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7th gear vs 8th gear

311 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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1 - 5 of 11 Posts
Even on my 20 Warlock 55-65 on a two lane road it often does better in 7th. I only have 3.21s so the down shift out of 8th is more common. And in Auto it tends to skip to 5th or 6th on hills. 3.55 is probably better but 3.92 may be over kill for my 2wd, except I’ll be towing a travel trailer by next year so it may be worth it.
When I swap a 5.7 or 6.4 (truck) into the Ramcharger I’ll probably step down to 4.10 from the 4.56 regardless of if I stick with the NV4500 or go to the 8HP70.

You should run the numbers with your tire size first, see where 8th puts you. 4.10's and 35's [34.6 in my case] works out to high 3.30's range. Pretty close to the 3.21's and 31.5's that were stock on these trucks as far as 1:1 RPM.
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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.
It was averaging closer to 26% throttle and the vacuum was correspondingly lower, as was the timing advance. I assume the cruise closed the throttle a bit just as the pic was taken.

Lots of factors to consider. How far below peak torque it is in 8th vs 7th is the deciding factor to me. Peak torque is just an expression of peak efficiency. I'll ante up and say it'll get better mileage in 7th as well.
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?
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.
It can be construed as that, yes. More fuel but fewer revs, or more revs with less fuel. It may be six one way and half a dozen the other.

I think my speedometer may be reading a bit fast meaning my fuel mileage may not be quite up to 13.5. I'm going to recalibrate it again to be sure it's accurate. Easy to do. Hold the button down until it indicates calibration mode, drive two miles, stop and punch the button again then cycle the key. Headed to Midland today to try and find boneyard hydroboost parts and an air filter box, I'll get on the I-20 service road and use the mile markers.
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