As a child you likely played a basketball game
called HORSE where you shoot baskets and each time you make a basket, you earn
another letter in the word, and the first one to spell out h-o-r-s-e
won.
This is not about that game, but about another
game where the one who makes horsepower (and torque), and uses them
best... wins.
Many places you will hear about comparisons between
torque and horsepower... this is yet another one.We don't want to argue for or against torque or
horsepower... we want it all for best performance. We just need to use them
correctly. To do this, we need to understand what they both are, and how they
alter performance.
If you are reading this, you are likely wanting to
"go faster"... what you really mean is you want to accelerate more quickly and
out run the next guy. You want to pull ahead out of turns, and down the
straight, so you cross the finish line first, or run a better ET when on a drag
track. SO you really want to "go quicker". This is acceleration.
It takes a given amount of force to accelerate a
known mass at a given rate.
The total weight of the bike, rider, fuel, oil,
water, riding gear, etc. all make up the total mass. If we know the mass of the
bike, and we know the force produced at the different engine speeds in each
gear, we can figure the rate that the bike will accelerate. Then we can look at
all of these details and how to improve them. This is the basic principal of
improving performance.
Torque is a rotational force - the force of the
expanding combustion gases acting to push down on a piston that is transmitted
to the rotating crankshaft through a connecting rod. There are losses to heat
and friction, but the end result is a measurable rotating force called torque.
It is this force that is used to cause acceleration.
Horsepower is the amount of torque at a specific
engine speed measured in revolutions per minute (rpm). It is a calculated
value of work, not a measured force. Horsepower is how we compare the amount of
work that can be performed by an engine. We can use it well, or not... that is
performance.
If torque and engine speed are known, horsepower
can be calculated with the simple formula HP = TQ X RPM / 5252, and if horsepower and
engine speed are known, torque can be determined by modifying the formula to TQ
= HP X 5252 / RPM. This means that horsepower and torque will always be the same
at 5252 rpm.
Examples:
52.52 lb-ft of torque at 12,500 rpm = 52.52 X
12,500 / 5252 = 125 horsepower
125 horsepower at 12,500rpm = 125 X 5252 / 12,500 =
52.52 ft-lb
Now let's change the camshaft and a few other
things in the engine and make the same torque, but at 13,500 rpm:
HP = 52.52 X 13,500 rpm / 5252 = 135
hp
Now let us do it again, but at 4000
rpm:
HP = 52.52 X 4000 / 5252 = 40
hp.
Notice that WHERE we make torque determines how
much horsepower is produced.
If you remember nothing else, remember that torque
is a force, and horsepower is a calculation.On a dyno, torque is measured with a load cell, or
by spinning a drum of known weight at a measured rate of acceleration, to
determine the force. Then horsepower is calculated using engine
speed.
What is interesting is that peak torque occurs at
the engine speed where it is filling with air best. Beyond this engine speed,
horsepower will continue to improve as long as rpm climbs more quickly than
torque is falling off - as we run out of airflow through the intake ports and
friction increases. Selecting the correct combination of parts, or modifying
them, can determine how far apart peak torque and peak power are made. We can
make them happen farther apart for a broad power band that is easy to ride, or
make them happen closer together, and a bit higher, for a bike that is "peakier"
and a bit harder to ride well outside of the power band. It is the engine that makes the best average
horsepower and torque within the rpm range where gear shifting occurs that will
have the quickest acceleration with all other factors remaining the
same.
Since many four-stroke engines work within a very
narrow band of thermal efficiency, we can look at the engine's displacement (how
many cc, liters, or cubic inches) and use this to determine a comparison of how
efficiently a given engine is at turning gasoline into torque. Many production
engines make 75 ft-lb of torque per liter at their maximum torque production,
and very good engines produce 90-93 ft-lb per liter, while excellent racing
engines have been known to produce as much as 95 ft-lb per liter on exotic
racing fuels or alcohol fuels.
In the examples above you may have noticed that
making 52.52 ft-lb of torque made 125 hp at 12,500 rpm and 135 hp at 13,500 rpm.
The torque is the same, so how do we make use of the higher rpm?
GEARING.
Gearing is a torque multiplier. Your transmission
and chain/belt/differential use gearing to multiply the torque output of the
engine and apply it to the road or track. The gear ratios determine the
multiplication factor, and the rpm limit determines the maximum speed that can
be attained (not including friction and drag). This is why your bike accelerates
harder in first gear than in fourth - fourth has less multiplication, but more
speed. If we use taller gearing, we can go faster at a given engine speed with a
slower rate of acceleration. If we make torque at a higher rpm (torque+more rpm
= more horsepower), we can use more gearing multiplication to accelerate even
harder. But as engine speeds increase, we lose more to friction, and the
stresses in the engine multiply. If we double the engine speed, we SQUARE the
forces that are trying to pull everything apart. So obviously we have to work
smart and stay within the the engine's limits or we launch parts into low earth
orbit in a slightly modified version of the big bang theory.
With a 600cc bike engine we can make a given amount
of torque. If we work out all the little details and get everything performing
as well as we can, we improve torque. If we make a little more torque at higher
rpm, we make more horsepower. The gearing we use to multiply the torque to the
ground can help us go quicker (better acceleration) or faster (better top
speed). If we get everything right, we can make a 600cc bike accelerate more
quickly than the next guy - sometimes even if he is on bike with a larger
engine. THIS is how we correctly use torque to improve performance in the adult
game of HORSE(power).
- POWER-TRIPP Performance
