Now you’re motoring, but now you’re more likely to need the assistance of a professional.
First thing to do is to carry out all the “Stage One” stuff that you want to do before you start, because they are the basis for the next round of modifications: there’s no point sticking hot cams in a strangled motor. By the same token, there’s no point getting carried away with the need for a Stage Two if you’ve not taken your bike to Stage One yet – you might be quite happy at first base.
Stage Two is largely about cams. You’ve got the means to draw fuel into the motor already, and to get the gases out. This is about how long you open the door to let the fuel through, and how wide you open it, and that is determined by what sort of work you want your bike to do.
There is probably more written on Harley Cams that anything else, and there is no shortage of people far better qualified than I to go through the absolute specifics so I’m just going through the general stuff … that way I can’t be blamed for your sticking a wholly unsuitable cam into your motor. What’s unsuitable? Something that makes your bike worse for the way you want to use it.
It is almost at odds with the perception of tuning that you can have a Stage Two motor that is actually detuned compared to the original but then tuning isn’t only about power, it’s about suitability for the purpose. You could make a touring Buell, a hot rod Electra or a lazy T-Sport by judicious use of cam profiles, matched to an efficient induction/exhaust system.
There is nothing especially clever about the principle of a camshaft, and all it does is transfer a rotary motion, which the crank delivers into a pushing action, that you need to open the valves.
A perfectly round shaft spins at half the speed of the crankshaft, driven by gears in the case of Sportsters and pre Twin Cam big twins, and a chain on the stock Twin Cam. On that shaft is one “lobe” per valve and as the shaft spins, anything that follows the track round the shaft and up over the lobe will rise and fall with it. That ‘anything’ is a cam follower, and it faithfully follows the track of the cam’s lobe, climbing the opening ramp, and pushing a pushrod up to a rocker shaft. The rocker shaft rocks when it’s pushed, pushing down onto the top of a valve, which is held in the closed position by heavyduty springs. Springs move on demand, whereas the lobe on the cam is immutable, so the valve opens and stay open for as long as the lobe on the cam is holding it there, but as soon as it rotates beyond that point the spring pushes the valve closed again, rocks the rocker arm, which pushes the pushrod down quicker than gravity would force it to drop and the cam follower rolls down the closing ramp for another circuit before the opening ramp comes round again.
A couple of quick things. The cam spins at half the speed of the crank because it only needs to do something every fourth cycle in a four stroke engine. A classic 2-stroke doesn’t have a camshaft spinning at the same speed as the crank because it doesn’t have a camshaft at all. And cam followers are the quaint old English term for what we now refer to as lifters: almost the same logic in the naming department as lifters lift, but then they also drop. They are also sometimes called tappets, probably because badly adjusted ones make a tapping sound, I guess.
Cams are specified by lift, duration and angle … and quantity: Sportsters and Buells have four with a lobe apiece, one for each valve, big twins up to and including the Evo had one with four lobes that run all four valves, and Twin Cams have two with a pair of lobes each, one per cylinder.
There is actually a fourth quotient, and that is the ramp. A steep ramp will take the pushrod to the maximum lift very quickly, and return it to rest as quickly as the spring can force it – as opposed to a gentle ramp which the cam follower will … well, follow.
A high lift cam will let more fuel through but they are generally used on fast spinning motors – the high lift allowing a good lungful of fuel and air compensating for the need for a short duration to give the valve chance to close again and be seated correctly before the next cycle starts. You don’t want to compress the fuel while the inlet port is open, because it’ll spit it back out again. If you’re playing with high lift cams, you’re more likely to use stronger valve springs to get the valve shut quickly, but there is a trade-off in that the harder the spring is pushing against the valve-train, the greater the potential for wear of the cam, follower and any bearings.
A long duration cam will give the maximum amount of opportunity for the fuel/exhaust to get in or out, but shutting the valve late increases the chance of the valve being open on the compression cycle. Better suited to slower-spinning motors in conjunction with a lower lift.
The angle will determine when the valve starts to open, and there can be an overlap built in according to what the engine is to do. It is possible to open the inlet port a little before the piston has reached TDC to make sure that it has opened sufficiently when it starts to descend, drawing fuel through; it gets away with it because the exhaust port is wide open and provides the easier route through. Similarly the exhaust valve won’t quite have had time to shut before the piston descends, but by then the inlet valve will be wide open and it will draw it through there rather than the exhaust valve that is slamming closed.
You will be delighted to know that you haven’t got to make your own decision on any of those elements, as every combination will have been tried repeatedly by very bright engineers. The resulting profiles represent everything from radical to realistic, wild to mild, and are well known for their characteristics. Hopefully you’ll have a better appreciation of why the engineer who knows about these things is asking you lots of questions – and if they’re not, be concerned: they may be good, but they’re not psychic and they need to know what you want.
If you want to play a greater part, you might want to consider the technology of the follower/lifter/tappet. Back in the old days of British pushrod twins, the cam followers were little more than hardened steel metal blocks that slid on the hardened camshaft lobes on a thin film of clean oil; they had a means at the top to locate a pushrod, and a means of adjustment. Meanwhile, Harley have used roller bearings to track the lobes for generations, and housed them at the bottom of their lifters. Not just ordinary lifters either, they’ve used hydraulic lifters since the end of the Knuckleheads: high technology at the heart of the big twin, but the fashion for decades was to replace them with solids. But times have changed.
Hydraulic lifters are self-adjusting, using clean engine oil to fill a chamber within the lifter body and a piston that provide the base for the pushrod. The size of the chamber is determined by the valve train itself, and the slack built into it. With the chamber full of oil, the lifter takes up the available slack and acts as a single unit of exactly the right size. But so does a screw thread, I hear you cry. Ah, true, but here’s the rub. An engine is made of metal and gets hot, and metal expands when it gets hot. So as you run your motor the hot bits expand and the cylinder head actually moves further way from the camshaft. How far can a motor grow? Not far but enough to make a difference. Try .040” on an Evo motor, when your valve clearances should be somewhere nearer to .002. With hydraulic lifters, as the engine gets hotter, and the gaps increase, more oil fills the bigger chamber and the slack is taken up.
It’s the opposite of your recollections, if your recollections are of old Brits, because the expansion of the pushrod is greater than the barrels, so Brit bikes rattle when they’re cold, and Harleys rattle when they’re hot – or they do if they’ve got solid lifters.
There is a downside, there always is. Hydraulic tappets are much more complex, and susceptible to dirty oil, and fall down as an engineering principle when the lifter becomes worn and oil can escape from the chamber because it screws up the adjustment.
For all their sophistication, mechanically adept luddites and power junkies missed the simplicity and economy of “solids” and they converted back, backed up by experience of failed units in days when engineering tolerances weren’t as fine as today, but for the majority of owners a set of hydraulic lifters were always better than a badly adjusted set of manual tappets.
Today there are a massive number of engineering companies offering a vast array of hydraulics, semihydraulic and solid lifters for your Twin Cam, Evo, Shovel or Panhead. It will come as no surprise either to note that you can also get high performance lifter blocks to house them, and these are not to be confused with cosmetic covers: if you’re going to seek the finest engineering tolerances in your lifters, you’re advised to make sure they’re sliding in a block engineered to the same standards.
While you’re in the motor playing with cams, it’s as well to replace the stock cam bearing with a better one, but aside from that – and the original Stage One mods – you’re about there. You might want to consider a different ignition module – but you should perhaps have accounted for that when you did the Stage One, giving yourself some elbow-room for further development.
You’ll note from Harley’s Parts and Accessories catalogue that 1550cc motors rear their heads quickly when talking about Stage Two, but that’s not a pre-requisite. Yes, a 1550cc big-bore would be nice, a 1700cc stroker would be nicer but that isn’t necessarily a Stage Two. It could be, but if you’re going to those lengths, it’s worth contemplating a little porting and checking of the rest of the lump, which takes us to …
To read an overview of Harley-Davidson staging please click here or click here to read more about Harley-Davidson stage 1 otherwise click here to see our Harley-Davidson stage 3 article
With thanks to Andy Hornsby of American-V magazine from which parts of this article have reproduced . Originally published 22/1/2004 © American-V magazine