Higher Saturn Overdrive Ratio

[gardenman]

With the MP2 5 th gear the ratio is .605 What RPM is that at 60 MPH ?
What if you were to get a custom gear ratio cut that would give a .510
final gear ratio. Which would be another 20% taller. What would that do for the miles per gallon do you think?
I talked to a machine shop and it would be expensive to do just one but most of the cost is for setup so it might be affordable if they made 10 sets. When I look at how much time some of the modifications take it might be worth it for me to go that route.

[cfg83]

gardenman -

Very cool idea. I went here for a comparison :

Fatboy Raceworks Graphical Transmission Gear Ratio Calculator

And came up with this :



Transmission One represents my MP2 5th gear swap. Transmission Two is identical except for the custom 0.510 5th gear.

I picked 1000 RPM as the "redline" because I wanted to know the minimum MPH needed to stay above the "lug line" of the engine. 1000 is roughly close to the idle RPM of the S-Series. Anything below 1000 RPM makes it hard to drive the car. This would actually force me to drive faster on average.

I plugged in other RPMs to figure out other MPH in 5th :

RPM    0.605       0.510
        MPH         MPH
1000   29.63       35.15
1200   35.55       42.18
1400   41.48       49.20
1600   47.40       56.23
1800   53.33       63.23
2000   59.25       70.29

It looks like ~1800 RPM would be your freeway RPM.

I am not in super hilly country, but if I were in pure flatlands, I might go for it.

CarloSW2

[dcb]

EDIT, updated graph, it isn't linear

Looking at the bsfc chart might help too: Brake Specific Fuel Consumption (BSFC) Maps - EcoModder

Given that you still need about the same power to go 60mph, I made a guess at your starting point (2000 rpm at 4/9ths thottle?) , marked by the arrow.

I then multiplied 78Nm by 2000, and got 156000, then I plotted 156000/rpm with the other dots. All the dots represent the same power level, rpm*Nm = 156000.

So the ideal for cruise would be to be smack dab in the middle of that 250 island, say 2500RPM @124.8Nm for this saturn. But our engines are all generally too big for that, Some consolation is that you can (with a stick shift anyway or some ingenuity) centre your shifts and maintain your throttle so that you accelerate in the bsfc peak area anyway (and stay off the brakes).


But for cruise considerations, and assuming no other mods than gearing, you want to pick the gearing that will get you the lowest bsfc number. It looks like you are about 1/2 way between 275 and 250 at 1500 RPM, so maybe a %5 improvement in MPG there (1-(275/262.5)). For the ratio above (1750 RPM) it looks like maybe 1/3 closer to 250 so maybe a %3 improvement. There is probably a more precise way to interpret the distance between rings, but you get the idea hopefully.

Of course getting your starting point right is the real trick