Breathing. It’s kinda important, to both you and your engine. Good car exhaust modification is a great way to remove the restrictions in the standard system and release more power. Plus a cool benefit is getting it to really sing when you stamp on the loud pedal! I have spent many a long time cutting out exhaust pipes and replacing them with my own better version. However, it is not as simple as replacing it all with the biggest pipes you can find, you can actually reduce the power in a naturally aspirated engine by doing that. There is a bit of a knack to it, in fact it is a science, and there is a formula that can be used to calculate the ideal exhaust pipe diameter… for this reason I would recommend that you buy exhaust parts that are made for your vehicle, as all these calculations have been taken in to account.
Generally there are four sections to an exhaust system:
- Back box/tail pipe/muffler, the very last section of exhaust where it exits the vehicle body.
- Centre section, this is usually “cat-back” meaning from the catalytic converter to the back box.
- Front pipe/front section and usually includes the catalytic converter.
- Manifold/header, basically the bit the bolts to the engine and joins to the front pipe.
It may seem counter-intuitive but unless you are going to replace the whole exhaust system in one go, it is actually best to start at the end. This is because we are going to be increasing the diameter (the size across a pipe) of the exhaust, and if you are doing in it stages, for example while you save up for the next part, you don’t want to have an area where the exhaust gas meets a step into a smaller diameter pipe.
Now for the actual back box itself, there are two main things to consider:
- The style of exhaust tip.
- The change it makes to the sound.
The style of exhaust tip: this is really down to personal preference, but some styles suit some cars better than others. A 3″ outwardly rolled tip would look great on a 90s Ford, for example. Whereas a 5″ slash cut would be better suited to a Japanese sedan, like a Subaru Impreza. Also consider how it will physically fit in your bumper. Often the recess for a standard exhaust won’t be big enough for something much bigger. You can cut the bumper to make the recess bigger, but sometimes it can be in an awkward place, such as a body line or there may be a towing eye or fog light in the way. Get a piece of paper, roll it up to about the same size diameter as the exhaust tip you are considering, tape it to hold it at that size, then hold it up to the bumper to give you an idea of how it will work out. You can sometimes tweak the position of the back box slightly, but not by much, so give this some thought.
The change it makes to the sound: Something very important to remember here, if you are going to change other parts of the exhaust, they too will change the sound and make it louder. So what you get with just the back box won’t necessarily be the end result. Often we want the car to sound “better” by making it louder but also a deeper and more aggressive tone. It is possible to play about with the silencers to change the sound, known as acoustically tuning the exhaust, something that supercar manufacturers do a lot. If you buy an off the shelf exhaust, it will already be “acoustically tuned” to what the people designing it thought was best. Look up videos on YouTube of the exhaust you are thinking about fitted to the same car as you have, to get an idea of how it will sound.
The usual design of the centre pipe is to fit down the tunnel, avoiding the gear linkages, snake around the fuel tank and over the rear axle, and join up with the tail pipe. There isn’t much room for improvement here, and this is usually the limiting factor for the exhaust pipe diameter. They can come with a centre silencer (muffler), and sometimes you can get a modified part that has the centre can delete. Removing this silencer can have a big effect on the tone coming out of the back box, sometimes better or worse. It can become tinny and raspy. Some people like this though, so each to their own.
If you are looking to mix and match centre and tail pipes then be sure that they will fit each other, this is not usually an issue on a flange joint, but can be an issue on a sleeve joint as the individual parts are made to fit the original exhaust size, not an aftermarket one. For this reason it is often better to get a “cat-back” or centre and tail section as one complete kit.
Okay now this is where we really start to make some gains. By this stage you will have you centre and tail sections of exhaust sorted out, maybe even fitted. You’ve got the exhaust tip sitting perfectly in the recess in the bumper, and it all clears all the other bits under the car so it doesn’t rattle and knock on anything. Maybe you have even been driving the car, finding that sweet spot to shift gear to get a pop or bang. Well be prepared to take it up a notch my friend, you know things are serious when you are changing out the front pipe!
The biggest thing to consider here is if you do away with the catalytic converter or not. For pure speed and power gains you would, but the law says you shouldn’t. There are ways around it, but that’s as far as I’m going to go on that one (for now ;)).
Keeping the catalytic converter: look for a front pipe that includes a sports or high-flow cat. It is possible to buy them separately and get it welded in. Some maps won’t run unless you fit a high-flow cat, so think about that too if you want to get a map in the future.
Catalytic converter delete: Now you are talking! something to consider if you are going this route is if you want to get a branched manifold and front pipe. This will open up yet more power gains, I will cover this more in the next section. If you opt for the single front pipe, because your car is turbo for example, then generally bigger is better. Obviously it needs to match up with the centre pipe so as not to cause any flow issues. Speaking of which, be sure to get something that is well-made as even just a slightly misaligned flange this far up the exhaust can be a major issue. Flow issues closer to the engine are bigger problems, spend a bit of money here to get something decent.
We basically have two ways here, turbo or naturally aspirated.
Turbo: at this stage, it really is best to speak to a dedicated engine tuner who knows the engine you have. There are tiny changes you can make at the point that will have major effects on how your car performs. This is also why it is so important to fit high quality parts in the rest of the system, if you speak to an engine tuner and you tell them you’ve got X Y and Z decent quality parts in your exhaust then they are much more likely to help you. They will be able to advise you on what material you should go for, and if you should wrap the exhaust or not. Sometimes heat wrapping an exhaust is bad.
Not a turbo: If your engine is naturally aspirated and you have sorted out the rest of the exhaust system you should have a nicely breathing motor by now. Fitting a branched manifold is going to free up the breathing a lot more. If we look at a four piston engine, just as an example, you can get a four branched manifold that will work its way down to one single pipe to join the centre section. There are different ways to do this though, and they each have a different effect on power and torque. You can get a manifold that goes from four pipes in to one, and off down to the centre pipe. Or you can get one that goes from four pipes, into two pipes, and then in to one pipe. You can get them with different lengths in between the “branches”, and of course pipe diameter is a big factor here too. This is where it is best to speak to an engine tuner who is familiar with the engine in your car. They will be able to advise on exactly the correct design of part for your application.
From Style To Performance
So there we have it, we have gone from deciding on a style of exhaust tip to making some quite serious performance improving steps. This is the addictive fun of tuning and modifying cars. It starts with a simple mod and before you know it you have the bug and you are making some major changes. It is all good though, because each step of the way you are making your car better. If an athlete needs to perform at their best they look at what they can improve in every aspect of their body, lifestyle, diet, etc. It is the same with your car, everything you do that improves the car, even if it is just keeping it clean (click here to read my post on Car Shampoo), will all add up towards producing more performance.
I’d love to hear your own thoughts on performance exhausts, and car tuning and modifying in general, so please leave a comment below.
Click Here For Part 1 Check back soon for part 3