I wonder what the maximum RPM that T coils can be used at. As I begin to see the end of my 5 main project I ponder the various accessories that I will need. Awhile back I picked up on ebay a KW timer elevator of a exceedingly sophisticated design. While it used a sliding brush to select each coil it used a set of points (with a built in condenser) to fire each coil. Normally it seems to be the timer that determines the rpm limit for T coils. It would seem that this timer would significantly elevate this limit. I have pondered inserting rare earth magnets into the flywheel so that the risk of magnet explosion can be eliminated. Obviously the lack of timing variations inherent in the T magneto will be a problem but perhaps not insurmountable. Perhaps Ron or some other smart electrical guy can give me a hint as to the practical maximum RPM the T coil system once the timer problem is addressed.
Only because it probably is the easiest to explain as to what you are going to run up against - lets talk about coils running on battery. If you read Ron Patterson's fine article on Coil operation you will see a couple of graphics showing how long it takes for a T coil to "ramp up and fire" when operated on 6V DC and then again a different graphic showing how long it takes to ramp up to fire when operated at 12V DC. It turns out that when you double the voltage on a magnetic, the field will build up twice as fast so it is no surprise that the coil ramps up and fires in exactly twice the time on 6V than it does on 12V. It would seem then that you can keep raising the voltage and thus speeding up the coil but there is a limit as to how fast since finally regardless of how much voltage you apply, the coil points are still going to take a finite amount of time to operate. While I have never actually measured that time, I would estimate it at being about .001 to .0013 Seconds. Since the timer dwell is 22.5 degrees of engine rotation, clearly then if the timer dwell is LESS than .001 seconds - the coil will not have enough time to ramp up and fire and the engine will miss. 22.5 degrees at .001 seconds will then = .016 seconds/rev or 3750 RPM. Understand that I am showing how to figure it out and just estimated the BEST firing time of a coil on Battery as being .001 seconds. On magneto the voltage doubles when the RPM doubles so the ramp to fire time gets faster as it needs to since RPM increase needs the faster time or else you get a timing lag and anybody who has tried to run their T on 6V with a properly setup set of coils has encountered this timing lag with increased RPM. But whether running on magneto or battery, eventually you will encounter the fact that the points will simply NOT operate faster and my estimate of .001 seconds is partly based on the typical maximum speed one can get from a relay with about the same size points in it. Hope this helps.
I build some special timers for the Montana 500 cars that will turn 3600 RPM without missing a beat. That is about 80 mph or so. After assembly, I run and final adjust them on a fixture connected to a 4 channel scope to check for contact bounce and dwell. The final test is on a 1985 Ford Ranger 4 cyl engine modified for timer & coils. I measure the maximum 'smooth RPM' that is achieved.The setup uses 12 volts on the coils since Ford neglected the Magneto on the modern engine.
Your limitation will not be the coils, but rather the timer, the engine's limitations (flying apart at high RPM), and of course, the safety of driving such a car at such high speeds.
FYI, that engine setup can be seen on my website at this link: http://www.andersontimer.com/Ranger.htm
I love the band-aid! It's a good fix...
John and Frank
Thank you very much. This is precisely the type of info i was looking for. Certainly 80 mph is plenty of speed! Obviously my engine will turn 3600 rpm all day and live. I realize a "True fire" would deal with the issue, still there is a attraction in attempting to retain the sometimes maligned stock ignition system
Actually as Ron Patterson pointed out - I BLEW the math. The timer has more dwell time than I mentioned. Notice the time that I posted. I should have been in bed. The coil limit really is probably about 1 to 1.3 mSec and that WILL limit the RPM but more from a REAL time effect on the timing than a misfire at that point. I was trying to calculate the RPM when the timing would be significantly lagging. Turns out that 1 REV of motor at 3750 takes .016 seconds so if the coil takes .001 to fire from the time that the timer says FIRE then the coil timing is retarding the actual firing by 22.5 Degrees at that RPM but the coil would probably NOT missfire at that point. Its hell getting old.
Les, what if you want to get max power in low or underdrive? What rpm do you expect to see max hp?
Stock direct: 40 rpm/mph
Stock Low: . 110 rpm/mph