Selecting gear ratios

[Ron Roberts]

In selecting a tranny for our rides, the common considrations are fitment to the engine, and the car (like the porsche being longer)but not much is given to gear ratios it seems. Drag racers pick a close 2nd and 3rd, but in a lot of applcations folks just stick with the stock ratios designed for under powered stock engines. I was looking at some info from Weddle, and it seems you can get about any combo main shaft you could want. So, Why have a 1st gear that redlines at 30mph when you have an engine that can take off in 2nd gear without a fuss? When an engine has gobs of torque wouldn't it be ideal to select gearing that had some longer legs?

Ron

[kdanie]

Yes, that would be good. Most people don't do that due to cost. You can easily drop $2500 on a custom built VW trans. You can easily do a Porsche 901/902/915 for that much $$ and have some left over. The ratios could still be off unless you do the reasearch on the trans you are looking at but 5 speeds is ALWAYS more fun.
ken

[Shad Laws]

Hello-

So, Why have a 1st gear that redlines at 30mph when you have an engine that can take off in 2nd gear without a fuss?

Selecting good gearing depends on a lot of things. First, the intended application... let's assume a high-powered street car for now. Second, the tire size. Let's assume ~24.25"-24.75", which is around the size most GL cars have. And, it depends on the rev-range of the engine. Let's assume a normal-ish 3k-6k powerband or so.

The first thing to do is select your lowest and highest gear. The tallest 1st gear I would use would be about ~12:1 overall ratio. This is accomplished on Porsche boxes with 3.875 x 3.18 (late 915) or 3.444 x 3.50 (G50, etc.). But, even this tall of a gear _does_ compromise some acceleration. In fact, most autocrossers prefer ~14:1. This is accomplished with 3.875 x 3.80 (VW), 4.429 x 3.18 (901, early 915), etc.

The highest gear is a bit trickier. If you still want your tallest gear to accelerate well at highway-ish speeds, then you really don't want anything more than ~3.1:1-3.2:1. This is accomplished with 3.875 x 0.82 (VW, late 915), 4.429 x 0.71 or 0.72 (901, early 915), etc. This puts the car at 3000rpm at highway speeds (~70mph or so). Realistically, anything taller is going to really decrease the acceleration of the car, but it may help gas mileage. There's really no reason to go higher unless you want gas mileage...

Now, with your tallest and lowest gears given, you can fill in the blanks for the middle gears. And then you discover something: the stock 1st and 2nd that VW or Porsche gave you are just about perfect. If you make them any tighter, you'll have to make the upper gears farther apart and acceleration suffers badly. If you make them any further apart, your upshift into 2nd will place you way outside the powerband. And, changing these ratios is very expensive. So, they are very infrequently changed :-).

For a 4-speed VW box intended for a moderately powerful and torquey GL-type of vehicle, I'd use the following gears:
3.875 x 3.78 = 14.7:1
3.875 x 2.06 = 7.98:1
3.875 x 1.26 = 4.88:1
3.875 x 0.82 = 3.18:1

Or, if you have a lot of torque and don't mind the slight low-speed loss in acceleration (and increase in price from aftermarket R&P), then you can do this:
3.444 x 3.78 = 13.0:1
3.444 x 2.06 = 7.09:1
3.444 x 1.32 = 4.55:1
3.444 x 0.93 = 3.20:1

Both of these combos are relatively cheap to gear up, compared to custom everything. Just MHO... all of the above is subjective and holds no absolute truths for anyone :-).

Take care,

[vujade]

Shad, do you have any idea what the gear ratios are on a 74 1303?

I have a 74 1303 tranny I was going to use in my 71 1302.
I heard the 1303's had taller gears.

[Ron Roberts]

In selecting gear ratios there are a few different considerations I guess. Just because the engine makes alot of power doesn't mean you want alot of tall gearing, which is what it sort of sounded like in my first post above. Even if you have 3 times the power between 1000 and 2000 rpm as a stocker you still want to consider the engines power band. And also the speed in MPH or KPH that you like to drive and shift. Also with the air cooled engine it is best to ere (or is it error) on the low side with your gearing. I brought this up because I have a mild 2017 with good low end power. The engine wants to shift into 2nd around 10MPH which is sooner than I like. If the engine liked to rev higher and was well ballanced I could rev it more without it feeling like it needed to shift. 3rd and 4th are pretty good for my engine.
3.875 R/P
3.8
2.06
1.26
.89
1st is almost useless and 2nd is just a little short. With a good T4 I could see the gearing being a little taller for non racing transportion. Shoot, if I wanted to drag race I'd go with a T1 and even shorter gearing!

Thanks for the replies
Ron