I've read conflicting reports regarding how plug gap can affect fuel efficiency. For example, I've read that too small a gap won't cause a clean burn, or that too large a gap can cause misfires due to the voltage being insufficient to jump the gap.
Beyond manufacture's recommended gap range, is there any hard data on optimizing plug gap for maximum fuel efficiency?
The generalized statements about the gaps being too small causing insufficient burn and too wide having a weak spark are spot on. As you widen the gap, you need to increase voltage to cover the gap. Also, as you increase pressure at the top of the compression cycle, you'll need to either increase the voltage output to the spark plug and/or reduce the gap of the spark plug. If you don't, the spark will not jump the gap as efficiently and therefor you won't get the best performance out of your vehicle.
I don't know if this will exactly answer your question, but is about the best I've found thus far (if I find more, I'll add to it), but it makes a lot of sense to me:
As a rule, a properly gapped spark plug will burn hot without being too wide at high rpm to cause a misfire. Ironically, the car manufacturer's recommended spark plug gap is not optimal! The recommended spark plug gap is designed to be adequate for cold starting and smooth driving on a car that is in need of an engine tune up. If you drive your car normally and tune the engine regularly, you can increase the spark plug gap by about 0.010" for better performance and better fuel economy. However, if you drive at full throttle most of the time, you should reduce the gap by about 0.010" for better performance. The spark plug itself, and the residue that forms on it, would indicate whether the gap is too big or too small. A light brownish discoloration of the tip of to porcelain insulator indicates the proper operation of the spark plugs with the gap being ideal or close to ideal for the most recent engine speeds. Thus, to check the spark plug gap at high engine speeds, you'd need to run at full throttle and immediately turn the ignition off without allowing the engine to idle. But ultimately, you'd need to run your car on a dynamometer to find the best spark plug gap, and the right ignition timing for your engine.
There is a lot more information on the page which gives indication about how the gap affects things, so it's worth a read in my book.
I would say, though, the ultimate gap range for one vehicle is going to be different than it will be for another. As is stated at the end of the excerpt, you'd need to run your car on a dynamometer to find the best spark plug gap. Any given car is going to be different. Some manufacturers may have the optimal gap set, while others will be a lot more conservative.
All-in-all, I've not found any empirical data which shows exactly what the difference is between gaps on any given vehicle, but as I've stated, every vehicle line is going to be different anyway. This makes it hard to come to a strict conclusion.
I ran across a scientific journal article which provides some statements with some hard data and some additional references:
Impact of spark plug number of ground electrodes on engine stability
On the other hand, it was believed that less amount of material near the gap is a major contributing factor for more rapid growth of the flame kernels. This means that larger electrodes increase the heat loss from the initial flame kernel while the rate of initial flame kernel development is adversely affected [2] and [3]. Herweg and Ziegler [2] found that reducing the contact areas between the flame kernel and the spark plug which can be achieved either by reducing the electrode diameter and/or increasing the gap leads to a faster flame kernel development.
In spite of the reduction of the electrode diameter contradicts to its durability, a fine end design has been adopted by different researchers [2], [3], [4], [5] and [6] to reduce surface interference of the flame contact between the electrode end and flame kernel.
It was reported by Hori et al. [3] that a fine ground electrode would increase performance but also noted that durability requirements make the construction of a fine-wire ground strap difficult.
The results presented in [3] and [7] showed that a spark plug with fine center and ground electrodes produced lower combustion variation with a reduction of approximately 3.1% in COV and 2.4% in the fuel consumption compared to regular spark plugs.
It is widely accepted that the early flame development deeply influences the subsequent combustion phase [8]. Accordingly, small differences in the kernel formation rate or location may produce significant in-cylinder pressure variations [9]. Pischinger and Heywood [10] found that the cyclic variation of flame propagation near the spark plug influences the amount of heat release at the spark plug gap and this greatly influences the so-called rapid burn angle.
Further down it says:
The results showed that the amount of crossover of the ground strap, the dimensions of the ground strap and the gap width are the primary factors affecting the spark onset, flame initiation and kernel growth.
So, if I'm understanding this correctly, combustion efficiency is adversely affected by quenching at the early stages of flame development. The two main ways to reduce quenching are by reducing the mass of metal around the flame kernel, or widening the gap ( which I guess also effectively reduces the amount of metal in contact with the flame kernel ). Even though they only give a hard number, 2.4%, in relation to the former, I get the impression you can imply similar results for the later, although it would be nice to have a directly measure hard number for that also.
Thank you for your interest in this question.
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