Bosch Mono-Jetronic

[Edward Hernandez R]

"I always assumed that manufacturers went open loop to run richer (more power) at WOT. Anyone know for sure?"

"Yes, it runs rich at WOT to obtain the best power.  I'm not sure if running *exactly* stochiometric at WOT is bad for the engine..."

Three reasons we run rich at WOT:

1) Power. Unless you have an ideal, perfectly vaporized mixure, running rich always makes more power than running stoich. Basically, there isn't enough time to burn droplets, and some fuel goes unburned, leaving "excess air" available. Running rich is a way of providing more droplets for this excess air that's available to burn it. Perfect mixtures have no droplets; all air and fuel is completely utilized during combustion, and therefore running rich would actually decrease power. But we can't prepare perfect mixtures yet...

2) Engine durability. Again, without ideal mixture preparation, running stoich at WOT is like running lean: longer burn rates, which raises combustion chamber temperatures, melting pistons, eroding plug electrodes and burning exhaust valves. Higher temps can lead to detonation(which can break ring lands) and can eventually lead to preignition(not the same as detonation; it's much worse).

3) Catalyst durability. Running stoich at WOT causes high cat temperatures for reasons cited above. High cat temperatures degrades their effectiveness over time, and  we need them to last a really long time. Running rich drops exhaust temps and help the cats last longer. High mileage recalls are no fun for OEMs.

Yes, rumors of EPA mandated WOT A/F ratios has OEMs worried, especially certain Asian manufactures who run extremely rich at WOT.