Originally posted by Ricky Benitez of MR2oc.com
condensed by me.
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A four cylinder engine has two cylinder events per crank revolution - i.e. each time the crank turns 360 degrees, two cylinders will have fired. Each time a cylinder fires, it only produces significant torque for about 60 degrees of crankshaft rotation. This means that by the time your crank has turned 1/6th of its rotation, the torque made by that cylinder is just about gone.
Additionally, each time a cylinder fires, it also extracts significant torque from the crankshaft as the piston goes back up, in order to compress the incoming intake charge over about 40 degrees of crankshaft rotation.
So it puts torque in and sucks a little torque back out of the spinning crank.
To see what this means in terms of instantaneous torque at the crank, take a look at the following diagram:
What this diagram shows is that for every 100 ft/lbs of continuous flywheel torque (which you'll see a percentage of at the wheels after you take drivetrain loss into account), each cylinder has to produce a peak of around 375 ft/lbs of torque.
That means that if your engine is producing 250 ft/lbs of torque at the rear wheels (or ~295 ft/lbs of torque at the flywheel), each of your cylinders has to deliver 1100 ft/lbs of instantaneous peak torque. If you're running torque levels higher than that, you do the math.
This is one reason why it's a good idea to build the cylinder head to flow and thus gain area under the curve across the entire upper RPM range. The other option is to turn up the boost to make higher power at a lower point in the RPM range by producing more torque. This latter strategy puts a greater amount of stress on the engine, most especially the rods and rod bolts.
For example, going from 200 rwhp to 300 rwhp when your power peak is at 5000 RPMs, requires a change of instantaneous torque from 925 ft/lbs to 1390 ft/lbs. That's a 33% increase.
Conversely, going from 200 rwhp to 300 rwhp at 7000 RPMs only requires a change of instantaneous torque production from 661 ft/lbs to 993 ft/lbs. While that's also a 33% increase from the original value, the final value is still another 33% short of the amount required at 5000rpm!
You can see that the torque requirement becomes exponential when you try to meet a similar horsepower target at a lower RPM.
In short, if you wish to make power, it is easier and safer to build your cylinder head and enhance your volumetric efficiency with items such as cams. This way, your motor will have the ability to breathe well enough at higher RPM's to meet your intended horsepower target.
Conversely, without upgraded cams or a way to improve VE at higher RPM's, you are restricted to making your power at a more midrange RPM.
That means you'll be raising the boost, the stress on your bottom end and your octane requirement, all in order to reach the same target.