Lotus Elan

3.3 Diff

PostPost by: Gopherit » Thu Jul 19, 2018 8:37 am

Has anyone run a 3.3 diff with the semi-close ratio gearbox? I know from previous threads that 3.55 with the close ratio box is not uncommon.
This, with the 2.5:1 1st gives a gearing of 8.875. The 3.3/2.9 gives 9.57 so shouldn't be too much of a problem getting off the line and the 4th would mean cruising at 70mph would be about 3,300 ( I use 185/70/13 tyres).

The car is primarily for cruising with the odd track day and has about 138 donkeys at its disposal and I am looking for more relaxed cruising.

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PostPost by: Chancer » Thu Jul 19, 2018 11:04 am

To my knowledge that ratio did not exist as a Ford English diff production item (happy to be proved wrong)

The tallest ratio was 11 pinion 39 crownwheel = 3.54:1
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PostPost by: Lyn7 » Thu Jul 19, 2018 11:15 am

I think the 3.3 diff ratio was an FIA homologated item. I have never seen one or met anyone who used one in a road car. Mind you these days it is not in the realms of impossibility for someone to produce alternative ones. Cheers, Lyn..
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PostPost by: Mazzini » Thu Jul 19, 2018 11:45 am

This company made some.

https://www.ntgear.co.uk/product/english-7-axle/

I have no personal experience of them but I was told by somebody that had fitted one that it wasn't as well made as he had hoped for.
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PostPost by: 2cams70 » Thu Jul 19, 2018 12:46 pm

I don't understand why you want to go that tall. Blunts the liveliness and rather defeats the purpose of having a Lotus T/C. If that's your aim you'd be better of with a 5 speed transmission, Even then it's a tough choice as the 4 speed has such a nice snickety snick gearchange. Better than the 5 speeds in that regard.
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PostPost by: Frogelan » Thu Jul 19, 2018 6:51 pm

@Chancer:

3.44:1 was homologated...yes it seems to be unobtainium.
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PostPost by: Gopherit » Fri Jul 20, 2018 6:50 am

NTG was where I was looking to possibly buy, so interested in any more info on their CWPs. I spoke to the MD sometime ago and they do seem quality conscious - had bought other CWPs from the usual offenders (eg Burton, Rally Design etc) and examined their quality. Burton's certainly made in Turkey and not good.

Re taking out the liveliness - we also run an Evante with 5-Speed so I know what to expect - all depends what speed you want to be lively at!

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PostPost by: denicholls2 » Fri Jul 20, 2018 2:54 pm

2cams70 wrote:I don't understand why you want to go that tall. Blunts the liveliness and rather defeats the purpose of having a Lotus T/C. If that's your aim you'd be better of with a 5 speed transmission, Even then it's a tough choice as the 4 speed has such a nice snickety snick gearchange. Better than the 5 speeds in that regard.


Drive much in the U.S., where most of the available roads for above 40 mph are Interstate highways with 65-80 mph speed limits (and 75 mph typical cruising speeds) and locations are 2-8 hour drives and you will long for the tallest gearing your engine can pull in top gear. 2500-3000 rpm at 70 mph is about right, fun can still be found in third. :)

My Europa's DCOE sits less than a foot from my ears through about 1/32" of fiberglass. It sounds nice for about a minute; listen to it for 8 hours at 4,000 RPM (highway cruising RPM for a four-speed) and not quite so nice.
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PostPost by: gus » Sat Jul 21, 2018 6:45 pm

I have done many miles on the highway, yeah its a bit loud, but never ever wanted anything that tall

Tall ratios also spread the gears out, I noticed this with a 3.64 in my seven, found flaws in the semi close that I never noticed elsewhere.

Cheapest solution for highway noise is to buy a bigger radio.

Slower 0-60 than a 2CV, no thanks
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PostPost by: promotor » Sat Jul 21, 2018 9:15 pm

I know of two diffs that were built using the 3.3 ratio (I didn't build them but know who did and trust them to do it properly) and both were noisy (on overrun if I remember correctly).
As I understand it when they were eventually returned to NTG the response was that they weren't really intended for road use as they were initially designed and built for use in racing and a large order had been made (and cancelled) for a number of these in a racing series. As such the noise was to be expected.

I also recall being told the backlash figures not being the same as Ford specs. Not sure how good on tolerance they were (and wouldn't like to buy one just to find out!)

As has been mentioned the taller diff will highlight the gaps between gears even more so it'd be wiser to have a torquey engine with a good flat torque curve across a wide power band IMHO.
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PostPost by: Frogelan » Sun Jul 22, 2018 9:40 am

@promotor

NTG is one of the few remaining gear manufacturers left in Britain and the others are not producing such tall ratio "English" diffs. One of the main difficulties is reproducing the exact shape of the CW & P teeth.

It is possible to get a REM finish done and I have always understood that diffs need careful and patient shimming.

I personally have several ratios and am looking foward to trying out my 3.4 from NTG (probably no earlier than late 2019 at the rate I am going!).

I agree with your points on torque and by implication, having a close ratio gearbox. If you sit down and do the maths with a 13" wheel, a 3.3 or 3.4 makes a lot of sense, unless your motoring is just on local B roads in Britain.
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PostPost by: promotor » Sun Jul 22, 2018 11:02 am

Frogelan wrote:@promotor

NTG is one of the few remaining gear manufacturers left in Britain and the others are not producing such tall ratio "English" diffs. One of the main difficulties is reproducing the exact shape of the CW & P teeth.

It is possible to get a REM finish done and I have always understood that diffs need careful and patient shimming.

I personally have several ratios and am looking foward to trying out my 3.4 from NTG (probably no earlier than late 2019 at the rate I am going!).

I agree with your points on torque and by implication, having a close ratio gearbox. If you sit down and do the maths with a 13" wheel, a 3.3 or 3.4 makes a lot of sense, unless your motoring is just on local B roads in Britain.


It's amazing to think how good Ford's original CWP's were (and still are if you haven't got a worn out one).
I don't know if NTG can offer REM but Quaife can do it (not sure if they only offer the service on a CWP bought from them in which case it's no good as Quaife only do down to a 3.54 ratio).
Diffs need the correct shim in and that requires attention to detail and not just slapping any old shim in like appears to be the case with some diffs I have seen.

I note that NTG's figure for backlash on their English CWP's are 0.004" compared with Ford's 0.005-0.007". They must be confident with their tolerances if they don't specify a top and bottom figure.

I like the idea of the 3.3 or 3.4 ratio I'm just not sure how well it will work with a 2.5:1 1st gear. Should be OK with a 2.97:1 box though.

Sorry for the thread drift, there doesn't seem to be much feedback anywhere on these CWP's so I wonder how many have actually been fitted and used?
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