Horsepower vs. Torque

June 19, 2015

 

 

This is a debate that never seems to end—which is better to have high horsepower or high torque?  In reality there isn’t a real debate and there is a clear winner.  Read on to see which comes out on top…

 

This is a slightly edited version of some analysis I did for a current racing series that was considering changes to thier power to weight calculations.

 

 

Torque is an instantaneous (or moment) of force.

 

Horsepower is a measure of the rate of work.  Specifically it is the measure of the rate that engine torque is applied.

 

So Torque is a force and horsepower defines how often (aka the rate) at which that force is applied.  This is why the formula exists:

 

HP = Torque X RPM / 5252

 

You take the force (torque) and multiply it by the rate (revolutions per minute).  You divide by 5252 to convert from angular (circular rotation) to linear force.

 

 

Horsepower is an excellent indicator of performance, because it gives you a measure of how much work (acceleration) can be done.  Torque can only tell you that when considered with RPM, which of course is exactly what Horsepower is.

 

Horsepower, not torque is measured on a dyno such as a DynoJet.  Specifically, the DynoJet measure the time to accelerate a drum of a known mass, said another way, it measures the rate of work done.  It expresses this as horsepower and then calculates torque from the measured horsepower.

 

Why all the confusion, then?  Take for example a 2.5L motor with a peak power of 240 HP vs. a 5L motor with a peak power of 240 HP.  In a battle like this the torquey 5L would win a drag race. People surmised that the 5L had an advantage because it has more torque, but another way to look at it is that the 5L has an advantage because it has a higher average HP.

 

How can I know this?  Because HP is a function of torque.  Thus more TQ = more HP.

 

 

Misconceptions about TQ and HP

 

The main problem with TQ and HP as people want to treat them as two separate things.  In reality they are two related things.  The thing to remember is that if you want to measure performance, than HP is always the best indicator.

 

Misconception #1

 

TQ gets you out of the corner – false.

 

Moving the car is continuous work—that’s HP.  The car that gets out of the corner better is the one that is making the most average HP accelerating out of that corner.  The bigger factor here is gearing.

 

 

Misconception #2

 

TQ gets you moving—false.

 

Again we are talking about work, which means HP is what we want to look at.  People will point to a diesel truck as an example of why this isn’t a misconception.  Diesel trucks make gobs of torque.  And there is no denying that a diesel truck will tow a heavy load better.  But why?

 

A modern turbo diesel may make 800 lbs ft of TQ at 1200 RRM and only has a peak hp of 400 HP at 3500 RPM.  A sports car might make 400 lbs ft of TQ and 600 HP, but can’t tow as well.  The reason isn’t specifically the TQ, but rather where the HP is made.

 

Let’s looks at the diesel’s HP at 1200 RPM

 

HP = (800 * 1200)/5252 = 182 HP.

 

At 1200 RPM is sports car is probably only making 100 lbs ft or TQ.

 

HP = (100 * 1200)/5252 = 22 HP.

 

Thus we can see why the truck can move from a dead stop and tow better.  It takes off making over 150 more HP than the sports car.  If the sports car could start from a stop at 5000 RPM, then it would be just as capable of moving the load.

 

 

 

 

Torque, a Competitive Advantage?

 

There are many ways of improving average horsepower today.  Examples include Variable Cam Timing, Variable Lift, Adjustable Boost Controllers, and detuning.  Even modern transmission have changed things.  Although they don’t change the average HP or breadth of the powerband, they do increase the average HP per lap.  This is done by letting the driver keep the car near peak power more per lap.  All of these things give a vehicle a competitive advantage. 

 

One often utilized tatic in race motors is the “detune.”  It has a dramatic impact on average power.  Detuning is an awesome competitive advantage in a limited HP class.  It allows a competitor to increase their average HP without increasing peak power.  Here is an example of an actual race legal detune:

 

 

 

This competitor has an average HP 277 HP across his entire power band--and it is legal!  Although the dyno only shows MPH, I know this is an BMW S54, so we are talking a 277 HP from 5200 RPM - 8200 RPM.

 

 

 

 

Now, consider this dyno:

 

 

 

Using his best run, this BMW 335i has an adjusted power of 282.7, ((277.77 + 287.62)/2) virtually the same as the car above.  But his average HP from 4000-7000 is only 255 HP.  (Please ignore the fact that this dyno because it is not using SAE correction, smoothing 5 like the previous example.  It is just for demonstrative purposes.) 

 

Assuming both cars weighed the same, which would you rather have, the Torque monster that has an average power of 255 HP or the detuned 277 average HP car?

 

 

 

More Analysis

 

To illustrate this point further I utilized the tool at:
http://vlsicad.ucsd.edu/~sharma/Potpourri/perf_est.html

 

This tool has been around for years.  It uses a simple but accurate physics model to determine performance characteristics of vehicles.  If you would like to play with it yourself, I found it works best in Internet Explorer with the site added to your list of Compatibility View sites.

 

 

For my analysis, I configured the 4 identical E46 M3s.  The all had a weight of 2920, a 200 lbs driver, 250 ms shift time, the same gearing, the same coefficient of drag, etc.  The only difference between the 4 cars is the power output of the motor.

 

Car 1
This is the modified and detuned S54 from above.

 

Car 2
This car utilized the 335i motor from above.

 

Car 3
This is a typical S54 with only slight tuning.

 

Car 4
My fictional 335i torque monster motor.  This has lots and lots of low end torque for illustrative purposes.

 

 

 

Results:

 

 

 

I would ask that you not get too caught up in acceleration times.  They are interesting, but aren’t very important to road racing.  Distance over time is more important.  This is why you can win a 1/4 mile drag race with a lower time and a lower top speed.

 

 

This chart illustrated the gaps between the cars.  This shows how far each competitor is a part after x seconds of acceleration.

 

 

 

 

This tool equates a car length to 15 ft.  Using that here is the approximate results after 14 seconds:

 

Car                    Gap to Leader(ft)    Gap to Leader (s)     Gap to Next(ft)     Gap to Next (s)

1 - Detune                     0                           0.00                               0                              0.00

4 - TQ Monster            34                           0.23                             34                              0.23

3 - S54 Typical             37                           0.25                              3                              0.02

2 – 335i                       60                           0.40                             23                              0.15

 

Assumed an average speed of 80 mph to convert gap into seconds.

 

 

After just 14 seconds of acceleration, Car 1 (S54 Detune) has a commanding lead.  The car with gobs of torque fared no better than a typical S54.  The car held to the current HP and TQ calculation is left in everyone’s dust.

 

Now remember, this is after 14 seconds of acceleration.  Think of the gaps after a 20 minute race!

 

 

 

Other Thoughts - Average Horspower is King

 

I also performed the average HP calculations on Cars 1-4 in my above example.  Here are the results:

 

 

                                    Detune              335i             S54 Typical           TQ Monster

Peak Power (HP)            282                   276                  282                        282

Peak Torque (ft-lbf)       279                   287                  240                        301

Average Power               278                   256                  263                        263


 

If you scale these power numbers to the gaps above, they scale nearly perfectly.  Thus the more average power you have, the better average performance you have.

 

So if you are looking for the fastest car on the track, forget about torque and focus on building the car with the highest average horsepower that is legal for your class.

Please reload

Featured Posts

Chasing down a Porsch 911 GT3 with a MINI Cooper?

February 5, 2014

1/1
Please reload

Recent Posts

August 18, 2015

Please reload

Archive
Please reload

Search By Tags