Exhaust Size Selection: It’s A Little More Complicated Than “Bigger Is Better”

Have you noticed that some automotive topics seem to circle around over and over again? Multiple articles from multiple viewpoints with multiple conflicting opinions leave your head spinning. Sometimes I think I can actually hear readers crying out Shut up, already! when I’m blogging about such subjects.

Here’s the thing, though. In our industry, there are two major factors keeping certain age-old debates alive and well. First, there are constantly new generations roaming into these automotive waters, meaning there will always be the need to teach the basics and fundamentals. And second, this industry is constantly evolving, meaning what was extensively informative a year or two ago can now be obsolete.

Today’s touchy topic of discussion is a great example of this: proper exhaust size selection. Basically, what was a golden rule decades ago, doesn’t necessarily apply to late-model vehicles. And while this doesn’t make it useless information, it does mean that there is more to learn—regardless of whether you’re a noob or a veteran.

Everyone Wants Sweet Pipes

So, why even talk about exhaust size selection? You can go straight to a supplier and pick up an exhaust system for your car that you know will sound and perform well because manufacturers have taken the time to design pipes with proper routing, diameters, and mufflers. However, despite this research and quality control, it’s still possible to wind up with something you don’t want or need. This is especially true if you’re having a custom exhaust system built for your applications.

Selection of proper pipe diameter is something that’s very critical for performance. Engines are air pumps and the more air they can move, the more power they can make. Most of us are taken up by the engine’s ability to draw in air but being able to push that air out does have a considerable impact. And, how do you improve the ability of an engine to help air leave the combustion chamber? Well, by adding bigger exhaust pipes.

But despite what seems like simple logic, there is a limit to the bigger-is-better mentality. Remember, you want some back-pressure to help with the velocity of the exhaust that’s leaving the combustion chamber. Factory exhaust systems often create too much back-pressure, resulting in a high amount of exhaust gases in the chamber, which hurts power. But too little back-pressure can actually produce the same result, by slowing down the velocity of the gases. This is why you need to install the right size pipes to help find that sweet spot of flow.

So, how do you go about proper exhaust size selection?

Well, two things are important to consider: power output and engine displacement. Sometimes these work hand-in-hand and sometimes they’re independent variables.

What I mean is that just because an engine is big doesn’t mean it’s going to be powerful. Look at late ‘70s versions of the 440, 454, and 460. Yes, they were big, but they barely produced enough power to get out of their own way! Conversely, just because an engine is small doesn’t mean it’s weak. (A modern F1 engine displaces only 1.6 liters and produces about 1,000 horsepower.)

Another thing to remember is that during the rise of emissions, a lot of good engines were choked down to make them more environmentally-friendly. In fact, it’s a solid argument to say that many of those large engines existed during the early days of the era because of their ability to still produce power on account of their displacement, and a good way to improve performance was by ditching the restrictive factory pipes.

While this is true, the ability to produce the power to move the air out is still bad enough on the intake end to put a pretty short cap on the list of exhaust sizes. (And you would have to work on that to see real gains.) This does mean that power is a bigger player in the game than displacement, but both are still relevant.

Carbureted vs EFI

Now, if you were working with older naturally-aspirated vehicles, you could walk away from this discussion now, hit Google images and find yourself a chart that holds the same information that is present below, and select the right pipe diameter for your build.

This information is pretty solid but, as you could guess, it’s relevant to older vehicle types. That’s not to say it isn’t still useful for modern engines and standards, but there are more factors that come into play.

To help us illustrate our point, we turned to Chris Thompson of TMG Performance Products, parent company of CORSA Performance Exhaust, Volant Performance, and dB Performance Exhaust. “Modern EFI-controlled engines can be tuned for different pipe diameters. As a result, it is not uncommon for these engines to have larger pipe diameters. On early carbureted engines, pipe size was very critical. A typical small block engine didn’t like more than dual 2.5-inch pipes but a big block was happy at 3 inches,” says Thompson.

This means that EFI allows a smaller engine or less powerful engine to take advantage of larger pipes than a carbureted engine of similar power levels ever could. And this is a good thing—because it not only provides the opportunity to move more air and make more power, but it also makes it easier for a builder to hit the sound levels they desire.

Daily Driver vs Purpose-Built

Okay, so my engine is powerful. That means, I should reach for the bigger pipes that match up to the power, right? Well, yes and no. This ultimately comes down to how you intend to drive your car. “An engine that will play at lower RPMs needs a smaller exhaust to keep the back-pressure up, in order to move the power band down. On an engine that revs at higher levels, the rule is the same, but the diameter goes up to accommodate the need,” explains Thompson.

Okay, so a race engine favors bigger pipes, while a daily driver favors smaller pipes? Well, again, yes and no. (Ready to slap me yet??) For this to become a set rule, we need to look at one more variable—whether or not your engine is boosted.

Gimmie A Boost

“With a boosted engine, the whole pipe diameter scenario changes drastically,” says Thompson. “Both supercharged and turbocharged systems require larger-diameter exhausts, but the routing and length of each will be different. It can vary depending on the manufacturer of the system.”

This is part of the reason why you will see massive pipes on boosted applications, regardless of power output. Now, diesels are a bit of another discussion as they respond extremely well to bolt-ons but, again, part of the reason you see massive pipes on these trucks is because of the turbochargers that are present.

Natural aspiration can only fill the combustion chamber up to its limit (which, most of the time is short of the maximum capacity) on account of the vacuum produced by the intake stroke. Air is being drawn into and pushed out of an engine by motion of the piston alone. However, boosted applications are essentially forcing air through the engine. This means, that it doesn’t need to rely as much on back-pressure—so you can really only help it by adding more room for the air to move through.

A restrictive exhaust has the same effect on an engine, as a straw would as you try to exhale. For the most part, you’ll succeed at getting air through. But your cheeks will puff up and the harder you blow, the puffier and redder your cheeks will get. By widening the straw (read: pipe) you reduce that strain.

But pipe diameter isn’t everything.

The ability to keep heat in the system is important, too. Heat works as a lubricant for exhaust gases and the more heat you can keep in the system, the faster that exhaust can move. This is why heat wraps and coatings are so popular. However, as Thompson points out, “The correct muffler is the most important thing to ensure this. Coating materials and wrap all affect the longevity of the system but have little to do with the performance.”

A muffler manipulates airflow and creates different sounds. This is done in a number of ways and, really, with today’s standards for mufflers it’s nearly impossible to say that one is truly superior to another. But we can say one thing: If you are working with a properly-selected exhaust system, throwing on a factory-type muffler that draws down air flow will make your hard work nearly pointless.

Now, you may be thinking $#%! mufflers, I’m just going to run dumps. But, while this reduces the issues of poor flow characteristics and increases sound level, it isn’t viable to every build. “There is a compromise on muffler dumps that are not tied into the ECU of the engine,” says Thompson. “The sudden change in back-pressure requires an instant change in the tune. On carbureted engines, performance is at one point or the other—meaning when the dump opens, the tune will not change for the decreased back-pressure and increased fuel needed.”

Basically, carbureted engines work with a fixed tune, set before you drive. Either it will need to be tuned to perform with a full exhaust or with dumps. It can’t change on the fly like a computer-controlled, or EFI engine, could.

You’re starting to see what I meant by some automotive discussions going ‘round and ‘round, right?

Well, there’s one more thing to consider in exhaust size selection: the use of X-pipes and H-pipes. If you’re running a dual-exhaust system on an engine with two banks, this is something that can help you level out your power curve. “H and X pipes work well in taking out the odd pulse that results from two cylinders on the same engine bank firing sequentially,” adds Thompson.

Ultimately, there’s a lot more to consider when it comes to proper exhaust size selection than the simple bigger-is-better mantra. And while it can quickly become an involved task, if you focus on these three characteristics, it becomes a simple problem with a simple solution: how your engine makes power, how much power it makes, and how you intend to use that power.