Stages explained, cont.
Still too big...
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, semi-hydraulic 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 …
There are those who would say that the first thing you should do to make a Harley work properly is to sort out the heads, but that is generally where "Stage Three" comes in.
The combustion chamber on Harley V-twins has benefited from better gas flow characteristics with every evolution, but the standard porting is not best suited to high performance. Porting is a subject in itself, and will be dealt with in the near future by someone who knows what he’s talking about, but we have reached the point where you’re really not going to sit in a shed with a bastard file and an heirloom toolkit. If you’ve got a fully equipped workshop – and we are talking fully equipped here, with lathes, milling machines and a space heater – you’ll know much more than I do already, and I’m amazed you’re still here.
This is where the engineers come in, and you have to put yourself in their hands. Sure, you can buy heads etc and stick them on yourself, but when you’re that deep in, you’re not going to put those heads in without seriously considering what you want from the bike. Well, if you’ve got any sense you’re not.
You’re in big money country now and a half-cocked Stage Three will be not much better than an amateur Stage Two – certainly not worth the additional expense of the parts. Yes, I know you can get ported heads off the shelf, but ported for what? More torque or more horsepower? Higher or lower revs? Fuel efficiency or straight line ability? Before you start you need to know where the power is needed, and what sort of power, to determine the size and shape of the valves: until then it’s merely a technical exercise.
And it’s not just heads, and that is why we now start to differentiate between engineers and fitters. A Stage Three motor really should be a blueprinted engine. It’s no longer enough that it is as good as an assembly line can make it, if you’re going to do it properly, it’s got to be as accurate as the original drawings: the blueprints. If the drawing has a dimension of 1.7701mm that’s what it has got to be, not +/- .005mm.
Production lines don’t do that, fitters can’t do that, mechanics would love the time to learn that. The only people who can do that properly are engineers – and even then, only the better engineers. The bad news is that there aren’t many left because there are few coming up through the ranks, and that’s because production lines have rendered a lot of basic skills obsolete and machine minders fill their steel-toecapped boots in industry.
A blueprinted engine will be less stressed than a production line example – even a good production line example – because everything will work as it should: as it was designed to do. The sort of engineer who will be capable of matching the specification will be more than capable of sorting out your porting, cam and carb requirements to make it better than the blueprint for your specific application, and that is the ultimate state of tune for your bike. Harley-Davidson produce motorcycles for the masses, an engineer will make a motorcycle for you … assuming you know what you want, and can communicate that to your chosen professional.