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Old 09-06-2013, 06:46 PM   #21 (permalink)
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Originally Posted by jkv357 View Post
How do you do gear-matching?
You need curves for Rolling-Resistance horsepower and Aerodynamic horsepower as a function of velocity superimposed on the net power curve of the engine as it traces velocity for the original configuration.
When after you've streamlined,you've reduced the load the engine 'see's' at any given velocity and it will over-rev unless you throw some taller gears at it.At this partial load,the engine falls out of its BSFC island of efficiency.This is where you can lose 60% of your streamlining benefit.
The trick is to lower the gearing numerically until,at your cruise speed the engine is pushing as hard as it was before with the higher drag.This is going to be when your engine is 100-200 above rated power rpm at it's new top speed.If you get that,everything else falls into place.
I'll need to bring some materials from home to refer to if I'm to go any deeper.
Wolf Heinrich Hucho goes into it in his books.
And Dr. Gino Sovran of General Motors Research Labs published a well defined SAE paper on the topic.
But the first thing is to know exactly where you are before you begin.Easy if you have your own dyno.
PS if you can get your eyes on a copy of SAE Paper#410139,"Is it Practical to STREAMLINE for FUEL ECONOMY?",by James C.Zeder,Chief Engineer,Chrysler Corp.,December,1941,you can see what Carl Breer's team had to go through to get the maximum bang for the buck out of their ecomodded 1934 Chrysler/DeSoto Airflow experimental car,Cd 0.244.

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Last edited by aerohead; 09-07-2013 at 02:58 PM.. Reason: add post script
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