This was one of those strings you pull on that turns out to be connected to a pretty big ball of yarn. I went looking for answers on this topic and ended up spending an hour or two reading about it each night, for almost two weeks! (Yes, I do have a lot of spare time haha) No big surprise here but the hotrod and race car guys have engine building down to a science and there's not much they haven't tried, dynoed, and written up.
I figured I'd post what I found out in case anyone else was curious about this. In short the answer I found is: there are several ways you can move your torque peak around to different RPMs, and you probably don't want to.
The first and most important thing i found out:
Rule 1. You Can't Get More Torque
Fill a 650cc can with evaporated gas and light it off and you will get a certain size explosion. Because you don't always get the can full right to the brim there will be some small variation in how big a bang you get, maybe 10%, but there's nothing you can do to make a *sigificantly* bigger bang from that same 650cc can. Think of a firecracker and a stick of dynamite. The firecracker will never bang anything like the stick of dynamite no matter how well made the firecracker is.
What's the point of this? The force you get from exploding that 650cc of air+gas is your torque. "Peak Torque" is the bang you get when the can is filled the most. You won't get significantly more torque than this unless you increase the displacement of your engine. (bigger can = bigger explosion = more torque). Bore and stroke ratio can't change this, revs don't matter, firing order doesn't matter, the length of your rod doesn't matter, only the size of the can. Again when I say "don't matter" I mean they don't make a *significant* difference. You can make a science of playing around with all the little variables and end up with 10% more peak torque but if you were hoping for more like a 50%(20 ft-lb) increase you need a completely different engine with a lot more displacement.
Have a look at this:
Code:
model Peak Torque
---------------------------
Yamaha XS650 40 ft-lbs
'01 BMW F650 39 ft-lbs
'08 BMW F650 44 ft-lbs
Suzuki DR650 39 ft-lbs
Suzuki SV650 47 ft-lbs
Yamaha R6 47 ft-lbs
Honda NC700 47 ft-lbs
Ducati 696 50 ft-lbs
That spans everything from single cylinder low rpm thumpers, to parallel twins, to v-twins, to inline 4 screamers. Even a couple 700s thrown in. They all make roughly 40 to 47 ft-lbs of torque because they're all exploding roughly the same size can. The super high tech R6 has massive valve area and all the science you can buy so it manages to make about 15% more torque than the XS650 does despite being 50cc smaller. That's impressive in it's way, but it also tells me that unless you redesigned every single part of the XS650 until it was just as high tech as an R6 you won't get more that 47 ft-lbs of torque out of it. BMW evolved their F650 over 7 years and managed to increase torque by just 12%. And they're engine geniuses, I'm not
So the engine in a full on 300Km/Hr race bike like the R6 actually "pulls" no harder than an XS650. Or not much anyways. There's only one difference between them that makes the R6 fast and the XS slow - the XS has to shift. Say you changed the sprockets to make both bikes have the same overall gearing then you sent them both down the strip. They'd be very evenly matched at first. The R6 can't make it's peak torque down low so the XS may even pull a slight lead... until about 35mph. At this point the XS can't move air into and out of the engine any faster so it can't keep spinning faster, it has to shift.
When you shift up a gear you literally throw away torque. (we're talking rear wheel torque here, the force that is actually accelerating the bike). That's the price you pay for wanting to keep going faster when your engine can't turn any quicker. When you shift the XS from 1st to 2nd you throw out 30% of your torque. Acceleration drops by 30%, or the bike "pulls" 30% less strongly than it did in 1st. The next gear you throw away another 14% of your torque, and so on up the range of gears until you hit 5th gear where you're down to just 43% of the pulling strength you started with. Meanwhile the R6 has just kept on trucking. It can rev to 17,000 rpm so it didn't need to shift. The XS has lost about 60% of it's strength but the R6 is still pulling just as hard as you both were back at the start. That's it. The only advantage the race bike has is that it doesn't throw out it's torque by upshifting until way farther down the strip.
This is why you don't want to move the XS's torque peak down to a lower rpm. The XS has a very flat torque curve already. You have about 35 ft-lbs at 2500 rpm. You could change the cam, intake length, maybe fill in the intake port and fit a smaller carb, and get your peak 40 ft-lbs right there at 2500 rpm. You've gained 14% torque so your bike will pull 14% stronger in the first couple seconds of acceleration, but to do this you've given up your high rpm ability so you now have to shift much earlier. Of course when you do, there goes 30% of your torque much sooner than it used to. So you got a 12% gain for a couple seconds followed by a much bigger loss all the way up to whatever speed you're trying to reach. Very bad deal.
If you want the bike to pull harder you can do three things, get more displacement, keep the revs up(stay in a lower gear), or put on a bigger rear sprocket. There's nothing you can do to the engine that will make the bike pull even close to as strong coming off a corner at 2500 rpm as it would have coming out of the same corner at 5000 rpm because for the same corner speed you have to be in twice as high of a gear to get the engine down to 2500 vs 5000. Higher gear = torque thrown out. In this case 50% of your torque.
Well most of you probably knew that, but hopefully it's useful to some others. I learned that each part of the engine - the cam, the intake, the exhaust, the port diameter, the valve diameters etc. - has an rpm range. The stock 447 cam seems to work best in approximately the 3000-5500 range. That's pretty good for what the bike is, not the best cam for peak power but not much given up on the bottom end either. Both the valves are a little big for the bore size, the ports are very wide, and the intake and exhaust are a complete crap shoot depending on whatever the owner felt like fitting. The engine would work the best and "fill the can" to the highest level when all of those parts are trying to work in the same rpm range so if you want to make your XS pull up to speed quicker it's probably a better idea to try to get the intake and exhaust working to the same goal that the cam is rather than trying to move peak torque around which will only make things more out of tune with each other.
I did mention you would see some 10% variation of torque based on how "full you get the can" (why you have lower torque at 2500 than 6000 with the same size can) but of course you're all correct that you can over fill it and force more than 650cc of mixture into a 650cc can. A turbo or supercharger will pump the most in but with careful balancing of the intake and exhaust lengths and diameters you can get a little "boost" in a naturally aspirated engine too by taking advantage of the inertia of the moving air in those pipes. This is how the R6 manages to get a 47 ft-lb peak from only 600cc, but it only works over a small range where everything goes into tune just right. Some bikes even have a moving part to change the length of the intake as you go up through the rev range so they get this boost twice. I think this is probably also a factor in the unpredictable results people get when they pull out the stock intake and fit pods. You're creating a shorter intake tract that doesn't fit the rpm range of the engine and with the wide ports the air already has less momentum than you'd hope for so I wouldn't be surprised if you run into reversion pretty quick with the carb right on port, pod right on carb set-up. Everybody seems to focus on the "DC" aspect (how much total airflow vs. how much fuel) while ignoring the "AC" aspect of how in or out of tune with the engine an intake of that length/width will be.
