I start getting a reading from my ZT-2 as soon as I start cranking my stone cold engine. I have it in the driver side on the top of the exhaust pipe just before it goes under the oil pan. How long on a cold start before it is hot enough to be giving me a reading worth a damn?

An another question regarding cold engine science - do we give cold engines more fuel because they need more fuel to reach 14.7, or do we give cold engines more fuel because cold engines like to run richer than 14.7?

 

give it about 30-40 seconds. I was wondering about this also, checked the website and they said it needs 15-30 seconds to warm up

immediately on crank my AFR is 15.0 to 15.1, then after 30 seconds it drops down to where it should be

 

I think the enrichment is needed as a lot of fuel is not atomized in a cold manifold and intake head runner. Until heat soaked we need more fuel. By soaking the surfaces with more fuel more atomized fuel gets into combustion chamber. my best guess.

 

I have their dash display. On cold fire up it shoes an 'introduction' then displays 'sensor warm up' before showing a value. That takes 30 to 40 secs. It comes up at 15.something afr then falls fairly quickly to 12.something reflecting the true afr.

 

I don't know about the zt-2, but LM1's, if powered by a spare battery, are pretty accurate. The extra fuel that is given is multi-purpose.
In order of importance IMO:
1) Fuel doesn't evaporate well and goes through the engine unburned when cold = warmup
tied with
1) Walls need to be wetted properly (port and cylinder), otherwise not enough fuel will be available to evaporate = cranking and transients
tied with
1) Piston, chamber, and cylinder walls warm up quickly, from a cold state to a quasi-steady state, which scews then above = afterstart enrichment

2) 14.7 is not very stable combustion when cold, due to cylinder to cylinder deviations, cycle to cycle deviations, and when there's a lot of wetness in the chamber - the fuel in the power burn may be leaner than 14.7, but later in the burn fuel from the walls gets incorporated and burns some, and skews the AFR reading to show 14.7.

3) Colder air is denser and IAT might not be present. I have found on the engines that I tune (small disp, multi-port sequential inj), that IAT is practically worthless for VE type tuning, and I've tested it at WIDE ranging temperatures. A coolant temp vs MAP table is sufficient, albeit a little rich at cold + high load + higher RPMs. I use this with the speed density calculation essentially cancelled out. I do use IAT for transients scaling, and for spark retard when hot.

If you have AIR injection working - disable it for the calibration process.

Tune the cranking for fastest starts, and try not to have it read richer than 10.5:1 at the first dip. When it gets to 13:1 at the first rich dip, you may find it not consistent, and may take a few revs longer to start.

Tune warmup to achieve 13.5-14:1 in steady idle conditions - open loop.

Go back, and tune afterstart to transition nicely from the 11.5:1 cranking to the 13.5:1 idle as quickly as possible without lean overshoot.

Again, make sure AIR is not letting any air in.

Do NOT assume that re-starting an engine that has been started in the past 20 minutes resembles the fueling needed for a "stone cold" engine. It will show much richer (a full point is normal) if it has been started recently. This is OK. It's from the warmer chamber, and that engines cool from the outside. There may also be residual fuels, but it's hard to say how much that adds after more than a few seconds. Stabilized cold starts are the key to getting it rich enough. Forced cooling (fans, chillers) is not fully representative either, but can speed up the process to get it in the ballpark.

TBI is not fun - so a cold idle reading richer may be necessary, and usually helps keep small load changes from causing problems. Low to mid 13's if it's above freezing. Below freezing - enough to keep it running smoothly without fouling plugs too much.