View Full Version : Help with Diff

11-20-2009, 09:47 AM
What diff would you use on a Spitfire with a 1300cc engine and a 4 speed tranny.
Will be driven around town with very little highway driving and most highway driving will be within 15 miles.

Would you use a 4.11 or a 3.89 and then why?

11-20-2009, 10:11 AM
This heavily depends on how you're gonna drive the car. Do you like hole shots? Are you looking for more "oomph" from dead starts? Do you drive agresively? Then go for the 4.11.

But for driveability, I vote for the 3.89. A 4.11 is a very geary ratio IMHO. With the 3.89 your revs should be somewhere in the high 3000s at about 60, and give you all the grunt you need around town.

The 4.11 makes any kind of highway speeds VERY revvy.

To each his own, but for casual, around town driving and occasional highway jaunts, I'd go with the 3.89.

Andrew Mace
11-20-2009, 12:30 PM
At 60 mph, the difference between a 4.11 (3820 RPM) and a 3.89 (3600 RPM) isn't very much. One other factor to consider is that a 3.89:1 diff complete will require you to change the u-joint flanges on your propeller shaft and axle shafts to the larger size, and you'll only have four studs to hold down your leaf spring (not really a problem). If you get a 4.11 diff (or already have one), odds are it's the older style with the smaller flanges as original on your Spitfire. Otherwise, either will bolt right up to the chassis.

Remember, the Mk3 Spitfire came with the "small journal" 1300 and a 4.11 diff, while the early MkIV had the "large journal" (and single Stromberg) 1300 and a 3.89 diff (ignoring, for the moment, the 1972 "Federal" Spitfire with its low compression 1300 engine and a reversion to a 4.11 ratio but in the newer case with larger flanges).

There's a chart on my web site (https://www.fairpoint.net/~herald948/database/gearbox.htm) showing various gearbox and differential ratios for most of the Triumph "small car" lines.