
lets say that my 100% increase of fuel resulted in guess of about 40% increase of power........