All you need to know about 4-stroke exhaust systems

We're going to take a closer look at how the muffler works on a 4-stroke engine.

Let's get down to business. It's all very well to talk about the muffler and say "it was better before", but our petrollettes of yesteryear have been transformed into valve monsters!
That's right... we used to buy a complete line for our pretty 125 two-stroke at 350€... Now you can add the number 1 right in front of it!

Why change your muffler?

First reason: to treat yourself! We're capable of buying the exhaust system of our dreams for the price of minimum wage, and we complain when we have to drop €20 on a bouquet of flowers for Valentine's Day... That's right! Motorcycles are a passion, and we want to have the most beautiful one in the garage (even if we don't have it at home) to impress our friends and force ourselves to believe that this pretty titanium line will save us 5 seconds per lap...

WRONG!!! It will be 1 second with a more torquey bike and with more extension depending on the exhaust line chosen. Expensive per second, isn't it? Well... I'll add a little more, YES the performance is there with a power curve different from the original.

From the moment I switched to four-strokes, I've always ridden AKRA, and I'm happy to report that the quality of the product and its performance are beyond reproach.

My latest purchase was a full titanium line for a 2016 250 CRF. This bike has the best chassis on the market (for my taste), but a "soft" engine compared to its competitors.

Indeed, with the noise restrictions, the CRF ends up with two exhaust outlets that muffle the engine. It doesn't make any noise, but... there's a serious lack of it on corner exits!

"Between an original line and an Akrapovic line, the gains in torque and extension aren't always the stuff of dreams".

Other motorcycle models have much freer mufflers, where the difference between the original and the "factory" muffler is much less significant.

To give you an idea, here's the power curve of the 2016 250 CRF between the original line and the Akrapovic line. You'll notice a more or less significant gain in torque and extension depending on engine speed (+1.1 Kw at 11600 rpm for power and +1.0Nm at 11600 rpm for torque). For an investment of over €1,000, it's a little disappointing all the same!

We can also talk about weight: the original line, made from a very rigid material, is much heavier than a pretty stainless steel or titanium pot. As far as I'm concerned, there's a saving of around 1kg with the titanium version (a pot for a road bike is much bigger, and the weight saving is much greater than my little kilo).

Very often, manufacturers offer two exhaust lines, with a stainless steel version and a titanium version. In terms of performance, nothing changes, but titanium is a lighter, more abrasion-resistant material than stainless steel. Before getting into the technical side of things, I'm very happy with my purchase, as it saves weight and power, but the downside is the product's fragility...

If the bike starts to roll over, start crying because the repair is not as simple as on a trigger pot... €€€€€€€

Composition!

The exhaust system of a four-stroke motorcycle is made up of two parts.

1 - The manifold

The first part to be found at the cylinder outlet, it extracts the burnt gases towards the silencer. Its shape may vary, but the outlet diameter must always be greater than the inlet diameter.

On a four-stroke engine, there's no need to expand the gases as on a two-stroke. For a few years now, the appearance of manifold-mounted bombs (see photo akra) has enabled you to gain torque while lowering your motorcycle's DB.

As with two-stroke engines, the longer the muffler, the greater the performance in terms of mid-range revs and extension (a short muffler improves throttle response and acceleration at low revs).

In simple terms, the faster you accelerate, the faster the exhaust gases leave the manifold without passing through the canister. On the other hand, at low revs (cornering, exiting a bend), when acceleration is light and controlled, more gases pass through the canister, increasing the travel and consequently the torque of your motorcycle over a given range of use.

2 - The silencer

As its name suggests, the muffler reduces the noise of our motorcycle. Much larger on a 4-stroke engine, it often comes in three finishes (carbon, titanium, stainless steel), depending on the manufacturer.

At a time when manufacturers are looking to reduce noise, they've come up with the famous DB killers. With this system, the noise of our motorcycle is attenuated by reducing the diameter of the exhaust outlet. Of course, this results in slightly poorer performance, but guys, we're not (unfortunately) riding a MotoGP bike...

Last but not least: mapping!

It's all very well to invest in a pot to gain horsepower, but if it's not tuned to the engine, your bike's performance won't be as optimal as it should be. With the advent of electronic fuel injection, you'll need a computer to change all that.

You have two choices: either go to your dealer, who should have the hardware to modify your mapping, or buy a programmable CDI box. What's the point? If we keep the same air/fuel ratio for the combustion mixture, the explosion won't be good because your engine will require more fuel than the original programming will provide.

For my part, after acquiring an AKRAPOVIC line, I invested in a programmable HRC box so as to be able to modify the ignition.

For this article, I've based myself on all-terrain motorcycles, with the performance of the racing lines that result from them. As you can imagine, performance on big sports bikes can be much better with a well-braked (or even "baffled") original line than with a complete line from a specialist manufacturer... Removable DBkillers or catalytic converters will give you more power and a nicer sound, but beware of almonds on an emission or sound check...

PS: I'd be delighted to discuss the subject with you in the forum comments. Don't hesitate to write to me. If you liked the article, please share it with your biker buddies, it counts for IXTEM MOTO. Thank you ;)