Transmission options

[Huskymaniac]

Originally Posted by Ironhydroxide

Well finding Gear ratio's isn't hard. and calculating ratios and speed isn't hard either... SO. I guess i need that Dyno for the Stage 1.

Even better, I found the original specs which say that peak torque occurs at 4400 RPMs. The stage 1 car on the dyno peaked at 4500-4600 RPMs. If you want that to correspond to, say, 75 MPH it would be a wicked low gear ratio. That would point to the 4.44 being your best option for fuel economy. But, again, I have no idea if it is true.

EDIT:

I'm sick but now you made me go and think. It occured to me that there may have been an assumption I was missing and now I may see the logic behind this claim that peak torque and peak efficiency occur at the same RPMs. Here goes:

Power = Torque * 2 * PI * RPM

Assumption: Rate of air and fuel consumption at a given throttle position is proportional to RPM or Fuel Consumption Rate = constant * RPM. Let's call that FCR.

Power is energy used per unit of time and fuel consumption rate is fuel used per unit of time. Efficiency would be energy obtained for a given amount of fuel so efficiency would be power divided by fuel consumption rate or Efficiency = Power / FCR = (Torque * 2 * PI * RPM)/(Constant * RPM) = Torque * 2 * PI / Constant

So efficiency should be torque multiplied by a constant and should peak when the torque peaks. This has a big assumptin built into it but maybe it makes sense.

[Ironhydroxide]

now you got me all thinking mathy and such.

I'm tempted to say there is a "double Peak" in the fuel efficiency of an engine. One where the engine speed is slow enough to counteract the extra use of fuel to keep it at said speed; and another where the engine is "officially" at peak torque, but is detrimented by the higher speed, and therefore more fuel consumption per length of time than a slower engine speed.

[Conn SVX]

I was once told that the most efficient motor would be a 1 cylinder motor running wide open.

[Ironhydroxide]

older working engines didn't have butterflys in the carburetors... they just loaded the engine down to keep speeds down.....

[Huskymaniac]

Some two years later I came across this interesting little article:

http://www.viragotech.com/fixit/Fuel...encyPower.html

So, at full throttle, max efficiency does indeed occur at the peak of the torque curve. However, most cars are not running at WOT at cruising speeds so that is no longer the case. To a point, lower RPMs is more efficient at cruising speeds as the parasitic losses are lower and throttle is higher due to the lower HP at low RPMs.

The key to fuel efficiency is to get the peak torque down to low RPMs. Because of the lower HP at the lower RPMs, the throttle will be higher AND you will also be closer to the peak of the torque curve. Variable valve timing and other design innovations have pushed cars in this direction. The new 2.0L turbo in the 2014 Forester results in great gas mileage for a car with 250HP because the torque peak is at 2000 RPMs!

Just thought people would find that interesting. Assuming the engine doesn't have issues (head gaskets or other), Subaru may have a winner here.

[Conn SVX]

Reason Diesel engines do so good . A ton of torque at low RPM