Regarding this issue of exhaust systems and their effects on the power curve there is a direct relationship on the torque curve.
The torque peak is moved to a higher rpm when an exhaust header with a larger I.D. is used, and the torque peak is moved to a lower rpm when an exhaust header with a smaller I.D. is used.
This effect is because the larger I.D. pipe moves the exhaust stream a little bit slower than the smaller I.D. pipe, and this has an effect on scavenging of the cylinder.
And the result is that the larger I.D. pipe is more effective at the higher rpms where the exhaust stream moves more quickly at those rpms, and the scavenging becomes more effective at a higher rpm range than before. It is capable of moving the exhaust out better at higher rpms, but is slower at lower rpms.
Conversely, the smaller I.D. pipe works to scavenge better at the lower rpm, giving more torque down low, but loads up and becomes more restrictive at the higher rpms.
Consequently, the larger I.D. size of the Hitchcock's pipe(after the reducer) moves the torque curve up the rpm range by some amount, and thus there is slightly lower torque at any given rpm than previously, UNTIL it gets to a sufficiently high rpm where it begins to work in its range of efficiency, and then it will outperform the smaller I.D. pipe at rpms higher than that rpm.
So, it's a matter of tuning effects that the pipe diameter has on the torque curve. It's not so much that it produces "less" torque in an ultimate sense, but that it produces this torque in a different rpm range, and it will feel somewhat weaker in the lower rpms, but somewhat stronger in the top rpms.
Scavenging effects are dependent on the speed of the exhaust stream, so the pipe diameter plays a part in determining what rpms the exhaust speeds get moving quickly enough to reach peak efficiency in scavenging.
If the pipe is too small, the power above torque peak will fall off and be weaker from restriction.
If the pipe is too big, the power below torque peak will fall off and be weaker from poor scavenging from slow exhaust speed.
The pipe size needs to be suitable for the intended use of the motorcycle, and since it will be one size, it needs to have some sort of compromise to suit all rpms, or it will impact ride-ability.
It is common that modifications, even slight ones, which are intended to improve upper rpm power, will have at least some minor negative effects on the lower or lowest rpm torque. This is because for power production purposes, it is advantageous to move the torque curve higher in the rpm range where it can be multiplied with higher rpms to produce a higher horsepower output.
Horsepower= Torque x RPM/5252
The higher rpm that we peak the torque, the more hp we make.
The potential down-side is that it might weaken the low rpms some.
Topic: Exhaust system and DynoJet Powercommander V (Read 11242 times)