Not long ago I was asked to present a little technical information at a “boot camp” held by my alma mater, California Harley-Davidson. The session was intended to be about 25 minutes long and was attended by experienced riders. Since I have about as little information as anyone, I decided to overcompensate by including no less than five topics in my short address… to be followed by a Q&A. Ha! You can probably imagine how quickly that plan came unraveled. For our purposes here, I can simply allow as how nobody much gave a crap about aerodynamics and its effects on fuel economy at high speeds. The relationship of unsprung weight to ride quality was a snooze too. And so it went. Then a comment made in passing, and regarding a subject genuinely near and dear to their little hearts, finally got a reaction. A big one, and not a reaction I would’ve predicted! The minute I said what I said… I got the look… the one that says simultaneously, “That was fascinating and I don’t have the damnedest idea what you are talking about.”
It went something like this: “There’s really no such thing as a true high-performance exhaust for Twin Cams.” Suddenly the tops of their shoes weren’t as interesting as the tip of my nose. The heads-up eye contact caught me in mid-flow and the next thing I blurted was, “And that’s because Twin Cams have no overlap… none!” I had pretty much figured that the concept of overlap would be familiar. I was wrong. The vacant stares told me that much… all “cow eyes” waiting for the other shoe to drop. “It’s a little like having the front door and the back door of your house open at the same time. Lets a nice breeze flow through,” I mumbled. Geez, could I have picked a more puzzling remark to add? I think not! Mind you it’s accurate as far as it goes, but doesn’t help folks connect the dots between breezes blowing through the house and why pipes don’t work on stock H-D Big Twins. Some of those folks took it at face value that it meant something that might actually matter. Others, naturally enough, knew better. After all, the pipe on their bike works a treat, makes all kinds of power… my nonsensical statements notwithstanding! By then of course, time was up and they moved on to the next table and more coherent and polished treatises on how to pack for a road trip, “proper” first-aid kits and other, more tangible, pragmatic and useful stuff.
Caption: By now you are familiar with Harley’s version of water cooling, which only cools the exhaust valve. Representing half of the hardest-working duo in the powerplant (the other being piston rings) the ability to transfer heat to the cylinder head is a major part of the valve’s job. In turn, once the heat is in the head, it better not stay there. Lack of cooling effects from air flow during overlap leaves most of that heat evacuation job to the little water-cooled doughnut around the valve. Arguably, the most beneficial in situations where there is little or no added airflow gained by moving down the road.
Caption: In sharp contrast to Big Twins, the XR1200 had unique oil-cooled heads, with very similar doughnut-shaped passages surrounding the exhaust valves. Since XR1200’s have adequate valve overlap (as do all Sportsters and Buells) one might accurately deduce that, in this case, the purpose was to keep the heads under thermal control at high-revving Autobahn velocities… for hours on end.
Since then, like most of us (I hope) who haven’t exactly nailed it, I have longed for a chance to do it over and do it better. To actually communicate the dynamic relationship between cams, exhausts and engine breathing. ’Course, might take hours, ’cause frankly it ain’t that simple. But, since you’re here, I sense an opportunity… heh! heh! I just hope it all works out this time, because it is kinda important. To begin with, Harleys seem to run pretty damn well these days… no overlap or not. Oops! Wait! There I go again! Let’s take it from the top instead; beats jumping right in.
Overlap is the angle, expressed in crankshaft degrees, during which both the intake and exhaust valves are open. This situation occurs at the end of the exhaust stroke and the beginning of the intake stroke. Apparently, not everyone knows that… or cares. But we’re setting the stage so bear with, OK? See, the next factoid is that lack of overlap I’ve been beatin’ you over the head with—as true as it is and as critical—loiters in mere camshaft “stat” numbers, and reads something like this:
Twin Cam Camshaft Timing Info
Stock 1999–’05 Carbureted:
Intake open/close: -02/38
Exhaust open/close: 42/-03
Overlap is -5.00 degrees and the Intake Duration of 216.00 degrees. The Exhaust Duration is 219.00 degrees. Your Lobe Separation Angle is 111.25 degrees. The Inlet Cam has an Installed Centerline of 110.00 degrees ATDC. The exhaust cam has an Installed Centerline of 112.50 degrees BTDC.
Stock 1999–’06 Injected:
Intake open/close: 02/34
Exhaust open/close: 42/-03
Overlap is -1.00 degree and the Intake Duration of 216.00 degrees. Exhaust Duration is 219.00 degrees. Your Lobe Separation Angle is 109.25 degrees. The Inlet Cam has an Installed Centerline of 106.00 degrees ATDC. The exhaust cam has an Installed Centerline of 112.50 degrees BTDC.
Intake open/close: -09/25
Exhaust open/close: 42/-03
Overlap is -12.00 degrees and has in Intake Duration of 196.00 degrees. The Exhaust Duration is 219.00 degrees. Your Lobe Separation Angle is 109.75 degrees. The Inlet Cam has an Installed Centerline of 107.00 degrees ATDC. The exhaust cam has an Installed Centerline of 112.50 degrees BTDC.
Caption: Once you have overlap, you need extraction. Quite hard to find. Do not confuse a fat and hollow “free-flowing” pipe’s lack of restriction with genuine extraction capabilities (tuned-length head pipes, preferably with a proper collector diameter) as they are not the same thing. Perhaps the most useless exercise of all is trying to get mufflers to do all the work. These might look “da biz” but if the head pipes aren’t right, no matter how it sounds, it don’t amount to much. Apparently, H-D concurs, as they have just released an emissions-legal SE system featuring 2 ¼” muffler inlets, “cats” in the “Nightstick” (Supertrapp) disc-type mufflers, and a full-round crossover pipe—allegedly worth 5-7 hp. Just ask for P/N 64800022 and bring money—$1,300, to be precise. Might be as good as it gets for a legal system.
So now you know! It must be said that, like everything else involving engines, it’s not good to focus only on one aspect of things at the expense of others. I said it… it’s complex. Also, cam timing has changed for 2014 and I don’t know the stats, but doubt they would allow nasty emissions. Now back to overlap!
Sure, it’s expressed in crankshaft degrees, but it really describes the window of time between the intake cam’s opening point (BTDC) and the exhaust cam’s closing point (ATDC). It can (and obviously does) vary between zero degrees (or less) on stock cams to as much as 70 to 90 degrees on some race motors. In general, most street engines should have 20 to 30 degrees of overlap and most performance cams can have as much as 60 degrees of overlap. Increasing the degrees of overlap tends to move the power up the RPM band. Decreasing the amount of overlap tends to boost low RPM performance. With inches to spare and emissions to contend with, it should come as no surprise why The Motor Company made the choices they made. (Leading, BTW, to the choice to water-cool the exhaust valve on certain models.) Okay… wait for it… increasing the overlap will also significantly increase peak power, but only if the exhaust system is properly designed to scavenge the cylinder. The corollary, pure and simple, is that with no overlap the exhaust has virtually no effect… as a tuning tool.
Caption: It pains me to show this, as these are obnoxiously loud and only work well for peak power… a lot like a drag pipe! (Yuck!) Still, if you have a jones to own a reverse-cone megaphone for your Harley, this is it. The infamous and popular Thunderheader. The main reason for its inclusion in this discussion is the keen diagram you see here, which does a decent job of illustrating how this type of “muffler” works. (That said, if you think this is still cutting edge, please re-read the caption about mufflers doing all the work and useless exercise.)
In Harley-land it’s practically a sacred cow that damn near any exhaust but the stock one will give a big boost in power. Maybe, just maybe… 20 years ago. The problem now is partly one of perceiving better throttle response as the same thing as more power, partly the fact that any muffler you can see through (and plenty you can’t) is in fact less restrictive than OEM stuff though invariably louder, and partly because we don’t really grasp the physics involved. Physics has counter-arguments, one being dynos might tell the truth, but rarely the whole story, another that there’s a difference between less restrictive and “tuned” because of… well… “The Pulse!”
All engines have pulses that resonate in and around them. The last century of engine development has been largely a matter of learning to make those pulses work for, rather than against, power production. That’s one big reason no four-stroke engine opens or closes its valves exactly at the beginning or end of its stroke. Somebody figured out early on that holding back the magic moment or causing it to occur earlier, for just a nano-second or so, assisted air flow through the pump, rather than confounding it. More airflow equals more power equals good stuff. Pulse management became the name of the game and you can bet it was (and is) not limited to stuff inside the engine. In the dark ages of carburetors, a hell of lot of improvement came with so-called tuned inlet length. Sonic speeds and proper air/fuel mixing could help generate “ram” efficiencies at or beyond 100 percent “cylinder fill,” and we need look no further than the XR750 to see a textbook example of it. These days, it ain’t that big a deal because of the speed and precision metering of electronic fuel injection—more often than not “masking” less than perfect inlet tuning. Performs much the same trick with this no overlap situation as well… hence huge EFI intake manifold diameters, but little in the way of alternate lengths.
The game has always been harder to play on the outlet end of an engine. Learning how to do it remains more art than science with four-stroke engines, but two-strokes? Let’s just say that when they started to kick ass at racetracks it was because of expansion chamber science… as formulated by a certain Walter Kaaden back in the 50’s. Once Yamaha and Suzuki got hold of the formula… it was all over.
Caption: Love or loathe ’em, Buells have overlap and (ah-ha!) “textbook” headers designed to take maximum advantage, which they do nicely. Lots of dyno testing on the factory race pipe for S1’s in the 90’s proved conclusively that this header design and collector diameter extracted all the power—something like 20 more than stock! That ginormous muffler was even better because it didn’t cut power or add (too much) noise. Although there are plenty of 2-into-1 “systems” to choose from for Harleys, few if any come anywhere close to this level of effectiveness. Two reasons: Harley guys wouldn’t want headers that looked like this, and stock Twinkie cams negate any genuine gains to be had from the “few” anyway!
Striving to achieve pulse “harmony” might be one good way to look at it, the trick being to build an exhaust pipe that doesn’t just allow the exhaust pulse to get out, but sucks it out. Moreover, it must help draw fresh mixture into the combustion chamber as well! Remember the front and back door analogy? Well, rather than a breeze, just think of a giant tornado-like vacuum occurring at exactly the moment both doors open. (Talk about “blowing your doors off!”)
Done well, matching the nature of the cam timing with the nature of the pipe’s ability to extract at the right moment means a huge boost in power. But it only works over a relatively narrow range of engine speeds—if it works at all. Old British race bikes, virtually all single-cylinder jobs seeking every increment of added urge, went to the edge and sometimes beyond with this notion, as embodied in the megaphone. At first an open-ended divergent cone sort of thing, they did bump peak power a ton, but only at max RPM. Couldn’t cruise one down the street because of a phenomenon called “megaphonitis.” Basically, a carbureted race bike wouldn’t carburate at certain points in the middle of the rev range… period! Can’t have that, can we, old bean? Enter the “reverse cone” and the beginnings of “acoustic wave” theory.
Caption: You’ll see plenty of these “cam wheels,” including the one in the SE catalog, and most have spec numbers already shown on ’em. This one doesn’t, for what I hope is an obvious reason. It’s kinda DIY oriented and gives nice visuals about what to expect from your overlap of choice. There are good cam calculators online as well, if you are as math averse as I am. For those who are not, it really ain’t tough. Overlap can be figured simply by adding the exhaust closing and the intake opening points. For example, a cam with an exhaust closing at 4 degrees ATDC and an intake opening of 8 degrees BTDC has 12 degrees of overlap. If this simple stuff isn’t known, you can still estimate it by using the advertised duration and the lobe separation angle.
1.) Add the intake and exhaust advertised durations
2.) Divide the results by 4
3.) Subtract the lobe separation angle
4.) Then multiply the results by 2
See… that didn’t hurt a bit, did it? Any questions?
The Brits (and everyone else in racing back then) had figured out the part about getting the exhaust pulse to assist performance by shaping the pipe so it would literally suck at the speed of sound. (Acoustic, get it?) The next problem was how to harness the second wave (and sometimes the third and fourth) that wanted to come back into the pipe once it reached the end. They wanted to get rid of megaphonitis by harnessing these reversed (or “reversion”) waves to stop the primary waves from overdoing things… and vice-versa, for that matter. Putting a “cone” at the end of the megaphone turned out to be a pretty good solution. The forms have varied a lot (picture shorty Sporty muffs from the 60’s or Norton Commando “peashooters” for familiar interpretations), but the principle remains. The first taper attenuating the positive pulse, the cylindrical “can” portion (if used) mitigating, and the final tapered cone angled to modify the behavior of the bouncing negative pulse(s)—with each engine type wanting slightly different shapes and dimensions to give best performance.
The reason any of this stuff works at all is because, amazingly, a sonic wave in an exhaust pulse will always revert at the end of the pipe. Not only because the pipe is the open end of the system, but also because the cylinder side isn’t! These acoustic waves beginning as “positive” pulses when the exhaust valve cracks open speed down the pipe under pressure to the end. Once the end is reached and the pressure drops away, the pulse reverts and becomes a “negative” wave heading back up the pipe, still at nearly the speed of sound! BUT… when it gets back to the cylinder it essentially ricochets off the wall of the barrel—several times! There is a hell of a lot of this high-speed traffic going on in there! A good traffic cop “pipe designer” knows how to utilize the physics of the situation to make more power within certain limitations we’ve already covered. The ongoing issue with stock Twinkies is the ricochet is not off the walls where it could be used to advantage, but off closed valves where can’t help a bit!
Seems to me (trying to be positive rather than negative) one could just keep playing at “Hype-performance” with their exhaust choice, or decide on a pipe that would actually work and some cam overlap to work with. Oh… and did I mention hotted-up engines ironically run cooler with their doors open?