Why Most Quad Tail Pipes Are Completely Pointless


Quad exhaust tips are prominent in modern car design, be them on simple city cars to the latest powerhouse hot hatches. But many of them are either completely fake or have no real function apart from aesthetics, as we'll explain...

CT News Editor Matt Robinson is one lucky man – he’s managed to get his hands on the new Civic Type R as a long-term test car (the jammy git). And yet after an inspection of the fairly obnoxious rear, a question arose; are those huge quad exhaust tips for show or for go?

Honda isn’t the only manufacturer which seems to love a multitude of exhaust outlets, each worthy of the Alex Kersten ‘Fist Test’. To begin with, it must be made clear that each exhaust tip on the Civic is functional; every tail pipe has exhaust gases flowing out of it and therefore has a function. Many manufacturers have even less class and decide to provide fake tips that only serve as a cover for a single exhaust outlet, with a close-up glance exposing the illusion.

In truth, most four-cylinder vehicles would only ever really need a maximum of two exhaust outlets due to the lack of power that they produce. Even with hot hatches merging into so-called ‘super hatches’ like the Type-R and Focus RS, 300bhp is still a relatively small amount in the grand scheme of things. This means that the amount of exhaust gases produced by each revolution of the engine is minimal in comparison to larger capacity powerplants.

Seeing as the Civic Type-R is turbocharged, this changes things up in terms of exhaust sizing. To increase the pressure and therefore the velocity of the air entering the turbocharger, the headers are often smaller in size than on a naturally aspirated engine. Thereafter however, ‘turbo-exhaust’ systems are designed to have as little back pressure as possible to maximise the inlet velocity to the turbocharger and therefore a designed to be larger in diameter. However, the effects on exhaust flow from turbochargers of the size found in the Civic Type-R would be minimal in the grand scheme of forced induction. This means that despite an overall increase in diameter over the entire system, the relative size of the exhaust tips found on the Type-R are definitely over the top.

The dimensions of an exhaust system generally follow the theory behind something called Volumetric Flowrate. This is simply the volume of gas that can be extracted from a system in a given time, which can be directly correlated with exhaust systems. It is calculated by the multiplication of the cross-sectional area of the exhaust tubing and the speed of the gas flowing through it:

The main aim of an exhaust system is simply to get the waste gases from the engine out to the surroundings as quickly and efficiently as possible. So if you want the exhaust gases to move quickly, you restrict the diameter of the exhaust to form a small area which increases the pressure of the exhaust gas and accelerates it down the tubing. Restrict the exhaust too much however and you result in inefficient turbulent flow which can cause back pressure. Alternatively, if the exhaust bore is too large, the speed of the gases leaving the engine will be dramatically reduced. A compromise between the exhaust size and efficiency must therefore be found.

In terms of the exhaust tips, the only real function is to influence the tone and volume of your exhaust note. This is because the relative speed of the exhaust gas flow is depleted by the time it gets to the outlet of the system, therefore influences on the actual dynamics of the airflow can be neglected.

Like the Type-R, most exhaust tips provide a small expansion from the exhaust tubing before it. Using the Volumetric Flow Rate calculation, this means the exhaust gases will slow down, reducing the tone of the exhaust noise and creating what can be an extremely satisfying warble from the tail pipes. I’m sure Matt will provide us all with an exhaust note video at some point on Car Throttle Extra. So to answer the question, the quad tips on the Civic Type-R are really just for show and for a specific exhaust note with no tangible gains in performance influencing the design.

Although let’s be fair, they look sick af. Long live needless additional tail pipes