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Re: 30 MPG out of a 5.0 Mustang?
Non mass air Mustangs also can adjust fuel ratio when conditions change and they do this using their MAP (manifold air pressure) sensors which detects manifold air density as well as by their built in air density sensor and exhaust O2 sensors. The reason you get better mileage at high altitude is that at high altitude, because the air is less dense, there are fewer molecules of air going through your engine for each revolution, than there would be near sea level. Fewer molucules of air, means that fewer molecules of fuel are needed to keep air ratios correct, so the computer automatically feeds less fuel to the engine during each injector cycle. At high altitude your engine essentially acts like a smaller cubic inch, lower compression, engine. Also, at high altitude it takes slightly less power to move your vehicle down the road, again because the air is less dense and creates slightly less wind resistance than does dense sea level air. That's the same reason that jets like to fly at 30,000 feet of altitude and higher even on relatively short flights..... because they burn way less fuel up there, as their planes are much easier to push through the less dense air.
There are a few things that will almost always increase gas mileage. Increasing compression ratios, spinning the engine slower, reducing exhaust back pressure, and leaning out the air fuel mixtures. All of these things listed here, with the exception of reducing exhaust back pressures, of course have their limits.
EFI engine are calibrated to have an air - fuel ratio of 14.7 to 1, which is ideal for producing exhaust gas mixtures that will enable catalytic converters to reduce emissions to the lowest possible level, however, the best fuel economy occurs at between 15.5 and 16 to 1.
During light throttle cruise conditions, actual cylinder pressures are very moderate compared to pressures at wide open throttle, therefore increased compression ratios are of more benefit to engine efficiency at light throttle cruise conditions than they are at wide open throttle.... which means that high compression actually is of more benefit to part throttle fuel economy than it is to WOT horsepower. The limit here is octane requirements neccesary to run very high compression ratios.
It should be obvious that a higher geared, slower spinning engine will use less fuel. There is an additional benefit to spinning an engine slower. To produce the same usable power at a lower RPM, the same amount of total air volume must travel travel through an engine, but if the engine is rotating slower, there will be less manifold vacuum and therefore more air density and cylinder pressure in each cylinder, which just like a higher compression ratio, helps the combustion cycle to be more efficient. The limit here is that gearing a car too high creates a vehicle that has no power at cruising speeds and you'll end up gearing down for every little hill.... bad for fuel economy.
Forget all the 'old wives tales' about engines needing backpressure to run properly. With a 4 stroke engine that is simply not true. Reducing exhaust back pressure reduces the amount of residual exhaust present in the cylinder when the next combustion cycle starts. For efficient combusition, the engine needs pure clean air, not air with left over exhaust mixed in with it. The only limit here is that very low restriction exhaust systems can often be somewhat loud, and perhaps in places, even be illegal. (removing catalytic converters) However, from an effeciency and performance viewpoint, the lowest restriction exhaust system is always the best.
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1969 Mustang Fastback 351W 4spd
1989 5.0 LX Coupe
2000 Kawasaki ZX9
2001 Ford Lightning
- too many toys - too little time -
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30 MPG out of a 5.0 Mustang?
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