6T5RUSH

I understand, in layman's language, the meaning of torque...it's that pushing in the small of your back feeling as you mat that right pedal, but I'm a little confused on that feeling as it relates to horsepower. Is it the horsepower number that allows the torque to be met? I know this may sound basic, but I am an easily confused guy. :crazy: You gearheads will set me straight on this. What I'm looking for is an understanding of peak as well as the power band of horsepower and torque. Know guys like Lars as well as SWDUKE will set me straight on this. Just curious.

Jim


[Modified by 6T5RUSH, 9:16 AM 3/10/2002]

Maximillian

In the most basic tersm, I always say it this way:

-> Torque is the amount of twisting pressure (like turning a wrench) that the engine can generate
-> Horsepower is a measure of how fast work (that twisting) can get done.

Example, a semi-truck engine (most diesels) produce tons of torque. However, they don't generally do too well at the 1/4 strip....because they can not do work quickly.




[Modified by Maximillian, 4:58 PM 3/10/2002]

kahuner

Torque is the "power to twist", and is not RPM dependent. So to move a heavy load from a dead stop, you need torque. Think of the difference between a 454 and a 283 ci engine. The 454 will have more torque. High compression, big displacement engines produce torque. Like a diesel.

On the flip side, to generate HP, you need RPM's. HP= torque x rpm divided by 5250. So big HP numbers are always at the expense of low end torque! Can't have both.

That's why you'll see cams advertised in terms of their power band. Idle to 5000 rpm vs 2500 to 7500 rpm. The first is a cam that is ground to produce low end power or torque. The second is to produce HP, while sacrificing low end power.

I have a feeling that you're thinking about a different cam in your car. If you're going to run it on the strip, which I know you like do, you could change out the cams and achieve more HP, but remember, you're going to have to wind-up the engine more and the engine needs to breath! (Possibly better intake/exhaust system may be necessary to realize any gains) And you need the matching rearend gears that will work with the cam. As you reduce torque, to make more top end HP, it takes a lower rear gear to launch the car.

Cheapest way to go faster is by adding cubic inches. It's why stroker motors are becoming so popular.

SWCDuke

Torque is a force acting angularly rather than linearly. Power is energy per unit time. Power is what accelerates the car and horsepower if a function of torque and RPM - (torque x revs)/5252. Power is ultimately turned into a drive thrust and for any operating condition power is proportional to force times velocity, or we could say that drive thust is proportional to horsepower divided by vehicle velocity. This is why gearing and and the shape of the power curve have a lot to do with the SOTP feel. The drive thrust is the "F" in Newton's famous F=ma equation. Or we can say that a=F/m, and the "F" can be computed from our power curve and gear chart.

Most guys just talk about "horsepower", meaning the peak value at high revs, but this is rarely available. When developing an engine configuration I make changes which primarily affect volumetric efficiency, and this translates almost directly to a torque curve. My objective for a street high performance smallblock is an "80 percent torque bandwidth" from 2000 to about 6000 RPM. This will provide good horsepower at all engine speeds and make for a strong and responsive engine across the range that we normally use on the street. The bottom end of the torque curve is primarly a function of valve timing, and peak power is a function of valve timing and flow efficiency. In simple terms one should select a cam based on minumum acceptable torque/power at the low end, and then work on the ports, inlet and induction system to achieve maximum top end power. Most guys select a cam based on how much top end power they want and don't give any consideration to the torque curve. That's why a lot of engines are overcammed and end up as stones in normal driving.

The flatter the torque curve (greater eighty percent torque bandwidth) the less "peaky" the engine. Valve timing is the key to achieving the desired torque bandwidth. A short cam will make the engine "torquey" with excellent low end torque/power. Too long a cam will make lots of top end power, but will result in poor low speed VE, torque, and power. The engine will feel soggy and unresponsive in most driving situations.

Lets say I have a smallblock that make 300 HP SAE net at 5500. Peak torque is 350 lb-ft at 4000 and torque at 2000 is only 250. Therefore, only 95 HP is available to accelerate the car at 2000 revs. Now I juggle the valve timing, basically make it shorter, and perhaps alter the indexing and find I loose five lb-ft at 6000, but gain 5 lb-ft at 2000. Peak horsepower drops to 295 - a reduction of less than two percent, but power at 2000 is up from 95 to 100 - a five percent increase. For a street engine this is a good compromise. I would probably keep jugglling the valve timing until I achieve 80 percent peak torque at not more than 2200. If power is insufficient, I would start reworking the heads, inlet and exhaust systems until I achieve the power I want, but you eventually get to the point where those last few horsepower get real expensive, so the amont of peak horsepower is basically a function of how much money you want to throw at the project.

An engine's peak torque is basically a function of displacement and compression ratio. WHERE that peak torque occurs and the bottom end of the eighty percent torque bandwidth is a function of valve timing - the cam, and once the cam is selected the amount of peak power I can achieve is a function of how much time and money I want to spend on the heads and valves, and the efficiency of the inlet and exhaust system. The inlet system (manifold and carb) is not terribly critical as long as the carb is not way undersized, but oversizing can cause metering problems that kill the low end. A key to peak power is exhaust system efficiency. I want minimum backpressure, but have to have acceptable noise limits, so on the car exhaust system development often pays better dividends in terms or peak power than changing the engine configuration.

Duke




[Modified by SWCDuke, 10:24 AM 3/10/2002]

6T5RUSH

Max, Kahuner, SWDUKE

Thanks for enlightiening me on this. Max, I'm not planning a cam change but I am trying to understand what I have and how it is dependent on my existing cam, intake, carb and exhaust setup. You're right about my once a year sessions at the Pure Stock Muscle Car Drags in Stanton, MI (now that's an oxymoron isn't it). I do enjoy that event but I realize like SWDUKE says you live with compromise or how deep are your back pockets. Kahuner, your definitioin does give me a greater appreciation to the term "nuthin' beats cubic inches".

Thanks guys!

Regards,

Jim

kellsdad

Here's a non-engine driven explanation just to add to the pile. Imagine you are raising a water bucket from the bottom of a deep well by turning the handle on a crank at the top of the well. If the handle is one foot long and you push on it with ten pounds of force, you will generate 10 foot-pounds of torque (turning force) on the crank to begin raising the bucket. Depending on the weight of the bucket, this may raise it at a rate of 1 inch per second. At this rate, the bucket will take a long time to get to the top, but you won't get too tired because you are not producing very much power. If you push harder on the handle, you produce more torque, the crank will begin to turn faster and the bucket will begin to rise more quickly and until it reaches, for example, one foot per second. If, at that point, you resume pushing on the handle with 10 pounds of force and the bucket will continue to rise at one foot per second. Now your arm will tire fast because you are producing twelve times more power than at the beginning even though you are producing the same amount of torque.

In this example, torque was needed to accelerate the rising bucket from one inch per second to one foot per second, and power was needed to keep the bucket rising once it got up to speed. Cars work pretty much the same way except we have to add in the effects of friction.


[Modified by kellsdad, 9:59 AM 3/11/2002]

kahuner

Yup; bigger is better! Just ask any woman.:D

Whaaat? Whaaaaat? What'd you guys think I meant?

I'm talkin' bigger diamonds, bigger house....com'on!

mrg

Yeaaaaaaaaaaaaaaaaaa, baby, yeaaaaaaaaaaaaaaaaaaaaaa .. ... :jester

joe58

To a drag racer, in very general terms, a torque increase will lower elapse time (ET) and more horse power will increase trap speed (MPH)

63Corvette

If you hang a 100 lb weight at the end of a 1 foot lever clamped in a vice, you will have 100lbs/ft of torque. If you substitute your hand for that vice, you will quickly realize what 100 lbs/ft of torque is. However, please note that NO work is being done! Nothing moved at any time.
Now horsepower is WORK done per time unit. Therefore. If you MOVE some weight in some time period, some work gets done....and you have generated horsepower, or watts, whatever unit of measure you like. Torque times rpm introduces the elements of work (torqueXRevolutions per MINUTE) divided by a constant (5252) = HP.