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Differential swap

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STL_Joe

STL_Joe

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I have a spitfire 4 with a 4.11 diff and am looking to swap out to a 3.89. Since the 3.89 has larger holes/flanges on the output from the diff, what is the better way to go? Switch the outputs from the diff to match the smaller axle flanges or change the axle flanges to the larger type to mate up with the ones already on the 3.89?

Trying to do this to get by not putting an OD in the car which is way more expensive.

Thanks
 
Easiest way -- actually the only way, IIRC -- is to swap out the u-joint flanges for the axle shafts and propeller shaft. Meanwhile, you'll also note that the later 3.89:1 diff only has four studs for leaf spring hold-down. If you're already using a swing-spring, that's no problem. Even with the original spring, though, I've been told that you won't have any problem with just four hold-down studs on the earlier fixed spring.

I would note also that you're only going to see about a 5-6% difference (reduction); I'm not sure it's worth the change for that. However, given the typical optimism of a Triumph speedometer, you might well find your speedo is now a bit more accurate! :smile:
 
I replaced the 4.11 with a 3.89 and knocked about 200 RPM at highway speed- we were ale to use parts of the original differential changng the "guts," not the entire diff. I wonder about using the small engine about pulling hills.
 
Using factory data (reposted here and doing some quick calculations:

4.11:1 (Spitfire 4 - Mk3)

60 mph = 3822 rpm

3.89:1 (Spitfire IV except USA 1972 and US-spec. 1500)

60 mph = 3593 rpm

3.63:1 (European spec. 1500 Spitfire)

60 mph = 3333 rpm

3.27 (most all GT6s)

60 mph = 2978 rpm

I also would be a bit concerned about whether an original 1147 is up to the task of hills with even the 3.89. OD top gear on a Spitfire 4 with 4.11:1 diff would give you:

60 mph = 3057 rpm....

but you still can easily drop back down to direct 4th gear for those hills.

If you do go ahead with the swap, I know I'll be interested in your reactions once you've spent some time with the new configuration!
 
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