When to shift? - PowerStrokeNation : Ford Powerstroke Diesel Forum
 
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post #1 of 5 (permalink) Old 10-17-2011, 05:14 PM Thread Starter
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When to shift?

So I was going for a long run (like with feet, not driving for a long time), and got to wondering ... If I am going, lets say 40 mph, for a while, if I am in 4th gear my engine is right at 1500 rpm (about the peak of the torque curve of some dyno sheets I have seen of about stock 7.3s), would I get better fuel economy staying in 4th or shifting into 5th and running at about 1100 rpms?

I have read, from many different sources, that an engine is most efficient at the peak of the torque cure. Would that translate to getting the best mpgs at that rpm, or would it be better to bet traveling the same distance with less rpms and the engine not being at its most efficient state.

Also, completely different topic, if you released all the pressurized air in every tire, ball, compressor in garages etc., would the air pressure of the atmosphere increase at all?

97 F-250 ECLB ZF5 7.3 4x4, 6637 Filter, MBRP 3" to 4" with 5" tip, HPO xover pipe, shimmed FPR, Boost and Pyro Gauges, PHP chip with stock, 60 hp, 80 hp, 140 hp, high idle, and no start
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post #2 of 5 (permalink) Old 10-17-2011, 11:55 PM
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Re: When to shift?

Running at peak BSFC is valid for boats and stationary equipment, mbut for motor vehicles you want operate at minimum fuel flow that will satisfy road load.

Diesels have very flat torque curve, thus a wide range of max BSFC.

Low engine RPM minimizes the work the engine hasto do pumping air in and out of the cylinders. The differnce is linear up to about 1900 RPM and gets hyperbolic above that.

2000 F-350 SC 4x2 6 speed
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post #3 of 5 (permalink) Old 10-18-2011, 01:54 AM
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Re: When to shift?

I usually upshift at 1300 RPM.

2000 F-350 SC 4x2 6 speed
3.08 gears Gear Vendor Overdrive
Slammed 4"
Rugged Air Dam
315,000 miles
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post #4 of 5 (permalink) Old 10-18-2011, 01:47 PM Thread Starter
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Re: When to shift?

Quote:
Originally Posted by Dave Whitmer View Post
Running at peak BSFC is valid for boats and stationary equipment, but for motor vehicles you want operate at minimum fuel flow that will satisfy road load.
I figured that was the case, but I was quite a few miles into my run and I just wanted it to work for my truck too.

97 F-250 ECLB ZF5 7.3 4x4, 6637 Filter, MBRP 3" to 4" with 5" tip, HPO xover pipe, shimmed FPR, Boost and Pyro Gauges, PHP chip with stock, 60 hp, 80 hp, 140 hp, high idle, and no start
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post #5 of 5 (permalink) Old 11-25-2011, 02:09 PM
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Re: When to shift?

Less RPM doesn't always mean less fuel is being used. If trying to be conscious of fuel mileage I drive by the Pyro. Whatever engine speed/gear selection yields a lower exhaust temp will get the best economy.

With my oversize tires (34.5") it takes more pedal to maintain the same speed in 5th gear (OD) than it does in 4th (direct) at anything less than about 65mph. I typically use top gear only while crusing above 65mph.


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