Discussion
If the new trumpets are shorter, you'll lose torque but should gain some top end, assuming the valve gear is taught and light enough and the heads are capable of flowing the extra volume without choking.
It's a function of the vibro-acoustics of the plenum system. If you lengthen the inlet runner, the dynamic pressure wave caused by stagnation pressure from the inlet valve closing on the flow last time round bounces back up the runner, off the plenum top and back down the runner again, and that process takes longer the longer the runner since acoustic velocity is reasonably constant in there. Maximum VE is achieved by having the inlet valve open whilst that pressure wave shoves more air in. Cam and inlet timing is all about matching things that take a set number of crank degrees to occur with those that take a set number of milliseconds to happen.
My old GSX400T (bike) ran like crap without the air box, better with it on.
Consider plenum mods carefully...
You'll get it to work, but you might need a session on the rollers and to tweak other things so you can get the top end.
If your new trumpets are longer, no head flowing required AFAIK, but the potentially increased VE at low RPM would need more fuel. I'd stay away from just winding the reg up as that will give you more fuel all over the rev range, and you want to adjust the SHAPE of the fuel curve, not just move it up and down.
It's a function of the vibro-acoustics of the plenum system. If you lengthen the inlet runner, the dynamic pressure wave caused by stagnation pressure from the inlet valve closing on the flow last time round bounces back up the runner, off the plenum top and back down the runner again, and that process takes longer the longer the runner since acoustic velocity is reasonably constant in there. Maximum VE is achieved by having the inlet valve open whilst that pressure wave shoves more air in. Cam and inlet timing is all about matching things that take a set number of crank degrees to occur with those that take a set number of milliseconds to happen.
My old GSX400T (bike) ran like crap without the air box, better with it on.
Consider plenum mods carefully...
You'll get it to work, but you might need a session on the rollers and to tweak other things so you can get the top end.
If your new trumpets are longer, no head flowing required AFAIK, but the potentially increased VE at low RPM would need more fuel. I'd stay away from just winding the reg up as that will give you more fuel all over the rev range, and you want to adjust the SHAPE of the fuel curve, not just move it up and down.
Good old bike actually. At about 6kRPM it transitions from a plucky sort of bugger-bugger-bugger-bugger noise and starts howling all the way to 10k. 45bhp is plenty with those brakes and that suspension!!! It's kinda golf GTI fast off the line.
Before I replaced the ruined airbox-to-carb trumpet seals which were leaking like seives, 4-6k used to be a black hole of torqueless woe.
Before I replaced the ruined airbox-to-carb trumpet seals which were leaking like seives, 4-6k used to be a black hole of torqueless woe.
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