| Indeed, the motor needs to be built (cams and head flowed, larger valves) and tuned to take advantage of 45-50mm systems, otherwise you'll actually loose power by using 45--50mm tubes on a standard motor in standard tune. It's best use 40mm tubes (o.e.m. Farne or spaghetti) on standard motor.
But do understand that the SP2, SP3 and SP4 have heathy cams, larger valves and motor state of tune to use 45mm headers, which is what they come with, either the original "Farne" style or slightly better spasghetti style.
Then we have the SP4S (SPS) and SP5 motors that can easily use 45mm and 50mm headers because they are at a high degree of tune (91 Corse' spec motor) to utilize these large diameters.
Imho.......the SPS/SP5 motors the 45mm headers will give less peak hp with more overall torque, whereas the 50mm headers will give more peak hp with less overall torque.
Therefore, I think the best exhaust for the SPS/SP5 is the 45mm Termignoni spaghetti system with Termi cans. |