A&Q about 350Z
Q:
This is an article a friend(Dan Jones) wrote.
Enjoy.
Torque Versus Horsepower - More Than You Really Wanted to Know
by Dan Jones
Every so often, in the car magazines, you see a question to the technical
editor that reads something like "Should I build my engine for torque or
horsepower?" While the tech editors often respond with sound advice, they
rarely (never?) take the time to define their terms. This only serves to
perpetuate the torque versus horsepower myth. Torque is no more a low rpm
phenomenon than horsepower is a high rpm phenomenon. Both concepts apply
over the entire rpm range, as any decent dyno sheet will show and as I will
show below.
Before I begin, I will give the short answer to the torque versus horsepower
debate is that torque is what accelerates a vehicle but it is the torque at
the rear wheels that matters and it is the horsepower of the engine that
determines how much engine RPM can potentially be traded for torque at the
rear wheels via gearing. Gearing is the reason you accelerate harder in 1st
gear than you do in 4th or 5th (there's also an aerodynamic drag effect but
that is relative less important for the purposes of this discussion). Please
understand it is the overall gearing provided by the transmission, rear end,
and tire that is important.
To begin, we'll need several boring, but essential, definitions. Work is a
measurement that describes the effect of a force applied on an object over
some distance. If an object is moved one foot by applying a force of one
pound, one foot-pound of work has been performed. Torque is force applied
over a distance (the moment-arm) so as to produce a rotary motion. A one
pound force on a one foot moment-arm produces one foot-pound of torque.
Note that dimensionally (ft-lbs), work and torque are equivalent. Power
measures the rate at which work is performed. Moving a one pound object
over a one foot distance in one second requires one foot-pound per second of
power. One horsepower is arbitrarily defined as 550 foot-pounds per second,
nominally the power output of one horse (e.g. Mr. Ed).
Since, for an engine, horsepower is the rate of producing torque, we can
convert between these two quantities given the engine rate (RPM):
HP = (TQ*2.0*PI*RPM)/33000.0
TQ = (33000.0*HP)/(2.0*PI*RPM)
where:
TQ = torque in ft-lbs
HP = power in horsepower
RPM = engine speed in revolutions per minute
PI = the mathematical constant PI (approximately 3.141592654)
Note: 33000 = conversion factor (550 ft-lbs/sec * 60 sec/min)
In general, the torque and power peaks do not occur simultaneously (i.e.
they occur at different rpm's). However, since the curves are functions
of each other, the curves will always cross at 5252 RPM when the units
are torque in ft-lb and power in US Horsepower. There's nothing magic
about this number and it will have a different value for different units,
say power in Watts and torque in Newton-meters.
To answer the question "Is it horsepower or torque that accelerates an
automobile?", we need to review some basic physics, specifically Newton's
laws of motion. Newton's Second Law of Motion states that the sum of the
external forces acting on a body is equal to the rate of change of momentum
of the body. This can be written in equation form as:
F = d/dt(M*V)
where:
F = sum of all the external forces acting on a body
M = the mass of the body
V = the velocity of the body
d/dt = time derivative
For a constant mass system, this reduces to the more familiar equation:
F = M*A
where:
F = sum of all the external forces acting on a body
M = the mass of the body
A = the resultant acceleration of the body due to the sum of the forces
A simple rearrangement yields:
A = F/M
For an accelerating automobile, the acceleration is equal to the sum of the
external forces, divided by the mass of the car. The external forces
include the motive force applied by the tires against the ground (via Newton's
Third Law of Motion: For every action there is an equal and opposite re-action
and the resistive forces of tire friction (rolling resistance) and air drag
(skin friction and form drag). One interesting fact to observe from this
equation is that a vehicle will continue to accelerate until the sum of the
motive and resistive forces are zero, so the weight of a vehicle has no bearing
whatsoever on its top speed. Assuming level ground, weight is only a factor
in how quickly a vehicle will accelerate to its top speed.
In our case, an automobile engine provides the necessary motive force for
acceleration in the form of rotary torque at the crankshaft. Given the
transmission and final drive ratios, the flywheel torque can be translated
to the axles. Note that not all of the engine torque gets transmitted to the
rear axles. Along the way, some of it gets absorbed (and converted to heat)
by friction, so we need a value for the frictional losses:
ATQ = FWTQ CEFFGR TRGR * FDGR - DLOSS
where:
ATQ = axle torque
FWTQ = flywheel (or flexplate) torque
CEFFGR = torque converter effective torque multiplication (=1 for manual)
TRGR = transmission gear ratio (e.g. 3 for a 3:1 ratio)
FDGR = final drive gear ratio
DLOSS = drivetrain torque losses (due to friction in transmission, rear
end, wheel bearings, torque converter slippage, etc.)
During our previous aerodynamics discussion, one of the list members mentioned
that aerodynamic drag is the reason cars accelerate slower as speed increases,
implying that, in a vacuum, a car would continue to rapidly accelerate. This
is only true for vehicles like rockets. Unlike rockets, cars have finite rpm
limits and rely upon gearing to provide torque multiplication so gearing plays
a major role. In first gear, TRGR may have a value of 3.35 but in top gear it
may be only 0.70. By the above formula, we can see this has a big effect on
the axle torque generated. So, even in a vacuum, a car will accelerate slower
as speed increases, because you lose torque multiplication as you shift up
through the gears.
The rotary axle torque is converted to a linear motive force by the tires:
LTF = ATQ / TRADIUS
where:
TRADIUS = tire radius (ft)
ATQ = axle torque (ft-lbs)
LTF = linear tire force (lbs)
What this all boils down to is, as far as maximum automobile acceleration is
concerned, all that really matters is the maximum torque imparted to the
ground by the tires (assuming adequate traction). At first glance, it might
seem that, given two engines of different torque output, the engine that
produces the greater torque will be the engine that provides the greatest
acceleration. This is incorrect and it's also where horsepower figures into
the discussion. Earlier, I noted that the torque and horsepower peaks of an
engine do not necessarily occur simultaneously. Considering only the torque
peak neglects the potential torque multiplication offered by the transmission,
final drive ratio, and tire diameter. It's the torque applied by the tires
to the ground that actually accelerates a car, not the torque generated by the
engine. Horsepower, being the rate at which torque is produced, is an
indicator of how much potential torque multiplication is available. In
other words, horsepower describes how much engine rpm can be traded for tire
torque. The word "potential" is important here. If a car is not geared
properly, it will be unable to take full advantage of the engine's horsepower.
Ideally, a continuously variable transmission which holds rpm at an engine's
horsepower peak, would yield the best possible acceleration. Unfortunately,
most cars are forced to live with finitely spaced fixed gearing. Even
assuming fixed transmission ratios, most cars are not equipped with optimal
final drive gearing, because things like durability, noise, and fuel
consumption take precedence to absolute acceleration.
This explains why large displacement, high torque, low horsepower, engines
are better suited to towing heavy loads than smaller displacement engines.
These engines produce large amounts of torque at low rpm and so can pull a
load at a nice, relaxed, low rpm. A 300 hp, 300 ft-lb, 302 cubic inch engine
can out-pull a 220 hp, 375 ft-lb, 460 cubic engine, but only if it is geared
accordingly. Even if it was, you'd have to tow with the engine spinning at
high rpm to realize the potential (tire) torque.
As far as the original question ("Should I build my engine for torque or
horsepower?") goes, it should be rephrased to something like "What rpm
range and gear ratio should I build my car to?". Pick an rpm range that
is consistent with your goals and match your components to this rpm range.
So far I've only mentioned peak values which will provide peak instantaneous
acceleration. Generally, we are concerned about the average acceleration
over some distance. In a drag or road race, the average acceleration between
shifts is most important. This is why gear spacing is important. A peaky
engine (i.e. one that makes its best power over a narrow rpm) needs to be
matched with a gearbox with narrowly spaced ratios to produce its best
acceleration. For instance, some Formula 1 cars (approximately 800 hp from
3 liters, normally aspirated, 18,000+ rpm) use seven speed gearboxes.
Knowing the basic physics outlined above (and realizing that acceleration
can be integrated over time to yield velocity, which can then be integrated
to yield position), it would be relatively easy to write a simulation program
which would output time, speed, and acceleration over a given distance. The
inputs required would include a curve of engine torque (or horsepower)
versus rpm, vehicle weight, transmission gear ratios, final drive ratio, tire
diameter and estimates of rolling resistance and aerodynamic drag. The last
two inputs could be estimated from coast down measurements or taken from
published tests. Optimization loops could be added to minimize elapsed
time, providing optimal shift points, final drive ratio, and/or gear spacing.
Optimal gearing for top speed could be determined. Appropriate delays for
shifts and loss of traction could be added. Parametrics of the effects of
changes in power, drag, weight, gearing ratios, tire diameter, etc. could be
calculated. If you wanted to get fancy, you could take into account the
effects of the rotating and reciprocating inertia (pistons, flywheels,
driveshafts, tires, etc.). Relativistic effects (mass and length variation
as you approach the speed of light) would be easy to account for, as well,
though I don't drive quite that fast.
Later,
Dan Jones
A:
Good post... ^^
Just a reply to whoever said they doubt that 2600hp is capable... several things.
A. It was NOT done on a chassis dyno (even though there ARE chassis dyno's that CAN handle this power)
B. Its a PRO STOCK DRAGSTER, its not a unibody civic. Its a fully roll caged, tubbed out car with a 115 octane burning motor.
I will take pictures for you, its running two demon 1150 carbs on top of a wieand 8-71 blower, pushing in 21-23 lbs of boost. He transfers this all through a TH-400 transmission that is air shifted.
In correspondence to what curtis is saying, there is someone mentioning that HP is always better. First- you say "Oh, I can just change the gearing in the transmission and rear end to compensate, and run the gear longers. Higher HP and running it longer on a specific gear have next to nothing to do with eachother. The powerband of a motor is entirely on how you set it up. You can make a motor with the HP range flat across the board or same with torque. If you ever notice, a lot of european sportscars have high HP numbers. On some reviews you will here them say "This engines are great for racing, but when it comes to daily driving, the lack of lower end TORQUE makes it unbearable"... but im with what curtis is saying, so before you question me, yell at him...
A:
the 383 is actually a bored out 350 with a 400 crank....
A:
Horsepower is a unit of power, and not an equivalent term. Also power does not represent the amount of work an engine can do unless it is for some given duration of time. Torque is representative of work, but work that causes rotation.
Torque is the amount of work the engine can perform. Torque represents a force at some distance, hence the units “lb-ft” or “Nm”. Power on the other hand is work per unit time, commonly in horsepower (hp) or kilowatts (kW). But since power is simply work divided by time the following equivalent units could be used:
1 hp = 550 lb-ft / s
1 kW = 1 kNm / s
An interesting point that you bring up is that you cannot produce work unless you have force and distance. With linear work you must apply a force to an object and that object must travel some distance. However, since torque involves rotation the outcome is slightly different. When applying a torque to a shaft, the shaft need not rotate for the torque to be non-zero. As long as the force is applied at some distance from (and not through) the centroid of the shaft a torque will be generated. Hence, at 0 RPM one could have X amount of torque with 0 hp.
I don’t think anyone here believes ‘power’ is a valueless performance characteristic. However, I think the point everyone was trying to emphasize is ‘horsepower’ ratings are often misunderstood and their importance exaggerated. Many people just don’t understand the relationship between power and torque.
A:
just found this, good info -
i want to copy and paste the whole page, but just this is importent for now:
the convertion for horsepower is: horsepower = torque * RPM / 5252
think of HP as a measurement of torque at a certain speed, thats why the peak horsepower range is commonly near the redline
A:
hey RandomTask i was realy talking about actual cars... and not hollowed out tube frames with a fiberglass body put over it...
ok well if we eliminate all the non regular gas burning engines what is the fastest car in the world... i mean there has to be one... and if u can get it the specs on the engine in it
A:
define "regular gas" you mean 87 octane? or any form of automotive grade gasoline. or do you just mean the fastest production car?
also you want the fastest or the quickest? two very different things.
the last time i checked the land speed record was 763.035 MPH, just over the speed of sound, but it was powered by two jet engines.
A:
87-100ish octaine gas... not meth... ya i know about the jet cars... but thats jet fuel :P and i guess both... fastest quarter fastest top speed...
A:
production car or upgraded car?
Or is the rule they must run on premium and under?
A:
as far as what engine is best there are a lot of other things to consider such as the engines center of gravity (a higher one will hurt handling) the engines weight, flat and v engines weight more than inline because they jsut have more material to them. also consider running duration, a daily driver needs to last a lot longer than a le mans car and an F1 car and especially a drag car. if you want a drag car, everyone will tell you a big block v8 and if they dont they are wrong(save for maybe a few odd exceptions). if you want a sports car your looking at a high reving, light weight engine. big blocks dont work so good for that (although the new ford GT seems to do ok, though it doesnt race).
A:
These are pretty much the fastest street legal cars you'll find, and they do it on 93 octane.
i want to copy and paste the whole page, but just this is importent for now:
the convertion for horsepower is: horsepower = torque * RPM / 5252
think of HP as a measurement of torque at a certain speed, thats why the peak horsepower range is commonly near the redline
A:
hey RandomTask i was realy talking about actual cars... and not hollowed out tube frames with a fiberglass body put over it...
ok well if we eliminate all the non regular gas burning engines what is the fastest car in the world... i mean there has to be one... and if u can get it the specs on the engine in it
A:
define "regular gas" you mean 87 octane? or any form of automotive grade gasoline. or do you just mean the fastest production car?
also you want the fastest or the quickest? two very different things.
the last time i checked the land speed record was 763.035 MPH, just over the speed of sound, but it was powered by two jet engines.
A:
87-100ish octaine gas... not meth... ya i know about the jet cars... but thats jet fuel :P and i guess both... fastest quarter fastest top speed...
A:
production car or upgraded car?
Or is the rule they must run on premium and under?
A:
as far as what engine is best there are a lot of other things to consider such as the engines center of gravity (a higher one will hurt handling) the engines weight, flat and v engines weight more than inline because they jsut have more material to them. also consider running duration, a daily driver needs to last a lot longer than a le mans car and an F1 car and especially a drag car. if you want a drag car, everyone will tell you a big block v8 and if they dont they are wrong(save for maybe a few odd exceptions). if you want a sports car your looking at a high reving, light weight engine. big blocks dont work so good for that (although the new ford GT seems to do ok, though it doesnt race).
A:
These are pretty much the fastest street legal cars you'll find, and they do it on 93 octane.
A:
gimme da specs! and does that alow for imports?
ya the fastest that can run on normal everyday premium-unleaded
A:
A fast quarter mile street legal car that runs on premium unleaded, for one runs a fine line. the fastest I can think of here is a street registered Big Block ford Falcon XB, running an nitrious injected 505ci V8. This guy drives it to the strip and back, his car runs 8.90s, but thats an extreme example. Something more practical but very fast to would be be XR6 Turbos. Basically a Ford Falcon(4.0l straight six) with a big aftermarket Turbo and injecters on it car runs flat 11s with only $10,000 spent on upgrades, so for about $60,000 you can get a nice daily driver and a stonking dag car.
A:
$60 000? Holy shit! You can buy a Z06 way less for that and run 11.9 stock with race slicks. Or for about 15 grand you could get a LS1 camaro with heads/cam or 150-shot, and few other minor things, and probably run high 10's on slicks.
And yes, the pump gas drags are open to anyone who can meet the requirements. But you probably won't find any imports there, the only way they can make any power is with huge amounts of boost and race fuel.
A:
I think he's talking Aussie prices, since you can't buy the newer falcons here in the US. 60,000AU is approximately equal to 45,000US.