Anyone on here used these 901 gear ratios ?

[andyjboy]

A, E, J, Q, X ?

How would this set-up be for a (mainly) street-driven car ?

A few more details:

A = 11:34

E = 17:34

J = 21:31

Q = 24:27

X = 28:24

The transmission would be mated to my 2.5 litre twin-plug (90mm x 66mm) engine.

[ErVikingo]

Andy,

Whats your powerband like? I can put it on my spreadsheet and give you some ideas (give me HP and Tq figures).

You are going with pretty low ratios IMHO. It depends on your type of driving.

[andyjboy]

Thanks Juan - I think I've probably got the same spreadsheet but don't really understand how to interpret it !

I haven't had the engine dynoed recently (I've made a number of changes since the last run) but these were the figures:

Power(bhp)/Torque(lbs.ft)

4500rpm - 156/182

5000rpm - 175/185

5500rpm - 190/181

6000rpm - 204/179

6500rpm - 212/171

7000rpm - 216/162

Maximum power = 217bhp @ 7400rpm (154 lbs.ft torque)

Maximum torque = 186 lbs.ft @ 5200rpm

[sithot]

I tend to look at gear ratios in relation to the difference from stock but as Juan stated, the setup really depends on your power band. Equal spacing is probably something to consider but I'm no expert so take that with a grain of salt.

A E J Q X is: -1, -4, -2, stock

I've owned a car with the A E I O U box and it's pretty slick but has no top end speed. That wasn't the nature as it was designed for short circuits.

A E I O U is: -1, -3, -3. -1

I'm not a fan of the early cars tendency to be light in the front end so I'd be perfectly happy with less top end.

A E K Q V is : -1 -2 -2 -2 I have this gear set. I know that it served a good friend well in his auto-X car. Top end is not a consideration.

Good luck,

Tom

[ErVikingo]

Andy,

I assume you are running a 7:31 final drive and 225/50/15 rears?

juan at alteregoracing.com

[JohnT]

Actually you want to have your 4/5 step to have the smallest RPM drop and your 1/2 the greatest. As the road speed increases you are facing greater parasitic loss of power from wind resistance and on the opposite end the gearsets with the highest reduction are the most fragile and should be used least. In a perfect world one would use the difference between peak torque and peak horsepower as a guide. In this case 2,200 RPM. The 1->2 shift would exceed this (say 2,500, 2600 RPM Drop). In most 901 boxes the killer is the 2->3 step. It's too big and acceleration bogs momentarialy (that's where the Alfa GTV's and Lotus Cortinas pull me between turns #2 & #3 at Sears Point every time!).

Just staring at the ceiling and relying heavily on SWAG I'd think something like-
1/2 = 2,500 RPM drop
2/3 = 2,200 RPM drop
3/4 = 1,900 to 2,200 RPM drop
4/5 = 1,400 to 1,600 RPM drop

This is very dependent on rear axle ratio and rear tire size. Bigger od tires or higher ratio (6:31) would call for narrower splits and smaller od or the 7:31 ratio would accept or need larger larger splits.

Then you have to consider top speed and on a road car, cruising speed. One can get ratios that maximize acceleration and make your upshifts sound like a F1 car and then run out of gears at 110 mph or end up trying to cruise at 75 or 80 hand then listen to the engine pounding along at 4500-5000 RPM's all day. The noise becomes physically tiring.

For a road car 5th gear has to be able to cruise and give the desired top speed so one would pick this one first and then work backward. And the problem will probably be 2nd gear. Unless you're ready to buy a 904 mainshaft, there is a limited number of 2nd gear options available. Certainly the part books list many 2nd gear options but when you check for availability you will discover that actually few of the ratios listed were made. Don't forget to look at 912 and 914 ratios to expand the options. For racing in the states (generally shorter tracks and lower speeds) old timers swear by the "Vowel Box" (A:E:I:O:U).

Hope this helps rather than confuses.

johnt

[andyjboy]

Thanks for the comments so far guys ...

Juan - yes, final drive is 7:31 but rear tyres will probably be 215/60 x 15

[tfiv]

I would think 2nd and 3rd would be too short (I'd sell that fancy 904 mainshaft and gear, it's not worth it for one ratio). I'm running A F J O S in my 2.0L. Whatever you decide, I would recommed getting perfect splits 1st through 4th and then a cruiser 5th. I've found that all of my fun on the street happens between 30 and 90, and I just want some quiet on the freeway trip home from Treffen.

[MarkS]

I would think 2nd and 3rd would be too short (I'd sell that fancy 904 mainshaft and gear, it's not worth it for one ratio). I'm running A F J O S in my 2.0L. Whatever you decide, I would recommed getting perfect splits 1st through 4th and then a cruiser 5th. I've found that all of my fun on the street happens between 30 and 90, and I just want some quiet on the freeway trip home from Treffen.

And you get that "quiet" with an S 5th !

[chris_seven]

1st to 2nd 2836rpm

2nd to 3rd 2085rpm

3rd to 4th 1893rpm

4th to 5th 1632rpm