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- Literally, like, so hot right now.

Exhaust pops & bangs explained by the experts

1y ago

64.9K

Have you ever been curious about that burble from an AMG? The pops and flames at car meets? I travelled to Litchfield Motors to find out how it's done, and just entering the car park had my heart racing. There were easily 20 GT-Rs there, mixed with a modified R8, 911 Turbo S, M3… the list goes on.

JDM Galore!

I was led through the workshop and introduced to Dan. Dan is the Technical Manager at Litchfield and he kindly sacrificed some of his valuable time to field all my nerdy questions on engine mapping and exhaust noise. Dan and I chatted about Litchfield as I’m taken to the dyno room, where Litchfield had prepared a (freshly washed, I might add) Nissan GT-R for us to play with.

At this point, I maintained my best poker face, trying not to display my schoolboy excitement as Dan told me more about the vast capabilities of their facilities. Eventually, I got my act together and started absorbing knowledge.

Purple = Fuel/Air mix, Brown = Exhaust gas.

We started with a little homework on engine cycles. Nearly all consumer cars have four-stroke engines. This means the piston has to move four times to generate power. These movements are named 'intake' (taking fuel/air mix into the cylinder), 'compression' (squashing the fuel/air mix), 'combustion' (the controlled burn of the fuel/air mix) and 'exhaust' (relieving the gas through the exhaust). However, it’s genuinely much easier to remember as suck, squeeze, bang and blow. I know…

The ECU in your car controls at what point in this cycle the spark plug is ignited (ignition timing) and how much fuel is injected at any given point, along with a host of other things. The altering of these parameters is commonly referred to as 'engine mapping'.

Keen to demonstrate the impact engine mapping can have, Dan hopped into the GT-R to demonstrate the difference between two Litchfield maps. Litchfield tailors the ECU mapping to each vehicle, based on its bespoke requirements. Different map options can then be selected by the driver whilst in the car. Here, Dan demonstrated the difference between a low boost and high boost map.

Pops and bangs can be attained through many different methods of modification. The main focus is around altering the timing of the car’s ignition map. On most cars, you want the combustion of fuel/air mix to happen just after TDC (Top Dead Centre – where the piston is highest in the cylinder). This means that the piston is only just on it’s way back down to the bottom of the cylinder – an explosion at this point gives it a (pretty significant) helping hand to force the piston down to the bottom of the cylinder as quickly as possible. The exact timing varies from engine to engine, but this is how the ignition is timed to produce maximum power.

Pops and bangs are generated when an explosion echoes in the exhaust. This is either fuel touching the hot exhaust before exploding, or an explosion happening earlier in the system and reverberating through the exhaust. The most common way of achieving this is by retarding the ignition timing so the fuel/air mix ignites later in the engine’s cycle. If timed correctly, the exhaust valve will be opening to allow gas – along with pops and bangs – to escape. This is what most commonly gives you the effect.

Those iconic GT-R tail lights are so unique

Many modern automatic sports cars will generate a pleasing burble or pop when changing gear – this is achieved in exactly the same way, but not just for the sake of producing that satisfying sound.

In a manual car, you depress the clutch and lift off the accelerator when changing gear. This reduces the load on the gears, allowing them to separate so a new one can be slid into position. In an automatic car, however, you can keep your foot buried in the carpet and flick the “+” paddle until you run out of cogs. This is because when you flick the paddle, the ECU retards (or sometimes cuts entirely) the ignition to reduce the load on the gears for just a split second. As above, this means either some unburnt fuel makes its way to the hot exhaust, or the exhaust port is opening as the explosion occurs in the cylinder.

Not everywhere can tune a Nissan GT-R. A 4WD dyno is a necessity.

Dan was keen to point out that there’s a limit to what is safe. All of that excess fuel and increased temperature isn’t always healthy for your catalytic converter or exhaust. This is kept reasonable when the car is moving with lots of air flowing around keeping things cool (well, cool for an exhaust).

If you’re that kind of reprobate who enjoys redlining their car in front of a crowd at car shows, exercise some caution and keep your popping and banging to a minimum. Luckily though, Dan tells me Litchfield have just the way to control your pops and bangs…

Yup. Litchfield have an app that lets you control pretty much any of your car’s functions from your phone, whilst driving. Boost pressure, boost in certain gears, traction control intervention and even the amount your exhaust pops and bangs can all be adjusted in real-time. All hopes of maintaining my poker face vanished at this point. Litchfield has even added features such as rolling launch control, for when you want to thoroughly embarrass non-GTR peasants on the move.

What are your opinions on this level of on-the-go engine trickery? Would you want this level of control in your car? Tell us what you think below!

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