Power vs economy

[R.I.D.E.]

These are my observations over a lifetime of studying engines.

My general observation is what makes an engine more powerful reduces fuel mileage.

In 1950 (year I was born) Allfa Romeo was the International Grand Prix champion, in the 1.5 liter class. This is the same size as my VX engine, about 93 cubic inches. The Alfa engine produced 390 horsepower. It was actually 2 46.5 cubic inch 4 cylinder engines bolted nose to nose with all accessories gear driven at the meeting point of the two engines, no fan belts were used. Not sure of the RPM but I suspect it was near 10,000. Two stage supercharging, DOHC and I believe 4 valves per cylinder. This represented the state of the art in automotive engineering at the time, much of the development began in Germany by Mercedes and Auto Union before WW2.

The fuel mileage was 2 MPG!

Modern NASCAR restrictor plate engines produce about 650 HP on the tracks that require restrictor plates. At 180 MPH they can get 6 MPG or about 3 times the Alfa grand prix engine of 1950. When you consider the total aerodynamic drag at 180 MPH, 6 MPG is pretty amazing.

We should understand that greater power does not mean greater economy. Greater efficiency can be measured by two objectives.

The first is the most power you can get out of a specific engine design, the second is the highest mileage you can get from the same specific engine design.

Greater power requires increased strength in components becasue the additional stresses involved require increased strength for reliability. Failure to do so means you have lots of power but not for long. Cooling the intake air charge allows increased charge density. Greater weight means greater reciprocating masses which cost more in energy.

Greater fuel mileage requires a completely different design tactic. Lighter pistons, rods, pins, fewer piston rings, lower tension valve springs, less valve lift, lighter valves, smaller intake manifold port diameters, even intentional increases in exhaust gas recirculation, all can contribute to greater fuel economy in the engine itself. Warmer air intake temps allow better atomization and less pumping losses for every power stroke.

Less mass in reciprocating components because you are not trying to achieve maximum power allow reductions in internal strength without sacrificing reliability. Smaller exhaust tube diameters also restrict the passage of air and fuel through the engine and increase efficiency to a point.

The limit to increasing efficiency in modern conventional engines is the point where your sustainable power is not suffecient to maintain a given speed up steep grades for longer distances. Eevn this can be addressed with things like nitrous oxide, supercharging, and other methods of increasing power for severe operational requirements.

I know a lot of pwople here already know this, but it seems like there are also a lot of people who think the old high performance enhancements, which basically allow more air and fuel into the same engine for more power would also increase mileage. This is simply not the case.

That being said there are improvements in engine evolution that have increased efficiency and power. This is where it gets controversial and becomes the source of much heated debate.

Look at the VX design specifics and you get a good idea of the differences between VTEC-C and VTEC. The average fuel mileage of those two different design philosophies represents an almost 100% difference. It would be good for those who are not sure of the difference to study the two designs, which ar basically the same engine to see what Honda did to make such a difference in mileage.

regards
gary