What's the Best Combination of Gears?

[Occasionally6]

The ideal gear ratio for any condition depends on how fast you want to travel and how quickly you want to accelerate to get there. Both of those dictate the power requirement. You use the gear to operate the engine at or near the most efficient load and engine speed that makes that power. (You need to measure, guess or calculate how much power is required to achieve the acceleration or maintain a particular speed.)

@Jeff88: We covered it in the other thread but those curves aren't a full BSFC map. They are a slice from one edge of one. A guide for what the map looks like, at least close to WOT, but not the full story.

Have a look for a BSFC map, even one from a different engine from yours, that includes lines of constant power. If you understand what that is showing, you will have a better idea of what different gear ratios do. Each line of constant power will pass through a number of efficiency islands. You would like to be in the high efficiency ones.

eg. here: http://ecomodder.com/wiki/index.php/..._2ZR-FXE_2010-

and read the comments under the Volkswagen Jetta TDI 1.9L ALH 1999.5-2003

The ideal transmission is a CVT, which holds the engine at the most efficient operating point for the requested power. Multi ratio transmissions are an approximation of that. The more gear ratio choices available, the closer the approximation.

For acceleration you probably want something close to an even spread. There is a case to close up the ratios (less difference between ratios) as you get to higher speeds, drag becomes important, and higher power is required to achieve the same acceleration.

If you spend a lot of time at particular speeds i.e. the speed limits, it may be best to select gear ratios for high efficiency at those speeds. It's not going to be perfect because real roads have rises and falls in elevation and wind direction varies, so the power required to maintain a speed varies. You need that CVT or have to accept that you wont always be in the efficiency sweet spots.

[cRiPpLe_rOoStEr]

Originally Posted by Frank Lee

The bias is towards maximum acceleration capability. I skip-shift my 5-speed m/ts, usually 1-3-5 or 2-4-5. As a flatlander with a car that has abundant torque, I could do away with the extra two gearsets and save the weight and loss of transmission efficiency by having a nice 3-speed. Perhaps my desire for only three gears would change if I lived in hill country and/or had a less torquey engine.

Depending on how wide is the gap between 2 random gears, there is virtually no advantage in getting a bigger amount of gears. Automakers are doing so because of some folks who buy a manual because it's either cheaper or the only option available for a certain model, but are too lazy to downshift when required

[Occasionally6]

Originally Posted by cRiPpLe_rOoStEr

Depending on how wide is the gap between 2 random gears, there is virtually no advantage in getting a bigger amount of gears. Automakers are doing so because of some folks who buy a manual because it's either cheaper or the only option available for a certain model, but are too lazy to downshift when required

Absolutely wrong. The increase in the number of gear ratios over time is due to the manufacturers chasing fuel economy (and possibly emissions).

Extra ratios cost real money. They also add weight - more than just the extra ratios would suggest because longer gear shafts deflect more so have to be made stronger or supported better - and some friction. For them to be specified there must be a tangible benefit.

As I posted the ideal is an infinite number of ratios so the engine can be held at the optimum load and rpm for a particular power demand. Pick any power demand. There is one engine load and engine speed at which that power is achieved most efficiently. That is where you want to be operating that engine. If there are a finite number of discrete gear ratios you will be somewhere away from that engine load and speed for most of the time the engine is generating that power output.

With too many ratios it is possible to spend so much time shifting that there isn't enough driving time remaining to justify them. That is ameliorated with the robotic shifting also more common in recent years.

[aardvarcus]

The recent increase in gear ratios in automatic transmissions is designed with fuel economy in mind.

Most manual transmissions available from the factory in small cars in the US in recent history are not designed with economy as the most important element; instead they are geared for performance or for cost. The manuals are either the “cheap” option or the “sport” option. They don’t have a wide enough spread between first and last gear, so the designers have to choose between performance and economy. So they put a low final drive in the car, and let the RPMs suffer at higher speeds.

Just as an example, the reason my car has the gearing it has is that you can go exactly 60mph in second gear at redline of 7800RPM. Thus when they advertise 0-60 times they have the lowest possible gearing to maximize acceleration while only needing to shift gears one time.

The “solution” is to put a lower first gear, taller overdrive gear, and a middle of the road rear end gear in the car, which nets you the best of all worlds. The downside is that now you have to increase the physical size of the transmission to fit these larger gears, the stresses on the gears are higher, and you end up with a much more expensive transmission that consumers today aren’t willing to pay for. Imagine a world where the manual is a more expensive option than the automatic, that would cut out all the “cheap” sales and leave just the “sport” sales.

[cRiPpLe_rOoStEr]

Originally Posted by Occasionally6

Absolutely wrong. The increase in the number of gear ratios over time is due to the manufacturers chasing fuel economy (and possibly emissions).

Extra ratios cost real money. They also add weight - more than just the extra ratios would suggest because longer gear shafts deflect more so have to be made stronger or supported better - and some friction. For them to be specified there must be a tangible benefit.

As pointed out by aardvarcus, having more gears is not so absolutely right if their ratio is too compromised, either for performance or fuel-economy.

[Occasionally6]

In the context of:

Originally Posted by Frank Lee

I skip-shift my 5-speed m/ts, usually 1-3-5 or 2-4-5. As a flatlander with a car that has abundant torque, I could do away with the extra two gearsets and save the weight and loss of transmission efficiency by having a nice 3-speed.

Originally Posted by cRiPpLe_rOoStEr

Depending on how wide is the gap between 2 random gears, there is virtually no advantage in getting a bigger amount of gears. Automakers are doing so because of some folks who buy a manual because it's either cheaper or the only option available for a certain model, but are too lazy to downshift when required

If there is a choice between only having gears 1 and 3 (say) and gears 1,2 and 3, where ratios 1 and 3 are the same, and 2 splits them, three gears will allow the engine to be operated closer to its highest efficiency than will two of them.

[freebeard]

If your aiming for a 10-second quarter mile, and each shift takes 1/2 second?

[cRiPpLe_rOoStEr]

Originally Posted by Occasionally6

If there is a choice between only having gears 1 and 3 (say) and gears 1,2 and 3, where ratios 1 and 3 are the same, and 2 splits them, three gears will allow the engine to be operated closer to its highest efficiency than will two of them.

But when the intermediate gear is actually not so intermediate, thus too closer to the previous gear and with a longer gap to the next (or with a longer gap from the previous and too closer to the next), which is what has been more usual, it ends up not increasing the efficiency so much...

[Occasionally6]

Originally Posted by freebeard

If your aiming for a 10-second quarter mile, and each shift takes 1/2 second?

Doesn't matter. The overall ratios will be different but the spacing will not.

[Occasionally6]

Originally Posted by cRiPpLe_rOoStEr

But when the intermediate gear is actually not so intermediate, thus too closer to the previous gear and with a longer gap to the next (or with a longer gap from the previous and too closer to the next), which is what has been more usual, it ends up not increasing the efficiency so much...

OK, the closer any two of the ratios are together the more like 2 (vs 3) ratios they are.

The flatter the BSFC map is, the less is the benefit. There is a law of diminishing return too, where shift/"idle" time is required for the ratio changes. Maybe 7 speeds is somewhere near enough?