I didn't realize Denso has a X24ESR-U.
I assume the "R" refers to a resistor version.
Thanks
Since the swirl-charge design in these engines makes the most of the longer spark duration, giving it will make for a nicer ride, especially at speed. 
Now, longer spark will exactly do what? I would assume once the mixture is ignited, there is no longer need for the spark while the "ball of fire" is spreading away from the igniting point. The spark lasting longer does not have anything else to ignite.
The "big secret" about the SOHC4 engines is what makes the long-duration spark so important.
These engines up to the F2/3 (they are different) are "swirl-charge" engines. This technology Honda adapted from their famous CVCC 2-chamber engines of the 1960s, which met 1980 emissions levels before ANYONE was required to reduce pollutants, while delivering almost 100 HP from a 1+ Liter engine. But, they also seemed to do it for another reason: more power and better fuel mileage.
The intake and exhaust ports in the heads are angled into the chambers, and there is additional shrouding, especially in the K0-K4 engines (less so in the later ones) placed specifically to turn the incoming air-fuel mixture, so it spins in and downward like a little tornado. As the piston rises toward compression, this accelerates greatly, approaching the speed of sound in the K0-K4 engines, as a spinning vortex. When the (longer-duration) spark fires, it ignites a long, curved arc of flamefront. The longer this flamefront, the stronger and more complete the burn will become, producing more power with less throttle opening.
There is another benefit from this "whirlwind effect" (that's what Honda engineers called it in the CVCC engines) which is this: the centrifugal force of this tornado presses the mixture out toward the cylinder walls to make it become a rich, thin layer while it makes the center portion very lean. This effect then cools the walls even as the extra-rich layer begins to become hot, finally igniting the leaner center portion at a time when a conventional engine's charge is beginning to burn out. This produces a longer push down the bore than a conventional ball-burn shape. In an undersquare engine like the 750 and 350F (and 250F), this translates directly into a stronger low- to mid-range torque curve. This is where the more-than-expected low-end torque in these 2 engines comes from, in fact. These engines are made, and timed, to produce max horsepower at RPMs that were considered stratospheric in the 1970s, yet you could drop 2 people and their touring gear aboard and roll off the miles effortlessly at low-midrange engine speeds.
The longer spark causes a wide arc to burn, nearly a complete "circle of fire" at RPMs above 3500 or so. I can write out the calcs if anyone cares, but suffice it to say, it's not hard to figure out. The airspeeds at 5500 RPM come out around 700 ft/sec inside the engine.
The inner evidence of this is plain when you see these engines' wear patterns. In the first case, you find a pretty thick layer of carbon on the head surface and the piston: this is from being wetted by the richness being pressed against those surfaces, and the extra cooling this provides prevents it from being burned off. If the engines are not revved enough, this becomes too thick and needs some help to remove it, or else the engines develop preignition spark knock: this was the struggle when the carbs were set at 120 mainjets with the needles in the center notch (K0 and Old Factory K1) and the 55 MPH speed limit hit the USA. No one was riding at 6000 RPM anymore, and the engines were clogging up badly with carbon, then preignition was bluing the pipes and exhaust valves. In the second case, the sharp ridges in the K0/1/2/3 combustion chambers slowly wear away from the constant winds. This can be seen when you have a young K1 and a hi-mileage K1 head apart side-by-side, and it's REAL interesting to study! Carbon bits moving at 700 MPH gotta hurt...In the post-1974 designs, the most notable change is the reduction of the sharp ridges and deep valve seat pockets that make this shaping: these later engines have smaller combustion chambers by about 0.5-0.6cc than the earlier ones, and often their compression checks come in at 140 PSI instead of the more common 125 PSI because of this difference. This improves MPG, which was part of how Honda got 50 MPG out of the K4/5/6 engines when the US was having the [in]famous gasoline shortages back then.
When the engines changed to have rounder (i.e., more hemi-like) chambers in the 750F0 bikes, the intent was to increase the low-end torque through a faster burn rate instead of a prolonged 2-stage burn. This is a hallmark of the hemi head designs: they can burn in a ball shape after ignition if the mixtures are even. But, the carbs and hoses and exhaust and ALL that other 'stuff' that cost so much to engineer in the earlier bikes needed to stay, lest the 750 become too expensive to make. Many people like to say this was where Sochiro himself "stepped back in" to work in the R&D group for the F0, and it may be true: he also designed the hemi-head 50/70/70/100/125cc engines, you may remember...in any case, the intake cam timing had to back off to prevent blowback up the intake tract with a hemi design, because the swirl, now largely gone, would not fight back against the exhaust pressures anymore, and the pressures back across the top of the pistons could then induce more "burnback" (aka backfire) up the intake tract at low RPM. This causes big flat spots (aka CB500/550 engines) that come to life like a switch was thrown once the burnback rate is pushed back into the engine by the intake velocities (air has push and mass of its own, you know...). (A side note: this is why the intake tracts of the mid-Fours is so long: it reduces the burnback - but that's a topic for my next book.) So, the cam timing changed to open later and stay open longer after BDC, all to create lower chamber pressure when the intake valve opened again. The 4-1 pipes were arranged to extract the pressures, too, with a collector arrangement and a LARGE muffler for low pressures. The tops of the pistons in the F0/F1 engines were domed a little bit to make up the compression difference: in the later F and K7/8 engines they were domed more to help prevent some of the blowback that still existed in the 4500-5500 RPM range, which impeded the power noticeably at around-town engine speeds in 3rd gear, where most city riders live (and, unfortunately, where 55 MPH touring also lived). The spark advancers in the F2/3 and K7/8 engines advance later, too, to help with this (and this helps ALL the SOHC4 engines today, as the springs have largely become weakened by years, rust, and heating cycles).
So, in a sentence...the longer the spark duration, the wider the 'ring of fire' that develops during the burn cycle, the less the blowback into the carbs, and the cooler the engine will run. I have tested ALL of these things on my bike(s) and know it to be true.
