The way i figure is that my 5.7 litre Hemi,a most excellent engine design, will always need X amount of fuel.the minimum amount needed to sustain combustion,and nothing will change the fact that for 5.7 litres there is a amount of air to fill and ,and a certain amount of fuel to be used.so i figure there is an ultimate limit for any engine size for fuel milage.its simple physics.right?(2) if you stick a fuel with more energy available per unit (BTU)?,then the fuel needed will be less,then conversly better milage from a given CI.I know theres fuels that the racers use for more power,couldnt we use a given fuel,say nitromethane,just an example,,with more potential energy per unit than gas, with less fuel used then milage would jump..right? at any rpm and load,the engine would be more fuel efficient,,right?  

You are kinda right here but several other factors play a role in engine efficiency too. First is compression ratio. The higher the CR the greater expansion ratio that takes place after ignition and it is during this expansion ratio the energy/work is coupled to crank. (one reason diesels get better MPG is because the very high CR they have required for operation also provides a bigger expansion ratio for energy capture). Next there is fuel octane, as you increase CR, octane needs increase as well. 87 octane is far from best fuel for a modern V8 and trickery via timing has to be used to tolerate it. This is typically via retarded timing curves which reduces peak combustion pressures and the point at which in relation to crankshaft angle they occur. Engine still runs but efficiency drops. Then there is the induction and valve timing. High output V8's with HP peaks at 5000 RPM and more and high RPM torque peaks as well do not operate as best efficiency at lower RPM's. Basically any engine be it gas or diesel will use the least amount of fuel per HP hour under moderate to heavy load when operating at or near its torque peak RPM. (also known as VE peak or Volumetric Efficiency peak) When operating much above or below this, efficiency suffers. Port size also plays a roll here because what works best at 5000 RPM does not at 2000 RPM because the velocity of mixture into cylinder plays a roll in its mixing and burning and small ports lead to sharper response and better lower RPM efficiency. Then there is your exhaust. The classic move here is to chop off exhaust and install duals. All is not as seems here. At lower RPM's the cams valve timing and port back pressure is such that gasses do not escape to quickly from cylinder so that more energy is transfered to piston. Letting it escape too quickly reduces lower RPM efficiency. Modern engines are well "tuned" from intake to exhaust but many fail to realize this. It is tuned as such so that various parts of system (intake, exhaust, valve timing , etc) come into and out of "tune" in RPM range to give the broadest possible torque curve. (some elements go out of tune as others go in tune at various RPMs giving a somewhat broader power curve)  

i wanted to concentrate only on the fuel. i was figuring that all things considered, the fuel is the key that every thing revolves around. i think if fuel "X" for example, if it has more has more potential energy stored per unit,less would be needed to produce the same power. its all about what happens in the combustion chamber.  

Well if you want to focus on fuel you need to focus on octane of it too. While higher octane fuel has slightly less BTU in it (I am talking about pure gas not a nitro methane mix) it allows for much higher compression ratios and more favorable timing to convert more of the energy to work. Gas engines do not capture energy from explosions but rather burns and the expanding gasses they cause. Higher octane fuel resists preignition and detonation which means more favorable timing curve can be used as well to capture more of the energy in the fuel. See it is not just about more powerful fuel but also about capturing more of the energy in the fuel. BTW, racing engine that run on alcohol do well because though methanol has only about 55% of the energy in a gallon of gas (so more must be burned) it also has a very high octane rating and can be used with very high compression ratios plus as the large volume of alcohol evaporates in induction system it cools air a great deal more than gasoline which result in a colder denser mixture in cylinder before compression and ignition stroke which lead to more energy being released and captured.  

it would be cool to have a self oxygenating fuel so yu could be rid of most of the valve train.you could have a wizbang injector thats high performance,be rid of the intake,valve train and that part of the cam,no intake whatsoever.just a exhaust valve. the injector would deliver the precise amount of hyper oxegenated fuel ,get fired by a c.o.p. ignition,  now your rid of a lot of mass and friction.smaller and more efficient engine! maybe an oxygen generator thingywooger..yea real technical speak,would help the fuel part.whatcha think?..maybe?  

They kinda do, it is called rocket engines. They carry fuel and oxygen. There is also a class of fuel used in some small rocket motors (like on maneuvering jets on shuttle, satellites  and deep space probes and such) that burns on contact when two parts of the fuel mix even in a vacuum. (very poisonous too) Germany during WW2 experimented with the Komet rocket powered plane that used two very unstable fuels that burned on contact to power it. They were prone to blow up or if the fuel leaked it could burn you and the vapors could kill you.