[hawk2100n]

Originally Posted by Old Mechanic

One of the most efficient engines on the planet has a bore of 3 feet and a stroke of 9 feet. Longer stroke works well for torque and horsepower at lower RPM.

In 1950 Alfa Romeo built a grand prix engine that was a 91 cubic inch (1.5) liter straight 8, which was actually two 4 cylinders bolted nose to nose. All accessory drives were gear driven off the junction of the two noses of the engines with two stage supercharging and 10k RPM red line.

390 HP at 2 MPG on the racing circuit.

It's like the difference between the old 225 slant 6 and a motorcycle engine that red lines at 15k RPM. The 225 had a stroke of 4.5 inches and a bore of 3.5. Lots of low speed torque. Geared right in a aerodynamic body with modern fuel delivery, the old slant 6 would still be competitive.

Best 5 speed gear ratios (overall) would give you red line engine RPM at 30,60,90,120,150 MPH in the respective gears. Considering a 6k RPM red line, that's 3k RPM at 75 MPH in top gear, just about exactly where the Civic VX final drive ratio was.

This doesn't seem to really follow the topic of the OP's thread which was about efficient engines which is a totally different subject.

regards
Mech

The info about stroke is dead on. It is one of the biggest things that an auto maker does. They have to start with the right tools to get the job done properly. If you at the latest civic, I believe they actually increased the stroke along with other numerous friction reductions in the engine to decrease consumption.

There are a lot of other things the designer can do to make an efficient engine. Gearing has been covered. But cam timing and variable phasing is responsible for big improvements. Also direct injection which allows higher compression makes the engine more thermodynamically efficient. Low drag accessories or electric operated ones like power steering and water pumps. There when you need them and off when you don't. Low friction coatings on the pistons and bearing surfaces; Teflon was one but newer compounds are common. Designing for lighter oils requires tighter tolerances and higher pump flow. Better fuel management and higher levels of atomization through high pressure direct injection and better nozzles. Better flow through the intake and exhaust. The list goes on but all of these things are areas that have been greatly improved in cars over the last 10-15 years and are responsible for reduced consumption of late but also maintain acceptable consumption in some of the very high horsepower cars you see commonly available.