® The truth about jetting for exhausts

One of our industry's most insidious misconceptions, and one that is promoted constantly by advertisements, as well as by discussions over accessories counters and Internet threads worldwide, probably tens of thousands of times each day, is the idea that installing an aftermarket exhaust system necessarily requires the changing of carburetor jetting. It just isn't so. Not every aftermarket exhaust affects carburetion. A number of complex physical things are going on in an exhaust system, most of which we'll touch on near the end of this article. First however, let's start by defining the three kinds of aftermarket exhausts.

Different exhaust types
First are those exhausts made solely for looks. No one claims anything but style regarding these systems. Think of the cruiser crowd and "shotgun" pipes. Second, there are those exhaust systems about which are implied (if not actually claimed) performance advantages. This is the largest category. The simple fact is, even these "implied performers" are, as with most aftermarket exhausts, like the first group marketed for asthetics more than for performance. Terry Vance (of Vance and Hines) and Denis Manning (Bub Enterprises) have both publicly acknowledged this, and it's far from a secret within the powersports industry that style and sound are where it's at with these systems, not performance. Most of what little performance is possible with these exhausts comes from a very different source than from the exhaust itself. We'll explore that in detail in just a bit. Finally, there is the third category, the actual tuned exhaust system, a very rare bird indeed and one usually found only in limited, hand-made quantities. We'll deal with that category separately. We'll also take a quick look at collector exhausts, probably the most popular design of all.

We already know what the "for looks only" exhaust is for, so let's examine the latter two exhaust types, the implied performer and the tuned exhaust, one at a time. The implied performer gets any performance it produces in some surprising ways. First, part of the increased vehicle performance, if any, comes from a significant weight savings. Though not measurable on a dynamometer, any weight reduction on such a light vehicle as a motorcycle or ATV offers impressive results. A 450 lb motorcycle for example that produces 100 hp has a power-to-weight ratio of 4.5:1. That is, the vehicle weighs 4.5 lbs relative to each horsepower. Installing an exhaust that is 10 lbs lighter changes this ratio to 4.4:1, which affects performance (mainly acceleration) in a way equal to increasing the engine's power by over 2 hp. There isn't actually 2 hp more, it's just that the vehicle performs as if there were, due to the comparable weight saving. Nearly every aftermarket exhaust offers this potential increase in performance. Even the first, for looks only, variety.

Pipe rejetting
But there is another, more interesting source of potential increased performance that the implied performer exhaust offers. This requires some explanation. Carbureted powersports vehicle manufacturers do not jet their carburetors perfectly, for two reasons. First, there are emissions considerations, mostly affecting the idle and midrange carburetor circuits. Consequently these circuits are jetted leaner than normal, and should be richened slightly for better performance. However, this rarely requires more than an adjustment to the idle mixture screw, and in some cases an adjusted slide needle height. Second, manufacturers ship their product to many different places across the globe. They purposely jet too rich on the main jet, the carburetor circuit that presents the most liability in terms of engine overheating. Therefore, all road-going powersports vehicles are jetted "fat" on the main jet. Contrary to what many people believe, but true nonetheless. Manufacturers do this because they cannot jet individually for each market, so they simply err on the rich side rather than the lean side, because the latter would present more problems. The fact is, engines are much more forgiving of a 3% rich condition than an equal lean condition, and just as importantly, few customers will notice a rich main jet, while nearly all will complain about a lean one. Make sense? (Incidentally, fuel injection has brought increased peformance to our market for several reasons, not the least of which is the fact that manufacturers no longer need to provide this rich margin. These fully electronic systems adjust themselves to the destination conditions automatically.)

A certain amount of low speed driveability, and even some high speed engine performance, is sacrificed by the manufacturer's stock carburetor settings. In the case of the idle to midrange leanness, careful recalibration can wake up the engine's throttle response and acceleration. As for the main jet, its richness reduces to a very small degree potential full-throttle performance, performance that is waiting to be tapped by the individual willing to take the time to carefully dial in the carburetion to the "nth" degree. Performance shops all over the world "dyno tune" to do this very thing. However, and the main point here, is that this dyno tuning is in many cases being done irrespective of whether an aftermarket exhaust is installed or not. That is, a considerable amount of dyno-based carb work is done on vehicles having stock exhausts. For various box stock racing classes, for example. The fact is, much of the performance increase resulting from replacing the exhaust and the subsequent carburetor rejetting is due simply to the rejetting, not the exhaust. The same or nearly the same result can be achieved by merely jetting out the lean and rich settings left by the manufacturer, without ever intalling the exhaust. In other words, you can rejet without the aftermarket exhaust and get virtually the same benefit as you would by jetting "to" the exhaust. In many cases, exactly the same benefit.

Remember two things about jetting. First, unless you're converting to alcohol or some other exotic fuel, never replace the idle jet. (At least on inline fours -- big bore singles are another matter.) When tweaking your carburetor, simply adjust the idle mixture screw for slightly more output. This is preferably done using an exhaust gas analyzer to dial in the mixture by the percent of CO (carbon monoxide) emitted by the exhaust (about 3 percent on 1984 and earlier models, half that on subsequent models). However, if skilled you can do it without one. Second, as far as the main jet is concerned, it is obvious that during box stock dyno tuning the main jet is decreased in size, not increased. The goal remember is to remove the rich margin. This doesn't fit what most of us are doing, and of course, adding an exhaust changes things, theoretically at least. With the increased flow capability of some aftermarket exhausts (many are actually the same or more restrictive than stock), some richness may be required. However, because that need is offset by the leanness that the engine actually wants, the net jetting result of intalling most aftermarket exhausts is the stock main jet. That's right, the run-of-the-mill "implied performance" type exhaust frequently requires no jetting change at all! In many cases, the stock jet will do just fine.

Aftermarket air filters
At least, that is, if no changes are made to the air filter. The air filter is what really makes rejetting necessary, not the exhaust. In fact the air filter affects the carburetor fully five times as much as does any exhaust system, even a good one. The ever-popular K&N brand air filter (and its many clones) is extremely freer flowing than any stock unit. So if you install one of these, count on going up two to three sizes on the main jet.

"Placebo jetting"
Interestingly, some customers just won't accept that their vehicle doesn't need rejetting. They insist that we (the shop) do it, even though they know it isn't needed. The misinformation machine that is so strong in our industry has persuaded them, and they're worried that if we don't change the jetting for them, their engine will burn up or be damaged in some way. Or more likely, they cannot reconcile in their minds the expense of the aftermarket exhaust on the one hand, and little to show for it in terms of added power in the other hand (which even the beginner knows accompanies a legitimate jetting change). This is an odd situation, and has resulted in a huge number of rejetting scenarios that just are not necessary. Call them "placebo" rejets, if you like. Just like the traditional sugar pill the doctors used to give out to hypochondriacs, folks who go around thinking they're sick all the time. And just as in the medical profession, in which it is documented that takers of those pills do get better, our customers do likewise. They go off with their newly-rejetted motorcycle or ATV very happily knowing that they "did the right thing," completely oblivious to the fact that it was unnecessary.

Tuned exhausts
But what about that third exhaust system type, the tuned exhaust? A tuned exhaust is a very rare animal, one that takes advantage of two interesting principles, expanded waves and reflected waves. Here's a short primer on exhaust wave physics. Sound waves traveling within the exhaust system are at various times positive and negative in terms of pressure. When a wave reaches the end of an exhaust opening, it expands, which makes it invert and become negative, or if negative, invert and become positive. The waves also bounce off any changes in diameter in the exhaust system (the more severe the change, the harder the bounce), reflecting the waves in a reverse direction. And since the speed of the sound waves is constant, the length of each section of the exhaust is carefully calculated so that the waves arrive back at the cylinder when the intake valve is still open, to coax as much mixture as possible in. It gets a bit complicated but the short of it all is that given certain diameters, angles, and lengths, exhaust pulses can be used to assist both the filling and scavenging (removal of exhaust gases) of the cylinder. This is what a "tuned" exhaust is doing, by definition.

There are four things to remember about exhaust tuning however. First, the science of this wave effect is not well proven outside the two-stroke world (although that is changing as more two-stroke technology is being applied to four-strokes). Much of it is still "cut and try." Second, wave effect is rpm-specific. The reflected wave can be timed to arrive only at a very narrow rpm range, making the effect not widely applicable to various rpm. Carefully chosen angles to some degree mitigates this problem (shallower angles reflect the wave more softly but make it last longer), but it's still a problem (the softer wave has less energy -- there's no free lunch). Third, once you add baffling of any kind, you negate the tuning effect. That is, no tapered cone (tuned) system in which the baffling is inside any of the tapers is truly a tuned exhaust. Hooker many years ago sold reverse cone megaphone exhausts for old Hondas, but unfortunately, they had baffling inside the megs. Although they worked nice and sounded good, it's doubtful they increased engine performance. Lastly, and perhaps most importantly, for wave tuning to work, the intake and exhaust valves have to be open at the correct moment, and in many stock engines this just isn't happening. The camshafts in most four-stroke engines provide too little valve overlap, for one thing, for the cylinder to be exposed when the returning wave arrives. The bottom line here is, only carefully hand-built so-called "reverse cone," un-muffled exhaust systems, fitted to engines with racing camshafts, are truly wave tuned.

Collector exhausts
But what about the collector exhaust, the all too familiar design that empties all its head pipes into one muffler? A sub-species within the implied performer exhaust type, this system offers no wave tuning effect whatsoever, although other kinds of tuning are sometimes (and sometimes, not) at work. One of these is extraction. The collector exhaust is more accurately named the extractor exhaust, because the joining of exhaust pressure at the collector generates in a pull on the head pipes, at least theoretically, resulting in improved cylinder scavenging. Extraction works, up to the point that the stock camshaft will permit it (remember), but there are problems. For example, extraction works mainly at high rpm, which is why these systems universally seem "pipey."

To sum up, the time-honored exercise of rejetting for improved power characteristics works whether or not it is done in connection with installing an aftermarket exhaust system. Most of the benefit of most of the performance exhausts that are available for vintage inline fours comes by the exhaust's effect on the vehicle's power-to-weight ratio, not the engine's power output. Real, old-fashioned, tuned exhausts for old bikes are extremely rare and you wouldn't be able to stand the sound level even if you had one.

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Last updated November 2022
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