Until the late 1990s, the CB750 was, to my knowledge, the only "big displacement" undersquare inline four production bike built. "Undersquare" means the bore is narrower than the stroke length. "Conventional" hotrodders complain that "undersquare engines can't breathe" like square or oversquare engines. This only illustrates that they misunderstand the great advantage of 'long holes'.
At low engine speeds, the longer bore gives greater torque for a given displacement, period. In most cases, long-bore engines were used for torquing applications, for this reason. Honda went it one better, though, by making a long-bore engine that could rev, REALLY rev. The rod ratio is actually over 2:1 ! When the revs begin, a special effect known as 'ram tuning' rears its pretty head, which forces more charge into the cylinder than one can get with a shorter stroke. This occurs because the vacuum is deeper at the midstroke, it lasts longer, and can be controlled more easily through the cam timing. Astute mechanics may notice that the 750 cam is a little late on intake closing compared to, say, a 350 twin or "modern" touring bike. This means that the power range starts later and stays longer than a larger-bore engine, and the resulting 'turbocharging' is free.
Next, the long-bore multi that revs has the extra low-end torque that makes it driveable while you're on your way to the powerband. This is no small thing, as riders of the CB500/CB550 know. The 750 has no "flat spot", like all other multis made. It never did. The long intake tubes, longer than the carb bore diameter, make up the midrange that otherwise starts to fall off in most fours before the powerband comes up. Then, the combination of ram-tuning in the cylinders and the long intake tract come together at about 6500 RPM to make the show start. The cylinders actually run richer at this RPM than below it, no matter how you tune the air inlet system. This is one of the great secrets of this engine, and why it is just as much at home with a touring load as it is between stoplights, once one understand the advantages. Add to this the higher controllability of mixing fuel for small bores, rather than large ones, and the smallest of changes come together in the biggest way.
Keep that in mind for your tuning efforts: you are working with small displacements, so don't listen to the V-8 guys. A lot of their stuff does not work with these bikes. Headers on small bore engines are highly overrated.
To be sure, Honda struggled with this design. It went from a 600+cc to a 700cc to the 736cc version in R&D. They got a lot of experience, though, from their famous 90cc engine family, which was frequently stroked to 105cc size. Their primary "engineering" reason for undersquare was publicly stated as "to control the width of the engine", but my Yoshimura friends assured me this was public camoflage to Kawasaki (who, by the way, lost out to Honda by 2 hours of being the first to introduce an inline 4 at the New York expo in 1969, but that's the "New York Steak" story, for another day). The CB750 was essentially a multi-cylinder Honda 90 with better metals and a very risky gearbox design (that proved out well, fortunately).
So, when you hear old Cycle magazine readers quoting that rag as saying "this is the bike that changed motorcycling", smile. That rag was speaking about riding to the drive-in on Saturday nite and coast-to-coast the next week. In truth, it changed the way all motorcycles were comparatively built from then on.
