- As some of you know, I once lived a normal life of the usual consistent toil and work needed to maintain many possessions. Although this system worked very well for most of my friends, for me it was like slow death. In time I established a way of life almost completely devoid of these pressures. I believe that, aside from the elimination of ANY. unnecessary possessions, this ideal is achieved by keeping things extremely simple. For the very definition of the word simple is EASIER, while the definition of complicated is undeniably HARDER. So did I want an easy life or a hard one? The choice became simple, and this ideal is probably what has kept me so loyal to Harley’s evo engine for so long. For not only does the evo offer the durability and longevity of any modern engine, it is also (aside for some of the very early stuff) the simplest design Harley ever built. Less moving parts and far easier to rebuild than a Flathead, Knucklehead, Panhead, and even easier is some ways to work on that a Shovelhead—which is what it was based on.
But lets move on to what I really want to say here…
For this article I’m gonna get into the cam design for the evo as well as the twin cam and show, in simple layman’s terms, what the actual benefits of the new design are. As you will see, although there are changes done to the twin cam that make it a far better performer, the twin-cam design itself is not one of them. So why then did they build it? Well, I’m gonna show you…
Although there are plenty of you gear-heads out there who already know this stuff, if you tolerate my baby talk through the first photos, I’m gonna eventually get to the actual reason the twin cam design was built. If you’re not quite sure about that already, you’ll soon find out. However, I’m gonna keep the beginning of this talk geared exclusively toward the non-gear-head. And although you non-gear-head guys may not be interested in tech stuff, if you bear with me for a minute you will also see, easily and obviously, what the real advantages to the new engine’s are, and are not.
I’m hoping that this article will be very informative.
The stuff I’m gonna get into here is not rocket science. In reality it’s only super simple Jethro and Elly May hillbilly engineering.
Let’s get started…
1 Since were concentrating on cams, I’m gonna talk a little about the valve system.
See those four chrome tubes on the side of that evo engine? Inside each one is a round metal bar, or push-rod, that is pushed up from below to move something in the top. The next photo will show these.
2 I’ve got the tubes open and you can see the push-rods. If I started the engine you’d see those things flying up and down pretty damn quickly. Now let’s get a little deeper into what’s pushing them up from the below…
3 Here I’ve removed the cylinders and push-rod tubes. Those two black towers are what the push-rods sit on top of. They’re called lifter blocks and I’m gonna pull the back one off.
4 There is is. That thing sticking out the bottom is a lifter and it moves up and down inside a tunnel in that lifter block to push up on the push-rod above it. Note the wheel on the bottom. It’s pretty important.
Note: I have other photos of the inside of these lifter blocks that shows them being pretty badly pitted up inside. And although they’ll work for now, I’ll keep an eye out to pick another set up some day and replace them if I’m ever in there again. All evo and older HD engines have easily removable lifter blocks similar to this. All but the twin cam. The TC’s lifter blocks are machined directly into the right engine case and if there’s a problem with them you must replace that entire engine case.
5 Here’s how it gets pushed up and down. That thing with the lobes on it is a cam, and those lobes are simply on a metal bar that spins. This is a cam. The next photo will show exactly what they’re doing.
6 Okay. Now you can see that the front lifter block is in place and the little wheels on the lifters inside it are sitting exactly on the lobe of that cam. When that cam spins the lobe’s gonna come around and push up on that wheel. This will move the lifter up, and then also push the push-rod on the top of that up. See, it’s hillbilly stuff man.
The next photo will give an even closer view.
7 Pretty damn simple isn’t it?
8 Now the lifter blocks are off and I’ve pulled the cone off to expose what’s inside the cam compartment.
The engine’s job is to turn a shaft that sticks out either side of it. The left side is ultimately hooked to the wheel, while this side is used to turn things the engine needs—like the cam. Anyway, that little gear shown here on the bottom comes straight out of the engine and simply turns that cam above it that we were looking at in the last photo. Pretty simple huh? That gear on the left is the breather which is also simple but we ain’t gonna get into that right now.
Here’s the catch, that little bottom gear coming off the engine (called a pinion shaft) has to be turning completely straight. It can’t be wobbling back and forth and up and down or it’ll break the gear above it—obviously. Because of this the crank inside the engine must be trued to very strict tolerances by an actual human being who must fight with it until it’s almost completely straight. Also, because the machining is not always exact on each engine, the big gear above on the cam comes in different sizes (undefinable to the naked eye) that must be fitted again by a human being to each engine. All Harley engines but the twin cam have been like this. This procedure probably costs extra money and HD has eliminated the need for it by taking a couple of cheap shots on the TC engine. I’ll show you that in a few minutes.
For now I’m simply gonna reach in there and pull that cam out…
9 There it is. See that little hole on the inside of the engine? That’s simply where the cam sticks into.
10 Very simple one piece cam with four lobes on it. This one looks in very good shape too.
Now, in the next photo, we’re gonna take a look at the exact same thing—only in the twin cam…
11 Look at that. More like the inside of a space shuttle ain’t it? That little gear on the bottom is the pinion shaft that comes off the engine. That bigger gear that the chain goes to is one of the cams (there’s now two cams with only two lobes each instead of four like the evo) which has another bicycle chain on the back side that goes to the other cam located in front of it. That too is space shuttle stuff. I’ll show it to you in the next photo.
Never before in HD’s history have they used bicycle chains inside an engine. Why’d they do it now? Well…let me tell you…
Remember how I said that the crank inside the engine must be made completely straight by a human being? It no longer works like that. Nowaday the cranks are pressed together by a robot which, although I’m sure it can knock cranks out like gangbusters and save a whole lot of money, seems seldom to be able to get them perfectly straight. Because of this that pinion shaft often wobbles round and round and up and down. This is called run-out and if it’s to far out of whack it’ll break gears won’t it? But it won’t break a bicycle chain. Likewise, the cam gear no longer needs to be sized by a human being as it does on the older engines.
Note the little red cam chain tensioner on the left. That’s the part that gave these engines all the problems between the years of 1999 and 2006. Seems like they got it pretty well worked out after that though.
12 Yea. There’s the back side. How do you like that? We’ve obviously left hillbilly town and entered the world of complex modern engineering. But what is the real benefit of all this? To you and me, there’s actually none; as you’ll soon see with your own eyes. I’m gonna get to that next…
Note the second little red cam chain tencioner.
13 Okay, we’re back to the evo photo again. This valve train is called a push-rod system obviously because the push-rods go up and down in there. But, take note that because these push-rods all have to come down from the top to meet a single cam at the bottom, they’re not all straight. In fact, if you look at where the front-exhaust push-rod tube that’s all the way on your right comes out of the engine, you’ll see it’s cocked off at an angle. Doesn’t come up straight at all.
Push-rod systems will not work at high RPM, especially if they’re not straight up and down, which is why the Japs use overhead cams so they can rev their engines up to 10.000 RPM. But most of us Harley guys don’t give a shit about that. We like the slow turning, lazy, V-Twin engines that will thump us up the hills and across the land.
Still, straight push-rod tubes are at least better than crooked ones—especially if you wanna rev it up. So the complex twin cam design was supposedly built so they could get them lifter blocks farther apart and tweak them push-rod tubes into a straighter angel. Well lets take a look at the twin cam in the next photo and see how that worked out…
14 There it is. You can see them lifter blocks are farther apart. But wait! Look close at that far right push-rod tube. That fucker ain’t coming out of there straight either. What’s up with that?
Well lets take a look at some of the older designs that they actually did get the push-rods straight, and see how they did it…
15 Here’s the Flathead. It’s real old stuff. What they did was simply put in four cams so the lifters went directly under the holes above them. That worked. But could they ever do something similar with a single cam like mine? Actually they did—on the very next engine, which came out sometime in the 40s. Let’s take a look at that design in the next photo…
16 The Knucklehead uses a single cam, just like the Pan, Shovelhead, and evo, but every damn push-rod is exactly straight! Check it out. How’d they do that? I’ll try to show you in the next photo…
17 What they did was; instead of moving the lifter blocks down below, they simply moved the holes up top out until each was directly over the it’s proper lifter block below, and wallah! A perfectly straight push-rod trajectory.
This is a great design that’s common ONLY to the Knucklehead.
18 Now lets look at the Panhead. Note that the front-exhaust pushrod tube is still at an angle—just like the evo. This is the first engine HD tried that on. Guess they decided to try putting the tubes at an angel and see how it worked out.
19 Shovelhead. Same thing. You already saw the evo. So why then did Harley Davidson REALLY decided to do this whole supposedly straighter twin cam push-rod thing? To answer that I’ll need to tell a little piece of HD, or motorcycle in generally, history:
As with all motorcycle manufactures, Harley Davidson has always been in fierce competition with, well, their competitors. In fact, in the last century there was originally over 130 American motorcycle manufactures; all but one eventually buckled under the pressure. For HD it was originally most often Indian. After their demise in the early 1950s the British came on the scene (Triumph, Norton, and BSA) and the Sportster was born to compete with these smaller, nimbler bikes. Next it was the Japanese who introduced reliability and longevity to modern motorcycle in the late 60s and especially the 70s. This development snuffed Norton, Triumph, and BSA out of business rather quickly and became the closest thing in history to almost put Harley Davidson out of business. In fact they were almost gone. Then, in the mid 80s, HD countered with the evolution engine (so named because it was supposed to have evolved to compete with the new era of motorcycles). Advertising for this new engine boasted that the evo offered greatly enhanced longevity, no overheating, and would not leak—problems that plagued almost all older bikes throughout history. Yet evo advertising said almost nothing of added horsepower; for that was not a majorly important factor at the time, and not what the evo engine was built to do—it was built to last.
Being of very early design the HD has always been one of the simplest machines on the market. One of my main concerns for the evo was: would it carry on this tradition? Well, as I’ve already stated, the evo engine and 5 speed transmission are both some of the simplest stuff HD ever built.
Modern longevity coupled with extreme simplicity; a very big plus in my book.
But let’s get back to why the twin cam design was built:
The evo debuted in 1984 and by 1989 had grown in sales to dwarf all the Japanese manufactures combined in sales for bikes over 800ccs, which is what HD sells. Problem was people wanted more power. Then, in the early 90s, S&S came out with a big 96 inch evo clone (before that S&S had only made parts. Now they had a complete engine). In time others began to make big evo engines as well; for you see, patent on the evo had never been that good and at the time were obviously not all that hard to get around. Aftermarket companies quickly became, and remain to this day, Harley Davidson’s newest form of stiff competition (although Polaris is trying to horn in on HD’s game as well).
So now HD had a choice: either build a bigger evo and go into competition with the aftermarket boys, or come out with something completely new and put better patents on it. And so the twin cam was born. In reality it had little or nothing to do with cam design improvements.
So what are the twin cam’s real advantages? First off the compression ratio was raised by one entire point, while cylinder to head surfaces were increased to lessen the possibility of blown head gaskets associated with higher compression. Next, consideration was taken to make sure the twinky’s heads flow better. Piston stroke was shortened by ¼-inch (a considerable distance), which probably contributes to a smoother ride. But the greatest advantage was in moving farther apart the 8 studs that protrude from the cases to hold the heads and cylinders onto the engine. This simply allowed the use of bigger cylinders and therefore added much larger displacements. For anything even near a stock evo (with the studs being so close together), increasing displacement by much is simply impossible.
So mostly they just made the twin cam bigger.
Other advantages are a larger wrist and crank pin to help handle the extra power. Aside from that there’s not much. Flywheels are actually lighter (granting quicker revving, although possibly at the cost of a little torque) while crank bearings remain the same.
So you see, the twin cams have nothing to do with performance enhancement. In fact, S&S now makes their V-111 engine in both twin cam and evo configurations and both put out exactly the same power and torque specks.
Because of my long history of riding these motorcycles and so much time spent traveling, I’m often asked what engine a person should go with. To this I almost always answer an 07 or up twin cam. For most simply wouldn’t be happy with performance on the older bikes. And under normal use, where the bike will spend most of its time in town and seldom see more than 50 or 100 thousand miles in its lifetime, the twinky is a far more satisfying ride. But they are now very complex, expensive to repair, and all but impossible to fix on the side of the road. But how many folks repair their own bikes these days anyway?
Although the twin cam has gained many substantial improvements, it is designed more for those who do not mind spending a lot of money, but do not really ride crazy miles. The evo was designed (possibly by accident) to go a lot of miles for very little money.
In my weird gypsy world that makes all the difference there is.
20 This is a photo the new 8-valve engine that comes in baggers now and is sure to completely replace the twin cam soon. For this engine they’ve returned to the single cam design, except again this cam will be driven by a bicycle chain instead of gears, and I have to assume this is again to help counter the occasional untrue crank.
This engine is rumored to come in 107” and use four valves per head.
This engine will undoubtedly offer great power and smoothness; both things I will probably never get to enjoy because one of my main concerns is also simplicity. And lets face it: the most common America (and HD too) belief these days is that “More complicated is always more better!”
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