[fishtail]

not interested in 'approximations' or even 'educated guesses'

thanks in advance!

 

[JW]

Oh, no...this is going to be a big topic...

Here's a link: http://www.zhome.com/ZCMnL/tech/torqueHP.htm

But I'm sure someone else here (like TEG) can explain things much simpler...

 

[HIBBoyScott]

That was interesting. Thanks for the link. So lemme see if I get this...

HP is basically meaningless? Torque is what really matters. And the higher the torque peak then the better it is because the car will accelerate up to that peak. So let's say that Car A has a redline of 5000 rpm and peak torque occurs at 4900 rpm. Car B has a redline of 5000 rpm, but the torque peak occurs at 3500 rpm. All other things equal (gearing, weight, drag, peak hp and torque, etc) then Car A will win a drag race because it doesn't have to shift as often to keep the torque on it's upward climb. Right? Is this only correct if both cars are launched at their torque peak? What if both are launched from idle? Ah, so confusing...

 

[Kroezer]

Here's and intersting way of putting it:

"Torque is the ability to do work, and Horsepower is how fast you can do the work".

For instance, if a man(equaling 100hp and 100ft/lb. of torque) were to carry a 100 lb. bag 100 feet, that would be his torque(ability to do work). And, lets say he would be able to make the trip with that bag in 1 min. comfortably, that would be his HP(how fast he could do it). Now if he had more a little more torque he would be able to carry a 120 lb. bag every min. comfortably. Now, add more HP, and he would be able to make the trip with that 120 pounder in 1.5 min. instead of 2 min. and still be comfortable.

I heard this analogy a few years ago, and it really cleared up any questions I had on the issue....Hope it helps.

------------------
If you can leave black marks from the exit point of one turn, to the breaking point of the next, then, and only then do you have enough horsepower.

[This message has been edited by Kroezer (edited August 23, 2000).]

 

[TEG]

HIBBoyScott wrote:

>> HP is basically meaningless? Torque is what really matters. And the higher the torque peak then the better it is because the car will accelerate up to that peak.

NO! HP is not meaningless - and a higher torque peak is not necessarily better.

What I like it a "nice fat, flat torque curve".

Basically what doesn't tell the whole story is when they quote a "maximum" HP or torque number. What you really want to know is what the HP and torque look like across the RPM range. Sure you can get a "peaky" engine which produces good HP, but only at a very high RPM, and then you find that it has small torque and low HP at low RPMs. For example a high revving motorcycle engine. The problem with having low HP/low torque at low RPMs is that you can "bog" the car at low RPMs before you get the revs up to the nice power band. This is why a screaming [email protected],000rpm 900cc motorcycle engine would not make a good replacement for a [email protected] 1800cc car engine. Sure the motorcycle engine could move the car along (with the same 120hp), but you would have so much trouble getting the engine to pull the car off the line and get it going up to the high RPMs.

Having gobs of low end torque is what you need if you want to pull a heavy load so that you can start off the line without "bogging down". For instance, an engine in a motorhome may only produce 160hp, but it would be something like a monster v8, or a turbodiesel and it would have tons of low RPM torque. If you tried to put a 240hp Honda S2000 motor in there it wouldn't work even though it produces alot more HP (because it has poor low end torque).

Here are some general rules (though not always true):

1> More displacement=more torque (although not necessarily more HP).
2> Higher RPM operation (high redline) can give more HP, but not necessary more torque.
3> Max HP is most important when calculating top speed.
4> Max torque is important when calculating slow speed acceleration and load bearing capability... (For a big heavy car you want torque, for a little sports car you can get by with low torque, high HP).

Comparing extremes you see that people complain that the Honda S2000 & Integra Type-R are a pain to drive slow in stop and go traffic because the engine is yearning for good RPMs and you are working the shifter like crazy. On the other hand a car like the Corvette has a huge V8 (compared to the little Honda inline 4s) and it can grunt along at low RPMs in 5th gear no problem, but some people complain that the corvette is too heavy and doesn't sound as exciting when you are trying to drive on a track at high RPMs.

 

[mrclam]

at high speeds you need a certain amount of hp to overcome drag, for instance the m3 can't overcome drag at 155 mph so its top speed is 155mph, drag limited. not redline limited

 

[ckolsen]

An important thing to remember is that Torque (and HP) numbers are at the crank, not wheels. So while a driveshaft may produce X torque, a good portion of that force is lost while being transmitted to the wheels. The more efficient the drivetrain, and the less unsprung weight you have, the better your performance. That's one reason why an Integra Type R doesn't need as much torque as an IS300 to accelerate quickly. That's also a reason why Honda uses smaller wheels on the integra's, less weight for the motor to spin around.

 

[ckolsen]

More important points:
(1) Doubling RPMs = Doubling displacement.
By doubling your RPMs, you are causing the same increase in combustion as doubling the cylinder displacement (or number of cylinders). (in reality, there are losses but in theory this holds true)

(2) The more cylinders you have, the higher your friction losses, and the greater the surface area for heat transfer(loss), so smaller engines are more efficient by nature.

(3) Doubling air density = Doubling displacement.
This is why turbos and SCs increase power, by increasing the air density (and thereby the air flow) in the combustion process. (Doubling air density requires 14.7 psi of boost, which is pretty high.)

 

[mrclam]

a comment on doubling rpms...i think taht's what vtec was designed to do, keep the power up after reaching high rpms because i think someone told me you get valve float and other stuff like taht at high rpm?

Originally posted by ckolsen:
More important points:
(1) Doubling RPMs = Doubling displacement.
By doubling your RPMs, you are causing the same increase in combustion as doubling the cylinder displacement (or number of cylinders). (in reality, there are losses but in theory this holds true)

(2) The more cylinders you have, the higher your friction losses, and the greater the surface area for heat transfer(loss), so smaller engines are more efficient by nature.

(3) Doubling air density = Doubling displacement.
This is why turbos and SCs increase power, by increasing the air density (and thereby the air flow) in the combustion process. (Doubling air density requires 14.7 psi of boost, which is pretty high.)

 

[JW]

The concept of VTEC and VVTL-i (but not VVT-i) is so that there are two different cam profiles which coexist in the same engine. For a thorough explanation of why it is beneficial for having two different cam profiles see this:
http://www.fortunecity.com/silverstone/lancia/58/technical_school/engine/vvt_1.htm

 

[Retracnic]

Ever since I've been into cars, the HP vs Torque debate has raged on. There is a lot of math behind it and I've heard a lot of strange analogies. In simplest terms torque is what you feel when you mash on the gas. HP is that same 'feeling' spread over time.
A pickup truck with a big V8 will feel fast when you first step on it. You are thrown back in your seat and the truck lunges forward. That's torque. But soon the excitement subsides and you realize you are in a dog-slow truck. That's because many large engines have high torque but low HP. Now lets hop in a TT Supra... floor it, we are now glued to our seat. Keep accelerating and we stay glued to our seat. Why? High torque and high HP. Now get in a Honda S2000... we start off and there's not much excitement, but as speed and RPMs build we are gradually pressed into our seats. That's because the S2000 has high HP but not much torque. So in simplest terms...

Torque is what slams you back into your seat, HP is what keeps you there.

 

[Simple Lad]

Refer to "Automotive Terminology" at the following link:
http://www.velocityjrnl.com/vel-si-autoterms-1.html