3500S diff ratio

Eddystone

New Member
Please can someone clarify what diff would be in a '73 ? James Taylor's P6 book says 3.54, whilst Autocar's road test says it's 3.07, equating to 24 mph per 1000 rpm. Then an owner tells me 44 mph at 2000 rpm, which is between the two, although inaccuracies may be creeping in. If it is indeed 3.07 then is there really much need for an LT77 conversion?
 
It's 3.08:1 for all V8s. 3.54:1 for all 4 pots.

There's is plenty of need for an overdrive gear if you regularly use motorways. In fact the gearing is all wrong really because there was no overdrive gear available.

Rover gave us a long final drive for the V8 but what that does is of course up the gearing in all ratios, hence the V8 is slower off the line than it could be. Which might handily for Rover protect an already weak gearbox.

Here's a chart I made before:

1678119226733.png
 
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Thank you, Peter, the book must therefore be incorrect, so that is very useful! So column 4 is presumably mph per 1000 rpm ?
 
Please can someone clarify what diff would be in a '73 ? James Taylor's P6 book says 3.54, whilst Autocar's road test says it's 3.07, equating to 24 mph per 1000 rpm. Then an owner tells me 44 mph at 2000 rpm, which is between the two, although inaccuracies may be creeping in. If it is indeed 3.07 then is there really much need for an LT77 conversion?

Which version of James' book, and what page?
 
Hi Chris,
I think the question's been answered now: all V8s have 3.07 diffs. But to answer your q, p 203 of Rover P6 Complete Story, 2020.
 
The needles on all of the V8s I have ever owned have moved together in 4th, so they both point straight up at 70mph, at 3,000rpm.

I've always thought the V8 was under geared personally, especially cruising along at 70mph which can be a bit noisy. I have a 2.88 diff which brings it down to 2,800rpm and then an LT77 box which gives a 5th gear 70mph of around 2,200rpm.
 
I think you will find the P6B and SD1 diffs are actually 12:37 teeth, giving 3.08333333... Most diffs give irrational ratios like this, but there are exceptions - my 928 has 12:33 teeth giving EXACTLY 2.75:1 ratio.
 
Hi Chris,
I think the question's been answered now: all V8s have 3.07 diffs. But to answer your q, p 203 of Rover P6 Complete Story, 2020.

The original 1993 version, Appx. A, page 172, states 3.54 for 4-cyl, 3.08 for 8-cyls.
 
I think you will find the P6B and SD1 diffs are actually 12:37 teeth, giving 3.08333333... Most diffs give irrational ratios like this, but there are exceptions - my 928 has 12:33 teeth giving EXACTLY 2.75:1 ratio.
Leant something today, never heard the word irrational used in this context before until I looked it up in the dictionary
 
Glad to make a useful contribution for once. Apparently the 'irrational' ratio avoids the same teeth meeting each other too much. Sounds a little dodgy to me.
 
P6B and SD1 diffs are actually 12:37 teeth, giving 3.08333333... Most diffs give irrational ratios like this,

I tried to bite my tongue;
As an irrational number cannot be expressed as the ratio of two integers. A gearbox ratio can not be an irrational number.
E.g. 3.08333... has the ratio 12:37.

12 divided by 37 is a Recurring Decimal. I.e. 3.08333.....

phew I feel better now.

As someone has pointed out, these ratios are chosen for wear qualities. The use of a prime number in a gear is desirable, in this case 37, as it will take the gears will take the maximum number of revolutions before the same pair of teeth interlock.

Gavin.
 
I tried to bite my tongue;
As an irrational number cannot be expressed as the ratio of two integers. A gearbox ratio can not be an irrational number.
E.g. 3.08333... has the ratio 12:37.

12 divided by 37 is a Recurring Decimal. I.e. 3.08333.....

phew I feel better now.

As someone has pointed out, these ratios are chosen for wear qualities. The use of a prime number in a gear is desirable, in this case 37, as it will take the gears will take the maximum number of revolutions before the same pair of teeth interlock.

Gavin.

= 0.324 ?
 
Maybe I should have described the number as imaginary? Couldnt find a way to indicate recurring decimal - dot over or under the last 3?
 
I had converted to the southern hemisphere system. Turn your calculator upside down! Dam these phones with flipping screens.

Makes perfect sense to me.

Every day is a school day, am loving this.

What I would like to know is..... who when working out a gear ratio decides on a figure that best serves the car in question depending on power, weight, carrying capacity, etc, then realises that by tweeking the gear sets can arrive at a ratio that will bring the same sets of teeth on the two gears together the least amount of times as possible ?
I bet people like these don't get out much, but it is beyond clever.

Why would you need to limit the amount of times the same teeth come together when everything is machined and set up up so precisely, and is running in a bath of hypoid oil ?

Is it something to do with cancelling harmonics that may lead to whine ?

The 64000 dollar question is - how do you work out when the same set will come together on a 3.08.3333333 : 1 gear set ?

The OP asked what ratios are in the diffs, and why fit an LT77.

It all depends on what you want from your car, my experience was, fitting an LT77 made the V8 car a great tourer, BUT, I had circa 290 lb ft of torque, so performance was brisk, a car with stock power may lack some sparkle ? Maybe having the need to drop between 5th and 4th to maintain progress when loaded up.
I would have been keen to see what my old car would have been like fitted with the 3.54 :1 ratio and LT77. It would have made track days more fun for sure.
I just looked out the window, in the time it took me to type this ramble the snow has arrived, on very wet ground everything is now white, looks serious !
 
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It all depends on what you want from your car, my experience was, fitting an LT77 made the V8 car a great tourer, BUT, I had circa 290 lb ft of torque, so performance was brisk, a car with stock power may lack some sparkle ? Maybe having the need to drop between 5th and 4th to maintain progress when loaded up.
I would have been keen to see what my old car would have been like fitted with the 3.54 :1 ratio and LT77. It would have made track days more fun for sure.
I just looked out the window, in the time it took me to type this ramble the snow has arrived, on very wet ground everything is now white, looks serious !

At the risk of repeating myself in other conversations...

Regardless that your car is tuned and more so for standard vehicles, there's also a practical limit with tall gearing in terms of usable performance on the road. If you are gearing up to 30mph+ per 1000rpm you are already over the speed limit before you hit peak torque in a standard car. So really it has only use as an overdrive or holding gear. It's not an effective driving gear in any condition.

I've long maintained the rear axle is a cludge because there isn't enough gear spread by not having a 5th or overdrive gear. Hence I believe as standard the driving gears are too tall and the P6 is slower off the line than it should be.

I have the ZF conversion and lock-up top is even taller than the LT77. When I drive through France I felt a slightly weird effect. At the Autoroute limit which is 130kph or 82mph, the car felt like it want to go faster. Presumably as it hit the optimum torque/power characteristics.

There's another downside too with the auto in that the gear is slightly too tall to consistently hold 80kph which is the national speed limit outside motorways, so it tends to hunt uncomfortably. Basically as the revs drop below 1500 and extra few RPM makes a difference.

I fitted 185/70 tyres which give a 6% drop. The car feels noticeably "lighter" on real roads and the pickup in most situations is more responsive. I reckon a somewhere around 8-10% drop would be ideal. If you look at ratios for SD1s and similar cars then this is borne-out.

So to me there are two questions and the engine is almost secondary to the roads we're constrained by. Ironically the best way to "tune" a P6 for the road in the very first instance might be to do a transmission swap to give you "legs" and drop the gearing before modding the engine at all.
 
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