Kids and their enormous exhausts on their tiny cars, we all know they’re the scourge of the road! But lets have a little look at the science of the situation and it should certainly make you feel a lot more positive about the whole situation. Believe me, there are reasons you should love to see them. At least if you’re a little bit evil and like to see idiots failing – and admit it, everyone loves to see others fail! This one doesn’t have many pictures, and it’s not much about acoustics but it has it’s place if that’s what you want to read, otherwise just take a look.
Now you all know the type of thing I’m talking about, and I’ll admit I’m on the side of the fence that says, “Oh for G*ds sake, shush” but bear with me because even if you like big exhausts you’ll find this interesting. You may even have a big exhaust on your car, in which case you’ll definitely find this interesting! I won’t bore you with pictures, but if a little car sounds like a subaru, or aspires to, then you know what I mean.
Big exhausts sound more “throaty”, deeper. This is basic physics. When low frequency sounds have wavelengths of 17 metres, a pipe of 17 metres is required to produce a resonance at that frequency (20Hz in this case). This basic theory holds and scales to say that bigger exhausts – like those found on a Subaru – resonate at lower frequencies so they sound deeper. Obviously as far as most people think and care to be honest and most people don’t care about this much theory! If we delve a little deeper though, I promise only a little, then problems will reveal themselves.
Everyone knows from being a kid, that squeezing the end of hosepipe makes the water squirt farther. This is because the water has to move faster through the smaller hole to maintain a constant rate of flow. I hope you’re still with me, because that’s as bad as it gets! The trouble with this little bit of knowledge is that if you make the hole much bigger, the water can’t move slowly enough to keep the hose full, and cavities of air will open up. In an exhaust system on a car, fresh air obviously can’t get in to fill the gaps in the hosepipes, but it will create gas starvation at the exhaust of the engine. “Big deal” I hear you say. However… Gases absorb heat and transport it away from the engine, everyone knows exhaust gases are hot (as well as toxic).
So a big exhaust will make your car engine hotter. Again, not a big deal you might think. But a hot engine will not run properly. It will put extra stress on the metal of the engine, the components in the engine, the cooling system, will result in a loss of engine power (that’s right, the car will actually get slower!) and will probably reduce the life of the engine. In extreme cases, it may even cause engine failure. Pretty serious (and expensive) issues if you’re a young guy without much money. So at this point, you’re either older and laughing, or younger and crying. If you want some real acoustics issues, these changes in temperature beyond what’s expected in the “normal run of things” will result in changes in the resonances of the engine block and it’s components. This will result in unexpected resonances in the engine which the mounts are not designed for, which will affect the engine, the transmission and even the frame of the car. A simple little exhaust can potentially compromise your engine, transmission and indeed your entire chassis! That would be a complete write off and the car would need replacing. Simple as that.
Now dealing with all these problems. They’re serious problems, but you’ll be pleased to know they’re just as easy to solve as they are to cause, albeit they are another expense. The very simplest solution is a larger air intake to account for the increase in airflow OUT of the engine. An excessively large intake however, will also result in an increase in engine temperature and all of the aforementioned problems. Easy to see why large companies make engines, and not the average people who live next door isn’t it?
An alternative to a larger air-intake which is much more effective, but also more expensive, is to have an engine cooling system which utilises a water-jacket. This ensures a constant flow of water through the engine block and back to the radiator to keep the engine block at a more controlled temperature, although can be susceptible to overheating, particularly on hot days when stationary in traffic for example.
As I’m sure you can appreciate by now, increasing the size of your engine exhaust is not a particularly smart thing to do. ESPECIALLY if it goes against the advice of the engine/car manufacturer. Otherwise you may get very familiar with this symbol!
Now we’ve scared everyone away to rip those huge exhausts off their cars, I’ll share a secret with you. The negative effects are calculable, they are even measurable. They’re only rarely a problem however, beyond the noise. So it seems that we’re going to have to put up with them for a bit longer. At least until Tesla and their electric cars take over the world!