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7th gear vs 8th gear

312 Views 10 Replies 4 Participants Last post by  inzane
First off, trying to take a pic of the tablet with my phone while doing 70 in a crosswind on a two lane farm to market road isn't conducive to award winning photography. Anyway....

I took Bumpy for a 370 mile shakedown with the new 410/8spd combo. In 8th gear with the cruise set on 75 it got 13.5 mpg, the engine turning 2100 rpm. I got on some back roads and set the cruise to 70, fired up the tablet for the Eddy EFI to take a look at things. I noticed something interesting when manually dropping from 8th to 7th.

Look at the numbers for RPM, throttle, vacuum, advance, and injector pulse width. The cruise was set, the only difference is 7th vs 8th gear. Throttle in 8th was averaging closer to 26% rather than the 24 at the moment the pic was taken.

One must also note the short/long term fuel trims and the AFR at the time. In 8th the long term is pulling fuel out but the short term is not despite the 14.4 AFR vs the commanded 14.6. In 7th the AFR is at the commanded setting, but both numbers are positive or adding a bit of fuel to the table. This is a function of the self tune and it is always at work modifying the fuel table to match the desired setting. These numbers are in constant flux so don't read a lot into them at the moment the pics were taken.

So which one would you say will deliver the best fuel economy? The higher RPM 7th gear with more vacuum, more advance, less throttle and a point less pulse width, or will less internal friction in the engine at the lower RPM in 8th gear negate those factors? Before you make comparisons to new vehicles, keep in mind those have dual runner intakes and a host of gadgets to optimize running at low RPM cruise. This 410 has a single plane and a slightly bumpy 110 LSA.

I plan to test this in the near future with an 1100 mile road trip through the East Texas hill country, the self learn will have plenty of time to optimize both gears.

So take your best guess, state why you think it's correct and let's see what happens. Correct answers will get a free trip to Granny Clampett's bottomless bar. Used to be topless, but she ain't as perky as she once was.

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I find the vacuum readings interesting. The lower reading to me would indicate the engine is working harder at the lower RPM. But the throttle I take it is relatively the same, going by the 24 & 23 readings. My gut feeling is the higher vacuum is the better mileage, but I cannot wrap my brain around the same throttle.

The trick question is how do you determine the sweet spot of an engine for fuel mileage, and for power. Some dynos will give the fuel consumption for the various RPM's at full throttle. But what if you only need the engine to produce 100HP to keep it rolling down the road? How do you come up with the best RPM, and decide gear choices.

Many vehicles will now give you MPG readings, but most are instantaneous readings, as in right now. Push the go pedal down a little harder, and the reading will drop. I drove one truck that would give you the average for the last 5 mins, or something. I found that more usefull, But I do not think it changed my over all driving habits. One could even argue that it was more of a distraction.

The gas engine Pacifica I had, would not use 8th and 9th, until after 75 MPH, except maybe for real light throttle, or even coasting down a hill. I could never get used to it shifting three times before I cleared an intersection. I had to kinda force myself to not try to feel the shifting, but I always ended up back there. I suppose in time I could have, but I got a great deal on the hybrid one.

Back to your question, I pick the 2450 RPM as the better mileage.
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Peak torque, Peak HP, and minimum fuel consumption are easy to find. but that is not the real world. Back in the 60's it was easy, most engines were giving their all to keep the vehicle moving, so you could look at the fuel specific consumption curve and get a good setup. Today we only push the engines when accelerating. So finding the cruise minimum fuel consumption is not so simple.

What does that number for injection mean?
Industrial engines used to come with a graph that showed the various curves. They said that to get the best fuel consumption, to operate the engine at the minimum fuel consumption, which is often at peak torque. But engines were genrally chosen for their HP.

But if you look at the Onan generators in many RV's they took something like a 20 HP engine, and ran it at 1800 RPM, where it only made like 11 HP, saying it was more efficient, quieter, and less wear on the engine. (so kinda the bigger engine running at lower power level theory)

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It is an expression of pulse width in MS, higher meaning a longer on time.
So then can it be thought of as a instantaneous fuel use number? But, I guess not because you need to take the lower RPM into account. HMMM.
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