Timing logic?

[Peer81]

Hello everybody,

I want to know something but can't find the answer on the net so here it goes.

In short, why do we only set initial timing at idle and not the initial + mechanical timing above 2500 - 3000 rpm like the older ones?

As I see it, initial timing is set by hand so is the same as the older dist.
Mechanical advance was set by weights and springs on ours is set by the 7 pin ignition module and can only be changed by a different ignition module.
Vacuum advance was controlled by the vacuumline to the intake on ours is controlled by the ecm.
It this about right?

I also found this topic from Sensei:
http://www.corvetteactioncenter.com...ry-forum/98701-81-ccc-performance-timing.html

So why not set timing to 36 - 38 degrees at 3000 rpm?

Greetings Peter

 

[RC81]

Since the ECM controls all timing there is no way to set it except at base idle. By advancing that setting if advances the timing thru the entire range. At least thats how I understand it. I was playing with mine the other day and set it at 16 degrees just to see how it would run. Didnt seem to have any problems but I decided to leave it at 14. Without a dyno run its hard to tell what setting produces the most power.

 

[Peer81]

Thanks now I understand!
I read here the EST spark module also has a backup advance curve build into it in the event the ECM is malfunctioning. That brought me off guard.
HEI Operation / Setting timing...'81 - Corvette Forum

Greetings Peter

 

[LannyL81]

I have read and I follow this: set the base timing at about 10 BTDC and with the CCC disconnected. Stop engine, connect CCC, then bring the engine RPM up to 2200 to 2500 and turn the distributor until the timing is at 34-36 BTDC. Clamp down the distributor.

Then the base timing is whatever it is.

Anyone else use this method? Or am I completely wrong here??

 

[Peer81]

I would say when you reconnect the CCC and then bring the engine to 2500-3000 rpm you've have base, mechanical and vacuum advance all in it so setting it to 34-36 degrees would be incorrect as with older distributor systems this it done without the vacuum advance. Just thinking in theory.

I would think leave the CCC disconnected and see if only the base and mechanical advance will also give you 34-36 at 2500-3000... But that is the question.

Greetings Peter

 

[LannyL81]

But since we have no mechanical advance, with the CCC disconneted then the ignition module would provide some additional advance, but I doubt that it would be anywhere near 34-36 total....it is just supposed to be a "limp-home" setting.

So to me checking it the way I do, checks to ensure that the CCC is providing 34-36 total advance.

 

[Peer81]

You're right, but 34 to 36 degrees is total advance without the vacuum advance. So how do you compensate for the vacuum advance that is also build in?

The thing I don't know is maybe the ignition module only has the "mechanical or rpm advance" table written into it. And the ECM alters or changes that mechanical (rpm) advance table with the vacuum reading it sees from the engine.

If not, and the ignition module only has a lipe home advance table with the ECM disconnected. And with the ECM connected the ECM is controlling the "rpm advance and vacuum advance" then you need to know how much total advance is at 2500 rpm (base, mechanical and vacuum advance combined) and what the vacuum signal is from the example engine to know if yours is right. In other words, I don't think this way it's possible to setup the timing right.

But now it's more theoretical then practical...

Greetings Peter

 

[Peer81]

Found some time today to play with my timing.
With the ECM disconnected it looks like the timing is fixed at .. (factory 6 BTDC or something) when I increased the rpm above ... rpm the timing jumps like 5 degrees back and that is it (revved to 2500 I think).

Greetings Peter