5 speed conversion

[RotoFlexible]

I note that Burton Power offers two "ZF gearbox to Ford 4-cyl" bellhousings, one with the starter on the left, one on the right, both aluminum. (Transmission and Clutch / Bell Housings / Lotus Twin Cam 8V). Aluminum and priced accordingly. I wonder if these work with the Alfa gearbox? An adapter plate is a lot less expensive.

[garyeanderson]

These are the photos that Rick sent me. I had to resize them to load.

[msd1107]

The quoted gear ratios for the Alfa 5-speed are the standard wide ratio gears. This gives a ratio drop of 66% between 1st and 2nd gears, rather wide (You are looking for something closer to 50%, at 66% the engine will bog noticably on the 1st-2nd shift. Shifting at 6500 brings the revs back to 3900, under the torque peak. On the street, shifting at 3000 rpm brings the revs back to 1800). And the .791 5th gear would give a potential top speed of 143 mph at 6500 rpm and with 155-13 tires and 3.77 diff (and 154 mph at 7000). You would need a very strong engine to pull such a 5th gear. It might be mere reasonable to use a 3.9 or 4.11 with such a tall 5th.

Looking at my old SCCA Production Car Specifications, Alfa homologated 3 alternative gear sets.

2.76, 1.78, 1.30, 1.00, and 0.85 is one set. This gives a 1st to 2nd ratio drop of 55%, quite useable. Shifting at 6500 rpm brings the revs back to 4200, a noticable and useful improvement. And 6500 in 5th gives 138 mph.

Another homologated set is 2.54, 1.70, 1.26, 1.0, and 0.85. The first 4 ratios are somewhat similar to our 4 speed transmisison, with an added 5th. the 1st to 2nd ratio drop is 50%. Shifting at 6500 brings the revs back to 4400. And the potential top speed at 6500 is 133. This probably is the ratio set to use if you can find it from some Alfa specialists, although the previous set would be quite satisfactory.

The last homologated set is 2.33, 1.58, 1.21, 1., and 0.88. These are similar ratios to the Quaife T9 semi-helical rally gear set. The 1st to 2nd ratio drop is 47% and the potential top speed at 6500 is 129. However, it is probably geared too high in 1st to be a good street box, giving 49 mph in 1st gear

The above information was produced using a spreadsheet that allow easy modelling of different car parameters. It is invaluable when considering some of these possible scenarios, particularly when considering different tire sizes, diff ratios, and engine speeds. Request a copy from me by sending an email to [email protected].

David
1968 36/7988

[garyeanderson]

There is more info at this link on ratios and tooth count and so forth.

http://www.velocissima.com/tech/tech.htm

and click on Alfa Gearbox Ratio Chart

[crannyr]

The ratio options are very worthwhile and helpfull. I can not say that I have experienced the bogging between gears but that is likely due to my engine being a 1710 cc that makes about 120 Ft Lbs at 3300 revs. Next time the box is out I may try an alternative gear set. Thanks