An important concept to understand is that an increase in power without altering the amount of fuel consumed results in an increase in fuel economy. That is, more power per unit of fuel translates to more distance on the road or a larger load over the same distance (e.g. towing a trailer or caravan). So when this article suggests a part will increase power, you can take this as an increase in fuel economy as well.
So what goes into an exhaust system? There are quite a few aftermarket modifications available in this department, some of which will help you reach your fuel economy goals, others which won’t. Below is an outline of the parts that can increase your car’s power and fuel efficiency.
|Typical 4-1 header|
In an unmodified engine each cylinder shares one common exhaust manifold, this means that when a piston reaches its exhaust cycle it pushes spent fuel into the exhaust manifold / chamber. If every cylinder is trying to push exhaust gases into the same chamber there is a resultant back pressure. This back pressure reduces power because it restricts free exhaust gas flow and puts a backwards force on the engine. A reduction in power directly correlates to a reduction in fuel efficiency and an increase in fuel consumption.
An exhaust header is designed to eliminate this back pressure by providing a separate exhaust pipe for each cylinder before it enters the catalytic convertor. Not only does this eliminate the build up of pressure but well designed headers have smooth bends rather than sharp transitions in order to reduce exhaust friction. When selecting an exhaust header you need to be aware that certain lengths of pipe are tuned to certain RPM’s. You need to find a header that suits the kind of driving you do. If you’re trying to save fuel you probably aren’t looking for performance parts tuned for high RPM’s but rather a header designed for round town driving at around 2000 RPM. The sound of your engine will also be affected by an exhaust header (for better or worse) so keep that in mind also.
Common headers are 4-2-1 (4 Pipes from 4 cylinders that joins to 2 pipes and then to 1) and 4-1.
Auto Anything sell a wide range of aftermarket exhaust parts at competitive prices. Check out Auto-lounge's Header Size Calculator to help you pick the best header.
A common misconception is that the larger the exhaust piping the better. This is not always true as a pipe that’s too large will reduce the scavenging effect resulting in a loss of power and an increase in fuel consumption. Muffler Mall has a pretty good definition of the scavenging effect:
Q. What's Scavenging Effect, anyway?
“The ultimate performance gains are to be found when an exhaust system, including the mufflers, creates a scavenging effect. This can increase horsepower, torque and fuel economy, all at the same time.
“Scavenging is when a [reflective] low-pressure wave comes back up the header pipe,” says Muffler Guru. “You want it to get back to the exhaust valve when the engine is in overlap. It creates a low-pressure area at the exhaust valve and aids in the induction charge, ... getting your intake flow started into the cylinder.”
The diameter and length of your exhaust pipes, your muffler design and your cam timing are all factors that affect scavenging.
“When you get this optimized,” says Guru, “you can get a boost in torque. Between that tuning and the intake manifold tuning, that’s how you can achieve over 100 perfect volumetric efficiency on a naturally aspirated engine.” A variety of high-performance mufflers are designed to enhance this scavenging effect.” - Source Muffler Mall
Selecting the right pipe sizing can also increase the scavenging effect while still increasing air flow within the system. The Auto-lounge Pipe Sizing Calculator can be useful when selecting the best pipes for your exhaust. Thermal insulation on your piping can reduce engine heating and allow cooler air into your fuel mix, increasing power. Be sure to look into the insulation of various pipes before you fit out an entire exhaust with them.
A catalytic convertor is required by law under most jurisdictions. The device reduces carbon monoxide emissions but introduces a back pressure into your exhaust as the engine is trying to push gases through a catalyst / filter. The back pressure created by the convertor can reduce power however removing or bypassing the part is illegal in most countries and can void engine warranties. We do not recommend that you remove the device, just be aware that it is there and taking effect. Some aftermarket convertors will decrease power more than others depending on their airflow properties and geometry.
Resonators, Glass packs and Silencers
These components all come after the catalytic convertor and have various effects on back pressure. Using aftermarket parts may or may not affect the back pressure of your exhaust system so consult your parts dealer when considering a purchase in this department.
There are many ways to tailor an exhaust system for optimum power and fuel economy. Make sure the modifications or parts you use fit with the type of driving you do, aren’t illegal to implement and don’t void any warranties. Auto lounge also has a Drone Length Calculator to help you eliminate resonance.