+1 to what the others are saying...
The lesson here, is that thumpers are make great torque over a wide power powerband... but not a high maximum HP...
Open pipes will maximize HP,.. but only by narrowing the powerband...
Multi-cylinder engines tend to make great HP, but over a narrow powerband.
This means they are faster,.. but they have to rev like crazy, and run at high RPM's to get that power.
The thumper,.. gets power over a wide RPM range.
It will pull strongly from idle, all the way up to redline.
Using an open pipe,.. kills that advantage...
It will work well in a multi-cylinder drag racer...
At a drag race, they will rev up to 7k RPM to launch,..(into the narrow powerband) and run fast,... but at idle, or low RPM's they have nothing...
Problem is.... a thumper reaches max RPM at 6500... the big piston and valves can't move that fast... so, they just don't make good rev'ers...
If you kill the torqueband in a thumper, you kill the advantage of it's design.
So,.. open pipes don't work well in big thumpers...
Thumpers don't make good drag racers.. but they are great in the twisty's...
It may sound sad,.. that a 650 thumper can't beat 650 in-line four... but, it's a different animal,..
While they are revving, and fanning the clutch madly to get power... we are just cranking out our throttle, making it look easy ...
It's like a turtle and hare thing...
... but I think of it more as a rhino a and hare...
...
If you open your pipe too much,.. the torqueband will become narrow, but you will never gain enough HP to make it worthwhile in a big single...