Will adjusting the timing affect the A/F ratio?
 

Just wondering on this. I have heard that I can get more throttle response from my SC by advancing the timing.

Will this change the A/F ratio?

I guess nobody wants to tackle this topic, as it can get complicated. To answer your question directly - no, the A/F ratio will not be affected by your timing.

You will however get the best performance with the most timing advance you can get without causing detonation/pinging/pre-ignition. This is the reason to check your timing at 6,000 RPM instead of just at idle. You want to make sure your timing advance is working correctly to get the most out of your car. You don't want to advance farther than the spec for your engine/year, as you may get detonation, and damage your engine. Just set it to the factory spec at 6,000 RPM and you're good to go (assuming that your advance is working correctly).

No effect on AF ratio. Walt is right, you don't want to play with the advance too much as you can get into engine-killing zone really quick. Only thing between the fuel and timing that are related is the octane rating. Higher octane will allow more advance.

stock timing specs on an 79 sc are pretty wimpy, i put 93 shell in my 82 and have no problems running full adv at 34 degrees. Makes for a very snappy throttle, just make sure your afr is correct BEFORE advancing.

A/F stays the same

When we dyno tune cars with new EFI installations, it is an iterative process..

Basically get the car to idle, then adjust for proper A/F at all rpm ranges... then fiddle with timing...

The ratio of air and fuel entering the combustion chamber a constant.. when you light it off can change.. (ign. timing)

i disagree. i have put the LM on my car and changed the timing and corrected the idle for the same as it was and seen the AF change. perhaps this is why retard is for emmisons. if it leans out the mixture, it reduces the CO out.

with my vacuum retard removed, and timing set for 30 deg advance, i set my mixture for, cant remember for sure, but it was around 13.8 or so. with my retard connected, now my idle timing has been reduced and my idle was reset for the same, and still the same 30 deg full advance, my mixture went lean and i was able to richen the mixture up for the same 13.8 that it was before. i have the printouts from the LM2 but i dont know how to post the file.

you want to set the timing first, then the AF mixture. they do interact. at higher RPM's, if you advance the timing, you have to richen the mixture to compensate for the apparant leaness due to the advanced timing having more time to burn the fuel plus the richer mixture is needed to cool the combustion chamber to help prevent pre ignition or detonation.

the only way to truely set the advance is on a dyno. as you keep advancing the timing, the power will increase to a point. that is optimum. you may be able to keep increasing the timing, without detonation, but you will rob power. the more you advance the timing, the sooner you have combustion. as that combustion happens sooner, it begins to resist the upward motion of the piston since it is happening 30-35 degrees before the piston reaches TDC.

having to use more advance is just a byproduct of a poorly shaped piston/combustion chamber.

here is a very good article about some of this. NGK's also has good info.

http://www.clubwrx.net/forums/tuning-electronic-engine-management/14426-engine-basics-detonation-pre-ignition.html

You may need want richer mixture as you push timing, the richer mixture is more resistant to detonation. So the real question is - what AFR do I need for the given advance? I have a 3.2L Motronic and don't push my WOT timing beyond 30BTDC at WOT. Part throttle is very different and values as high as 50BTDC are not uncommon for good throttle response.