[sickpuppy318]

Hey all, this site is frickin awsome!
I learned some stuff about efficiency maps while learning to use thermal efficiency maps of specific turbos to match them with your engine. I was trying to pick a turbo for my tercel.

The deal with the graph in post 60 is that anybody can look at a graph and interpret what it is supposed to mean. But what can you do with it?

That graph was ment to demonstrate the efficiency enhancing characteristics of a turbocharger. The two different conture maps correspond to equal displacement engines. The Y axis is not torque, but rather a theoretical estimation of cylinder pressures.

Someone said that the efficiency island is for unachievable cylinder pressures at cruising speed. That the extra efficiency cant be realized in real driving, and that the only efficiency gains from a turbo would be from downsizing the engin.

That is only partly true... A large enough engine would see no gain from a turbo, because the pressures with enhanced efficiency would produce too much torque to cruise with (BMEP = 4pieTorque/VOLUME).

But if the engine had such little volume that in order to achieve cruising level torque output the engine needed extream cylinder pressures (above efficiency island), then the addition of a turbo would increase curising efficiency. Flat ground, lower RPMs.

Or so says the graph in post 60.

Here's my interpretation of the graph (in color!), except here i've graphed two instances of Pressure at cruisng torque at each engine speed. Think infinitly slow accel., infinitly close to steady state torque. Torque stays constant (same aero drag or whatever), but i've included two different engine volumes. Now the graph showes four engines, NA/Turbo X Big on bottom and small on top. The voluminous engine has low pressures at its steady torque level. The tiny engine has high pressures, so high that they're inefficient. The turbo would increase efficiency for the tiny one.

Say your metro had tiny hockey puck wheels that enabled it to cruise slowly but in its best BSFC. Or fifth gear dissapeared. And to boot you had a big billboard on the roof. In this instance the turbo would increase efficency without downsizing the motor.

Or maybe you were just trying to eco-mod your old 22R powered motorhome. Here a turbo would increase cruise efficiency and maintain part throttle efficiency, without downsizing the engine.