I am going to combine Vekke's “Constant Throttle” technique with Julian's “Throttle Stop" technique. Both use throttle limiters for on-road testing of the aerodynamic drag effects of modifications.
Earlier this year, Julian recommended making a simple “throttle stop" to insert under the gas pedal. His goal was built to reliably set the throttle plate opening for a consistent power output. Stepping on the gas would offer a consistent baseline speed under controlled conditions, for example perhaps 18% throttle for 65 MPH. After an aerodynamic modification, any increase in speed at 18% throttle would indicate a decrease in aerodynamic drag.
Vekke wrote about something very similar in 2015 in two or three separate posts. He called it “Constant Throttle," or “CT" for short. In one post he suggested placing “a stopper under the gas pedal" that could hold the vehicle at a specified speed, but he switched to placing the throttle limiter on the spring throttle on the intake manifold. Once he had a speed he liked from the limiter, he inferred changes in aerodynamic drag from recorded changes in engine RPM (not speed).
Both approaches have advantages for the testing I want to do. Julian's is elegantly simple and quick at the expense of a little precision and safety. Vekke’s is somewhat more precise and safe at the expense of slightly more complexity. Bolting the throttle limiter firmly in place on the intake eliminates subtle movements of the stopper under the gas pedal that might change throttle position unexpectedly. An intake mount also eliminates any chance the stopper could get under the brake or clutch pedal in an emergency. (See my last post in Julian's throttle stop thread for more on this).
There are also benefits to monitoring both speed and RPM. A good GPS can be reasonably accurate and can automatically track speed averages. Monitoring RPM digitally from the engine sensor means having a second independent data source to compare. If everything goes correctly, percent changes in speed and RPM should be similar.
Here are some pictures of my throttle limiter, made for free from salvaged materials. Part of it is permanently mounted, replacing one of the throttle cable bracket bolts (picture 1). This allows the rest of the device (picture 2) to be quickly and securely mounted or removed in 30-45 seconds (picture 3). The result is 16.1% throttle (picture 4), which seems to correspond to about 60 mph in 5th gear.
It is a little ugly, and it could be simplified further, but it works perfectly and it eliminates my safety and precision concerns about under-pedal throttle limiters.
PS: “Constant Throttle" seems like a better name than "Throttle Stop" because it highlights the purpose of the technique: hold the throttle plate opening constant. But I will probably use both and throttle limiter and Great Flying Spaghetti Monster occasionally.
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Nice solution!
