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Section : Rear Axle
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Posted By : Phil Kinsella, 20-Feb-2007, 12:57pm
Okay, I am fitting a J type single rail box to an 1147cc engine in a Herald. I am happy with this area and it is under way. What I am unsure of, is which
is the best diff to fit? I have various ratios available and was going to fit a 3.89:1 which came from a Spitfire - Is this the best one to fit? I wish to
achieve a decent relaxed motorway cruise.
Also how do I work out the ratio of a diff please?
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Posted By : Ian Osprey, 20-Feb-2007,
04:13pm
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Posted By : Chris Taylor, 20-Feb-2007,
07:12pm
NO NO NO! Unless your diff is knackered, why change it? With an overdrive and standard Herald 4.11:1 diff, you will be getting 20mph per 1000rpm engine
speed, which is as much as an 1147 engine can pull up any kind of gradient.
IF you were wanting to reduce engine revs at speed WITHOUT fitting an overdrive, then fitting a 3.89:1 diff would make a small but worthwhile
difference, but you run the risk of having the car overgeared if you fitter a higher ratio diff AND the overdrive. Keep it standard: with the exception
of the GT6, Triumph used the same back axle ratio with or without overdrive.
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Posted By : Phil Kinsella, 20-Feb-2007,
11:26pm
Okay then....that may be a No to that idea. When you say overgeared I am unsure what is meant - What will that mean when driving the car compared
to its standard set up?
The car currently is quite sedate to drive, which is lovely, but on longer journeys the engine sounds like its screaming it whatsits off.
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Posted By : Chris Taylor,
20-Feb-2007, 11:50pm
Overgeared means a ratio that is too high for the engine to pull, like trying to climb a hill in 4th gear. If you've got a fifth gear, (which
overdrive provides) then you can always change down, but if you achieve the same thing with a higher back axle ratio, your options for changing
down are more limited. Think of having to change down to 3rd where currently you use 4th.
You should find that adding overdrive with your existing diff will do what you want. If you do that WITH a 3.89 or 3.63 diff, then you will
find the road conditions in which you can use the overdrive will be less and less.
A 1200 would just about pull a 3.89 diff ratio, but the drop in revs would not be as much as using an overdrive, AND acceleration would be a
bit slower in ALL gears.
The engine is revving quite a bit faster than people are used to in modern cars. That is a fact of life in many 1960s cars, which were designed
for pre-motorway roads, and need to rev quite fast to produce power. With the Herald family at least you have the option of fitting an
overdrive.
You could also try removing the engine fan and fitting a thermostatic electric one (Kenlowe type). This won't stop the engine revving fast, but
it WILL reduce the noise from under the bonnet quite significantly.
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Posted By : Phil Kinsella,
21-Feb-2007, 08:33pm
Excellent that makes sense. Thank you for your assistance. I will post how it goes.