SUCK
If you remember, old Nick Otto had his intake valve open just at the moment when the piston was poised to descend on the induction stroke. What he didn’t know, however, was that he lost some of the induction efficiency this way. The valve isn’t closed one moment and open the next, hey presto! Controlled by the ramps of the cam, it has to be lifted off its seat and accelerated up to its maximum lifting speed. It then slows down to a stop when that maximum is reached, where its motion turns around. It then re-accelerates up to its maximum closing speed and then slows to settle into its seat again. Therefore, there is more efficient induction when the valve is already pretty well open as the piston starts pulling the mixture in. Additionally, to get the best suck from the descending piston, it would be advantageous when the valve is at maximal lift when the suck is maximal. That is just before the mid-point down, at about 70-80° after TDC (ATDC). So, for either profit, the valve best starts opening before TDC (BTDC). How much before also depends on that scavenging process mentioned. We have already decided to keep the exhaust valve open, overlapping with the open intake valve to produce the scavenging of residual exhaust gases and to start drawing in fresh charge before the piston actually starts to descend. It can’t open too early or else the piston ascending on the exhaust stroke would push the waste gases down the intake tract. This too has to be timed just right.
An additional element is the mixture itself. Even though it is gaseous, it can’t just gallop into the cylinder the moment the stable door opens. It has its inertia to overcome before its flow gets into its stride. That is another reason for having the valve open in plenty of time for scavenging waves to start the flow moving and for the descending piston then to take over. But once it is flowing, the properties of that mixture can change. At high flow rates it starts ‘thickening up’, whose momentum is difficult to stop. But who would want to stop fresh charge from getting into the cylinder? The more the merrier for extra power! So, although Otto closed the intake valve when the piston reached BDC, leaving it open longer allows a ramming effect that gets more charge into the cylinder than just that drawn in by the swept volume of the piston. Again the timing for closing the intake valve has to be just right. Past BDC the piston has started to rise and compress the mixture. If this rising compression pressure exceeded the ramming pressure before the valve closed, charge would be pushed back down the intake tract. So, it closes after BDC (ABDC), but not too long after.
A rough estimate of this ramming effect can be calculated. From the engine specifications – tabulated in Table 1 – one can see that the swept volume of each cylinder is 60.96/4 = 15.24 cu.in (249.8 cc), combustion space of the GL1000s is 1.86 cu.in.(30.49 cc) and the combustion pressure is 171 psi (12.03 kg.cm2) The compressed charge of mixture started life at atmospheric pressure (14.7 psi, 1.0332 kg.cm2). We know from the formula (1) above that Pressure A x Volume A = Pressure B x Volume B, so an estimate of the charge volume before it got rammed into the cylinder is given by:
Induced Volume = [(171 x 1.86/14.7) – 1.86] = 19.78 cu.in (324.1 cc) (5)
If the induction had been left just to the piston moving down the cylinder, we couldn’t have got more than 15.24 cu.in (249.8 cc), the swept volume, into the pot.
Clearly, too, this ramming effect is greater if the induction flow velocity is increased. In order to get into the intake manifold, the induced volume of air has to pass through the venturi of the carburettor. If that venturi is made slightly smaller, then for the same volume of air to pass through the smaller diameter it has to move faster. This apparently does lead to better cylinder filling through ramming – and that improves the torque produced by the engine. However, there are obviously limits to this. Make it too small and the restriction may increase the speed of flow, but there is not enough time to pull in enough air - and the engine is starved of mixture. Again obvious is the fact that the changed air-flow speed also changes the low pressure that draws the fuel in through the jets. Therefore, there has to be a different jetting.
It seems that you can also change the engine performance by doing the opposite – opening up the venturi. The larger venturi slows down the air-flow speed but offers little restriction for the mixture to be sucked in rather than being rammed in. This is apparently relevant for getting peak horsepower when the engine has a wide open throttle (WOT) at high rpm. Because of the very brief valve-opening times, the availability of unrestricted mixture is more important than its speed. The problem of this system is at low engine speeds. The low velocity of the air passing through the venturi makes it difficult for the carburettor to get the mixture right and stable. Therefore, high WOT horsepower may be obtained but low speed response may be ragged.
Then there is the question of whether there are any resonant waves in the intake tract as there are in the exhaust. Well, yes, there are – except that the dynamic situation is a bit different – and very complicated! The exhaust tract has high-pressure gases passing from the valve to the atmosphere. The intake tract uses low pressure to draw mixture in from the atmosphere to the valve. Consider the engine running, and the mixture is flowing well through the intake valve into the cylinder. The intake then closes. The pressure waves generated bounce off this closed valve and run back along the intake tract. They pop off the open inlet end of the intake tract, create a reversed pressure wave that then runs back towards the valve. This resonance continues with a series of decaying amplitude pulses in the closed off system. This is just what happens in an organ-pipe. If the dimensions of the intake tract are well designed, this can have a very positive effect on cylinder filling when the intake valve does open. Again the frequency of this resonance influences the point in the rpm-range that peak power is obtained.
You may have seen set-ups that offer flexibility in this tuning. As the roving camera moves around the pits at motor races, you sometimes catch sight of some magnificent 8 or 12 cylinder engine. Most prominent is the battery of carburettors, with their highly polished trumpets standing out – indeed just like the pipes of some baroque organ. A wonderful sight! These bell-mouths may be 10-12 in. long – but they can be exchanged for others of different length. This would be done to modify the intake resonance, and with that the power band, to suit that particular track. So, I learn that changing the overall dimensions and design of intake systems can dramatically change the pattern of power just as – and maybe more than - exhaust changes.
Matters get complicated when the intake valve opens. The organ-pipe tuning breaks down and is replaced with so-called Helmholtz resonance. The mixture coming through the narrow intake port into the ‘big’ volume of the cylinder also starts resonating. (Larry Atherton likens this to the result of blowing into a whisky jug. There, the resonance process creates that deep sound that is sometimes used to provide the rhythm for a Southern States music band.) Harnessing this effect properly in an engine intake system can also bring dramatic improvements in cylinder-filling and performance – adding power, throttle response and fuel economy.
By the way, can you believe that until recently I went through Life knowing that it is important to keep the tappets - the valve clearances – well adjusted, but without knowing why? Alright, I always thought the clearances were there to take up heat expansion. Make the gap too small and at best the valve train is over-stressed, at worst a valve-stem is bent; make it too wide and the engine sounds like a cement-mixer as the valve and cam-follower wears excessively. But the recommended gap is specific, so that the cam-timing is correct in the warm engine. If the cam-follower has too little or too much distance to cover, then the valve operation is screwed up. Is there hope for me?
WHAT’S IN A WING?
As you can imagine, when I got this far, I didn’t want to see another dish of cornflakes in my life. I didn’t want to see another cornflake packet in the house, even though it meant I couldn’t complete my collection of plastic dinosaurs. However, what I did want to see was how these various design elements related to the specifications of the four-cylinder Gold Wings. I therefore collected up all the specs (of models of interest to me) in Table 1.
I didn’t get very far with my comparisons before another one of my preconceptions was shattered. It has been often said and written – and I have done that, too – that Honda didn’t really know what they were aiming for with the first GL1000. It was further said that the first American owners told them what they had produced – a tourer of the superlatives. Yet the specs seem to say that Honda did indeed know what it was doing. Look at those ’75/77 specs – the 32 mm carburettor, the valve lift of way less than 24-28% of the valve diameter, the cam timing, particularly with that very short intake-exhaust overlap of only 10°. All these specs are saying, "Conservative … conservative" … with a big C!
Honda did not give this engine any sporting pretensions at all. It is all laid out to give modest, unstressed performance – of a tourer – from Day 1. That didn’t change over the development years. The first changes occurred with the ’78 model – not to make the Wing a rice-rocket but to make it slightly more conservative. The cam timing is more conservative and the carburettor goes down to 31 mm. The resulting slight improvements in performance were in low-down torque rather than horsepower. Obviously, upping the capacity to the GL1100 and then the GL1200 couldn’t avoid increasing the horsepower, even though the carburettors were now down to 30 mm. The jump to 1085 cc, for instance, was an 8.6% increase in capacity. It brought a 6% increase in horsepower, but a 10% jump in torque.
Taking a different tack for a moment, engine specification and performance were not the only factors to change during Wing development. Weight changed as well. From the ‘75/77 to ‘78/79 GL1000, power didn’t change much, but weight went up by 9 kg (20 lb). Both of them, however, were still superior to the all-singing, all-dancing ’87 1200 Aspencade. Notably, the GL1100 provides both the best and the worst power:weight ratios of all 4-cylinder Gold Wings. The naked GL1100 carries the least fat per pound. Saddling it with the full fairing and luggage gave this Interstate Wing more pounds per power than any others.
In spite of this picture seeming reasonably clear to me, I still wanted some back-up evidence. So I raided my shoebox archive again, looking for the specs of bikes that were around at the time the Gold Wing was being designed. I couldn’t find full data, as Table 2 shows, but enough to make a comparison. To many, the ultimate sports tourer of the day was the BMW 90S. (I have long lusted after one!) At first sight, this boxer twin has a similarly conservative layout to the boxer-four Gold Wing – at least in respect to cam timing. However, look at that carburettor – compared with the Wing, its 38 mm Dell’Ortos are cavernous. And the valve heads – they’re the size of dinner plates. It may only produce 67 HP, but with 132 lbs less weight to carry, it left the Wing for dead in the power/weight ratio stakes.