Cylinder Head Modification Part 1

To many, cylinder head modification is a black art. Indeed, it is a complicated undertaking, especially if you are attempting it yourself, but do you really know why you want to take a grinder to such an expensive piece of hardware? The obvious answer is “I want more power,” but that answer is far too general in terms of determining what to grind, and where. Much has been learned, through trial and error, as well as through the application of sound engineering principles. In the days when the V-8 was king, and Chevrolets, Fords, and Mopars ruled the streets, a new breed of men was born – those with grinders in their hands and a hunger for power in their hearts. Much of the technology they developed on these two-valve pushrod motors has been refined and re-applied to the modern four-valve overhead camshaft engines of today, and with success. Many of those involved with 60’s and 70’s muscle cars are today applying their prowess to the sport-compact market – but is there more to be gained?

When talking about cylinder head porting, everybody talks about flow and velocity. Flow is, basically, the amount of air that passes through some portion of the cylinder head, usually through an intake port or an exhaust port, and measured at differing amounts of valve lift. Flow is often measured in cfm, or cubic feet per minute. Velocity is simply the speed of the airflow. Both are important. Both are related. Knowing how they relate to one another and how they affect performance is the key to unlocking the cylinder head’s potential. Flow numbers can be deceiving, however, and don’t tell the whole story. You’ll see many different tuners talking about flow numbers at different levels of vacuum, at different valve lifts, so it’s hard to equate them. We’re not going to talk about hard numbers here – we’re going to talk about what really happens with airflow inside your engine.

The key to making power is to create energy in the combustion chamber. This energy works to drive the piston down in the cylinder and cause the crankshaft to rotate. The more air/fuel mixture you can get into the cylinder, the greater your potential for creating more energy, and more power. That’s why turbochargers and superchargers work so well – they cram more fuel laden air into the cylinder while the intake valve is open. In a naturally aspirated vehicle, the intake mixture must be drawn into the cylinder by the pressure drop (suction) caused by the piston moving down in the cylinder. The mixture is then compressed by the piston, and ignited by the spark plug. The burning mixture expands, pushing the piston down again. The next time the piston comes up, the exhaust valve is open, and the spent gases are forced out through the exhaust system. Then it all begins again, and I’m sure you seen the diagrams and heard the explanations a dozen times or more.

Your engine is actually an air pump, and it is very easy to determine what the maximum airflow requirement of your engine is. Let’s use a hypothetical engine to illustrate what we mean. I like using round numbers, so we’ll use a displacement of 100 cubic inches, which is about 1640 cubic centimeters – a little over 1.6 liters. This is a size that most sport compact enthusiasts can relate to. We’ll assume that it’s a 4 cylinder engine, so each cylinder displaces 25 cubic inches. (Six and eight cylinder guys can multiply our answers by whatever factor they need to get to their displacement). Since we’re going to assume that most engines have one intake and exhaust port per cylinder, we only analyze one cylinder at a time to get a feel for what’s going on.

Let’s set the rev limiter at 8000 rpm, and figure out how much air can possibly flow into one cylinder at that engine speed. We’ll round things off to whole numbers as we go, to keep it simple.

25 cubic inches per cylinder x 8000 rpm = 200,000 cubic inches per minute.

We’re talking about a four cycle engine, so the intake valves are only open every other crankshaft revolution, so we divide by two.

200,000 ÷ 2 = 100,000 cubic inches per minute.

Seems like a lot, but we have a little more math to do. We’re going to convert from cubic inches per minute to cubic feet per minute (cfm), since that is a very commonly used unit. There are 1,728 cubic inches in a cubic foot.

12 inches x 12 inches x 12 inches = 1728 cubic inches

So -

100,000 cubic inches per minute ÷ 1,728 cubic inches per foot = 58 cubic feet per minute (cfm)

That’s how much air the we need, every minute, for a 25 cubic inch cylinder to be completely filled on every intake stroke at 8000 rpm. Multiply by 4 and you need about 232 cubic feet of air for all 4 cylinders. That is roughly equivalent to a box that measures just over 6 feet on every side. For those of us who grew up racing V-8 musclecars, a little easy math tells us that a 400 cubic inch motor would need over 900 cfm of air at 8000 rpm (if you could rev it that high without floating the valves or ventilating the engine block with a connecting rod).

That intuitively feels right to those of us who used to race small-block V-8’s. To us, a 650 cfm carburetor on a 302 cubic inch motor that never saw more than 6000 rpm was enough for good performance. These days, we think of throttle bodies and fuel injection, but the same theories apply.

As usual, more later. I'll put up a hubpage again and suggest some selected reading when I get a chance.