|®||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
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.
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
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.
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.