Here is a great post we had on the Harley Reddit, regarding this:
"WALL OF TEXT FOLKS, but then again, there's a LOT to talk about on this subject, and I happen to be wide awake and thinking about this shit, mainly because I had occasion to watch someone on a quiet bike almost get sammiched between two cars because the idiot driver changing lanes didn't have any idea the bike was right next to her. Had the bike been LOUD, I'm sure that one of the drivers would have been inclined to actually LOOK into the lane next to her, if only to see if the noisy bike was actually in her blind spot. Bottom line? If it's loud enough to piss people off, they definitely know you're there. BUT, this is not advocating "straight pipes". They make your bike run like shit compared to a good muffler pipe, and most straight pipes sound like shit, no matter how much you love loud bikes.
In the last year or so people here have told me they had been told that their bike ran better with straight/drag pipes than with muffler pipes. I'm willing to bet that a LOT of you folks believe this too. Sure, they run better in some parts of the rpm range than stock mufflers, but a decent set of muffler pipes or a 2 into 1 exhaust system will outperform drag pipes on a streetable bike, just about every time.
Some bikes will benefit from drag pipes, but most of them are on the drag strip, which is why they're called drag pipes in the first place, but those motors are monsters, and those pipes are tuned to a specific length to avoid harmonic and pressure reversion, a very real issue with drag pipes.
There are very good, sorta-easily explained reasons why drag pipes don't work perfectly, despite the intuitive belief that "straight pipes means getting the burnt combustion products out of the motor faster, which obviously means best performance". This primarily has to do with exhaust pressure turbulence and imbalances caused by harmonic reversion, having to do with the actual sound waves created by the combustion gasses, and pressure reversion, which I'll address in a bit.
S&S has a short video discussing carburetor tuning with drag pipes, but what the video really also shows is a bike with drag pipes on a dyno, and how horribly the pipes perform in the midrange with whatever jetting they tried. They also tried a couple of attempts at "garage expedient fixes", to address the mid-range sag in the horsepower curve that occurs in just about every street motor with drag pipes on it.
Take a couple minutes and watch it. It's enlightening.
But now I wanna talk about pressure reversion, and the principles behind why a muffler is a good thing, rather than why drag pipes are bad things.
Exhaust comes out of the head in pulses- exhaust valve opens as piston comes up pushing gasses out of the cylinder, gasses escape creating pressure in the exhaust port and pipe, exhaust valve closes, gasses stop coming. When the piston is coming up on the exhaust stroke, there's high pressure created in the exhaust port and the pipe near the head, not just because of the piston pushing those gasses out, but also because those gasses are still burning and expanding as they come out of he head. BUT- when the exhaust valve closes, that pressure coming out of the cylinder stops, creating a vacuum behind the fast moving exhaust pulse in front of it. At lower rpms, this causes the exhaust gas speed to slow down and even reverse inside the pipe momentarily, and when the next pulse comes out, it's not gonna flow at optimal speed out of the pipe, instead running into part of the previous exhaust pulse, running back to fill the vacuum created when the valve closed.
At high rpms, this isn't so much of an issue, primarily due to the number of times in a second that the exhaust valves open (at 6000rpm, that's 100 50 times per second), keeping that pressure pretty much constant and pushing it right out the back of the pipe. Optimal exhaust function would have all the exhaust gasses heading back at the same speed, fed at increasing pressure and speed by the increasing engine rpms producing that much more exhaust gas. The problem is that there's just not enough exhaust gas in a reasonably stock motor to do that with a straight pipe at all rpm ranges. Use a smaller pipe and you get backpressure, but can't get the gasses out fast enough at higher rpms. Use a pipe big enough for higher rpm function and you end up with pressure imbalances throughout the pipe at lower-mid rpms.
At lower-mid rpm ranges, there's enough time between pulses that a vacuum forms behind the exhaust pulse, which slows down the exhaust speed sufficiently to cause following pulses to get jumbled up inside the pipe, creating all kinds of different pressures at different distances from the exhaust port on the cylinder head. This is inefficient as the next pulse needs to fight this imbalance, trying to get out the tail pipe. Stock exhaust systems restrict the flow of exhaust gasses, which causes the exhaust pulses to stack up and close those gaps efficiently, even at lower rpms, eliminating the vacuum and resulting in much more efficient engine function at lower rpms ("gobs of torque" is how some folks describe their bikes with the stock pipes). But that same stock exhaust restricts the flow enough at higher rpms that the exhaust pulses stack up so heavily that you end up not being able to get the exhaust gasses out of the motor completely, and a big part of each pulse ends up back in the cylinder, which affects the combustion of the next cycle negatively.
Also more restrictive exhaust systems are quieter, less restrictive systems louder. This feeds the mistaken belief that if properly designed peformance exhaust is louder than stock exhaust, then LOUDER exhaust must be better than the better designed system. And since straight pipes are they loudest, they must be the best pipes. Nope.
The solution is an exhaust that restricts flow enough at lower and mid rpm ranges, but flows freely enough at higher rpms that the gasses never back up into the cylinder. Properly designed muffler pipes do this very well. 2 into 1 pipes like the ProPipe and the Thunderheader are designed to do exactly this with as much efficiency as can be designed into an exhaust system that still sound pretty damned amazing. Supertrapp does this perfectly or at least allows you to tune your exhaust pressure to fit your exact engine for peak performance, but many folks don't like the exhaust sound, since it's a lot quieter than other muffler pipes or the 2 into 1 systems designed for better performance.
Now we get to just he issue of noisy exhausts. Let's face it, most of us love our bikes and one of the things most people will tell you is "I LOVE the way it sounds". This is NOT necessarily a matter of ego or vanity or even compensating for some inadequacy, as Harley hating folks have claimed over the years. There's been studies that have shown that women will even respond sexually to the sounds made by a high-performance motor, and whether it's a V8 Camaro or a V-twin Dyna, properly designed exhaust systems on long stroke engines sound like they're high-performance motors, even if they're relatively mild compared to what they could be. And despite the fact that this study only examined women's responses to these sounds, men are biological creatures who are going to respond similarly in many cases. So all those haters out there can ʞ©$ɟ off- chicks dig hot rods and a Harley has been an E-ticket to pussy for many many years for exactly this reason (and I haven't even addressed the "girls love the bad boy" bit that's also true for many women, though clearly not all).
I'm all for loud exhaust, both because I love the sound of a properly designed and tuned exhaust, and because I really do believe the "loud pipes save lives" mantra. I've spent most of the last 20 years living and riding extensively in rural mountain areas, and one of the most dangerous road hazards a motorcyclist can encounter (other than drunken hillbillies who think driving a pickup truck while wasted is fun) is Bambi and his buddies in a deer-only street gang, attempting to get other deer who want to join the deer-gang to engage in the initiation ritual of playing chicken with internal combustion vehicles on mountain roads, typically at night when they know the cars and trucks are less likely to see them and hit the brakes. It often ends badly for the deer, and sends many a car to the body shop when Bambi's buddy doesn't quite make it out of the way in time. On a motorcycle, however, this can be fatal for the person piloting the motorcycle, too. Hitting a 100lb object at 40, 50, 60, or even 70 mph will ruin your ʞ©$ɟing day.
Well, the fact is that in 20+ years of riding in the mountains, the ONLY times I've encountered deer and other wildlife in the road, daytime or nighttime, have been when I had quiet mufflers on a bike, either when I'd put them on to head to the police station because I needed to get a pipes ticket signed off, or when my Road King was new and I hadn't installed the ProPipe for a few days yet). For the other 19 years, 11 months and 20 days out of 20 years, the only deer I'd see would be at night when I'd come around a bend to a long, unlit straight away and I could see the eyes reflecting back at me, way off in the distance. They'd be gone long before I was within a quarter mile of them, scampered off the road and into the surrounding terrain, as quick as can be. Loud pipes DO save lives. Period.
But what about those busybodies who say "so ʞ©$ɟing what, most people with loud pipes are riding in urban/suburban areas, and I can hear them all the ʞ©$ɟing time and it's annoying. As far as I know, the folks I hear ranting about "loud pipes saves lives is bullshit" a) have never ridden a bike with loud pipes, or at least not for any appreciable length of time to actually form an opinion, so how the ʞ©$ɟ would they know, and b) they don't seem to give a shit about what the truth might actually be, if it disagrees with their self-righteous attitude about loud pipes.
But let's look at other cases- I have a Vance & Hines ProPipe on my Road King, and it's loud throughout the rpm range, and remarkably that much louder when I'm into the throttle. No question about it, the ProPipe, like the Thunderheader, is a LOUD performance exhaust system. But the simple, obvious fact is that in urban, congested areas, if your pipes are loud enough that people who don't like them can be pissed about them, then they're also obviously aware that you're there on the road with them, too. This would suggest immediately that the loud exhaust noise coming off your bike is definitely helping drivers be aware that you're there, instead of ignoring your existence because you don't fill up a traffic lane or their rearview mirror. I typically ride a bit faster than traffic, since it allows me to put a bit more focus on what's going on in front of me in a 120 degree cone, and take a bit of attention off the stuff going on behind me and to an extent, next to me. But the fact is I can't count the number of times people have begun to change lanes into me who have corrected their direction just as soon as I was close enough that they could hear me, obviously because they clearly hadn't seen me but they had heard me.
I split lanes in traffic here in the San Francisco Bay Area just about every day, sometimes for 6 or 7 miles at a stretch on my commute, and people ʞ©$ɟing suck... the polite drivers on the peninsula notwithstanding (they actually hug the left side of the diamond lane 90% of the time, with the people in the lane next to them hugging the right side of the lane, because there are more bikes commuting on the peninsula than in the East Bay and North Bay). It seems that people in rush hour traffic are generally incapable of signaling a lane change before cutting sharply over into the lane next to them that may be crawling a tiny bit faster than the one they're in at the moment. I'm usually not splitting at any speeds high enough that I can't grab my brake and slow down fast enough to avoid a collision if someone in front of me chooses to do a snap lane change without signalling (there are exceptions, but that's for a different discussion).
That said, every single day there are people here in the east bay who move over to the left or right, depending on their lane, to let me pass, as soon as I'm close enough to them that they can hear me, despite the fact that I don't see most of them doing this for the Japanese or European bikes I see splitting lanes either in front of me a ways or the rare times I'm in a car. Since I'm spending 100% of my attention on the people immediately in front of me on both sides. I look at the head angle of drivers (if they glance up at their mirror or check their blind spot, it's indicative of making a lane change, even if they don't signal). I look at their their signals, which you can't really trust, but some folks actually use them. AndI look at their front tires (to detect a lane change before the car even moves sideways). For the most part, it's obvious that they're not checking their mirrors because there's just no reason to do so, sitting in slow or stopped traffic. But also just about every single day I will have cars three or four car-lengths in front of me make snap lane changes, signal or not, but if I'm only one or two lengths behind them when they start this lane change, they almost always cut back into their lane before their inside tires have completely crossed the dotted line, and it's not because they suddenly looked back. It's because they heard me.
Anyway, if you don't like loud pipes, that's your right. If you love loud pipes, more power to ya. But if you want straight pipes because you think your bike will run better, you're mistaken unless you have a BIG inch, high-performance motor. Japanese and Italian MotoGP race bikes and their street bike counterparts have large mufflers on them, not just because the law requires it, but because they work better with them on there. And so do most Harleys, no matter what model or year. If someone tells you "loud pipes saves lives is bullshit", well, now you might have at least something with which to rebut their claims. It's not "proof" in an academic sense, but my 33 years on Harleys, almost all of them with loud pipes, gives me a pretty good sense that- in combination with great attention placed on your surroundings and potential hazards, loud pipes do indeed save lives. They've saved my life more than once, and they'll continue to do so in the future, unless the deer and driver populations around the planet all go deaf."