arghx

iTrader: (3)

Here are graphs from the stock timing maps out of an FD. Someone in the 3rd gen section disassembled the computer and put this together. Disregard the unit labels (they don't mean anything useful), just look at the shapes of the curves. Here is the leading graph:



The graph reflects actual ignition timing, not timing advance relative to the baseline -5 L -20 T that the 13B uses. As you can see, as rpm increases the plug is firing earlier--this is the equivalent to centrifugal advance. As manifold pressure increases (vacuum decreases or the engine goes into boost), timing is retarded. Since it is computer controlled there are little "dips" and other topography in the map that Mazda felt useful as a way to reduce emissions, improve driveability, or protect the engine from detonation. You can't do that with a dizzy. Here is the trailing timing map:



The biggest difference is in the vacuum/load/manifold advance curve. (looking cross the Y axis). The trailing timing dips a lot more under higher loads to maintain split between leading and trailing. That's mostly a safety and emissions thing.

Hopefully that clears things up a little bit. It works the same way on a dizzy, look again at the 12A advance curve:



But in that graph ^ the vacuum and centrifugal advance are all relative to the baseline TDC and 20 ATDC values. And there's also way less precision in the timing advance control. For example, on rotaries with electronic spark advance there are completely different routines to control ignition timing at idle based on accessory load. The electronic spark advance cars can also deviate from the timing map when knock is detected. But on the old dizzy's all you can really do is use a vacuum solenoid to make changes.

JoeNoble

iTrader: (6)

I don't know much about all the technical stuff but i believe it was kentetsu that said try it both ways and see what your car likes. That sounds like the best idea to me.

j9fd3s

iTrader: (3)

the simplest way is to just set the timing @wot and then hook up the vacuum advance like stock, to add timing @ cruise

kenetsus results might be ok, but his method is shall we say unique? (and overcomplicated)

BTW you seem to have asked a good question

Kentetsu

The National T-Bucket Alliance article is, of course, a load of BS. I figured that would be obvious to anybody that took the time to read it. I was just showing that you can find stuff on the internet to back up any argument, no matter how ridiculous.

"kenetsus results might be ok, but his method is shall we say unique? (and overcomplicated)"
I don't understand how this is over complicated, or unique? Move hose from nipple A to nipple B, check and set timing. Seems simple to me.

Really, the only difference we are talking about here is whether you want the advance at idle, or not. As I see it, advancing the idle is simply for emissions, as it lets you run a little leaner. But, in my opinion, having the advance start to kick in once you start opening the throttle gives the motor a bit more punch.

Anyway, its not like I've got a dog in this fight or anything. I just found something that seems to give great results, and so I'm sharing that information. I have no need to force anybody to agree with me, if you know what I mean.

j9fd3s

iTrader: (3)

yeah totally. you're probably getting similar timing numbers, but you're just doing it a different way

theNeanderthol

iTrader: (1)

wow cool info arghx! thatnks for that