Back pressure and performance:
Changing the exhaust muffler impacts the performance considerably. This doesn’t include the catalytic converter either (if your bike has one – the older, the less likely). This is because manufacturers will tune the fuel injection system or the air to fuel ratios at the carburetor according to the restrictions in the exhaust system. Removing the baffling and having a screaming exhaust may sound tough, but the lack of back pressure can result in the engine running lean. This can be a loss of power up to 10% and a hotter engine, which equates to more wear and tear. Tuning then becomes ever more important to return the power back to the engine, but you’re still left with a very loud exhaust.
The other thing that you will lose with an open and unbaffled exhaust is torque at the lower RPM range. Therefore the back pressure significantly impacts the torque curve of the engine. Low-end torque is particularly important for bikes that scoot around town or go through tight twisties as this is where the power and adrenaline rush is felt. There’s a reason why manufacturers spend big money to maximise the efficiency of the exhaust and still consider emissions and noise.
Source: Competition Conversions, SA
So how to reduce noise but keep the performance up?
The main aim of an exhaust muffler is to reduce the noise output that the high pressure engine gasses created during combustion. At the same time baffles and deflectors provide back-pressure so the engine isn’t expelling them too quickly either. If you’ve changed your stock muffler and replaced it with a straight pipe or a muffler that provides no restriction, baffling or sound deadening, you’re cutting your torque short and annoying everyone around you in the process. it’s essentially a lose-lose situation.
Baffles divert sound waves from one area to another. As they rebound from one baffle to another or get absorbed by sound deadening material, the energy in the sound wave is dissipated. Depending on the make-up of the muffler this can be achieved in multiple ways – often with a series of smaller compartments, perforations and sound deadening material.
Below is an example of an aftermarket exhaust purchased by a client that offers no baffling. We used Computational Fluid Dynamic (CFD) analysis to show how exhaust waves at the muffler outlet exit when unrestricted. You can see that it essentially exits freely. The exhaust is very noisy to say the least. The pretty red colours indicate high velocity.
Given the challenge of using the same muffler but upgrading the internals we’re faced with minimal space to provide baffling and sound deadening. The diagram below shows the proposed design. You’ll notice it has two baffles – one at the end, the other at start of the muffler. Through the baffles is a 32mm in diameter tube with perforations and deflectors. Surrounding the perforated tube will be sound deadening material. What you will notice is the bouncing around of pressure waves due to the baffles and the deflectors. The sound deadening material – typically a special fibreglass mat wrap – will absorb the sound waves before they eventually exit the exhaust.
What is the right amount of back pressure? Unfortunately, without a series of dyno tests you won’t be able to dial this in perfectly, but we generally work back to about 20-30% restriction from an open exhaust (simply based on what we’ve observed most manufacturers doing). The other important thing here is that we can check the combustion by inspecting the spark plugs. But we’ll discuss that in another article.
Here is the CFD Analysis run again but with the modification and its effect on the direction and velocity of the exhaust gasses.
We’ve used the same formula for several projects and found that inserting a baffle can reduce the noise level by about 3-4 dB(A). If you decide to add some extra sound deadening material then you can reduce it a further 1-2 dB(A). This may not seem like a lot, but an important thing to consider about noise is that loudness follows an exponential curve. So the difference between 60-65 dB(A) isn’t the same difference in loudness between 100-105 dB(A). Remember that for most old bikes the limit will be 100dB(A).
Have a similar project yourself?
You have two options; 1. Buy a baffle insert, modify it and insert it (can do the trick sometimes – but often not well) or 2. Build one from scratch and make sure it fits in perfectly, No rattles, no whistles or other strange noises. Read on!
Building a Motorcycle Exhaust Baffle:
- What material do you need?
- Mild steel round pipe (size will depend on your build and exhaust). Try to follow what the manufacturer would have had originally. 26-32mm diameter is common.
- Sheet metal (2mm will suffice – but try match this with the pipe thickness).
- High quality sound deadening material.
- Stainless perforated sheet to finish it off.
- What tools do you need?
- Angle grinder.
- Drill and hole-saw kit.
- Plate roller – or a round pipe that matches the existing outlet size.
- Die and tap kit.
- Set of spanners or hex keys.
- Step by step process
Step 1: Measure twice and cut once! – Measure the total length required for the muffler insert and cut the pipe to size.
Step 2: Mark and cut the perforations and deflectors with an angle grinder. Push in the deflectors just over half way. The more perforations the better. But here we’re using 3 large perforations.
Step 3: Using the right size hole-saw cut out the ends.
Be Sure to measure the inside diameter of your muffler entry and the inside diameter of the exit.
For the entry side, you want it to be a few millimeters away (so it’s floating). You’ll then have lots of air going through to the sound deadening material from the front. The rest of the air will bounce against the flat plate and make its way through the perforated and deflected pipe.
For the exit, you’ll need to make sure it is flush with the outlet of the exhaust muffler.
Step 4: Measure the exit diameter of the muffler. You’ll need to get the circumference by multiplying the diameter by Pi (3.14…) and cut out some flat plate to roll. Then roll the flat plate and weld it at the join. It should fit in the outlet nice and snug (it needs to be snug to avoid vibrating and whistling).
Note that you if don’t have a roller you can attempt to shape it by hand or use an oxy to bend. Even easier would be using a pipe that corresponds to the exit inner diameter.
Step 5: Now weld it all together to create an insert. It should slide in with just a little bit of effort. On the insert weld on a threaded nut. You’ll be drilling a hole in the underside of the muffler in the next step.
Step 6: leave the insert in the muffler and move the whole unit to a bench together. On the underside drill a hole for a bolt. 6mm is fine. Remove the insert and make sure it’s aligned. Check to see it slides through and bolts up nicely.
Step 7: Wrap the pipe with a few layers of sound deadening insulation and slide it back in. Get your 6mm cap head bolt and your hex key and tighten.
Bonus Step: If you want the outlet to look fancy, you can cut out a bit of perforated stainless sheet and shape it (We used a plenishing hammer but you can use a normal hammer). Then weld it in place. See the before v after shots below.
Over time the sound deadening material will breakdown and become less effective. You’ll be able to remove it and replace it by undoing the bolt underneath.
Before v After
That’s all for this tech article. Be sure to follow us on Instagram and facebook @jaxgarageau for more content on builds and other works. Youtube channel coming soon!