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Old 12-30-2007, 07:47 PM   #11
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once you get to a speed, feather the gas just a little. if the rpms move (wihtout the speed changing) the TC is not locked up. if you can feather the gas and the rpms don't move, the TC is locked up.

The above formula seems to work for my car as well as where it takes the least throttle input to maintain speed.
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Old 12-31-2007, 05:21 PM   #12
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thanks both of you. after experimenting, i've found 4th gear TC lockup is ~50 mph. this apparently is my sweet spot.
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Old 01-17-2008, 11:52 AM   #13
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Obviously slower is better, I would try constant gas 65, hopefully it won't deviate past 55-75 that way.
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Old 09-04-2008, 12:47 PM   #14
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Quote:
Originally Posted by Pete View Post
Bowtieguy,
As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s). Piston speed (ft/s)= 2*stroke(inches)*rpm/720.

Piston speed = 2 x Stroke in inches x rpm / 720

For the 3.8 GM V6 this is between 1740 to 2100 RPM.

Cheers , Pete.
Old bump...

I've had Pete's post about piston speed in my sig for a long time but never actually did the math. Here's the math for my 2008 VW Rabbit:
2 * 3.6 inch stroke * rpm / 720 = piston speed (optimally 16.4 to 19.8 ft/s)
simplifies to this very convenient number:
7.2 * rpm / 720 = piston speed
which results in numbers like this:
7.2 * 1640 / 720 = 16.4

So, 1640 to 1980 RPM would be the most efficient piston speed. That's a measly 38 to 46 mph in my "high" gear (not really very high at all, but the highest I've got).

I guess that means I should experiment with normal cruising instead of engine-on P&G at that speed, though I usually EOC at that speed (which would have to be better than even the most efficient piston speed).

Also, I wonder if it means I should downshift to keep in that range when cruising at slower than that. There's a heavily enforced 30mph road on my way to work where I have been cruising at 30mph in 5th (~1300rpm), but maybe I should try 4th and see how that affects me. I could actually watch the fuel rate meter for this purpose. I think I'll try that, I may have never tried it at 30mph.

Okay, so my 2002 GMC 5.3 V8's stroke is 3.62204724 inches...almost exactly the same. Now if I could only convince the transmission to shift earlier under heavier throttle...
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Old 09-05-2008, 07:18 AM   #15
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Results:
I don't think the calculation works for me. I didn't use it quite as intended but my test should have confirmed it if it's true.

My VW is in its ideal piston speed sweet spot as figured above at 38-46 mph in 5th, or closer to 30mph in 4th. I have some portions of my commute where I hold a steady 30mph, and tried 4th and 5th in them while watching fuel rate. Fuel rate jumped almost 50% when I downshifted to 4th (after stabilizing my speed again) to bring it into the "ideal" RPM, while on level land.

There's one place where I hold a steady 30mph uphill, and in 5th I'm at WOT to hold that speed without accelerating. On that hill, 4th with partial throttle to hold the speed read about the same fuel rate as 5th @ WOT.

I guess the ideal piston speed sweet spot is not significant enough to overcome other FE issues such as gearing and aerodynamic drag...
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