What is Horse Power? How Does it Relate to Dyno Figures?

[Gadget]

WHAT IS HORSEPOWER AND HOW DOES IT RELATE TO DYNO FIGURES.

Let’s start with a few basic rules according to SIMON.

1.The No Magic Rule
This is rule number one; a ‘how to tell bullsh*t rule’ which goes like this; ‘there’s no magic’. In other words, if something looks too good to be true, then it usually is. Example: If 10 people owned XR6 Turbo’s with all running similar boost, air temperatures, similar mixtures, cam timing and RPM limits, nine out the 10 would make (for example) 300rwkw, but the final one makes 400rwkw. In this case we know something is amiss.

In other words nitrous oxide, methanol or some other contributing magic is required to provide this vastly higher figure. Because there is no magic there must be a logical explanation.

2.Some Basic Laws of Physics
These rules apply to us all, no exceptions.
RPM does increase piston speed, high charge temperatures are a problem, recommended pressure/flow rates of turbochargers need to be adhered to, camshaft timing does matter, fuel pump/line size/injectors do have pressure flow rates, and they can be measured. When these are measured in the same manner, the results will always be constant.

3.Compromise
Everything falls into this category, whether it’s price, reliability, economy (and everything else whether mechanical or socially); there will always be compromise.

4.The Nothing for Nothing Rule
6 of one, half a dozen of the other.


HORSEPOWER

In recent times horsepower, big horsepower for that matter, seems to have just become a number. You hardly pick up a magazine these days without seeing 500 to 750kW on the cover. So now lets have a look at that means.

One horsepower, what does it do?
One horsepower lifts 550 pounds (250kgs), 1 foot or 305mm vertically in one second.

Now before you all go okay, grab a calculator and do some sums whereby you, for example, multiply the one horsepower by 500, then maybe 1000, and then have a look at how these amounts of power will lift an 1800kg XR6 vertically and in how long.

PLEASE do your own calculations, however here are a couple of possibles:

A)1035hp would lift your XR6 (as above) 400 metres vertically in approximately 9.5 seconds.
B) 1035hp will lift a Melbourne City tram 11 feet in one second, or alternatively it will lift 11 trams one foot in one second.

To further etch this picture into your mind (noting that this really only applies to people living in Melbourne), I suggest you take a spin into have a look at the city tram depot and check out the size of 11 trams. Get the picture.

Engine horsepower is calculated by multiplying the torque by rpm and then dividing this number 5252.

An example would be 800 ft-lb at 6800rpm divided by 5252 = 1035hp. (For the record, these figures relate to the ‘1000hp engine’ we have on our engine dyno).

To make the same horsepower (as described above) at 5800rpm (the factory imposed BA Turbo rev limit) would require you have 938 foot-pounds at 5800rpm divided by 5252 for 1035hp. This is approximately a 17% increase in torque.

Coming from the other direction, (5800 to 6800rpm), you guessed it, it’s a 17% increase in rpm.

Why?
RULE 4: Nothing for nothing

Let’s face it; an engine is basically an air pump. To make power or to increase power it requires more and more air through it or across it, depending on the way you look at it. Rev an engine harder with less boost, rev it slower with more boost, it all equals air flow. If you want more power you need more airflow.


HORSEPOWER V TORQUE

For some people it’s all about horsepower, while others say it’s torque that accelerates a car. But of course you can’t have one without the other. Indeed, I have never seen a car without much power win anything, however I have seen races won without stacks of torque. Why? Gearboxes and diff ratios make torque, they don’t make power. EG. A Yamaha FZ1000 motorcycle.

It is torque that usually breaks things and in the case of the XR6 Turbo, culprits include autos/manual transmissions/clutches and as some have found out; CON RODS.

RPM also has its problems. Increased piston wear is probably the biggest, although most other engine components will suffer. But if we go back to our original example of 800 ft-lbs, we’ve already increased torque by around 250% over standard to achieve the 1035hp and have increased the rpm by only 17%.

Alternatively, achieving the same power at 1000rpm lower (5800rpm) we have an increase of 270%, and this can’t go on forever. Therefore an increase in rpm has to come. However, if we increase rpm by 1000, we are only looking at an increase of approximately 17%, and this is a long way from the 270% torque increase.
See Rule One: NO MAGIC.

Having dyno-tuned a huge diversity of engines over the past 15 or so years, an odd calculation came to me one day and I have been using it ever since. You see, people often talk about good engines making around 100hp per litre of capacity which is fair, however this figure really doesn’t apply to turbo or super high RPM engines.

After looking at the many engines I’ve tuned over the years, I concluded that another good way of looking at power per litre was to change this to torque per litre, per absolute boost pressure by RPM. Seems pretty close and works well on both naturally aspirated and ‘boosted’ engines.

Examples below:

Yamaha FZR1000: 80 ft-lb x 1-litre x 1 atmosphere x 10,500rpm divided by 5252 = 160 hp.
3-litre F1 engine: 80 ft-lb x 3-litres x 1 atmosphere x 19,000rpm divided by 5252 = 870hp.
5-litre V8 Supercar: 80 ft-lb x 5litres x 1 atmosphere x 7,500rpm divided by 5252 = 571hp (pretty close contrary to popular belief!)
Nizpro BA: 80 ft-lbs x 4-litres x 2.5 Bar Absolute (23 psi or 1.5 Bar boosted pressure) x 6800rpm divided by 5252 = 1035hp.

N.B. Absolute Pressure. Remember that we live on a planet that has 14.7 psi or 1-Bar of pressure at sea level. (Remember the Absolute Pressure number as this will come in handy in a later article on Inlet Manifolds and Exhausts).

Have a good look at all of the examples and you will see that all numbers are in fact quite close to fact.

This formula will come in very handy for you as once again it relates to the No Magic Rule, but more on this in our next article.

I hope that the information provided assists our forum readers and I look forward to answering any questions you may have.

Written by Simon.

Posted by Matt Dunn