Getting the right stainless steel muffler for your café racer, scrambler or bobber.

Purpose Built Moto has an ever-expanding range of mufflers to help our community of riders and garage builders achieve the custom bike of their dreams. With a custom café racer or scrambler though comes a lot of grey areas for guys with little experience with building bikes but willing to give it a go. Exhaust size and Muffler selection is one of those grey areas. What size headers are right for your bike? How long do they have to be? What is back pressure and do I need it? What type of muffler do I need? All questions I will try to explain in this article.



So you’ve purchased the bike and cut it to pieces and thrown out the old muffler with a pea shooter hole in it that makes it sound like the Jetson’s mobile. What now? If you want to get the best finish possible you’ll choose to replace the headers with a custom system, if you’re going as far as an oversize piston kit + upgraded carbs it may be well worth it. The rule of thumb here is that output has to match input and vise-versa. If you have better air/fuel flow, higher compression and/or bigger capacity, you need an easier outflow ie. A custom built system or an off the shelf upgrade if it’s available.

When it comes to modifying the exhaust it can affect different engines to varying degrees. Some can breathe wide open with next to no back pressure, as long as it’s tuned right – Harley engines are good examples. Other engines if left wide open crumble in bottom end power and have nothing until you’re peaking in the rev range, 4-cylinder 4 strokes are another example.

The logical aim in modifying your exhaust would be to open up the power curve to its full potential and match any intake upgrades you’ve done all while achieving the desired growl you want. I won’t go into much more detail as I’m not a complete expert, but here is a more common example I see a lot.



The desired bike you’re working on, say its an air-cooled 4-cylinder. It’s had the airbox removed, A set of K&N pod filters fitted and the carburetor upgrade or re-jet with an oversize piston kit. If you were to leave the stock exhaust system as it was, chances are you won’t gain nearly the potential benefit you could. A good analogy is to imagine water trying to pass through a funnel. You’re pouring water into the funnel slowly and it’s passing through fine (standard engine and exhaust), as you speed up pouring water in the funnel it reaches a point where it will start to pool up as it can’t flow any faster (upgraded carb, piston kit, standard exhaust). The water coming in is the fuel/air from your carburetor, and the flow out is your exhaust. If the flow out is too restricted it will choke up the engine and you lose power. The only way to get the flow right is to increase the size of the outlet so you can achieve the right amount of flow ie. New exhaust system. This works in the opposite direction too, not enough fuel/air and too large an exhaust. You want to hit the middle ground here. If you want a well-rounded bike you don’t want to go too large either.

So you see now that each mod you make on the intake will have to have an equal modification on the exhaust.

Once you have you’re header pipes sorted and chosen the configuration

• 4-into-1

• 4-into-2

• 4-into-2-into-1 (my personal favourite)

• 2-into-1

• 2-into-2

I could go on. But now you need to choose your muffler.



I am a little Biased I’ll admit but often the cheap Emgo pipes you can buy from nearly every parts store and on ebay are very underwhelming. They tend to give your bike an overly loud and hollow sound, desirable for some but hardly ideal on the performance scale. Exhaust builders will normally relish the opportunity to build you a complete system with matching mufflers but you’re going to need deep pockets. The middle ground is to find a good slip-on muffler suited to your bike’s engine size and have the headers made to suit. In some circumstances, you can even get a pretty decent result just with a good slip-on muffler added to your standard headers.

Let’s get into some muffler details. A muffler has 2 primary purposes,

1. To provide backpressure to your engine by slowing down the exhaust gas flow, and also to give some anti-reversion properties to support the headers (if you want to go deep on this google “anti-reversion 4 stroke engine”

2. Muffle the sound coming from the engine and smooth the noise out.



A lot of the cheaper mufflers you’ll find will provide purpose 1, but fail at purpose 2. They achieve back pressure through an open hollow can and a baffle. Normally a fixed set of circular steel pieces in the centre of the muffler or a removable perforated tube. If you remove these you’ll get an overly offensive and crackly sounding engine. Not great for performance either. Whatever muffler you choose, If its overly different from your stock set-up tuning will be needed.

If you’re looking for a higher quality component, you’ll want a muffler that consists of the outer can, an inner perforated tube and packed with fibre or stainless steel wool. This construction provides better flow, smoother sound reduction and if sized correctly for your bike the right amount of back pressure. This is how your high-end mufflers are built, the Akrapovic, Yoshimura, and FMF among others all use this technique.



So does Purpose Built Moto’s muffler range. I looked around for a muffler that looked right and had the performance qualities I wanted for my pride and joy but I couldn’t find one. So I started making them. With an all Stainless steel construction, Fibre packing, and sized to universally fit most if not all bikes with little modification. As with all things I do I aim for a simple, clean and functional aesthetic. Also supplied is a removable conical baffle, made from perforated sheet, this can be installed or removed and trimmed down to achieve your desired noise reduction and also back pressure. These can be installed either at the coupling where the muffler meets the header, or welded into the end of the muffler.



You can see the full range of our mufflers for bikes anywhere from 200cc up to 1200cc in any shape and size. Available in 1.5″- 38mm, 1 5/8″- 43mm, 53mm and 2″ 51mm Dia. Inlets. If you have any questions please send through an email.

I hope this has shed some light on the art and science of your café racers exhaust system and allowed you to make a better-informed decision. Happy bike building!

Tom.