Hello All. I was wondering what is better torque or horsepower? Why are bikes built for either? I understand more HP makes a fast bike, but don't you FEEL the torque on a bike when you get romp on it? Why is it that HP or TQ drops off as rpms increase? Thanks for the help. Be safe! J.T.
Tq Vs. Hp?
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You have probably heard this before and from what little I know. Torque is the amount of twisting force an engine can produce. Horse power is the rate at which it can be appiled. With that said, horse power and torque are derived from each other, and are essentially the same entity. Torque and hosrse power numbers are effected by the flow capacity of the cylinder head and its surrounding components.
Hence.... up on the pipe or sweet spot equals max numbers and it takes gearing to keep it there
Hence.... up on the pipe or sweet spot equals max numbers and it takes gearing to keep it there
A engine produces TQ. HP is a calculated figure based on rpm.
It all depends at what engine speed you will operate. In order to increase HP you need to increase the breathing ability of the engine, bigger ports, bigger valves, more cam, etc. Basically you have to make the engine able to rev sky high. The constant in the mathematic formula is 5252 (this is why at that rpm TQ and HP are always the same). If the engine is able to rev higher the rpm have a larger effect on the calculation then the TQ.
The more you increase all these things in order to gain HP on top, the more TQ you will lose on bottom.
Compare a VRod with a bagger, they have the HP but relatively puny TQ, put the VRod engine in a heavy bagger and it will fall on it's a$$ unless you constantly downshift to keep the engine in it's designed powerband.
On a old technology bike that is severely chassis limited TQ is the name of the game. It is not uncommon for a stock bike to pull a modified one on a top gear roll on for exactly this reason. A lot of guys that want big engines do not understand these basic facts and are unwilling to adjust their riding style.
It all depends at what engine speed you will operate. In order to increase HP you need to increase the breathing ability of the engine, bigger ports, bigger valves, more cam, etc. Basically you have to make the engine able to rev sky high. The constant in the mathematic formula is 5252 (this is why at that rpm TQ and HP are always the same). If the engine is able to rev higher the rpm have a larger effect on the calculation then the TQ.
The more you increase all these things in order to gain HP on top, the more TQ you will lose on bottom.
Compare a VRod with a bagger, they have the HP but relatively puny TQ, put the VRod engine in a heavy bagger and it will fall on it's a$$ unless you constantly downshift to keep the engine in it's designed powerband.
On a old technology bike that is severely chassis limited TQ is the name of the game. It is not uncommon for a stock bike to pull a modified one on a top gear roll on for exactly this reason. A lot of guys that want big engines do not understand these basic facts and are unwilling to adjust their riding style.
Thanks, gentlemen. Well, I must admit I'm still a little confused. Bigger heads, pistons, and breathing better allows for a bigger 'explosion' in the cylinders, better intake and exhaust, thus giving more hp. Right? So, what gives the engine torque? I understand 2DEUCE, that a bigger combustion crates more power to turn the engine, thus increasing torque. Right? So, when modifying an engine, which is more desired? Going on what HIPPO stated, with a bigger engine at higher rpms, hp is increased. So, why does the engine lose torque? It seems that it would be a direct correlation. Just going on limited knowledege on the internal combustion engine and physics. J.T.
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Horsepower = Torque X RPM/5252 torque can be developed at low RPM's (more displacement, more torque) but Horsepower increases with RPM's. Some engines like the V-Rod don't really come into their own until the RPM's really get up there because they have low torque and Rev high therefore their power is derived from RPM's instead of torque. The standard old V-twin gets it's power from torque thus develops it's power in the lower range of RPM's. Like HIPPO said you can have low-end torque or High-end Horsepower. If you can figure a way to get both you'll be a very rich man.
Building power is an extremely complicated process if you want to get it all right. It is easy to say that you need more air in/out for more power, longer stroke/large bore for more power. But when you get on the dyno (or hit the throttle), there may be a gain, but you are going to have these two lines that dip around everywhere.
You have truely understand what cam duration does to the engine, what lift does, why you want the proper sized intake and exhaust (what people falsely report at "backpresure"), proper intake and exhaust length, fuel delivery, egnition timing, and a lot more. After you learn all of that, then you have to figure out how to put it together to get what you want.
Best method for those who do not want to go through the effort of the above (99% fall into this catagory), see what others have done, see what worked and what didn't. But look at a system, not a part. Just because pipe A worked good on this guys bike, doesn't mean it will work just was well with your cam.
You have truely understand what cam duration does to the engine, what lift does, why you want the proper sized intake and exhaust (what people falsely report at "backpresure"), proper intake and exhaust length, fuel delivery, egnition timing, and a lot more. After you learn all of that, then you have to figure out how to put it together to get what you want.
Best method for those who do not want to go through the effort of the above (99% fall into this catagory), see what others have done, see what worked and what didn't. But look at a system, not a part. Just because pipe A worked good on this guys bike, doesn't mean it will work just was well with your cam.
Talk the torque...
In simplest terms, torque is a measure of the force created by the combustion process, as transmitted via the crankshaft through the transmission and ultimately to the rear wheel. Horsepower is a measure of how quickly the engine and drivetrain can deliver that force.
It is possible to have an engine that creates an enormous amount of torque, and yet have relatively low horsepower. Consider one of the earliest forms of "engines": a waterwheel. Say you see a waterwheel that has a diameter of 12 feet; with a rotating mass of 400 lbs; and that is rotating at a maximum velocity of 12 RPM. Such an "engine" has a torque output of: 6' (radius) x 400 lbs mass = 2400 lbs/ft of torque. (By way of comparison, a Twin Cam SE Stroker 103CI engine puts out a peak of about 110 lbs/ft of torque). You'd think that with this much torque you'd have a pretty good horsepower figure, right? But according to the formula (torque x RPM)/5250 you only end up with 5.48 HP.. not very impressive. A bike powered by an engine with similar torque and HP characteristics would accelerate like hell until it got to about 6 mph or wherever the 12 RPM "redline" occured (provided you could prevent wheel spin..but thats another story) and then it would max out. The secret then is being able to produce torque at high RPMs...
You also asked the question why do torque and horsepower both drop off after a certain point ..
As an engine increases the number of RPMs it is doing, several things are happening. The most obvious is that more fuel and air are being combined per unit of time.. this is what leads to the steady increase in horsepower output in the early part of the range. Torque also increases because the pistons are being hurled downwards at higher speed (Force = mass x acceleration). But as RPMs increase greater inefficiencies start to creep in. As engine speed rises, there is more friction loss, not only between the pistons and the cylinders, but also in the crankshaft and cam bearings. These are mechanical inefficiencies. You also lose more energy to heat loss - thermal inefficiencies. Lastly you will reach bottlenecks in the intake and exhaust systems. The engine cannot combine the fuel and air as efficiently, so more is wasted. Ultimately you reach a point of diminishing returns - putting more fuel and air into the engine creates more friction than the energy its combustion provides. It may be possible for the engine to keep increasing its speed (RPMs), but the resulting torque is so low that net horsepower also begins to drop.
Succesful engine design therefore seeks to achieve two aims: as high a torque figure as possible, across the widest possible range of engine speeds. Engine parts are designed to minimize friction losses at high speeds, but also utilize high compression ratios and large cylinder volumes to create the torque.
In simplest terms, torque is a measure of the force created by the combustion process, as transmitted via the crankshaft through the transmission and ultimately to the rear wheel. Horsepower is a measure of how quickly the engine and drivetrain can deliver that force.
It is possible to have an engine that creates an enormous amount of torque, and yet have relatively low horsepower. Consider one of the earliest forms of "engines": a waterwheel. Say you see a waterwheel that has a diameter of 12 feet; with a rotating mass of 400 lbs; and that is rotating at a maximum velocity of 12 RPM. Such an "engine" has a torque output of: 6' (radius) x 400 lbs mass = 2400 lbs/ft of torque. (By way of comparison, a Twin Cam SE Stroker 103CI engine puts out a peak of about 110 lbs/ft of torque). You'd think that with this much torque you'd have a pretty good horsepower figure, right? But according to the formula (torque x RPM)/5250 you only end up with 5.48 HP.. not very impressive. A bike powered by an engine with similar torque and HP characteristics would accelerate like hell until it got to about 6 mph or wherever the 12 RPM "redline" occured (provided you could prevent wheel spin..but thats another story) and then it would max out. The secret then is being able to produce torque at high RPMs...
You also asked the question why do torque and horsepower both drop off after a certain point ..
As an engine increases the number of RPMs it is doing, several things are happening. The most obvious is that more fuel and air are being combined per unit of time.. this is what leads to the steady increase in horsepower output in the early part of the range. Torque also increases because the pistons are being hurled downwards at higher speed (Force = mass x acceleration). But as RPMs increase greater inefficiencies start to creep in. As engine speed rises, there is more friction loss, not only between the pistons and the cylinders, but also in the crankshaft and cam bearings. These are mechanical inefficiencies. You also lose more energy to heat loss - thermal inefficiencies. Lastly you will reach bottlenecks in the intake and exhaust systems. The engine cannot combine the fuel and air as efficiently, so more is wasted. Ultimately you reach a point of diminishing returns - putting more fuel and air into the engine creates more friction than the energy its combustion provides. It may be possible for the engine to keep increasing its speed (RPMs), but the resulting torque is so low that net horsepower also begins to drop.
Succesful engine design therefore seeks to achieve two aims: as high a torque figure as possible, across the widest possible range of engine speeds. Engine parts are designed to minimize friction losses at high speeds, but also utilize high compression ratios and large cylinder volumes to create the torque.
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