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Downgrading my gearing


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Hi guys

I've just bought a std tranny box to replace my Ashcroft HRTC as the gearing is far to high great on a motorway but no good for town work as 1st is like 2nd and so on, i've tried swapping back my std diff which it does pull the gearing down but not quite enough so so going this way should better.20200914_190036.thumb.jpg.127c12d4507088ce3f7a3a01b08fe5d9.jpg

20200914_190141.jpg

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Remind me, were you running HRTC and 3.54s together, or did you try them separately? So, having gone back to 4.71 diffs, are you intending to refit the3.54s as you refit the standard transfer box?

The effect of the diffs is a 35% step up, the HRTC is said by Ashcroft to be 32%, so if if dropping the diffs back down wasn’t enough, then refitting them and dropping the transfer box but having the tall diffs is going to be marginally worse.  Are you returning the whole transmission to standard?

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3 hours ago, Snagger said:

Remind me, were you running HRTC and 3.54s together, or did you try them separately? So, having gone back to 4.71 diffs, are you intending to refit the3.54s as you refit the standard transfer box?

The effect of the diffs is a 35% step up, the HRTC is said by Ashcroft to be 32%, so if if dropping the diffs back down wasn’t enough, then refitting them and dropping the transfer box but having the tall diffs is going to be marginally worse.  Are you returning the whole transmission to standard?

Both fitted but I'm a bit confused now as I thought 3.54 diffs was 28% and HRTC 32% according to the Ashcroft chart anyway :)

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That's a big difference in gearing straight away

A 6.50x16 is 27" in Diameter, 205/80x16 is 29" and a 235/85x16 is 31.7"

We reckon on tdi conversions, that a Series will pull either OD and 3.54's or 235/85x16 and OD.  More than that and it's over-geared

To be fair, it's much the same rule for a 2.8 V6 or a 3.5 V8

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11 minutes ago, Nonimouse said:

That's a big difference in gearing straight away

A 6.50x16 is 27" in Diameter, 205/80x16 is 29" and a 235/85x16 is 31.7"

We reckon on tdi conversions, that a Series will pull either OD and 3.54's or 235/85x16 and OD.  More than that and it's over-geared

To be fair, it's much the same rule for a 2.8 V6 or a 3.5 V8

Yeah to be honest i'd forgotten about my tyres so yeah your probably right so i will do the tranny box swap and see what happends i've still got my old 4.7-1 diff a lot of work i know but hey ho 🙂

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It would be worth working out the gear set first. You may have a better gear spread by keeping the tranny gears and dropping the diffs down

If you use the Ashcroft gear ratio calculator https://www.ashcroft-transmissions.co.uk/calc/ratio_calc.html

Peak torque is at 1,800 rpm through 2,200 rpm so work on being at the right gear through the rev range

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The 28% figure is for overdrives.  That was published for Fairey types a long time ago, but Roverdrive give the same figure, which seems a remarkable coincidence.  I wonder if it really is the same.

I have 235/85s and overdrive.  That isn’t really enough for motorways, running out of gears before the Tdi runs out of torque, but is pretty good around town.  Even with the overdrive disengaged, the 3.54s were dreadful in town and it’d run out of torque on the motorway with the overdrive in.  But your 88 doesn’t have a drag inducing roof rack, which really does kill top end speed, so your situation will be slightly different to mine.   
 

That’s why I got the 4.1 diff gears - with the overdrive as a fifth gear, they should give comparable 1st and 5th to a Tdi Defender.

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🤒

1 hour ago, Snagger said:

The 28% figure is for overdrives.  That was published for Fairey types a long time ago, but Roverdrive give the same figure, which seems a remarkable coincidence.  I wonder if it really is the same.

I have 235/85s and overdrive.  That isn’t really enough for motorways, running out of gears before the Tdi runs out of torque, but is pretty good around town.  Even with the overdrive disengaged, the 3.54s were dreadful in town and it’d run out of torque on the motorway with the overdrive in.  But your 88 doesn’t have a drag inducing roof rack, which really does kill top end speed, so your situation will be slightly different to mine.   
 

That’s why I got the 4.1 diff gears - with the overdrive as a fifth gear, they should give comparable 1st and 5th to a Tdi Defender.

See mine is the opposite its great on motorways but on normal roads i can't go into 4th below 45 so most of the time i'm in 3rd and 1st is like 2nd thats with the 3.54 in if i fit the 4.71 in 1st is more usable but still high ☹️

Edited by Cornish Rattler
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As a tangent - the intermediate gear in the Ashcroft HRTC set is actually the Suffix B S2 intermediate gear (521330), which is no longer available ...  unfortunately they only have enough of them to sell as sets, so to get one I’m going to need to buy the whole HRTC gear set 😥

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1 hour ago, Nonimouse said:

Personally I'd prefer a Fairey (ooer) then it can be used as a splitter

Yeah i had them in all my series landys and they was great, my last one a 200tdi S2a swb with std gearing and o/d it was great the only down side to it was it would run out of gears before it would run out of puff so with this one i wanted it to have higher gearing to help keep the engine revs down on motorways so though an HRTC and a 3.54 diff would do the job which it does really well but now 1st is like 2nd and so on which i half expected but its just to much hence why i want to down gear my gearing ☹️

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The series box is an LT76. Later modified into an LT77, than an R380

The LT77, coupled to a 1.22Tbox and 30" diameter tyres, runs 70mph at 2.600rpm (give or take) and 60mph at 2,200rpm. Meaning it's at the sweet spot at 60mph. The best place for fuel economy and grunt. More than that and it gets louder and progressively faster, using fuel like it's going out of style.

A 200/300 will happily sit at 3000 rpm all day and night, but that's fuel inneficient and lacking in overtaking oomph

A Series 111 LT76, Coupled to a AshTrans HR TB, running 3.54's and 75x16's is hitting 2,600 rpm (ish) at a nadge under 70. It'a series, 70mph is quick enough, so at 60-65 it's barely sipping fuel and running like a dream

with a standard box, t box, 31's, 3.54's and a Fairey  - 70mph OD top is 2,400 rpm, and nigh on 3k rpm at 70 in fourth no OD - making it the most drive-able of the two (inmho)

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17 hours ago, Cornish Rattler said:

🤒

See mine is the opposite its great on motorways but on normal roads i can't go into 4th below 45 so most of the time i'm in 3rd and 1st is like 2nd thats with the 3.54 in if i fit the 4.71 in 1st is more usable but still high ☹️

I had the same with the OD and 3.54s - driving around in towns, 4th was too high, but it really suited 3rd with overdrive.  The trouble was that driving with that combination, especially up some of the steep hills, was too much stress on the gears and stripped the teeth.  Ed suffered the exact same failure under the same conditions on his 88.  I’m surprised you haven’t had problems yet, but if you don’t have to drive long, steep hills, you may not have needed so much engine torque while in 3rd.  That why, with the 4.1s, I will only use the OD as a fifth gear in high range.  I don’t think there will be any gear box over stress in low range, as the output resistance is so much lower.

The lower SII suffix B low range is a nice mod, Steve, but far from essential.  I’m not sure that buying a whole HRTC is worth the price unless you really need it, but maybe you can strike a deal with Cornish? 😉. In my case, they’ll almost perfectly offset the effect of the 4.1 diffs in low range, returning the final drive ration in low to roughly standard SIII values.

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1 hour ago, Nonimouse said:

 

with a standard box, t box, 31's, 3.54's and a Fairey  - 70mph OD top is 2,400 rpm, and nigh on 3k rpm at 70 in fourth no OD - making it the most drive-able of the two (inmho)

My roof rack does make the performance suffer, so a clean SII or SIII might be happy on the motorway with that, but mine needed the pedal all the way down all the time.  It just doesn’t have the torque for the drag and gear ratio.  But town driving, even with the overdrive disengaged, was a pain - first is just too high with 3.54s.  Pulling out at junctions was torturous, worse than the 12J on a standard transmission.  It was ok once going, using one gear lower than normal, though a bit revvy.  Overall, I went from 28 mpg with the standard transmission plus overdrive to 26 mpg with the 3.54s on a mixed cycle commute.  I just spent more time trying to accelerate and more time in a revvier 3rd+OD than 4th.

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5 minutes ago, Snagger said:

My roof rack does make the performance suffer, so a clean SII or SIII might be happy on the motorway with that, but mine needed the pedal all the way down all the time.  It just doesn’t have the torque for the drag and gear ratio.  But town driving, even with the overdrive disengaged, was a pain - first is just too high with 3.54s.  Pulling out at junctions was torturous, worse than the 12J on a standard transmission.  It was ok once going, using one gear lower than normal, though a bit revvy.  Overall, I went from 28 mpg with the standard transmission plus overdrive to 26 mpg with the 3.54s on a mixed cycle commute.  I just spent more time trying to accelerate and more time in a revvier 3rd+OD than 4th.

It's interesting just how much a collection of bits of thin steel tube can really much up the aerodynamics of a brick. I love my roof rack, but the reduction in noise and the added 2 mpg means it stays on the drive unless needed

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It really is.  My 109 managed 85mph with a 12j on a flat section of highway without the rack (standard gearing plus overdrive), but it’d struggle to reach 65 in the same conditions with the rack.  And the difference in noise was significant, not just because of the aerodynamic noise, but the engine working harder to overcome the drag.  It cost about 2-3mpg at 60mph with the 12J, but I have never driven it without the rack since fitting the Tdi, so I don’t have figures for that.  I’d expect about 2mpg for driving over 45mph.

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57 minutes ago, Nonimouse said:

It's interesting just how much a collection of bits of thin steel tube can really much up the aerodynamics of a brick. I love my roof rack, but the reduction in noise and the added 2 mpg means it stays on the drive unless needed

Our 127 Ambulance came with a yeeuuge roof-rack, it's amazing the difference it made removing it - it was the equivalent of having a fat bloke sat on the roof and really didn't help cornering!

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  • 3 weeks later...

Got the g/box out today so as i'm off for a week me and my bro are going to strip the new to me tranny box and see what its like, everything seems to work manually on the bench and no play anywhere so hopefully it should be ok if it is ok we will put it all back together using new seals and gaskets then swap the tranny boxes over then re-fit the g/box 🙂

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Std trannybox swapped over to my g/box now just got to swap a few studs over as the std trannybox had a few missing and fit the sump plate on and its ready for going back in once i've replaced the clutch as it span whilst we was out on our last metal detecting trip due the the high gearing coming out of an awkward junction ☹️

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“Tranny box” could be quite a subject.  Are we even allowed to discuss such a thing in these “enlightened and liberal” times? 🤔😜

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