Racing Fuel Systems

I just recurved my distributor and my AFR readings have changed.

My previous curve was 16 cranking, 27 at idle rpm, but only a slow advance to the low 30's by 4000 rpm. (And manifold vacuum advance up to 16 additional degrees above 12"Hg, which also added about 10 deg at idle). The advance on the heavy spring was so slow that the timing actually reduced at mid- to high rpm due to the delay in the electronics. I just got the darn 950 dialed in, too. Idle around 13.5-14 AFR, cruise 14.5, WOT 12.5.

Then I put the light on it recently and found the above (thought I had 35 by 4000, but even that was only around 32 at a 3000 rpm cruise). So I recurved. Now I have 24 initial (cranking or idle), and 36-37 as high as I want to stand next to it holding the throttle and a timing light Idles around 35 with the vac can connected, 8.5" Hg.

The idle AFR seems to have leaned out about half a point, likewise the cruise. I haven't had it on the highway for some WOT trials yet. But the off-idle and low-speed cruise feel great with much improved throttle response
I assume this change in AFR is normal, but not something I have seen much written about. Thoughts?

I had similar when changing to a full flow muffler from a more restrictive muffler. I assumed the engine was scavenging a little better and sending some oxygen straight though on the overlap. Could this be the case with improved ignition timing? Higher combustion pressures fly out the exhaust faster taking a little more intake charge containing oxygen with it during overlap. If this is the case is the cylinder really seeing a different air/fuel ratio with the overlap event distorting true AFR?

I am curious too if your plugs looked different. If combustion is more complete with the same AFR will they look as though the cylinder is leaner?

As I understand it, the meter measures the amount of oxygen with a dial that's calibrated in AFR. I agree that the readings can be misinterpreted and I don't tune only to the AFR reading. A misfire can actually read lean, because all the unreacted oxygen on that stroke comes out in the exhaust. Along with the unburned HC but the sensor doesn't actually read that.

When combustion is richer than stoichiometric, there is less O2 in the exhaust, since there is not enough O2 to burn all the fuel, so the O2 is all used up, and the meter reads below 14.7. Conversely a lean mixture has a surplus of oxygen in the exhaust since there is not enough fuel to react with it all, so the meter reads above 14.7.

So... wouldn't an improvement in the "burn" reduce the surplus of unreacted oxygen, and cause a richer reading?

The added timing has made the engine more efficient. It needs less fuel to make the same HP. You are seeing this at idle & cruise in the form of the t/blades not being reqd to open as far, hence slightly leaner. Forget about O2 meters & enjoy!

Doesn't sound that amusing, all things considered...

But is there likely to be a large change to my timing curve? Seems like 24 to 36, all in by 3000, is a good conservative setup.

Likewise, do you anticipate substantial changes to the carb tune?

What iron heads are you using now? If they are open chamber and the new aluminum heads have a closed chamber (especially if the closed chambers result in an actual quench area), the best total timing setting may end up retarded a couple of degrees from what you have now. Only testing will tell, though.

I wouldn't expect the head swap to mean the carb tune will need to change. You should be picking up a noticeable amount of power across the effective RPM range, but the driveability won't necessarily change.

Now watch it play out that the ignition requires no changes, but the carb hates the new heads...