Power '201'

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Advanced power theories and ideas

I'll assume that everyone that reads this has a decent understanding of the 4 stroke theory, and understands the basic physics behind how an engine works.

We've already established that engine power is almost directly controlled by the amount of oxygen you can put in a cylinder.  I explained that a 350 moves 350 cubic inches of air, a 454 moves 454 cubic inches of air, etc...  If displacement was the sole characteristic of engine power, there would be no aftermarket heads, cam shafts, or other parts.  Let's look a little deeper into the dynamics of making power.

I'm going to use the Chevy 350 as an example.  It is probably the most commonly hot rodded engine in existence, and there are literally over 1 billion of these engines out there.  There is a plethora of aftermarket products available for them, and a million different ways to make power with one.

In a perfect world, every cycle of your 350 would move 350 cubic inches of air.  Unfortunately, the engine is just not capable of completely changing air on every rotation.  Older engines, with less refined head technology normally only change out about 80% of the air inside them.  That means that about 20% of the air in you 350 at any given time has already seen one combustion cycle and has nothing to add as far as power production is concerned.  In fact, all the old air is doing is taking up space that could be occupied by fresh air, fresh air that contains the oxygen you need to make power.  The capability to change out old air for new is called volumetric efficiency.

This is part of the reason changing heads or cam shafts create more power.  Performance heads allow the engine to cycle more of it's own air out and allow more fresh air in.  With more air, comes more power.  As you improve your volumetric efficiency, you increase power.  As engine technology advances, engines are slowly having more and more volumetric efficiency as time moves on.  The LS1 engine from GM has a little over a 90% volumetric efficient in stock form.  That is amazing for a production engine, and it shows in the power they produce.  Very few factory V8's make anywhere near the power per cubic inch that LS1's do.  It's not uncommon for an LS1 with an after market cam and headers to make near 400 horse power at the wheels.  The reason these engines can accomplish this is because of the outstanding volumetric efficiency the heads afford.

Greater than 100% volumetric efficiency

So far, it would appear that once you had achieved 100% volumetric efficiency, that your engine would be tapped out. No matter what changes you made, your engine would never make more power because you were using every gram of available oxygen to burn gasoline and make power. While that sounds good in theory, it is actually possible to exceed 100% volumetric efficiency. That's right, an engine can move more air than it can hold. Here's how:

There are 2 key characteristics of air that allow volumetric efficiency to exceed 100%. Number 1 is that air can be compressed. You can put 10 lbs of air into a 5 lbs bag so-to-speak. You can not compress a liquid, but air can be compressed virtually any amount as long as you have the equipment. Even your body can compress air. Close your mouth and puff out your cheeks, you are compressing air inside your mouth. That compressed air is what is pushing out your cheeks. The second characteristic of air is that is has mass. Some people think air is weightless, and while air does weigh very little, it does have weight. Like all things that have mass, once they begin to move they have a tendency to stay in motion. Newton's first law of motion states, "An object at rest tends to stay at rest and an object in motion tends to stay in motion at the same speed and in the same direction unless acted upon by an unbalanced force."

Picture your intake manifold. Now pretend you are an air molecule being sucked inside. You travel through the throttle body or carburetor into the plenum or intake manifold, then you gain speed as you are pulled towards the intake valve. Suddenly, right before you pass the intake valve, the valve slams shut. You continue to fly towards the back of the valve and slam into it. The reason you kept moving after the valve was closed is because you have mass, and you were in motion. You wanted to stay in motion, but the valve prevent you from entering the cylinder.

If you were to time things right, you could take advantage of the fact that once the air is moving towards the cylinder it doesn't want to stop. Picture your piston moving down on the intake stroke. The intake valve opens and air is pulled inside the cylinder. As the piston accelerates downward, the air being drawn into the cylinder also accelerates. The air enters faster and faster, gaining momentum the whole time. The piston reaches the bottom of the stroke and starts to come up again. There is fast moving air in the intake manifold heading toward the cylinder, even though the piston is moving up, that air is still entering the cylinder. The air in the cylinder compresses slightly, the intake valve closes, and now you actually have more than 100% cylinder filling.

For simplicities sake, let's assume a 350 makes 1 horse power for every cubic inch of air it moves. So, your stock 350 with 80% volumetric efficiency is making 280 horse power because it is moving 280 cubic inches of fresh air per revolution. Adding heads and a properly timed cam shaft brings your volumetric efficiency to 95%. Now you are making 332.5 horse power, for a gain of 52.5 horse power. After some research, you select an outstanding head and cam package that brings your volumetric efficiency to 105%. Now you are making 367.5 horse power. That's the power of volumetric efficiency, the more efficient the engine, the more power it will make.