Hi ALL ~!~!

There must a reason why issit a NA engine gotto be high rev ? ? ? since RPM kills motor why issit there is still so many racing engine builder wants to build these kinda engine ? ? ? ? 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 ? ? ? :dunno: 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 ? ? ? ?

ThaNkzZzZ ~!~! :thumbup:

 

Whoa there. Chill with the extra punctuation and smilies please.

Normally aspirated motors don't necessarily need to rev high, but small displacement normally aspirated motors do. It has to do with the equation for horsepower, which is:

hp = (RPM * lb-ft) / 5250

So to make more horsepower you need more revs or more torque. If you don't hav the torque (i.e. from displacement or forced induction) then you need to increase the revs.

 

Racing engines don't have to last longer. Depending on the kind of racing, the engine can be rebuilt from once a year to several times a day. That's great for racing, if you can afford it. But it's not practical for street use.

 

i wouldnt say rpm are totally useless. both honda and toyota have engines that are designed to rev high...i know the celica gts/matrix rxs(i believe it is) have the 2zz-ge that redlines at 8100 and fuel cuts at 8400. and my buddies jdm h22a fuel cuts at i believe 8100. rpm isnt necessiarly a engine killer, but you do have be beef the engine up to regularly run that kind of rpm. back in my v8 days ive built a gm 350 for my brother that had a 8500 rpm cam in it...of course there is alot of **** you gotta do to make it run that kind of rpm but its possible and, if done right, can be very streetable

 

Which means High rev is for making good horsepower number ? ? which is actually more for boosting purposes ? ? ?

 

By definition, horsepower is just a function of torque and rpm. (HP=(TQxRPM)/5252) A typical well built NA engine will make between 1.35-1.55 HP/cubic inch. Unless it is a professionally built dedicated race engine, you will probably not make more than 1.4 hp/CI. So if you are torque limited, the only other option is to increase your rpm range to make more horsepower.

With a 1.8l engine (110CID) x 1.4 = 154ft-lbs. That is a pretty decent number. The question is where are you going to make that number and where is you torque going to start falling off faster than HP can increase (peak HP). If you are making 100ft-lb at 8000rpm then you are making 152hp at 8000rpm. But if you can make 100 ft-lb at 9000 rpm the you will be making 171 hp. Same torque at a higher rpm si what makes HP, make sense?

 

Basically just look at the math. Horsepower is the product of torque and rpm.

To get more power out of a motor without increasing its torque, you have to make it rev higher. The way you do this is make the motor withstand the stress associated with spinning faster.

To get more torque out of a motor without revving it higher, you have to increase its torque. The way you increase torque is to increase the breathing capacity of the motor through any number of ways including bolt-on mods, cams, more displacement, etc.

 

These recitations of simple mathematics are misleading. They imply that all you need to do to make mad horespower is zing your motor up to 10 grand. Not so. What'll actually happen before you start bending valves is that horsepower will drop off. It takes a lot more than RPM to boost horsepower. A motor is essentially a big pump; it moves air. But a pump can only move so much air. You can move the pistons faster and faster, but if no more air can make its way into the cylinders, you're not going to be able to make more horsepower.

It's not that hard to design a motor that generates x torque at y RPM. But just because it makes a certain amount of torque at one RPM is no guarantee that it can do it at another RPM. As RPM goes up, the motor eventually reaches the limit of its ability to breathe. Beyond that point, torque falls off as there is less and less time to move air in and out. Simple mathematics allows the horsepower number to go up briefly as torque falls, but since it's torque that makes a car go, the motor's performance is decreasing. And pretty soon the horsepower drops as well.

 

That's all fine and good... generalized as hell, and I'm totally stumped as to what it has to do with the question at hand.

 

Yeah seriously.

RPM and torque are obviously not independent variables. Just to explain the general principle it's simpler that way.

Relax, this isn't exactly what I would call some kind of "academic debate."