so does it means that for 2 same car with different engine, one make more torque at lower RPM and second make lesses torque at a much higher RPM. second engine will produce a higher Horsepower number but comes to running the 1st engine will run better issit? which is why bigger horsepower cars is not always faster? am i correct to say so ? ? ? ?

I got 1 more questions . why issit i try 2 different brand of 98 octane pump gas but 1 feel more powerful then another ? can anyone explain to me why ?

ThankzZ for all the replies ::

 

Maybe I didn't put it in easy enough terms but you cannot just rev your engine higher to mystically make more horsepower. Again, by definition, horsepower is just a function of torque and like MrFatBooty alluded to, the best ways to increase torque are to increase breathing ability with improved heads, cams, or tuning or by increasing the engine's displacement.

As for what car will be quicker, that all depends. If I have two cars that weigh the same and one makes 150 ft lbs at 5000rpm and the second one makes 125ft lbs at 7000rpm you are wondering if the one making less power will be slower. The answer is no, not if you gear the car right for that engine. If the first car has a final drive ratio of 4.56, then multiply by the torque 150 and the tires are actually seeing 684ft-lbs which is what will accelerate the car. We change the gear for the higher revving car to a 6.37 keeping tire size constant and multiply by 125ft-lbs and the tires are seeing 796 ft-lbs which will accelerate the car quicker than the higher torque but taller geared car. So really it just depends on the whole setup. A little too simplified but that is how it works.

 

It seems that most are having trouble getting the general idea. If they don't get the general concept, they aren't going to grasp the specifics. Why don't we look at the questions at hand?

Q. "There must a reason why issit a NA engine gotto be high rev ?"

A. The answer to this question is that the premise of the question is false. High-RPM operation is not a requirement for a normally aspirated motor.

Q. "since RPM kills motor why issit there is still so many racing engine builder wants to build these kinda engine ?"

A. Because a racing engine only has to last until the end of the race. The specific reason why they build high-RPM N/A motors is because the rules limit the amount of engine displacement and power-adders. Without those limitations, they'd be using cubic inches, not "cubic dollars" to boost output.

Q. "After market Cam tends to have long duration n higher lift which shift the power band to a higher RPM and still many people buy . . . why ?"

A. Many people simply don't understand that the scalar number for peak horsepower is the least meaningful specification for street use. They buy parts that offer the highest horsepower numbers, not knowing that in order to produce that level of torque, and therefore horsepower at a high RPM, the cam is causing the motor to make a lot less at lower RPMs. It's a common beginner's mistake.

Q. "its like why people don't build a engine with the power band below 8000 RPM so they need not rev so high everytime which in the end made the engine last longer ?"

A. The reason why people who know what they're doing build high-RPM motors is because of rules that don't allow them to build horsepower more cheaply and easily. Their competition will do whatever they can to get the most out of a motor within that set of rules, and so must they if they want to be competitive. People who aren't bound by any such rules do it because they're stupid, they want to emulate real racers, they think it's "cool" etc.

That takes care of the questions. Now, getting back to my point, adding RPM to increase output has its limits. The faster a motor turns, the less time there is for each of the 4 cycles in an Otto-cycle motor. 2 of those cycles involves moving gasses into and out of the cylinder. Less time means less gas movement. Less gas movement means less output. So obviously there is a limit after which adding RPM produces less output, not more.

I think the people who have posted the horsepower formula fail to understand that torque is not a static number. A motor that produces 150 lb-ft of torque at 5000 RPM will not make anywhere near 150 lb-ft at 8000 RPM. Therefore the assumption that more RPM == more HP is false. You can tune a motor to move the torque curve up and down the RPM range, but that's a function of tuning, not RPM.

 

It is much easier to explain and understand my point using static numbers. Of course, unless you have an electric motor which reaches peak torque instantly, you will be dealing with a torque curve and a corresponding horspower curve. I am well aware of how rpm and power are related as I own a machine shop and engines that I have been involved with have set NHRA Comp Eliminator national records, NMCA national records, won NHRA, NMCA and SCCA national events, hardly a beginner.

Let me give an example that is as easy to understand as I can possibly make it. Torque is force and horsepower is force per unit time, also called "work". Say you have Arnold Schwartzeneggar and Mini-Me each with a shovel and they each have to dig a ditch 1 foot wide and deep by 20 feet long. Arnold can remove a 1' by 1' by 1' amount with each shovel full, plenty of lifting force, and he can remove that amount every 30 seconds. Mini-Me can only remove .5' by 1' by 1' at a time, not much force, but he can repeat this every 10 seconds. What is Arnolds rate of work and how long will it take him to finish the ditch? 1CuFt/30 sec or 2CuFt/min, and he will finish the ditch in 10 minutes. What about Mini-Me? .5CuFt/10 Sec or 3CuFt/min and he will finish the ditch in 6.67 minutes, QUICKER THAN MR TORQUE ARNIE! Basically what you are seeing is that with limited torque it is possible to produce more work than a higher torque competitior if you have the combination set up right. That is why NA and other engines for that matter try to get to the higher rpm ranges, to produce more work, given the limits on their torque production. Warren Johnson was quoted as saying that the engine that makes the most explosions will win the race. Basically saying that rpm is good if you have an engine designed for it. Is that easy enough to understand?

 

woa, im fizzled, man this is worse than trig.........

 

You're not kiddin! It's hard to build an engine with just dollar signs though. It takes a lot of math to get it right and make good power.

 

:werd:
That's been pretty much my point.