Tall vs Short Gearing for FE

[SVOboy]

I've gotten in this fight twice in the last few days, and it's rather annoying.

If anyone cares to give their thoughts, please weigh in in a logical manner, cited sources are always good, as well as FE stats.

Thanks all!

[omgwtfbyobbq]

Based on what I've read...
Assuming taller gearing means the engine is spinning slower at some given speed. In order to make the same amount of power at that given speed, with the engine spinning slower, more fuel, and more importantly, more air must be in the cylinder, which reduces the difference in pressure between the volume of the expanded cylinder and the crankcase durig the intake stroke, which results in less force needed to turn the crank. These are pumping losses, and are a problem to some extent with every spark ignition to date. Lower engine speed also results in fewer friction losses since the engine turns less per a given power output. High EGR rates as well as forced induction also reduce pumping losses.

From this article.

Originally Posted by Car and Driver

Driving the BMW at 75 mph in fourth gear yielded 26.4 mpg. In sixth gear at the same speed, it got 29.7 mpg, a 12.5-percent improvement. In lower gears, the engine simply spins faster, generating more friction and sucking harder against a partly closed throttle.

I think the spinning refers to friction losses and sucking refers to pumping losses. The way to test this is to drive a nice round trip with no stops at the same speed in two different gears... If going from 4th to 5th increases economy, then going from 5th to a taller 5th should have the same effect depending on how much taller the new gear is. GM is also planning a new series of six speed automatics, with taller OD gears.

Originally Posted by Green Car Congress

The company recently introduced three new members of the six-speed family. Most of GM’s six-speeds feature a wide, 6.04:1 overall ratio compared to 4.0 of typical four-speed automatics.

[JanGeo]

Yes more fuel per stroke but less strokes per second so you can come out ahead a little if you are on the right point on the thermal efficiency curve.
One thing factors into the rpm that I didn't appreciate which explains a lot - the Synlube site has a friction vs speed graph of metal surfaces rubbing with oil vs synlube and you all should look at it. It really explains a lot about what is happening when we lug and over rev the engine.

http://www.synlube.com/howsyn.htm

Synlube1.jpg

[krousdb]

Too bad there are no values on the X-Y. Graphs can be misleading without actual values.

[omgwtfbyobbq]

And engine operating temperatures. Although in defense of synthetic oils I've heard they are especially suited for colder climates compared to dino.

[MetroMPG]

Originally Posted by SVOboy

I've gotten in this fight twice in the last few days, and it's rather annoying.

It's a no brainer, really. You just have to show them the evidence.

I'm curious to know what arguments people are making that lower gearing is better.

[krousdb]

Some anecdotal evidence would be what Honda did. One the high FE 92-95 versions of the HB (CX and VX) they dropped the final drive to 3.25 from 4.06 on the DX. They also reduced weight and CD and used different engines, but if shorter gearing was better for FE, why did Honda do the opposite? The EPA figures show that the CX and VX FE is clearly better than the DX.

Since Honda engineers are much smarter than I am, I will let thier actions speak for me.

[MetroMPG]

EDIT: crap. Sorry Dan, I got confused reading your post. Your example is of course right. Deleting my earlier nonsense.

[MetroMPG]

Edit: la, la, laaa... deleting my confused & confusing reasoning...

[krousdb]

You mean because of the lower speeds? The DX would be better as long as you stay below 45 MPH. Above that, you would be looking for another gear. Believe me, I have had both.

[JanGeo]

I think you need to look at the LOD reading at various speeds - I see 50-60 %power generation a 50-70mph speeds in 5th in the xB which is probably a sweet spot for the engine even though it is turning 3075rpm at 60mph in 5th - not very tall at all. Also being able to climb the max incline allowed at highway speeds is also a factor I am sure.

[JanGeo]

Originally Posted by krousdb

Too bad there are no values on the X-Y. Graphs can be misleading without actual values.

Yeah knowing the sweet spot in the middle would be nice but I have to believe that it would be the full range say 0 to 6000 rpm would be my guess. Anyone want to spin a engine with an electic motor and measure the torque required to run through the entire rpm range?

[SVOboy]

The point presented to me is that something like an si transmission would be better because it would get you up to speed fast and with less throttle input. *shrug*

[MetroMPG]

What did you say to that?

I would have offered:

1) when accelerating, more throttle at lower RPM can be a good thing = higher load = gets the engine into its most efficient BSFC zone for acceleration.

2) even disregarding (1), once up to top gear the SI tranny is now a penalty. The engine probably still revs away crazily (relatively speaking). If it had a different top ratio, that would be great, but it's probably not as good as the DX gear/FD combination (could be wrong there - I don't know the Honda details).

[JanGeo]

It's all about the TOTAL number of revolutions the engine makes that determines the energy wasted. . . . along with all the other factors like gearing losses at high loads etc. . .

You all should get some eCycle CMG (brushless Communtated Motor Generators) installed onto your engines and get rid of the starter and alternator and make your vehicles into instant HyBrids.

[MetroMPG]

Originally Posted by krousdb

You mean because of the lower speeds? The DX would be better as long as you stay below 45 MPH. Above that, you would be looking for another gear. Believe me, I have had both.

No, I was confused. You're absolutely right.

I misread your post and spewed some gibberish. So I edited away my confusing reasoning in my earlier messages.

Carry on...

[SVOboy]

I told him that in my observations throttle makes little difference if the rest of the way after is balanced well for the throttle you used.

He is of the camp that throttle has everything to do with fuel economy and engine revolutions nothing.

He said it didn't matter if it went through less revs because you'd be accelerating for less time.

[MetroMPG]

Also, did you show him my RPM vs MPG graph? Would seem to contradict his point that RPM doesn't matter.

[SVOboy]

Perhaps I should say that he believes rpm doesn't matter in acceleration. Anyway, I don't think he's budging on his opinion, I just don't know why he has it, perhaps because it's most convenient for him and wanting a fast car,

[white90crxhf]

i bet with the same driving habits i have now i'd get close to 60mpg with my hf tranny. btw the acceleration difference isnt that much between hf & si transmission.(on an hf motor anyways)

[omgwtfbyobbq]

Couldn't someone just swap the gears between trannys to get a 1-4 that are good for acceleration and a really low 5? Best of both worlds?

[SVOboy]

You're not going to see much of a gearing difference with a si 1-4 but an HF 2.95 final,

[philmcneal]

hm maybe that's why the fit had such short gearing? gears for the city?

makes me wonder if nissan's 6spd for the versa is more suited to my cause, i could care less for highway.

[SVOboy]

All new honda sticks have short gearing,