[Ecky]

You're 100% right about catalysts - running leaner (15-16:1) will damage them if you don't occasionally dump a little fuel in them to purge them. Honda's lean burn engines will occasionally run rich for 10-20 seconds to "purge" the cats and keep them healthy.

So, I assume you're reading your AFR using your standalone wideband gauge?

To my knowledge one way to get a leaner air fuel ratio is to offset the O2 sensor voltage with an interceptor. If you adjust it so that the ECU sees the correct voltage for 14.7:1 when it's actually 16:1, the ECU will hold that air fuel ratio. However the ECU will only do this when it's in "closed loop". Any time it goes into open loop operation it will fall back on its fuel tables and ignore the sensors.

I'd like to give a word of caution about this, however, and suggest that a much better and safer way of going slightly lean is to use aftermarket engine management and a wideband sensor for the ECU. Reason being, exhaust temperature rises and peaks around 15.5-16.0 AFR. It's fine to run these AFRs when the engine is at low RPM and low load, but if you stay lean while accelerating briskly (e.g. trying to pass someone on the highway or getting up to speed to merge) you can burn your exhaust valves. Cars that come from the factory with lean burn do not run lean under heavy loads or at high RPM, and you have no easy way of preventing this with a simple O2 sensor interceptor.

Regarding any HHO gains, the typical answer on this forum which is backed up by a lot of scientific study on the topic is that the amount you should expect under most circumstances is either zero, or negative. Making hydrogen gas is an energy-lossy process - it's low efficiency, so using energy to split water and then recombining oxygen and hydrogen back into water in the combustion chamber will net less overall energy and hurt fuel economy. It shouldn't matter in practice though as realistically, the amount of HHO you can produce in a car is going to be such a tiny percent of intake mixture as to disappear into background noise.

I've read a few studies suggesting that there are a few edge cases where the addition of hydrogen can be beneficial to fuel economy, but it's almost always still negative in terms of cost per mile because of either the cost of buying hydrogen, or the lossy nature of generating it yourself.

Under stoichiometric combustion, modern engines release greater than 99% of energy stored in the fuel, so there are basically no gains to be had there. Most losses occur as heat is lost through cylinder walls and goes out the radiator, through friction in piston rings, bearings, gears moving through viscous oil, etc. making these better places to look for improvement (not to mention tires and aero drag), so things like reducing RPM (taller gearing to reduce piston speed), running thinner oil, higher temperature thermostats to reduce temperature gradient, etc. are better places to look for improvements. HOWEVER, under some extremely lean conditions, in some combustion chambers which are not designed for lean combustion, when you've leaned out sufficiently that gasoline does not burn fully (which ruins fuel economy) and the engine is running rough/poorly, the addition of hydrogen can partially restore some of that lost combustion efficiency. Running lean when your gearing is poor and your engine is not properly loaded can reduce pumping losses and the addition of hydrogen *might* help keep combustion efficient. You're probably better off just not running that lean though if you aren't injecting hydrogen you supply with a tank.

I could not tell you offhand what the efficiency of a 5 amp dry cell is, so the following back of the envelope math is not to be taken too seriously. However, if we can assume a very generous 50% efficiency of the dry cell, you'd get maybe ~30 watts worth of hydrogen from a 5 amp cell. Multiply that by 33% efficient combustion and at highway cruising speed, hydrogen generated will make up in the ballpark of 0.05% of "fuel" in the combustion chamber. I'd wager you'd need in the ballpark of 20-100x this to see any appreciable effects, so it's no wonder your fuel economy is the same. Look into a 200-1000 amp dry cell or tank hydrogen if you'd like to play with it and see if it can help stabilize very lean air fuel mixtures.

My 2 cents.