HP, TQ, RPMs

[Shrek]

In an engine which is better also knowing you can change gear ratios to rev to redline?

[sinspawn1024]

The simple answer? You want as much horsepower as possible.

However, the simple answer only gets you so far.

It is important to distribute the power across the rev range. For example, an engine can be made to have very large diameter, short length exhaust headers, short stroke, wide bore, and short wide intake trumpets, and that engine will have a relatively high top end power, but it will be absolutely gutless in the low and medium rev range, making it useless until you're right above redline (making starts particularly difficult).

Alternatively, you can use a long stroke, long trumpets and headers, multiple collectors, etc. and make a very high torque engine that will start very well and accelerate very well from low speeds, but will lack top end power (muscle engines like those found in American sport- and muscle-cars tend to go this route, and use enormous volume to make up for the lack in top end power... that's why the Corvette needs 6.3 L to make the same amount of power that a BMW can make with 4.0 L, because the 'Vette makes much more torque).

Most manufacturers go for something in the middle. It won't have as much power at low rpm or as much power at high rpm, but it'll be pretty evenly distributed so that the engine doesn't seem to lag or fall off.

[Callum]

I think i once read Mr Enzo saying something along the lines of:

"Horse power sells cars, torque wins races"

[Scotracer]

I think i once read Mr Enzo saying something along the lines of:

"Horse power sells cars, torque wins races"

And that doesn't make any sense as Power is derived from Torque.

Horsepower is the theoretical ability of an engine to do work/produce thrust. You want as much as possible.

[Jersey Tom]

They are all equally as important.

Volumetric flow rate is pretty essential though (function of RPM and displacement). More air you can bring in, more fuel you can burn. Want to be able to turn fuel into energy as quickly as possible.

[Scotracer]

They are all equally as important.

Volumetric flow rate is pretty essential though (function of RPM and displacement). More air you can bring in, more fuel you can burn. Want to be able to turn fuel into energy as quickly as possible.

And if you run through the equations it isn't out of the question to equate volumetric flow rate with Horsepower. Horsepower is a function of torque and RPM. Torque is, in essence, a function of displacement. I know you know all of the above but I need someone to be able to check my lines of reasoning

Torque is important but it is pretty irrelevant without knowing the RPM at which it is produced. And when you know that, you have your power. So that's why I say HP is the most important.

[xpensive]

When Power is Torque times Angular velocity, as long as you have a seven-speed and quick-changing gearbox as in a modern F1-car, "torque" means nothing really. What you want is your engine to run as close to the maximum power-output as much of the time as possible.

With a CVT, you wouldn't have to care much about how wide the "power-band" is either.

[Jersey Tom]

Though quick-change 7-speed boxes aren't completely common in general.

In any event, an important point is that peak values are not as valuable as area under the curve...

[Ciro Pabón]

HP can be derived from Torque and RPM (as xpensive and JTom point out), so you only have two options.

First option: in Europe they build cars with engines the size of a pencil sharperner, turning so fast that sound like a cat being skinned while someone scratches a blackboard.

So, if you buy an european car, the higher the redline, the richer you are. ALL European cars have huge RPM dials with the red line marked as prominently as possible, to show all the girls the revving speed of your manhood. If your car is European and don't have a huge RPM dial, is a shame: throw it away as soon as you can.

Second option: in USA they love huge engines the size of a medium oil tanker turning as slow as their Congress, so, if you buy an american car, check this: the higher the number in the sticker with the capacity of the engine, the higher the number in the sticker with the prize of the car. ALL American cars have huge stickers right by the frontdoors to show all the girs the huge capacity of your manhood.

Finally, those are not options, but I have to mention them:

In Asia the most important part of the car is the exhaust, so, don't worry about torque, revving or HP of asian cars: they are all the same.

In South America merely by having a car (it doesn't have to run: it simply has to have the approximate shape of a car) you can get laid, so most south american cars are a misery without HP, torque or revvs.

In Africa they don't have cars, so they don't use engines, so they don't worry about HP or torque or anything. I assume they don't need a car to get laid: they are happy, truly.

So, this is the engineering conclusion: the most important of your choices is... the girl. To choose the Torque way or the RPM way you have to rely on the local customs about how to get laid: by the redline or by the sticker, pick one.

My recommendation: if you want a car with a lot of HP, find one with either large torque (a Wartsila large oil tanker engine, for example), or with high revving (an old Cox aircraft model engine, for example). The ideal would be a car with high torque and high revving (like an engine for a model of a large tanker developed by Cox and Wartsila).

[Belatti]

=D>

Still, Asian cars are way revy than Europeans... wich USD 20,000 European car can throw raw 9,000rpm like an Honda Civic?

In South America merely by having a car (it doesn't have to run: it simply has to have the approximate shape of a car) you can get laid, so most south american cars are a misery without HP, torque or revvs.

True. In general I used to get laid with my 70HP misery after making an Am-Eur wannabe in a Porsche look like a snail under heavy traffic. They key? Our cars have barely comfort gadgets, so they weight like a feather... Colin Chapmans way?

Now Im an Eur wannabe but without the revs: mine is the small displacement-turbo way!

Back on topic: narrow powerbands can hurt traction, even with good gearboxes and ratios. Then, there are some parts at some tracks where you drive with some cars and where you cant change gears that easy. There, torque and wide powerband rules.
Other thing is top speed: you only need HP from your engine for that, so if you are thinking about a land speed record or police getaways in freeways, you know, screw torque.

[xpensive]

I think Belatti has very good point in that it depends on the application, but I would still not go a long with separating the two. As long as Power is Torque times Angular velocity, you can always trade one for the other.

The "width of the torque-band" is another matter though, that is always useful in order to make the situation more convenient.
Not having to whatch your revs all the time, freeing your right arm to entertain your mistress while driving perhaps?

[Flummo]

Horsepower makes the car move.

Torque is used to tighten nuts, bolts, and to twist transmission parts to scrap metal beyond recognition. (Yes, torque is simply the twisting force, and twist something hard enough and you will break it. Raising rpm does not make the transmissions life difficult in the same way...)


What I want is as high power as possible, over the biggest possible part of the engines rpm-range. Not always a simple equation.

[xpensive]

Horsepower makes the car move.

Torque is used to tighten nuts, bolts, and to twist transmission parts to scrap metal beyond recognition.

Good point, torque without Rpm is nothing, give me a decent wrench, piece of matching pipe and I can easily show you 1000 Nm on that rusty bolt-head. But it won't be power.

[Scotracer]

Horsepower makes the car move.

Torque is used to tighten nuts, bolts, and to twist transmission parts to scrap metal beyond recognition.

Good point, torque without Rpm is nothing, give me a decent wrench, piece of matching pipe and I can easily show you 1000 Nm on that rusty bolt-head. But it won't be power.

Already stated by yours truly:

Torque is important but it is pretty irrelevant without knowing the RPM at which it is produced. And when you know that, you have your power. So that's why I say HP is the most important.

Yes I am that awesome

[xpensive]

Not the same, you completely missed the bolt-anology Scot. Sorry, try again?