Originally Posted by smokeshow

Ignoring some of the smaller details, there are two main contributing factors to determining how your equivalence ratio needs to be set during engine warm up. There is some overlap in their authority and capability but they can be mostly addressed individually to ease your calibration process. One is how much fuel can vaporize before it enters the cylinder and the other is how much of the fuel present in the cylinder can actually contribute to burning and generating torque. I'll summarize each one as it pertains to a stock LS1.

1) GM has historically used an intake valve temperature model for PI engines (and DI too, actually) to estimate how much extra fuel needs to be injected due to what isn't vaporizing. As you well know...only the fuel vapor is combustible. When I was at Chrysler we used a pretty clever model that estimated the actual mass ratio of the fuel that is vaporized and available for combustion called Burnable Vapor in Suspension (BVS). I still like to think of it using this model - the BVS ratio is used to calculate how much extra fuel must be injected to make up for the discrepancy caused by some fraction of the fuel not vaporizing during cold/cool engine temps. GM doesn't make this term explicitly calibratable, so you have to rely on the IVT gain tables to make up the fuel difference. For example, if the IVT, IAT, injector tip temp, air flow rate, compression, fuel RVP (more below), etc all come together and make it so that only 2/3 of the fuel injected is actually vaporizing, the IVT gain at that point must be 1.5. So with that initial 2/3 multiplied by 1.5, you're back up to 100% of the actual burnable fuel that is needed.

2) The other big player is figuring how much of the fuel that does vaporize actually burns and isn't lost to things like cold ring seal and other cold engine heat-sapping dynamics. Cold rings are an obvious one...some of that vapor will just push past the compression rings and be lost to the crankcase, so that must be accounted for. However the biggest component here is the fact that fire doesn't like cold. It is harder to initiate combustion in the cold, and since a cold engine saps away combustion heat to the heads, cylinder walls and piston, less heat is available to sustain the combustion as well. So more fuel must be injected to make sustaining combustion possible in an environment not well suited to burning stuff. The key takeaway: decreased temperatures reduce flammability. With a lower flammability limit, you have to provide more fuel. That's what open loop EQ ratio tables do - they provide more fuel to move the fuel concentration above the lower flammability limit based on ECT and IAT. For example, if the engine and conditions produce an environment where the lower flammability limit of your fuel is 20% but the amount of fuel you're injecting only nets a 10% concentration, you'd need to double that open loop EQ ratio to get to at least the minimum amount of fuel required for combustion. And then you'd need to find the optimum EQ ratio within those flammability limits to keep combustion variability minimized.

Another variable outside the control of the software and calibrations is the Reid Vapor Pressure of your particular fuel which is determined by the alcohol content of the fuel and when (on the calendar) you fill your tank up. You'll be familiar with the summer/winter fuel blends...this fuel change can have dramatic impacts on startability and warmup combustion stability if not also accounted for. There are various ways to handle this, but for another time.


That's probably way too much information but it should cover all the fundamentals for cold start and warmup fueling for a production ECM. Holley will do some things differently, but the physics remains the same. Just find the **** available to turn with the Holley software that accomplishes something analogous to those topics above and it should run while cold, no problem.