I am currently working on a 1.6L hayabusa and would value your opinion whether to use 4 or 8 injectors?
The throttle body we designed for the Gen1 engine produced superior torque and power over the standard throttle bodies and during its design, we specifically worked on testing it against 8 injectors on the dyno. The end result meant that there was no advantage in running 8 injectors.
The Gen2 engine is produced with 8 injector set up as standard. We have tested the design on both 1000cc engines and the larger capacity Hayabusa versions. The standard throttle body set up with 8 injectors only increases performance when the upper injectors are used above 7000rpm and gives an increase of approximately 5bhp. The reason I believe the manufacturers have gone for 8 injectors is to allow them to move the lower set closer to the engine for emissions and then the upper set in an attempt to regain the performance that would be lost in doing so. When tested against the twin throttle body set up we have designed to fit the later spacing of the inlet ports of the Gen2 engine, we found we had a gain in performance everywhere over the standard Hayabusa throttle bodies.
So provided you are able to replace the standard throttle bodies with our throttle bodies, they will always give an improvement.
I have got a Hayabusa 2004 Gen 1 engine with nothing special on it at the moment. I am led to believe that running the 4 short airhorns is a better option, but please correct me if I am wrong.
The use of a combination of air horn lengths has a tendency to smooth the torque curve, the longer air horns promote lower to mid-range torque and the shorter air horns improve the upper mid-range torque to peak bhp. If your engine is solely being used at higher rpm on the standard 1299cc engine, then potentially 4 short air horns will be the preferred option. If you are using a long stroke engine in the future, then 4 longer air horns would be the best option for that engine because the engine does not rev as high.