Tire Tracks: Hot for Teacher

It’s unmistakable.  The drum intro to Van Halen’s Hot for Teacher is undeniably good even if you hate rock and roll.  The reason for this is the geometry of the human ear.  Certain frequencies, both pleasant and offensive, resonate in the ear canal of most people, unless, of course, you have the cauliflower ear of a wrestler or mixed martial artist, in which case I have no idea what the hell you hear.  This drum intro is pretty damn close to one of those frequencies.  The same explanation can be used for harmony.  Certain notes only sound good together because of the ratios of the human ear.

In our world of increasing fuel economy standards and social responsibility, we’ve forgotten the most important part of the automotive industry:  passion.  The two aren’t necessarily mutually exclusive, but building a car that evokes an emotional response takes resources currently being used to make a pickup truck get 30 miles to the gallon.  Passion just doesn’t come from the bodywork, it’s the feel and sound of  a vehicle.  Everyone seems quick to dismiss the once mighty V8 as a relic of the past, with its poor fuel economy and excessive power.  The question is posed: Why not have a 4 cylinder engine with 300 horsepower that gets better fuel economy than an equivalent power V8?  First, real world usable power is more than the peak number highlighted in car magazines and should be reported as an average power over the entire rpm range.  Which is why I ran down a higher horsepower base Porsche 911 Carrera in a slightly modified Mustang not too long ago.  The 911 driver was shocked enough that it almost unspiked his $100 haircut.  But -- and I can’t believe I saying this -- I’m not here to talk about power.  The sound of a V8 comes as close to a harmonic chord as any engine configuration we’ve ever created.  Which brings me back to Van Halen.

The drum intro sounds a lot like the over-cammed idle of a big block V8.  There are a couple of things that largely determine the sound of an engine.  The most important is primary firing frequency, or the explosions that make the pistons move.  The four stroke combustion process has each cylinder firing once for every two rotations of the crankshaft.  Thus, on a V8, for every revolution four cylinders are firing, creating a fourth order harmonic.  As the rpms rise, the cylinders keep firing faster and faster, but it is always fourth order, or four times as frequent as the revolutions.  The other dominant sounds are the exhaust gas pulses coming from the headers and the valve train noise from the cylinder heads.  There are a lot of things that can change these frequencies, such as type of exhaust headers and how many valves per cylinder you are running, but the approximation of 2.5 and 1.5 orders, respectively, is not bad and fairly consistent.  The explanation of why these orders work out like this would probably make most of you stop reading, so you’ll just have to trust me on this one.  The point is, when combining harmonic orders of 4, 2.5, and 1.5, it’s pretty close to a musical chord.  The sound itself is determined by the type of crankshaft you are running, which is why a Ferrari and muscle car will always sound different, but they both are hitting the same chord.  There are a lot of other components in the engine making noise, but these three tend to be the most dominant.

Most engine configurations, excluding rotary engines which have a very different anatomy, have the same approximate 2.5 and 1.5 order content.  A V6, however, with two fewer cylinders, has a 3^rd order harmonic to go with the other two, which creates a near perfect dissonance, which can explain why a V6 sounds great to some people and thrashy to others.  A four cylinder engine with a 2^nd order in between the other constants, then, is like pushing three piano keys all right next to each other and expecting it to sound like a chord.  However, I will say there is something appealing to me about the antisocial sound of an inline four at redline, it seems like the automotive equivalent of punk rock.  The firing order content also explains why a V10 sounds like a UPS truck with a hole in the exhaust.  In higher cylinder engines, there are so many spark plugs firing that it begins to mask, or alias for the technically inclined, the upper rpm range, which makes the engine sound smooth, but not as harmonic.

Proper exhaust tuning is essential to actually hearing the order content that creates this chord.  The wrong muffler or resonator can make your car sound louder, but it'll also remove the frequency band that gives it the right growl.  OEMs spend quite a lot of time and money making sure a performance car or truck has the right sound without adding power-robbing backpressure into the exhaust systems, so think about that before taking a SawzAll to your current system so you can add a coffee can sized exhaust tip to the back of your car.  And before you throw out your V8 to get the latest electric plug-in vehicle that only shoots baby squirrels from the exhaust pipes, think about Van Halen.  Because I don’t know about you, but I’m definitely still hot for teacher.