Internal combustion engines are basically an air pump, with the addition of a combustion cycle where fuel is mixed with the air and exploded to create a self-sustaining series of events. The faster the engine runs (RPM), the more air and fuel it consumes.
As RPM increases, there is less and less time is available for these events to transpire. Filling the cylinder, and expelling the burnt gasses needs to happen in a shorter timeframe. As RPM increases, time decreases. (F=1/t)
So the induction and exhaust systems need to be able to complete their task in less time. Making them more efficient is paramount so they can keep up with demand and time constraints.
Think about that for a second; All modifications to increase horsepower (making more Torque at a higher RPM) Are centered around trapping the most amount of air in the least amount of time.
Once the speed of the engine reaches the point where it cannot completely fill the cylinders with air and fuel, The output diminishes. Filling the cylinder with as much air as it can possibly hold is known as Volumetric efficiency. Stuffing 10 lbs. of air into an 8 lb. bag is where you can gain significant amounts of torque.
Longer duration camshafts, more efficient ports and runners help extend the engine's operating RPM range by allowing the cylinders to fill more completely while the events are happening faster and faster. (There's just less time available). That's what increasing the engine's efficiency at higher RPMs is all about.
Peak Torque occurs when engine's volumetric efficiency (the point where the engine "traps" the most air) is at it's highest. See the diagram below. The point where peak VE occurrs is determined by many many things. Cam timing (Intake close) intake runner cross sectional area are Two of them. (There are many, many more)
When the runner's cross section is sized properly for the RPM and displacement of the cylinder, air velocity is optimized. Too big of a runner results in higher CFM capacity, but the sir speed suffers.
If the runner is too small, The velocity can actually get too high. This will cause "sonic stall" Yes, you read that right. If the velocity exceeds the speed of sound (1100 ft/sc) bad things happen.
(Port velocity on the flow bench is entirely different then what's going on in a running engine. They're Two entirely different things)
Airspeed is what stuffs the cylinder, NOT the CFM rating! Remember that!!! The port still has to be capable of moving enough air, but it's the air speed that counts.
VEs in excess of 100% (over filling the cylinders) can be obtained by utilizing the energy contained in the moving air stream to over fill the cylinder. This is known as 'inertia supercharging'. Utilizing this "ram effect" will give you a really broad torque curve.
CFM can give you a theoritical horsepower number (at the crankshaft), but without the proper air speed (velocity) all bets are off! If it's too slow, the engine will be peaky and will not produce the torque where you need/want it, or not at all. If it's too fast, turbulence in the port will choke off the flow. It's a balancing act, you have to get it right.
(potential) HP = .26 x CFM@28" For example: if you have an inteke runner/port flowing 300 @ 28",
300 * .26 = 78hp (156 for Two cylinders).
This is a pretty close estimate when compared to dyno results. You must account for drivetrain losses however. These are typically 10-15%. So take about 12% and you're going to be close.
The darkened areas are where the air speed is the highest.
Typical (generic) Cylinder head; This illustrates where the airflow restrictions are, and what percentage of gains are to be had at various points of interest.
A couple of things worth mentioning:
- The port walls that everybody looks at and marvels over (1.), are insignificant in the grand scheme of things. Funny how that works. People spend tons of time trying to make that look pretty, (they're wasting their time).
- The biggest gains are had 1" below and 1" above the valve seat. The exit into the chamber 'pressure recovery area' (8) is often overlooked. (Theres allot going on right there)