For those with knowledge and experience, I apologize ahead of time. The teacher gene is presenting itself again, hoping to pass along what I know somewhere down the line...........

That may be true of the exhaust itself, but not sure if the case with the combustion process INSIDE the chamber during the ignition phase.

Most I/C engines will spit flame out a bare port, (I used to watch this in the Ford engine test cells quite often - Right before the call to go in and clean up their remains when they blew up, but that's another story).

Mixture, as well as valve and ignition timing are key to that part of the SUCK, SQUEEZE, BANG, BLOW process. The former is called a stoichiometric ratio; that means as close to the perfect amount of fuel is present, ie. not too rich or lean. Valve timing, which considers not only when in the cycles they are open, but for how long and includes overlap: when both are open at the same time. This is in part to allow for the energetic exhaust flow to help pull in more intake charge. Of course, there is a compromise here since this relationship depends on how well both the intake and exhaust systems are designed and that, like any musical instrument, is RPM dependent. In olden days this timing and overlap were fixed, set up in an attempt to get the most power and performance in a desired RPM range. In our 2-stroke engines this is in the size and height of our intake and exhaust ports and is where you would see expert "Modders" show their skill.

Currently, along with ignition timing and mixture, auto engine control systems can actively control both intake and exhaust valve timing on the fly, (unfortunately this adds quite a bit to the complexity and cost, as well as presents a most common failure point).

So, valve timing is important, but can be strangled if breathing is restricted. MANY hundreds of hours of testing and design have gone into intake and exhaust system flow, valve placement and combustion chamber design, all to optimize atomization, (the process of reducing the fuel particulate size down as small as possible and mixing it UNIFORMLY throughout the combustion chamber).

NOTE: For anyone interested, an extreme example of this can be found in the story of the F1 engine used in the first stage of the Apollo Saturn V

In the end that can be the show stopper to how lean you can go before things start to go drastically wrong. Fortunately, with computational fluid dynamics, good computer modelling and other means a lot of that can be worked out before ever casting metal, (or plastic).

A bit of an aside here: There has to be enough fuel to present a flame front that burns at a fixed and controlled rate, where igniting each molecule of fuel takes heat away from the combustion process behind it. Counter-intuitively, too LITTLE fuel can actuall cause the process to heat up too much. Not only is this where a lot of pollutants produced, but that overheating can mean the process becomes uncontrolled. In the worst case the fuel self-ignites and/or the flame front is turbulent and too violent, (overstressing internals). Increasing compression ratio magnifies this and is in fact how diesel engines can ignite without an external system.

As another aside, diesel ignition is in itself violent since the heating and ignition usually occur at once, rather than as a flame front, meaning the physical stresses on internals are much higher. I was actively in the industry when GM decided to try and make their venerable 350 into a diesel. You can imagine how well it went when they tried - without any reinforcing - to take an engine designed for compression ratios in the low teens and increase that above 20:1,

Back in the 80s I had a second business prototyping, installing, testing and certifying alternate fuel systems, and my friend, (and member of our club) was head of Chrysler's entire alternate fuel program. My full size Jeep was my flagship advertising medium and took 2 years to complete, (including my own design digital dash and 4-wheel disk brakes) Part of that effort was spent simply getting a mechanical distributor to follow the completely different ignition curve required.

Anyway, the point of all this is, regardless of fuel, the basics of good breathing, proper mixture and ignition timing are still relevant.

ps. I remember the first time under a Mazda Rx 7. The heat shielding ran all the way to the back bumper! Only then did I remember the reports on the OS rotary engine. It was 30 sized, but even if able to spin over 17K, it burned fuel like a 60. I imagine like its full-size brethren, most of that was still combusting on the way out the exhaust.

pps. This brings up what I hope is my final aside: I have built a few YS 120s, most from boxes of loose bits, and was surprised to learn there is little information available regarding anyone converting these supercharged engines to diesel. Imagine this would require a lot of experimentation simply to find compatible fuel system components, since the diaphragms, O-rings and seals definitely aren't.

The other thing is: would it be worth the effort and risk. Yes, they are fuel hogs for t heir size, but this is in comparison to their power output. I was thinking the whole purpose of diesel conversions now is to reduce consumption, but it also allows to spin a much larger diameter prop at lower RPMs. At some point the internal physical stresses will be pushing close to the limits, making this an effort of diminishing returns. I already built an OS Gemini 160 from two wrecks, the first having thrown one of the early gen rods out the side. OS usually is better than that, but they are on the third completely different version, in both materials and design. A bit of an expensive mistake.

O/K. Caffeine is kicking in, so back to our regularly scheduled program...............Beuller...................Be uller............