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Advice on what Diff to use

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tomgt6

Jedi Warrior
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I have a Herald with a 1147cc engine and a 4 speed transmission in it and the diff is the 4.8 diff. (does anyone need some stumps pulled) I am trying to decide if I should put in a 4.11 or a 3.8 diff.

I don't know if the 3.8 will bolt right in or not. I assume the front plate will have to be change on either.

If I did go to the 3.8 with the 1147cc engine what issues would I have? Thanks all for your advice.
 
well,
i don't know squat about heralds, but the ratio you choose will depend on how and where you plan to use the car. If its a lot of hyway and you want good mileage and want to keep the revs down, go 3.8. If you have winding mountain roads and prefer a quicker accelerating car, go 4.11. I would go with 4.11.
Rob
 
As I noted to Tom in a PM, even an 1147cc Spitfire engine might be a bit strained for acceleration driving a 3.89:1 in the heavier Herald. Also, the 3.89:1 likely is from a MkIV or later Spitfire, which means you'd have to change the flanges on the propeller shaft and axle shaft u-joints to bolt up to it...plus you have the four-stud spring mount. I'm told that older-style springs will hold down fine with just the four studs, though.

For whatever it's worth, when Triumph offered a twin-carb conversion for the Herald 1200, which essentially was the Spitfire engine, they usually specified a 4.55:1 diff! I've driven a couple of "twin-carb" Herald 1200s in various stages of tune (albeit both near-stock Spitfire spec.), and they were very nice with the 4.11:1 diff; I think it's probably the most reasonable compromise between acceleration and cruising with that setup. With a 1296 engine in typical Spitfire tune, I think you might be ok with a 3.89:1...but you still need to change all the flanges as above. (Keep in mind that the 1972 Spitfire MkIV, US spec. version, with its whopping 48hp from the low-compression 1296, reverted back to a 4.11:1 diff for that year only. The torquier 1500s were able to handle the 3.89:1 diffs again the following year.)
 
My 13/60 has a 1:4.11 and I get quite decent acceleration, and it is still reasonable on the highway at least up till about 75mph where the front end starts lifting up from air pressure under the front of the car. At 80mph it starts becoming a bit scary. I haven't found out what its top speed would be.

Even at 75mph, with stock wheels the engine is only at 4681 rpm according this gears / speed calculator. Which is not unreasonable and well below the redline.

https://www.mintylamb.co.uk/gearspeed/

Considering that the 1200 is a bit less powerful, I don't thing that the 3.89 would be a good choice. It will be needlessly slow in the city, put extra strain on the transmission, and you won't really gain much on the highway. I wouldn't be surprised however if it lowered the actual top speed since the engine wouldn't have sufficient torque to handle the lower ratio.
 
This all makes sense. With all the good advice I will will buy a 4.11 diff and be happy about it.
 
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