Hey HondaMan,
Just wondering if these were still available?
and if so did you ever release the ones with teh built in timing light that I read about a few pages back?
If not, I'm horrible with electronics and was just wondering is there any difference from setting the timing via strobe light vs static timing with a multimeter?
I also read somewhere that you shouldn't use the ignition if you have aftermarket coils without the resistor pack, is there anyway I can tel if they are aftermarket or stock? I just got a 1974 CB750 (just discovered it has a '78 F3 engine in it). I did purchase an aftermarket ignition switch though since the key was no longer working with the one that came with the bike, does that matter?
Lastly, I received a motogadget m-unit and plan on overhauling the whole electrical system once I've gotten the bike tune properly, are there any conflicts with using the m-unit and your HMI?
Also I noticed a resister option available if you have a 3ohm set up, what's the reason to go to a 3 ohm instead of keeping the stock 5ohm ignition coils?
There are many riders who believe a "hotter" spark is better than a "stock" spark. Dyna started making these high-output coils back in the era when cars used points and often had barely enough spark voltage (GM cars, in particular) to make them run. When their points burned, the voltage fell off rapidly. Companies like ACCEL, Borg-Warner, and Jacobs started making higher-voltage coils in those days (1960s-1980s era) which made those cars run better, especially in cold weather. This bred a "common knowledge" sense among fledgling wrenches that "hotter" sparks were somehow "better" in all engines, and in the raging days of the 1960s-1970s era, kids my age "distrusted the establishment" in any form, including big business. Over time, this created a mindset that anything "stock" from any big company was made as cheaply as possible, so ANYTHING aftermarket, especially if it was expensive, must be better. Right?
This mindset still hangs around, although it survives largely by word-of-mouth over beers (or similar diversions). Honda, in the case of the SOHC4 bike, did a stellar (in 1970s tech) job of matching the spark duration to the long-stroke 750 engine, a design feature they learned from their own CVCC car engine of the same decade. It only takes about 4000 volts to fire these engines, and the coil voltage was set at 7500 volts to survive poor gas and maintenance, fouled plugs, or somewhat-worn parts, like plug caps (which have resistors inside). This allowed lots of margin. More importantly, they set the inductance of the coil high enough that the spark lasted an amazingly long 1.5mS, where most cars (and earlier Honda bikes) had sub-1mS spark duration. This was done because, especially in the CVCC and SOHC4 bike engines, the air-fuel charge enters the cylinder at a sharp angle, then spins to almost 700 MPH during the compression stroke in a whirlwind pattern (on purpose). The reason: this pushes the fuel molecules toward the outside of the chamber and makes the center leaner: when the spark lights it the richer portion lights easily, and as it burns and heats up it later starts off the leaner center portion. This 1-2 step burn extends the length of time the fuel burns, making it push longer on the piston. The undersquare SOHC4 750 (and 350F) engine design amplifies this even more, allowing a much wider powerband with the 187cc piston than would otherwise be possible. Honda was apparently concerned that while the Four could spin faster than any competitor of the day (9k RPM, easily) and make lots of HP at those speeds, it should also have good low-end torque for around-town riding. A long spark thus improves the low-end torque noticeably.
In cars of the old type, with points, Jacobs could bring out 10HP more on a 350 cubic inch engine, for just installing their coils: they tripled the typical spark duration by using heavy iron (and lots of it) cores and low wiring impedance, but with more resistance in the ballast resistor (aka "Resistor Pack", here) than OEMs did, and by using resistor wires and plugs where OEMs then used either one or the other of these items.
So...applying these techniques to the SOHC4 bikes: Honda used a long iron core (150mm+ length) and very fine secondary wires, superbly wound, to pack extra inductance into the 750's new coils. Then they installed 7500 ohms in the plug caps, to slow down the discharge time even more. The result was terrific! The spark does not droop more than 7% from 100 RPM to 11000 RPM when the parts are all new. But, the resistor caps don't last forever, nor do the rubber sleeves of the coils: such was the 1970s tech in Japan! In the case of the "F" bikes, Honda needed more top-end spark, as the primary drive ratio was lowered in these engines, making them spin about 800 RPM more than the early engines at equivalent hiway speeds. The carbs were also set leaner (to meet new EPA regulations at the time) and this required more spark voltage ("hotter") to start it reliably when cold. This led to the post-1975 coils with shorter spark duration, but 800 volts more spark, so they used 10,000 ohm sparkplug caps to try to stretch it a little more: these caps quickly burned out.
...that's where the problem started. Within typically 8k-10k miles, the plug caps were burned past their limits, but no one seemed to know they should change them: instead, the popular "fix" became the old adage again, "...use a 'hotter' spark, because it is 'better'...". Dyna offered their 3-ohm, high-output coils in those days, and simply plunking them into a bike with bad plug caps would make them run (especially when cold) better...for a while.
Oddly, today this whole mindset persists, despite hundreds of articles, books, kits, and other things that prove it wrong. Old habits die hard? At any rate, just keep this in mind: a LONGER spark will make any engine run better, which is why the multi-spark ignition systems are standard fare on any race vehicle now (and many EFI cars today!). So, go for the LONGER duration spark, not a HOTTER one: in any physical size of coil, only a certain amount of spark energy can be made. If it discharges quickly, the voltage is higher: if it discharges slower, the duration is longer instead. And, even at 10:1 compression ratio, it still only takes 6kV to fire these engines at 10k RPM, so even the OEM coils will make plenty of voltage.
So, IMHO the final answer to the question: "Why 3 ohms instead of 5 ohms" is: that's the wrong direction!
