How does TB IV advance?

[NHGuy]

So I was trying to understand how a Thunderbolt ignition system advances and I can't work it out in my (small) brain.
If the distributor and the cam have a 1 to 1 relationship how can the amplifier add advance?
Does it cause the coil to fire stronger &/or earlier to make the spark jump ahead inside the cap? So does the spark jump a little back at low rpm and a little forward at higher engine speeds?
Scratching my head.
I'm googling this.

 

[Bt Doctur]

Re: How does TB IV advance?

done electronically using a "hall effect " system

 

[NHGuy]

Re: How does TB IV advance?

Sure the sensor is Hall effect. But if the rotor always goes by thecontacts in the cap at the same angle how the does the spark get there 22 degrees early at higher RPM.
Feeling a little dense here!

 

[achris]

Re: How does TB IV advance?

I've spent a lot of time in the workshop with a distributor, amplifier and oscilloscope... Basically the module measures engine speed by the timing of the pulses from the sensor (inside the distributor), frequency being the inverse of period. Based on the engine speed and a small 'map' (much like that in a fuel injection ECU) it will fire the spark a calculated time before the next pulse from the sensor. There is no movement of the rotor in relation to the shaft like the old mechanical advance. The width of the end of the rotor and the post on the inside of the distributor cap is enough to allow them to still be in alignment regardless of the advance (remember the maximum advance of any of the TB-IV modules is 24 degrees on one of the V8s).

As I haven't been able to read the program code of the module I can't be sure of how the calculation is done, but if I was to re-engineer it, I'd use a cosine wave...

HTH,

Chris.....

 

[Bt Doctur]

Re: How does TB IV advance?

http://www.wellsve.com/sft503/Counterpoint3_1.pdf
That mumbo jumbo is why I prefer the old fashioned points ign system with the weights.

 

[NHGuy]

Re: How does TB IV advance?

Yup Chris, I was kind of suspecting the width of the rotor tip might be a clue.

 

[achris]

Re: How does TB IV advance?

Also remember that 24 degrees is crankshaft degrees, the rotor will only be 12 degrees advanced... I've just gone and measured the length of the rotor and the width of the end of it... It's 1-1/2" long and the width is just a hair over 1/4".... Plugging those numbers into my triangle calculator reveals the distance between no advance and full advance, at the end of the rotor is only 7.9mm. Given that the post inside the cap is also about 1/4" wide, plenty of room....

HTH...

Chris.......

 

[NHGuy]

Re: How does TB IV advance?

Also, thanks doc, good reading for a motor geek.

 

[achris]

Re: How does TB IV advance?

http://www.wellsve.com/sft503/Counterpoint3_1.pdf
That mumbo jumbo is why I prefer the old fashioned points ign system with the weights.

:facepalm: Living in the dark ages.... Points were 'old hat' when I built my first CDI system, in 1981!!! Merc has considered points a dinosaur since 1984, when the first of the Thunderbolt systems was introduced on the V8s... and even earlier on their outboards.



Chris.....

 

[Bt Doctur]

Re: How does TB IV advance?

Well ,even in the dark ages I can still make fire with 2 sticks. My 72 Sportster still runs points , never an issue.

 

[achris]

Re: How does TB IV advance?

Well ,even in the dark ages I can still make fire with 2 sticks. My 72 Sportster still runs points , never an issue.

And you have to replace them every, what 10,000 miles, and the timing drifts as the points and there little rubbing block wear. And there's the problem of the springs breaking or the weights sticking on there little shaft. All of which are a right royal pain to get to and fix. Nah, give me electronics any day. The spark is stronger, and has a faster rise-time, the plugs stay cleaner and last longer. The engine starts easier and idles smoother.. My previous engine (1994 4.3LX) was checked to be 8 degrees when I installed it, and on every service since I have checked the timing, and you know what, after 20 years, it has not moved! And the advance curve has remained the same... THAT is what I call reliable!

How many sets of points, and condensers, have you had to replace in that time? I suspect by now you would have paid for an electronic conversion several times over.... And that's not accounting your time to fit the new points, set the gap, (and if you do it right, check the dwell, and adjust) then set the timing...



Chris.....

 

[MikDee]

Re: How does TB IV advance?

The Best thing they ever came out with is electronic ignition! Especially on cars! I had a 74' Chevy Nova that I built up for my son, and it would fall on it's face with the stock points ignition, until I put in a worked HEI Dizzy. I was actually running AC-MR43T plugs (and they worked the best of all), This was about 20yrs ago. Still, if my boat came with points, I wouldn't bother to change to electronic ignition, unless I was having an unusual problem with them? They're good for up to 5k rpm, the most that boats are gonna run at. My last boat had twin 205hp 4.3 Cobra's and I didn't think it was necessary to go for the expense of elect. ign. it ran just fine as was. But, if I was looking to build up the engine for more performance, I definitely would upgrade to Elect. Ign.