POWER
Trying to illuminate the shadows of my ignorance in other areas, I occasionally succeed in bringing a little light on the matter – at other times I only throw my ignorance into high relief! For instance, I became aware that I tended to believe an implicit old wife’s tale. I had grown up in a pretty feline world of motorcycles. There was the long-stroke Panther single and the Triumph Tiger twins. The Panther was regarded as a ‘thumper’ – a torque machine best at hauling family sidecars in pre-Mini days. The Triumphs were looked upon as spitting and scratching horsepower demons. So, implanted in my mind was the idea that torque and horsepower were two different animals – and you couldn’t have both animals at once.
Then, I recall my very first clear memory of a motorcycle – when I was about nine or ten. One of the neighbourhood lads owned it – a beautiful leaf-green and gold pre-war, two-stroke Scott, with burnished brass fittings. (Was that the birth of my interest in motorcycles Gold in name and colour?) I acquired street cred and stature just by hanging on to the fringes of the gang kicking tyres about this and other machines. Obviously I was well brought up enough not to mix in with the conversation of these important men, my elders, but that gave me all the more time to let my eyes linger over that machine. With that serpentine colour and slinky shape, it just had to handle well. Unfortunately that expectation was not met – for soon after the lad ran out of road at a large urban roundabout, and buried his unhelmeted head terminally in an ill-placed tree.
Then there were the ubiquitous British twins. Is that when I first learned the power enhancing effect of accessories, such as the twin carbs on the SS Super-Sports versions, with their seductive bell-mouths sucking in imaginary power from the air. Perhaps these were the origin of so-called Mark I tunings. These appeared mostly on cars. Glue a strip of chequered sticker tape on the side, add the letters ‘GT’, and the car was sure to go 5 mph faster! Certainly the gaping shark jaws I, as an 18 year-old, painted on the Avon fairing of my 350 cc AJS added to its predatory power. I was convinced that this predatory power helped me eat my way through city traffic.
Could it be that these well-informed ignorances still affected my attitudes to the early Wings? I mean, they were tourers, weren’t they. Tourers don’t have gut-wrenching horsepower to hurtle themselves and their riders up to warp speeds. No. Honda had made the choice for torque – and that excluded horsepower … didn’t it? Recently, realizing that I didn’t have many answers – mainly because I knew even fewer questions, I decided to leave the safe plateau of ignorance and start down the slippery path into the eternally deep pit of knowledge.
However, I seldom need one good reason for doing anything that stresses my brain-box – I need several! A second reason came along when I realised that keeping classic bikes on the road was becoming more and more difficult. Basic originality was becoming increasingly difficult to sustain because of the unavailability of parts or the funds to pay for the few remaining. There are constant discussions about the wish or desire to replace parts with non-original (non-OEM) parts. This is not so problematic with cycle parts, but what happens with non-OEM engine parts? Here, most discussion centres on carburettors and exhausts. Now, in my TUT mentioned above, these are in the Suck and Blow part of the equation and so could affect Power. I wanted to know how. I wanted to be able to judge what non-OEM parts could affect the performance of my old Wing – for good or bad.
I had the good fortune to stumble across aids to my scholarship – ones that offered the possibility not necessarily of knowing a few more answers but of being puzzled by a few more questions. These were the publications of the Californian-based Motion Software (www.motionsoftware.com). The basis product is a PC-based engine simulation software, Dyna2000, now in version 3.08. Coupled with this are Larry Atherton’s various handbooks that not only guide the user but explain the technical background to engines and their simulation, in very easy-to-read text (down-loadable from the website, as PDF files). Importantly, they blew away many of the pre-conceptions that I had carried around with me for decades. Additionally to this, I excavated one of the shoeboxes under my bed to come up with a few of useful cornflake packet cut-outs, such as:
J. Bradley, The Racing Motorcycle: A Technical Guide for Construction. 405 pp.,
Broadland Leisure Publications, York, UK, 1996.
G. Cocco, Motorcycle Design and Technology. 215 pp., Vimodrone, Italy, Giorgio Nada
Editore, 1999.
P.E. Irving, Motorcycle Engineering. 326 pp., Los Angeles, CA, Clymer Publications,
1973.
P.E. Irving, Motorcycle Technicalities. 97 pp, Sydney, Turton & Armstrong,
1973. (Articles by "Slide Rule", Motorcycling, 1932-1944)
P.E. Irvine, Tuning for Speed: How to Increase the Performance of Motorcycle
Engines for Touring, Racing and Competition Work. 6th edn., 260 pp., Sydney,
Australia, Turton & Armstrong, 1987.
J. Robinson, Motorcycle Tuning: Four-Stroke. 2nd edn., 178 pp., Oxford,
UK, Newnes, 1994.
P.H. Smith & J.C. Morrison, Scientific Design of Exhaust & Intake Systems.
3rd edn., 274 pp., Robert Bentley, Cambridge, MA, 1971.
J. Stoffregen, Motorradtechnik: Grundlagen und Konzepte von Motor, Antrieb und
Fahrwerk. 322 pp, Vieweg, Wiesbaden, FRG, 1995. (In German)
Oh! Yes. There was a third reason for pursing this knowledge. I wanted to upstage you lads and lasses. I was also a young lad when I discovered a very effective alternative to knowledge – confidence! Let me take a current situation to illustrate this. I would be King, a highly respected king in all the bar-rooms of Miami or Palm Beach these days. I would stay quiet while the arguments – and maybe the bottles – went to and fro. At an opportune moment, when words failed the others or the blood had to be staunched I would, for example, firmly say something like: "I think the solution can be found in Bumsrusher vs. The State of Florida, 1892 … mmm … November, I think."
This is, of course, total nonsense. However, uttered with the confidence of papal infallibility, few will question it. (That is a Power of a non-mechanical sort!) If some delicate soul does admit to a lack of conviction, like "O.K., smartass, wotsthat gotta do wiv it?", and backs up his query with a fist the size of a football, then you may have to reveal a few more details of the case. "Well, as anyone knows, it was concerned with the legality of building a privy behind a cat-house – someone’s right to dig a hole and for someone else to fill it with sh… oops! … manure while everyone up at the house is screwing themselves or each other. I think an intelligent person like you can only get the flavour of the case by reading the original."
So, as you see, there were many reasons for laying aside my favourite hobby – laziness – and getting the gray cells into gear. One of the first things I learned (better late than never) was that the internal combustion engine is nothing more than a self-driven gas pump. You could test this by disconnecting the ignition and connecting up an electric motor to the crankshaft. By spinning the crankshaft you would pump air from outside into the intake tract, through the pots and out of the exhaust. This is a pretty useless exercise pumping atmospheric air into the atmosphere because all that this pumping does is to consume power – giving a pumping loss. However, what if, in the middle of this pumping action, you gave the system a judiciously placed bang in the butt? You may not only overcome those pumping losses, but you may be able to connect the system up to itself – to eliminate that electric starter motor that got the thing turning in the first place. You may also end up with some excess power that, with the addition of a couple of wheels could drive that engine down the street and into the mountains. Now, it was when that slippery path led me to this point that I began to get interested. I thought I could be on the verge of inventing something. (I would call it a motorcycle.) But I still didn’t really know the difference between horsepower and torque. So, back to the armchair.
TORQUE TALK
I recalled Mr. Davies, our second-year physics master at the local Grammar School nearly 50 years ago. (He was affectionately known as "Tosser", due to his habit of playing ‘pocket billiards’ in front of the class. But that’s not why I remembered him at this juncture.) No, Tosser introduced us to engines – as known to physics. It seems that an engine is a device for doing work, usually by the conversion of one sort of energy into another. Clearly, the internal combustion engine does this, releasing chemical energy to apply a force to a wheel at the back of some mechanical contraption. If this force is strong enough to move the weight of the vehicle, then the engineers say that work is done – and they can measure that. The units they use for this measure – foot-pounds (ft.lbs) - give a clue to what is being measured. If the force moves one pound of weight a distance of one foot, then one foot-pound of work is done. However, if that movement is in a circular direction, as with most engines, then a new name had to be found for that capacity for doing work – and that was torque.
(Don’t think for a moment that ‘work’ is a man-made process, arising from his machines. You stumble across Nature ‘working’ all the time. Think of that Pacific tectonic plate shoving against the American West Coast. It may only move millimetres per year, but those forces move billions of tons of good ol’ USA sideways and a few Rocky Mountains into the sky. Now, that is work, however slowly accomplished.)
THE POWER OF HORSES
However, humans being what they are, they are not just interested in whether a thing happens or not – they must know how quickly it happens. With engines they wanted to know the rate at which the work was done. The 18th century Scottish engineer, Jimmy Watt, came up with a suggestion. He reckoned that a horse could pull 200 pounds over 110 feet (22’000 ft.lbs) in a minute. Therefore, he assumed that any engine that could do this had the power of one horse. Later, he must have gone over the border to test English horses, because he adjusted the power of a horse to 300 lbs over 110 feet. That measure of horsepower (HP), 33’000 ft.lbs per minute or 550 ft.lbs/sec is still used today – except by us funny Europeans who want to use more logical metric units. No sense of tradition, us lot!
So, horsepower is only the rate at which work is done – and is directly related to it by a time factor. It is no different for torque though, because the forces are circular, the factor of pi comes into the equation as well as the rate at which the circular motion takes place. This, of course, the number of revolutions per minute, the RPM. So, by jiggling the numbers, the conversion to horsepower can be made with:
HP = Torque (ft.lbs) x RPM/5252 ft.lbs/min (4)
Although HP is directly related to torque, the relationship is not linear. This is due to that RPM/5252 multiplication factor. It changes as you move over the RPM range. To show this, imagine an engine producing the following torque band, in ft.lbs at various rpm:
75.0 @ 4500, 80.0 @ 5000, 75.0 @ 5500, 70.0 @ 6000, 65.0 @ 6500, 60.0 @ 7000
With the torque peaking at 5000 rpm, my trusty Texas TI-58 calculator (also a late 1970s classic) gives the following power curve, in HP @ rpm:
64.3 @ 4500, 76.0 @ 5000, 78.5 @ 5550, 80.0 @ 6000, 80.4 @ 6500, 80.0 @ 7000
The HP peaks at 6500 rpm. Now, assume we want to have more torque around at lower engine speeds, say, for sidecar pulling. We tweak the engine differently, so that it produces the same torque curve, but shifted 500 rpm lower down the rev-band so that the 80 ft.lbs peak at 4500 rpm. The HP power curve would then look like:
68.5 @ 4500, 71.4 @ 5000, 73.3 @ 5500, 74.3 @ 6000, 74.3 @ 6500, 73.3 @ 7000
Now, I am beginning to get the first insights into engine manipulation. All we have just done is to change the engine speed at which it delivers the same amount of torque to the crankshaft, and we have markedly changed the HP-curve. Gaining low-down torque has resulted in a ‘loss’ of about 6 HP at its peak. That same amount of work, but at lower rates – or rpm – results in the rated HP being less and tending to come down the rev-band. Obviously, doing the reverse - sacrificing the low-down torque by pushing that peak 500 rpm up the rev-band to 5500 rpm - does result in more peak power (86.6 HP @ 6500). However, the cost would be less tractability at those lower rpm.
So, there are no free lunches. HP or torque can be gained or lost up and down the rev-band. The simple conclusion would seem just to be a matter of choice. Unfortunately life isn’t that simple. The engine may be tweaked to get more of this or that, but the bike may not be able to use the extra something. When the basic power curves were obtained, a smart manufacturer would design the gearbox to make optimal use of that power over the rev-band. A tweak to produce a new power curve may not suit that OEM gearing. Technically it is not so difficult to get a bit more torque or horsepower here or there from a standard, suit-all-tastes engine. Tweaking the gearing, however, is a major (a.k.a. expensive) undertaking.
OLD NICK
My intense researches have now reached the point where I want to know what sort of tweaks or manipulations can shift the power curves around. That meant that I needed the starting info on the specifications of the four-cylinder Wings (are there any others?). So, it was back under the bed, to search through the workshop manuals hiding in that shoebox. For the Wings of interest (to me), I assembled the available data in Table 1 following. Before we get on to that, though, we have to think about Old Nick who conceived of the four-stroke internal combustion engine. That does not mean that I am a hairshirt-wearing tree-hugger who sees the Devil in all things mechanical. I am talking about Nicolas Otto. Although he made the crucial breakthrough in design in 1876, with the 4-stroke cycle providing that all important compression stroke, he didn’t see far enough into the Suck and Blow bit of power-making.
To be fair, perhaps the dynamic behaviour of gases was not well understood then, nor wave dynamics mathematics, nor even something as simple as organ pipes. Nick seemed to have seen the Sucking and Blowing to take place with static lumps of gas, and not entities that changed their properties when moving and changed them again when moving at high speeds. Well, his engine itself was no turbo, turning over at about 160 rpm. So, his Suck and Blow were timed maybe just as I would have timed them. Otto’s intake valve opened just when the piston was at its top dead centre (TDC) point, so that the immediately descending piston pulled in the fresh charge. It was closed at the piston’s bottom dead centre (BDC) point, so that when it ascended, it could compress the induced charge. When it reached TDC for the second time, at the point of maximal compression, the mixture was ignited so that the expanding combustion gases could push the piston back down again to the second BDC. It was then time to get rid of the spent gases, so the exhaust valve was opened at BDC so that the then ascending piston could drive then out and be ready for the next four strokes to begin. Seems neat and logical, doesn’t it? No trade-offs. However, now we have to start looking at those other properties of moving gases, to see whether any trade can actually increase power. Let’s start back to front, looking at that exhaust, Blow stage.
BLOW
Clearly, the efficiency of the engine depends on removing those spent gases, to make way for fresh charge. If the exhaust valve doesn’t open early enough to give time for them to get out, power will go down. But what is early enough? At first sight, opening the exhaust valve before BDC (BBDC) should cause a loss of piston-pushing power from the combustion stroke because the excess pressure goes down the spout. However, releasing the exhaust gases at high pressure allows them to get up speed, swoosh down the pipes and then, further encouraged by the rising piston, get out of the way for fresh charge. This leads to power gains. So, the trick is to get a positive balance from the loss of power due to early exhaust opening and the power gains this brings. It was found that exhaust valve opening some considerable time before BBDC does give this positive balance. But when do you close the exhaust? For that we have to get into wave dynamics – about which I know nothing!
My recent reading showed that I was 60 years out of date. I still entertained a concept that was disproved decades ago. I always thought that cylinder emptying was helped by the momentum of the slug of exhaust gases. Rushing down the pipes it sort of ‘sucked’ the residual gases out with it, on its shirt tails. That ain’t so! Of course, contained within the pipe, the gases flow down the pipe with a positive wave of high pressure. However, a peculiar thing happens when they pop out of the pipe. Wave-dynamic processes cause a wave of negative pressure to flow back up the pipe at supersonic speeds. This does not necessarily affect the movement of gas particles still flowing out. (This might be understood by considering a smooth-surfaced flowing stream. Going with the flow is a cork. Tossing a stone ahead of the cork causes the characteristic ripples to spread out. These also spread upstream. They cause the cork to bob up and down, but they do not impede its flow downstream.).
Now, if the exhaust valve is still open when that negative pulse arrives back at the cylinder, it is this that pulls out more exhaust gases, scavenges them. So, the longer the exhaust valve stays open, the more chance this effect has of working. Nonetheless, the timing has got to be just right. The speed of these dynamic waves is pretty constant. However, the times when the exhaust valve is open depend on the engine rpm. Therefore, this exhaust-induced increase in power may operate only over a limited range of rpm.
A further point to consider is that the time the gases need to pop out of the exhaust-pipe and for the returning pulse to arrive back at the cylinder obviously depends on the length of the exhaust system. Still further, the nature of the exhaust ending (or the pseudo-ending created by a voluminous expansion box) can also have an effect. Fitting an open pipe that suddenly exits into the atmosphere produces a really definite, powerful pulse that returns up the pipe. This would have strong and definite scavenging effect – over a very narrow rpm range. With these powerful positive and negative pulses resonating up and down the pipe, we would also have a rather impressive trumpet effect – somewhat ear-shattering perhaps. If the exhaust header pipes reach the outside through a series of stepped pipes of increasing diameter – and maybe through a large collector box, then that one single pulse will be broken down into a series of lesser pulses spread out over time. Therefore, any scavenging will be less pronounced but available over a wider rpm-range.
Can this scavenging wave in the exhaust be used to any other advantage? It surely can. If the intake valve happens to be open when it arrives through an open exhaust valve, the negative pulse can also help to draw in fresh charge, filling the upper spaces and driving even more waste out. This occurs when the exhaust valve closes after the intake valve opens – giving an overlap when both valves are open. However, leave that exhaust valve open too long and, yes, power will go up because the scavenging is maximal. On the other hand, there is the danger that fresh charge also goes down the exhaust pipe – wasting fuel (which is also power) and worsening the emissions profile with unburned hydrocarbons. Therefore, the exhaust valve closing time has to be just right as well.
Of course, with the design of the exhaust system these factors can be tuned for a particular effect – to obtain something positive or avoid something negative. A non-OEM exhaust system could enhance a positive effect. It could destroy it. It might introduce a negative effect. It could produce more noise – and be illegal in some parts of the world. (Again addressing one of the old saws about vehicle noise – Loud Pipes Save Lives – any believer in this clap-trap should read Patrick J. Hahn’s recent article Loud and Clear in the electronic magazine The Interactive Motorcyclist (www.activebike.com).