A performance exhaust is a lot more than just sound and looks. We show you why with the Fast Car exhaust guide rammed full of the best tips, tricks and advice…
Why would you want a performance exhaust? Most standard exhaust systems, in fact we would be inclined to say almost all, are restrictive to some extent even on a standard car, and pose a huge restriction on a tuned engine. Manufacturers have to adhere to strict noise considerations, not to mention ease of fitting, ground clearance.
Sometimes companies even deliberately make them restrictive so the standard car isn’t too powerful, and for these reasons performance gains can be easy to achieve. The gains on an otherwise standard car could be as low as a few bhp (though generally 10% is the minimum) but can often be 20bhp+ on some turbocharged engines and often improve spool up too.
There are a few other additional benefits including much lighter weight in many occasions, and on particularly restrictive systems the reduced backpressure could in theory actually help reliability a little. Read on to find out more about performance exhausts…
DOES A FREE FLOWING SYSTEM HAVE TO BE LOUD?
There is a simple answer to this, no. Despite the common preconception, big power and big noise do not run hand in hand, and if you had the money and space you could easily build a 1000bhp exhaust system that would pass every trackday noise limit in the land.
The reason most free flowing exhausts are loud is it’s a lot cheaper, easier, and lighter, to make a loud exhaust, as it needs little or no silencers, not to mention the sound is often an attraction to many buyers.
Silencers are not a restriction as long as they are straight through versions, which means the path of the exhaust gas through silencer is almost totally unrestricted, despite making the exhaust volume much lower.
The main challenge with building a quiet exhaust is the price. A straight three inch piece of pipe may be £25, but a three inch silencer of the same length may be £150, and when you may need four of them to pass the noise regulations, plus the extra pipe work and labour costs, it’s easy to see why many performance exhausts are loud; if they weren’t many couldn’t afford them!
TRACK NOISE LIMITS
This is a big talking point among trackday goers, because a large amount of cars fail the limits and therefore can’t go out on track. Generally speaking, tracks have a noise limit of between 95dB and 105dB static, with some having drive-by limits as low as 85dB, low enough that even some standard supercars fail the test.
Some circuits have purely static tests, meaning once you are on the move you can be as loud as you like, but the majority will still black flag you for being too loud on the move despite you passing the static test. Some circuits have very (and we mean very) occasional “Noisy days” where there are no dB limits, and some occasionally turn a blind eye to cars noisier than the advertised limit on special events.
But this is rare, so if you ever want to hit the track you really need a quiet exhaust system. Also be warned that what sounds louder to the human ear isn’t always louder according to the decibel meter. The low rumble of a turbocharged car often breaks the noise limits on track days far easier than a screaming N/A engine; never presume your car is quiet enough or too loud, get it checked.
The final thing worth mentioning is more and more trackdays are being banned at locations all around the UK due to noise complaints, complaints that would never have happened if the exhausts weren’t too loud in the first place, so it’s wise to fit a quiet exhaust regardless of the current rules at track or airfield.
As already mentioned, trackday noise limits are strict and are getting stricter all the time, and even on the road a loud exhaust attracts a lot of the wrong kind of attention from the police, VOSA, DVLA and so on. So a quieter exhaust system can be a really good idea.
The trouble is you don’t want to lose any performance, and you may not be able to get a full custom made ultra-quiet system, so what are your other options? Well, there are many, some cheap, some not so cheap; some free flowing, and some far from it!
These bolt inside the tailpipe and do exactly what the name implies – bung up the tailpipe, which quietens down the exhaust. Although many people don’t realise this, a bung is hugely restrictive unless your exhaust was far too big in the first place, and can often cripple a car’s top end power output.
Bungs can literally be fitted and removed in minutes, but when the majority of places you want maximum performance is also where you need to be quiet, their use is somewhat limited. As a quick emergency fix, the tailpipe bung is hard to beat, but ideal it is not.
I’m sure you’ve realised silencing via a restriction is a bad idea for performance. You need to do something better if performance is your goal. The most obvious and effective solution is extra silencers, meaning no lost performance while still lowering the volume. Getting an extra silencer or two welded in to your current exhaust system is an easy job for any competent welder or exhaust specialist.
If you don’t want to go this far and don’t mind your car looking a bit odd, some people fabricate removable extra silencers to attach to the back of their car for track days. A real silencer is far more expensive than simply creating a blockage, but it is the price you have to pay to maintain performance.
An option which actually comes as standard on many supercars and is getting popular as an aftermarket tuning option, is a bypass valve. This, unsurprisingly, bypasses your quiet, but restrictive exhaust at either the flick of a switch or at a certain boost pressure, letting the gas out of a less restrictive but louder exhaust, generally a straight pipe. Again, this keeps static and slow speed volumes to a minimum, but won’t stop you being black flagged for excessive drive-by noise.
Working in a similar manor to bungs are flapper valves, which are controlled either electronically or mechanically. When closed, they restrict the exhaust system and drastically lower the exhaust volume.
Unfortunately, when closed these are even more harmful to power than bungs, and we have seen turbo cars unable to reach even half their desired boost pressure with one of these fitted and closed as they are so restrictive.
The most overlooked way of reducing your exhaust volume is also one of the cheapest and least restrictive, and that’s cleverly placed exhaust outlets. Have you ever noticed a lot of race cars have turned down tailpipes so the exit actually faces the ground? This is so the sound waves hit the ground and are either absorbed or deflected everywhere, effectively and noticeably reducing exhaust noise.
Some people take this even further by having a side exhaust exiting from the opposite side of the car to where they know the noise meters are placed. A variation on this is to run one silenced side exit either side of the car, splitting the exhaust gas, and therefore the noise, 50-50 between each side.
It’s not uncommon to see a tuned car emitting flames from the exhaust, and the frequency this happens can be affected by the exhaust system. If you removed the entire exhaust from a car and revved the engine while looking at the exhaust ports you would see flames emitting from them on any engine, but the exhaust length and silencers make these flames invisible at the tailpipe.
On a more powerful car, especially one mapped to still inject fuel when off throttle, the flames can sometimes reach out of the tailpipe, especially on cars with shorter exhausts. Things like silencer boxes can reduce the chance of the flames a little, but conversely an exhaust that spits flames generally burns up the silencing material a little faster, reducing its effectiveness after a while.
As cool as exhaust flames look, the police really don’t like them, as our Stav once found out in the past when he was handed a £750 fine for “Endangering the public”, and “Dangerous emissions from a motor vehicle” amongst other things due to his exhaust flames…
DO ENGINES NEED BACKPRESSURE?
Once again, the simple answer is no, despite the common myth saying otherwise. Fitting a huge exhaust system to certain cars may have bad effects, but this isn’t due to reducing backpressure, it’s other bad design elements. Backpressure is never a performance enhancer; you want as much exhaust gas out of your engine as quickly as possible, which means, especially on non-turbo cars, careful choice of pipe lengths and sizes so the exhaust gas moves as fast as possible out the exhaust.
On turbo cars it is simple, as manifold design has less effect and a huge post-turbo exhaust will only give gains with no losses. On non-turbo cars, although we won’t go into detail as it’s a hugely complex subject, any low down power lost isn’t down to lack of backpressure, it’s down to the diameter and scavenging effect of the new exhaust design moving the power band to higher in the rev range.
Most people will have heard to the term screamer pipe, and it basically is a separate exhaust pipe coming from the wastegate of a turbo car. The “screamer” part of the name is due to this pipe normally being unsilenced and incredibly loud. As a wastegate doesn’t open until full boost a screamer pipe wouldn’t cause any issues when driving normally or for MOT time, but the noise is almost assured to get you black flagged on track.
There are two solutions to this, either simply plumb the screamer pipe back in to the rest of the system, which would quieten it down but potentially lose you power. The other option is to keep the screamer pipe separate, but add a silencer or two to quieten it down. This option is the the best for performance and noise reasons, but it adds significant extra cost and weight of the silencers.
Almost any car built in the last twenty years will need a catalytic converter fitted to pass an MoT emissions test, but unfortunately due to their design they are generally not very free flowing; exactly the opposite of what you want for performance. There are various ways of getting around this, namely a sports cat, a de-cat pipe, or a cat bypass valve.
A sports cat is the most effective solution, and basically means a larger freer flowing cat than the standard item, increasing flow without affecting your emissions. Unfortunately sports cats are expensive and at high power levels can still pose a restriction, possibly necessitating the need for parallel sports cats, further increasing price.
The cheapest and simplest option is a de-cat pipe, which removes the cat altogether and replaces it with a straight piece of pipe. Unfortunately this means it will fail any emissions testing so you will have to refit the car come MoT time and hope you don’t get pulled over for a random emissions spot check.
The least common, but possibly the best overall solution is to use an electronic or vacuum operated bypass valve to let the exhaust gas pass the restrictive silencer when needed, but travel through it when the car is being checked for emissions.
There is always a lot of discussion about the legality of exhausts, especially about where on the car they exit, and unfortunately it is a real legal grey area. What one policeman or MoT tester may be happy with another may not. The reality is excessive noise is likely to get you noticed by the authorities, as will ones exiting from unusual places.
If you want the least bother, make your car as quiet and subtle as possible. But as long as they are quiet and safe enough, side exit exhausts, and even front bumper exits, are not illegal, and some vehicles come with both of these options as standard.
There are three types of materials commonly used in exhaust systems, mild steel, stainless steel, and titanium. Mild steel is by far the cheapest and is quite durable, but due to its susceptibility to rust and stainless steel prices lowering, mild steel exhausts are rare.
Stainless steel is the most common type of exhaust material due to its resistance to rust (although lower quality stainless steel can still rust), but can be brittle, especially on exhaust manifolds on turbocharged cars.
The final material is titanium which shares similar properties of stainless steel but actually weighs nearly half as much, making it the material of choice for serious race cars wanting to shed every last kilogram. The disadvantages to titanium is that it’s very hard to weld properly, but most of all, cost. Titanium They’re generally around four times as much as a stainless system, so full titanium exhaust systems are rather rare and for big budget performance cars only.
Although many people associate the words “custom exhaust system” with a one-litre Saxo fitted with a brace of six-inch tailpipes, the majority of big power cars will have had at least some of their exhaust system custom made.
For many people custom exhaust components are fitted due to changes in the car’s turbo setup, a lack of adequate off the shelf parts, or to make their current system a little quieter to pass track noise regulations.
There are countless custom exhaust manufacturers around the UK, all with varying equipment and skill levels. So make sure they can build you the exact part you want at the correct size to avoid confusion, particularly when it comes to larger bore piping, as not all companies are able to do it all.