ok, another stupid question.



if best power on gasoline is 12.5:1 afr how come everyone with an NA motor makes best power leaner than 13:1?



shouldnt that be too lean?

I coulda swore that 14.7 was the best for power and emmisions. Don't remember where I heard that though.

I've wondered that myself. To start with the mixture you find best power at isn't as simple as saying this is where best power will be found with gasoline. It's very engine dependent. What works for one probably doesn't work for another. A good 4v pentroof head will be able to make best power leaner than a large bore, 2v dinosaur breathing through a carburetor. It all has to do with how the fuel and air molecules mix and how much of the available oxygen is combined with the fuel. Could be that the rotary produces a good swirl or tumble effect and better mixes the fuel with the air allowing all available fuel to be burnt with less air. That's really the reason you need to run fatter than stoich in most combinations in order to achieve best power. Running just enough fuel to be able to combine with all available oxygen atoms. When you run at stoich in most any engine you are leaving oxygen molecules that have not found fuel molecules to attach themselves to....you could of burned more fuel and made more power. If the charge is fairly static and there is not good mixing of the fuel and air you'll need to run fat in order to achieve that complete burn using all available oxygen. If there is good mixing then less fuel is needed to attach to all available oxygen.

In my limited experience and knowledge, I've always felt that power would increase as the AFR got closer to stoich, with the downside being an increased possibility of detonation. The critical detail in tuning any engine would be to find the best compromise between safety and maximum power.

Believe JD is talking specifically about an NA engine that would not be knock limited.

yep it has to be something like that. i think my tr3 would make more power by richening it up

14.7 is the point of stoichiometry, or in fewer ten dollar words, it's the point where there's exactly enough fuel for the amount of oxygen and vice-versa.



For best power, you want to run a bit richer than that for various reasons. It runs a little cooler, and since you can never perfectly atomize fuel, it ensures that every oxygen molecule can find fuel to combine with. Adding more fuel is easy, adding more air is not!

I have been messing around with a inovate motorsports LC-1 wide band A/F system ( or Lamda) Of couse they say that 14.7 is stoich and is the best ratio for gasoline and air, But they also say that most engines can make a little more power running slighty rich....



On a turbocharged car it should be tuned rich for saftey, because the extra fuel cools the combustion chambers... I've heard of high boost rotarys running somthing like 10/1 A/F ratios.. does that sound right???

yep. my STOCK 91 non turbo @wot was 10:1 by 7000rpms.

going by my atomization theory, rotaries have high enough turbulence that it helps the combustion.



also, 12.5:1 is just a rule of thumb. lots of piston engines make best power in the 13-13.5:1 range as well. I've heard of some funky technology ports and chambers that make best power in the 16:1-18:1 range.

Well, everyone seems to have a little piece of the puzzle.



The best milage you could normally find would be a diesel engine. Compression ratios up to 24:1 one. So the available oxygen is dozens of times more than in a NA gas engine. The other less obvious advantage is that the temperature of the oxygen is in hundreds of degrees, before the fuel is injected, at pressures that increase the fuel temperature even before it enters the chamber. This system works so well that the fuel need not very exotic at all. In fact anything that will burn can be used as fuel.



Even fluids that will not burn at sea level pressures will burn quite well in a diesel. So any gasoline engine seems a bit silly compared to a diesel. It is also true that the closer the gasoline engine gets to diesel like compression ratios, the more effiecient it becomes.



So why don't we just jack up the compression ration to the roof, and start from there? Well we are headed that way now. The new high pressure electrical injectors used in newer diesel engines can be used in gasoline engines as well. So injecting fuel later in the cycle, so it is not in the air being compressed, and less exposed to the heating of the air from compression, holds off the detonation normally connected to high compression and the gasoline engine begins to look more like a diesel each day.



So what?



Well the piston engine that has been well designed, has a large area of piston that will fit tightly against the cylinder head at TDC. this is called squish area. So, when the piston closes in on TDC the mixture burning near the spark plug is sprayed with fuel air mixture from the edges of the chamber most remote from the plug, because it is forced out of the squish areas where detonation would normally occur.



The rotary has nothing in the way of a squish area. The opposite is true. The rotor face is just close enough to the housing just about everywhere to cool the mixture but not close enough to drive it anywhere suddenly. So, the rotary has a big problem with unburned hydrocarbons (fuel) but not a big problem with detonation. The quenching effect is so prevelent that the flame front dies off before viable mixture near the apex seals burns. So you get a nice fireball out the exhaust pipe that shows you fuel that should have burned inside the engine.



With no squish area inside the engine and little turbulance to maintain a viable mixture, excess fuel is required to assure that the oxygen available is consumed. The fuel is flopping about in great clumps from lack of heat energy and there will always be an unhappy amount of fuel that is not finding oxygen to bond to. Note that the Lemans 4 rotor had 3 spark plugs in each housing.



So the ideal is only that, and is not attainable in anything but a 25CC test engine in a lab.



In racing there is not yet a demand for emissions control, so we can operate with only an eye to performance, and if HC and NOX are off the scale, it matters not. The rotary as we know it will not ever perform well with a liquid fuel. However, a fuel that has been excited mechanically and, or, with heat could make major improvements in effieciency.



These same mechanical short comings become (to some extent) advantages when the engine is boosted above atmospheric. The cold chamber surfaces allow for more compression (boost) without detonation. Higher compression means more energy in the mixture and more blobs of fuel finding oxygen to bond to. Along the same line of thinking, a low boost mechanical supercharger combined with very high pressure injection into the housing after port closing could make for a very viable emissions friendly package.



The point is, that ideal is not now available to anyone. Rich of ideal and much richer than ideal is the actual rule. In a very general truth, power increases (as you would think) as you lean toward the ideal, until all of these factors combine to stop you short of your goal.



Lynn E. Hanover

Wasnt there a (US delivered only) rx7 model in the Gen 1 years somewhere with a 12a that ran extra lean for emmissions purposes? Something like 14 - 16 (maybe only on decel though) ????

Lean burn 12A was available in late '79 in japan and late '80 in US in the new P815 model 1st gen. It ran at 14.8:1 AFR cruise using its high energy constant dual plug ignition system and used twin bed catalyitics and a split air system air pump. Sounds like all rotaries since except it had a carb :P



It was just called lean burn then because it was replacing the old rich running thermal reactor system.

Oh right. Yeah the old thermal reactor system must have run rich as. I was reading in my rx2 workshop manual that on thermal reactor models trailing ignition was cut at mid rpms so to allow a more unburnt mixture to burn properly in the TH - weird!

yep its only lean burn compared to the thermal reactor system. in real life, it runs more like mid 13's