Hey all! Welcome to the next installment in my series of writings.
I have often heard the words 'horsepower' and 'torque' tossed around and I got the sense that maybe the person using those terms was not 100% sure of their definition.
To many, horsepower means nothing more than 'my bike is fast'. Torque is 'that other thing that means my bike is fast', and bigger numbers means that I am cooler than the guy/girl next to me. While this may be a fine definition for some, let's take a little more in-depth look at horsepower and torque to see exactly how it applies to our real-world riding.
To begin, lets define both those words but to do so requires a little further delving into one of my favorite subjects; physics. DISCLAIMER:
This will be a simplified viewpoint as it relates to our subject. If you are a physics nut(job) please do not become offended by my shorthand.
Horsepower and torque are both units of measure for power produced by an engine. Ok, so how do you define power? Power is simply the rate of doing work. Well...what the heck is work? The rate of moving an object. So power is the unit of measure for moving an object. For motorcycles, we measure this with torque and horsepower. Stick with me, it gets easier, I promise!
First, let's quickly clarify something. If you are discussing the power plant of a MOTORcycle (misnomer) or any internal combustion vehicle, you are talking about an engine
. An engine converts fuel to power. A motor is an electrical device. Lets get those terms straight, shall we? Torque
Put simply, torque is a unit of measurement of a twisting force and is measured in foot pounds or pounds-per-foot (lb-ft). If you have a foot long wrench on a bolt and set a 10lb weight on the end of it, you are exerting 10 lb-ft of torque on the bolt head.
Torque is most easily understood by motorcyclists as the force exerted on the rear wheel and it is directly related to acceleration
. More torque=faster acceleration. Horsepower
As it applies to the motorcycle world, horsepower is a unit of measure equal to the force necessary to move 550lbs one foot in one second. Who came up with this? Not a clue, but it doesn't matter. For the point of our discussion think of horsepower as 'top end' speed. The maximum velocity that you can travel is directly related to your horsepower (we are removing gearing from this discussion for the sake of simplicity).
The equation for measuring horsepower is pretty easy: (torque * RPM)/5252. So, if we are putting down 50 lb-ft of torque, at 10,000 RPM we would be producing 95.2 horsepower. What the heck does all this mean?
Ok, ok. In very simple terms if your top speed is 180mph
, that is a function of your horsepower. How quickly you reach your top speed
is determined by your torque. There is some overlap of course but that is far beyond the scope of this discussion. So which is better?
The simple answer: both. Ideally you want as much of both of them as possible but to answer this you need to determine what your use for the engine is.
In a technical track where you are spending lots of time accelerating through the gears, the more torque you can produce the better. In a longer, high speed sweeper type track, a higher horsepower engine will serve you better.
Have you ever watched a WSBK or MotoGP race and heard the announcers say that this particular track favors the Yamaha or Aprilia? The Yamaha is an inline 4 engine...they generally produce higher horsepower but less torque. The Aprilia is a V4 engine and produces more torque and less horsepower. If you look at the track layout, the longer tracks will favor the Yamaha and the shorter, more technical tracks will favor the Aprilia. The Yamaha has a higher top speed but the Aprilia reaches its top speed quicker
Ok, so now that the remedial learning is done
we can now focus on how the heck do we make use of this information?
Well, first we need to measure our torque and horsepower, and that is done on a dynamometer
or 'dyno' for short. The six reasons why you need to get your bike to a dyno
The dyno is a lot more than just a tool for measuring horsepower. It is also a tuning instrument used to get the absolute most out of your engine. Ever have your bike stumble in lower gears? The dyno can cure that. Ever feel like you were going fine but at certain RPMs the bike kind of falls flat on its face? Yep, the dyno can fix that.
I have never understood why people will spend $1000 for an exhaust and $800 for a power commander but not spend the $200-300 for a custom tune from a dyno so that you can actually make the best use of that shiny new exhaust and PC. That is like buying a HD television but not paying for HD cable service. Sure, it may work but you aren't getting your moneys worth.
1) Your bike will have more power after visiting a dyno
. Getting a custom map done fine-tunes your power commander or bazzaz unit to your riding conditions (temp, humidity, riding style) and really 'wakes up' the motor.
2) Your bike will run smoother after visiting a dyno
. From the factory our bikes are tuned to pass emission standards which unfortunately takes precedence over actually running right. Power delivery is easier to manage and makes the bike more fun and friendly to ride.
3) You can cause harm to your bike if you do certain modifications and do not get the bike tuned
. As I just said above, bikes are tuned from the factory to meet emission standards which means they already run lean (fuel to air ratio...less fuel, more air). If you install an aftermarket air filter (still more air) and an exhaust system (still more air) then you are making the bike even more lean. The leaner you make the fuel/air mixture, the more heat it generates and any engine builder can tell you that heat is the enemy. Unchecked it has the potential to do bad things to your internal engine components. THIS IS NOT A SCARE TACTIC. It doesn't mean that if you do not get this done your bike will suddenly explode. It does mean that you are shortening the lifecycle of the engine and adding increased wear into your system.
4) You can get improved fuel economy
. Even though we generally create more power by adding more fuel into the mix, you will usually end up with increased fuel economy due to the increase in power. If you needed to run 20% throttle to maintain 70mph on the highway, you may only need 15% throttle after having it tuned. That translates into fuel savings.
5) You know how accurate your tachometer is
. Every bike that I have seen has been off with the tachometer. On a dyno you will know what your true RPMs are compared to your displayed RPMs. This information will be useful when you start looking at dyno charts to get the most out of your engine.
6) You cannot get the most out of your bike without knowing exactly where your power and torque is
. This may not be relevant for street riding but for track riders and especially racers, this is the most important benefit of dyno tuning. How to use your power
So, now that we have established some definitions and gotten our bike to a dyno, we get the dyno results sheet which looks something like this below.
This is an exact graphical representation of the power created by our engine, in this case my own Kawasaki ZX6R race bike.
The top line is the horsepower that the engine creates. The line below it is the torque that is generated. On the bottom of the chart you will see numbers 3-16, that is your engine RPMs x 1000.
This gives us a matrix to look at. Go to the 6 on the RPM scale and look up that horizontal line. You will see that the torque is somewhere around the 39 mark which means our HP is ~44. Well heck, that's not a lot! We can clearly see that there is more horsepower as we move to the right of the chart, which means that as our RPMs climb, we produce more and more power to a point
. This gives us an idea that this particular engine needs to be kept in the higher RPM range to get the most out of it and that is true if we are talking about top end speed.
Lets take a moment and look at that torque line. You can see that the highest amount of torque is right about 11,000 RPMs. Since we have already established that torque=acceleration, if our goal is to accelerate we want to be keeping our RPMs in a range where we have the highest torque...and ignore the HP numbers for now. I know this goes against all the internet-tough-guy-forums and bro-science thinking where all they throw around is large horsepower numbers in an effort to seem ultra-cool. Trust me on this. Torque is where it is at unless you are in your top gear and pulling down a long straight.
So, since the goal is to accelerate as quickly as possible, where do we want to keep our engine RPMs? Right here:
For this particular motor, there is a 'sweet spot' where the torque is nearly perfectly flat. That means tons of available power to accelerate between the 11,000 and 13,000 RPM range. You can see that after 13,000 the torque starts to fall off. What this means is that you want to shift this motor at 13,000 RPMs to drop it back into the sweet spot. You keep doing this until you are in 6th gear and then you have no choice but to continue to rev the motor.
This is what I mean when I say 'short shifting'. It means that you do not continue to rev your bike until you hit the rev limiter. You shift earlier than the limiter to keep the bike in that sweet spot. If you have ever watched any of my videos that show the dash, you will see that I always shift earlier than the rev limiter.
**NOTE** This is where knowing your displayed RPMs vs actual RPMs is critical. On my particular bike we discovered that the tachometer reads about 1000 RPMs higher than actual. What this means is that even though I want to keep the engine between 11,000-13,000 actual
RPMs, the numbers displayed on the tach would be closer to 12,000-14,000. Those numbers are what I shift with when looking at my tachometer.
So, we know that to get the fastest acceleration we want to keep the RPMs in that sweet spot shown above. What about when I am in 6th gear on a long straight? This is where horsepower becomes more important than torque. How much actual power is there to push you and your bike through all that wind resistance?
About 12,500-14,000 RPMs is the ideal range when I am looking for top speed as that is where my peak horsepower is made. If the track that I am at requires me to rev past that 14,000 mark, it is time to look at changing your gearing to keep your bike in that rev range for top speed.
You will note that after 14,000 RPMs the horsepower starts to fall off drastically...and so is the torque! What that means is that under no circumstances do you want to rev this engine past 14,000 RPMs. You are just not making the power that you want to make.
One final note: All bikes are different!
Here is a dyno chart from an older Yamaha R6.
Looking at this chart you can see that it really doesnt make good power until much higher in its RPM range, but that the power continues right up until redline. That means this engine is one that you want to take to the rev limiter on every gear.
This is why it is critical for you to dyno your own engine. Even in the same make, year, and model the engines can vary in how they deliver power.
So, I hope that you stuck it out to the end of this as it has gotten quite long and that you have found the information useful!