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Had a discussion with my bro in law about efficiency vs. HP gains yesterday. As with most conversations with him (he is a bowtie minion) things got heated.

In any case, when a gain in MPG is observed does this ALWAYS mean that HP goes up also? I have my opinion, but need some good info to back me up.

Any feedback, links to articles, etc is greatly appreciated!!!

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If a motor is rich and you lean it down it will probally make more HP at the same rate of fuel consumption (Brake Specicif Fuel Consumption optimum fuel consumption air fuel ratio at steady state accorduing to the Society of Automotive Engineers is 14.25 to 1 or .42 lbs per HP per hour How about them numbers, lay them on him ) as before with less throttle opening, hence better fuel milage. If the motor is lean and you enrichen the mixture to make it best AFR it will increase the HP and possibly the fuel mileage, maybe not Bottom line is the slower(RPM) you can make the motor run at a specific speed and not have to go to full throttle it will use less fuel per mile at the lower RPM Lots of things involved in making good fuel mileage, not just HP