Well, it's a lazy Sunday morning, and i felt like writing something.
So, considering spring is coming up soon, and people may want to "perk up" their Bullets for this year's riding, I thought I'd devote a little time to the subject.
What is "power"?
In an engine, "power" is basically divided into two aspects.
These are "torque" and "horsepower".
Strictly speaking, "torque is the "torsional force" or rotating force around the center axis of the crankshaft. Just like if you put a torque wrench on the crankshaft nut, and used the leverage of the torque wrench to turn the engine.
Horsepower is the work done by the torque, including time, which is usually described by rpms. And horsepower is a mathematical function of torque and rpm, specifically torque x rpm, divided by 5252(a constant).
Looking at this function, we can see that since the dividend in the equation is 5252, then torque will always be higher than horsepower up until 5252 rpm, and horsepower can then go higher than torque above 5252 rpm.
So, in our Bullets, it's easy to see that torque is the main aspect of power production that we will use in our daily riding. We cannot use rpms that are much higher than 5252, because of the strength limitations of the parts in the bottom-end of our engine. However, we can go a little above that, and usually 5500-6000 rpms would be considered the safe maximum limit for us.
And given this function of torque and horsepower, we can now address power production with these things in mind.
How do we increase torque?
Torque in an engine is the result of 2 things primarily. Those things are the leverage exerted on the crankshaft, and force exerted on the piston.
The leverage exerted on the crankshaft is dictated by the stroke length in our engine design. Specifically, it is the distance from the centerline of the crank, to the centerline of the crankpin. This distance is 45mm. But, you say that our stroke is 90mm? Yes it is. The 45mm goes up and down around the centerline, so the distance of 45mm is multiplied by 2 to get total stroke. But the actual distance involved for torque production is 45mm, because that is the distance of the crankpin from the crank center. This leverage is increased with longer stroke, and decreased with shorter stroke. It is fixed by the engine design, unless you change to a stroker crank.
The force exerted on the piston is the combination of the combustion pressure in the chamber, times the area of the piston crown. So you can increase this number by enlarging the bore/piston size, or increasing the compression pressure, or both.
And this is commonly done with a big-bore kit and high compression piston.
Anytime you increase torque, you automatically increase horsepower, due the the nature of the mathematical function. But only at the points in the rpm range where that torque has been raised. It is entirely possible to gain torque in one part of the rpm band, and lose some in another part of the rpm band. So we want it to be broad enough in the torque curve, to provide good useful riding results. Some racing engines make power only in the highest rpms, and are very poor at low or midrange rpms. Conversely, some other engines pull like a tractor at low rpms, and can't reach high rpms very well at all. It's all part of how the engine designer picked the parts for the application.
Horsepower on the other hand, always follows torque. You can't have hp without torque. BUT, there is a special circumstance, where breathing efficiency allows torque to drop slightly, and the rpms can still cause horsepower to increase, as long as the engine can continue to pull in sufficient air/fuel mixture to increase rpms.
The "knee" in this curve is known as "peak torque rpm". It is the point where the engine is operating most efficiently for torque production. Everything is coming together just right at this rpm. As we accelerate from idle to peak torque rpm, the torque continuously increases until we reach peak torque rpm. Then it may hold on to the peak torque figure for a while, or it may begin to drop off at higher rpms. Most times it will slowly decrease for about 1000-1500 rpms, and then drop off precipitously when breathing capacity reaches the limit. Even during the decreasing slope of the torque, engine rpm can increase and thus still produce higher and higher hp, until the breathing capacity of the engine is reached, and this is "peak horsepower". The longer the engine can provide breathing capacity to rev higher, the longer the area from peak torque to peak hp will become.
So, there you have your 2 curves in power production of the engine, with the critical rpms for peak torque and peak hp. The rpms between the peak torque rpm and the peak hp rpm is commonly known as the "max power range".
How does the engine continue to make more power after it reaches the torque peak? Why doesn't it just stop there, if it has passed the place of best efficiency?
It's because the engine has capacity to breathe in more air/fuel mixture, at a rate which can be multiplied by rpms, even after max efficiency(peak torque) is passed. And it can continue to do this as the torque slowly declines, until that breathing capacity is limited enough to not be able to overcome the torque decline any more.
So, what we'd like to do is reach a high peak torque figure, and hold onto it as long as we can while the rpms rise further. And we'd like to place this range within the ability of our engine's structural integrity, so that we can access it without blowing the crank or rod. And in our case, this might be a torque peak of 4000 rpm and a hp peak of 5500 rpm, or maybe even a torque peak of 4500 rpm and a hp peak of 6000 rpm. The latter is pretty high, and probably reserved for a sport rider who likes to push the limits.
What can we do to help raise torque, keep a broad and useful curve, and increase hp too?
Well, we are pretty much limited in mechanical leverage by the stroke, unless we get a stroker crank. So that's about set in stone.
But we can get a big-bore kit and hi-compression piston pretty easily, and that can help out all thru the entire range.
But, those are only parts of what we can do.
How can we go further?
We can increase the breathing capacity of the intake and exhaust systems, and also increase the breathing capacity of the cylinder head. We know that a somewhat bigger carb, and a free-flow air filter can help, and many of us have done that. So does removing the very obstructed exhaust system, and replacing it with one that can let the gases out of the engine with less impediment.
This seems to make sense, right? We can bring more air/fuel mixture in, and get the exhaust gases out, better. And it works. To a point.
What is this "point" of which I speak?
Remember a few paragraphs back, when I mentioned that it's possible to gain torque or hp in one rpm range, and lose it in another. That is the issue that I'm referring to.
It's possible to set up the engine breathing so that it is very big, and flows alot at higher rpms, but this makes it flow slow and lazy at lower rpms. And that means that at lower rpms, not alot of air/fuel mixture is getting in, because of slow lazy flow that was created by making it all big enough to flow at much higher rpms, and that's what makes racing engines do poorly on the street. Conversely, if we make the breathing small enough to give us really high flow speeds at lower rpms, giving us great low-rpm torque, our ports(or carb or manifolds) can't flow enough mixture at the higher rpms to develop the power at the high rpms. So, for street work, we need a balance.
What is the "balance"?
The balance is to have breathing which can be fast enough to give good flow and good torque down low, and still be able to breathe well enough to allow the engine to access the high rpms. This means not going too far in either direction. Not too small, and not too large. So, we don't want an overly large carburetor, or overly large intake manifold, or over large ports. Nor do we want them too small.
We want it to be like "Goldilocks". Just right!
So, we select are carburetor size, manifold size and port size, so that we are good for our intended range of use, which is about 2000-6000rpms. It does us no good to set up the engine to do 9000rpm. We'll never get there without blowing up, and it will just wreck our low-speed running.
Happily, our India-made head has an intake port which is very nearly sufficient for our highest rpms, and doesn't really need enlargement. But, the shape of the port is poor, and while it may be able to pass enough air, it doesn't do it with good velocity. So what we need in our India-made intake port is improved shape, and not necessarily bigger size. The size"could be" bigger, IF the shape is improved sufficiently to get good velocities with that bigger size. That's not easy to do, but with the poor shape that we have in there, it could be possible, IF you really know what you are doing.
The same goes with the carb and manifold. If the carb is too big, then it just slows everything down, and you can't get the overall flow you need because velocity suffers. So, putting a big 38mm carb on our stock ports isn't going to help us. But, going bigger than the 28mm carb might certainly help us, because our ports are 32mm, and that makes the carb a restriction.
The whole idea is to get more air/fuel mixture in, which means sufficient volume, AND sufficient velocity, which combine to get most mixture in during the time the valve is open. And do it well enough at all our rpm ranges, so that we don't suffer weak spots in our riding range.
The more mixture that enters the engine at any given rpm, the more the combustion pressure is going to be exerted on the piston crown, and the more torque is going to result. So, it stands to reason that we want to get as much in as we can, at all the rpms that we plan to ride. Since this increases torque, it also increases horsepower, and everybody's happy.
So, in summation, we must strive to modify the engine within the expected range of use(rpms) to provide the best breathing possible in the bulk of that rpm range, and use that to produce more force exerted on the piston. And we can combine that with a piston that "squeezes" that mixture with more compression, to improve the result. And we can use a slightly bigger piston to give more area for that pressure to work on.
Of course, we could discuss how cams assist this, and the exnaust extraction effects, but I think we've bitten-off a pretty good chunk here, so we can chew on this one for awhile.