Quote:
Originally Posted by paintme205
I have seen some people mention their is an ideal RPM to cruise on the highway based on the torque curve of the engine.
Others seem to say the lower the RPM, typically the better.
I'm cruising around 2800-3000 RPM's now in my festiva at 65-75 MPH.
Peak torque is at 3000 RPM.
I'm considering Taller tires to get RPM's closer to 2k while cruising.
Good idea or bad idea?
It looks like changing to 155/80r13 tires in front would lower rpms 7% and also reduce tire width by .4 inches compared to my factory festiva wheels. If I lower the car a 1/2 inch, wind resistance / aero should remain close to the same.
If I got super motivated, I could try and find some LRR tires.
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Here is an example of an engine map Hucho published for a small-displacement engine from 1976.
*The brake mean effective pressure is seen at left.
*The horsepower per square-inch of piston area emerges from the top.
*The RPM/piston speed is on bottom.
*The road load power (Aero and Rolling Resistance) is evolving from the lower left as a function of RPM.
*The islands of brake specific fuel consumption lay across the road load pathway.
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For a number of thousands of dollars and a dynamometer,you could construct a BSFC map for your car and then you'd have a chance at predicting what would actually happen for any given modification.
Without it,you're kind of shooting in the dark.
*It could just be possible that Ford has already 'optimized' your powertrain for 'real-world' driving transient conditions,and that what you have is already worth keeping.
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When you start comparing 'rules of thumb' to published BSFC maps,you discover that there are no hard and fast,generic,one-size-fits-all solutions for gearing.
Mod at your own peril.