[technicalities]

[TECHNICALITIES]

GORDON H. JENNINGS

SOMEDAY, I will learn not to make promises. As some of you may recall, this column concluded last month with a promise to reveal all about modifying a Honda Hawk engine for road racing — and I am not prepared to do that just yet. The problem is that we think we have a means of installing a substantially larger intake valve, and this will change the entire character of the engine. The whole question will be settled very soon, but until it is, there is no point in going into the matter any further. With a bigger intake valve, the porting will be changed, and there will be a corresponding increase in carburetor throat size. Also, and this is of primary importance, when we have made the engine breathe better, it will no longer be possible to run a 13: 1 compression ratio. Only the relatively poor high-speed cylinder filling provided by the stock valves prevents severe detonation with the engine set up as it is now. So, give me a bit longer to investigate this big-valve business; a complete report will be coming very soon. Now then: on to other things.

We have been getting some interesting mail (from the technical standpoint, that is) here of late. One letter is from a reader who takes me to task over some remarks I made in a recent discourse on 4-valve cylinder heads. In this, I stated that 4000 feet per minute of piston speed (mean) could be considered a safe upper limit for reliability in a racing engine. Then, I applied this limit to two hypothetical examples having radically different bore/stroke ratios. For this, I was taken to task, and quite rightly so. The actual stresses involved are not a direct function of piston speed, and in engines having an abnormally short stroke, relative to bore, mean piston speed alone is not a very accurate measure of operating stresses. I plead guilty to having oversimplified an example in order to make a point; but, even though the example was flawed, the point was essentially correct.

Actually, stresses on the crank-rod-piston train cannot be determined by any single factor; they must be calculated separately. Insofar as the piston is concerned, mean piston speed has virtually no bearing on the question of load. It is, instead, entirely dependent on piston acceleration, which is quite a different thing. Piston acceleration is determined by crank speed, stroke and, most important, the ratio of connecting rod length to stroke. With a very short rod, connecting rod angularity is high and the piston is yanked abruptly into motion as the crankpin swings past center; the longer the rod becomes, the more gentle is piston acceleration.

Loads on the crankpin, and on the rod’s big-end bearing, are a mixture of simple centrifugal force (as the crankpin tries to throw off the rod) and inertia. The centrifugal loads are easily calculated; inertia loads imposed by the mixed up-and-down and rotating motion of the rod and piston are more difficult but not impossible. Gas loadings, too, can be calculated fairly closely. So, in the final analysis, it is possible to build engines that will run reliably at 4000 ft/min. piston speed in a wide variety of bore/stroke ratios — but not unless there is some adjustment in bearing size and the ratio of rod length to stroke for very short stroke, very high speed engines.

(Continued on page 14)

In the same letter, the Ferrari Grand Prix engine was offered as an example that would prove the ability of the twovalve cylinder head to function at ultrahigh revs. Of course, the Ferrari engine in question does run quite happily at 11,000 rpm, with two-valve heads, but these valves are not nearly as large as the 2.125" diameter intake valves I had specified in my original hypothetical engine. It may be significant, too, that Ferrari is known to be experimenting with 4-valve cylinder beads; presumably for the reasons I outlined in the December issue’s “Technicalities.”

Another reader asks why virtually all motorcycle engines have their exhaust ports on the “forward” side, where the exhaust pipe must be curled around underneath instead of running straight back — as would be possible with the exhaust ports on the back side of the engine. There are three basic reasons: first, the exhaust port and its immediate surroundings are the hottest area on the engine, and it is a good idea to keep them up front where they get a flow of cooling air; second, it would be quite difficult to bring exhaust pipes directly back from the rear of the engine without placing them where they would be a serious bother for the rider — from the standpoint of either simply being in the way, or as an unwanted leg-warmer; finally, even for a relatively high-speed engine, pipe length must be at least 32" for best performance, and a pipe that long running straight back from the cylinder head would overhang the end of the motorcycle — even if no thought was given to a megaphone or muffler.

Recently, the AMA’s Competition and Technical committees held their annual meeting, and we were fortunate enough to get a copy of the minutes. If we understand things correctly, it would seem that the AMA is thinking about adopting a 45 cubic inch limit for “heavyweight-class” racing — without any of the usual over-allowance for flat-head engines. If this were to replace the present 45 cubic inch flathead/30.5 cubic inch ohv limit, it would be an improvement in some respects; but not in others. There is much to be said in favor of eliminating the present rule, which serves only to pump life into a type of engine that has long since disappeared from dealer’s showrooms. On the other hand, it is blatantly obvious that 45 cubic inch overhead valve engines would soon be developed to the point where everyone would have so much power that the problem of getting it down to the track surface will take on more significance than extracting yet more power from the engine. Why then have a displacement limit at all? It seems certain that everyone would be able to wring 70 or more bhp from 45" ohv engines, and that much horsepower would have to be applied so cautiously to most existing tracks that no one would work very hard to get more. Therefore, the AMA might as well make it easy on everyone and eliminate the displacement limit entirely.

(Continued on page 16)

I also note without much pleasure that the rules barring such technical innovations as double overhead camshafts and more than two valves per cylinder are still in effect. There was a time when it was unthinkable that anything but an all-out racing engine would have these features, but that time is past. Indications are that very soon these “racing engine” specifications will be common on most sports/touring bikes, and then what will the gentlemen of the AMA have to say? Even now we know that Honda has a twin-cam 350 running in prototype form (with an undisclosed number of valves) and our man in Japan tells us that there is a 500 as well. Italy’s motorcycle industry is beginning to think in these terms, too, and anyone who wants to remain competitive on the world markets will have to follow. I hope that the day does not come when manufacturers will be forced to build special single-cam, two-valves-per-cylinder engines just for

racing to conform to the AMA’s archaic regulations.

The flat track lads will now be able to shift gears while racing, as the regulations have been revised to permit this. Actually all it means is that now everyone can do their shifting legally. Unfortunately, a motion to require a rear wheel brake was defeated, for reasons that I must admit entirely escape me. Generally speaking, I think it is correct to say that a motorcycle is a somewhat more safe conveyance when it has brakes; why the situation should be greatly different in flat track racing is not too clear. TT racing has become little more than flat track with right as well as left turns, with a jump tossed in, and the fact that TT bikes are fitted with brakes does not appear to create any problems. Quite the contrary, the brakes often extricate riders from potentially very sticky situations.

Speaking of brakes, Yamaha has gotten approval from the AMA for the use of the Gunter disc brake on their road racing bike. Why they should need a different brake is beyond me, as the Yamaha TD-l’s drum brake is marvelously effective and very light. Perhaps they are planning to make the TD-1 so fast that more braking power will be needed.

Curiously enough, while the AMA will let them use the Gunter disc brake, it turned thumbs-down on a proposal to allow a special road-racing frame. If you think about it, it would be more logical to reject the brake and okay the frame. The frame is made by Yamaha, while the brake is not, and the frame would very likely cost about half as much as the complete Gunter brake system. It is obvious to me that, once again, logic has fallen victim to tradition. •