Life with a longer diff

Life with a longer diff

Author
Discussion

adam quantrill

Original Poster:

10,547 posts

178 months

Monday 3rd December
quotequote all
My "new" diff has been in now for a few months and is much "longer", resulting in lower revs when cruising at a given speed.

I've found that sometimes 5th just isn't good enough for acceleration and have to "drop a cog" or even two for some fun. Max revs in thirs is now around 90 I calculate.

On the other hand my mpg has improved by about 20%, which saves some juice for a bit of fun now and again, at no extra cost!

RCK974X

2,510 posts

85 months

Monday 3rd December
quotequote all
What ratio is it actually ??

Not quite the same, but when I used to build kits, I would often go up a ratio (or two) if available, as it's no hassle to change down for a quick pass of something.
As you say, it typically pushes the mpg up noticeably. But then kits tend to be a lot lighter than the donor vehicle....and the wedges aren't, much.

The 280 of course had a 3.07 anyway, but only with 4 speed box. (mostly...it's TVR....)

Did you fix up the speedo (or drive gears) to match ??

adam quantrill

Original Poster:

10,547 posts

178 months

Tuesday 4th December
quotequote all
I went from a 3.31 to a 2.88.

I haven't changed the speedo drive yet but have a few different coloured drive cogs to play with.

Yeah the Jag weighs 1825kg and the wedge 1100kg so quite a difference if you look at it that way!
Even the SDi weighed in at a (claimed) 1440kg.

magpies

3,803 posts

118 months

Tuesday 4th December
quotequote all
just remember it is illegal for the speedo to read slower than actual road speed - maybe unlucky to be found out though

adam quantrill

Original Poster:

10,547 posts

178 months

Tuesday 4th December
quotequote all
I have heard this rumour from time to time but never had it substantiated. (That is, what is the law that would be broken? If you can cite the statute that would be useful)

It went through the MoT just recently with no complaints (at least, no complaints about the speedo ;^).
Advertisement

adam quantrill

Original Poster:

10,547 posts

178 months

Tuesday 4th December
quotequote all
OK you seem to be thinking of "The Motor Vehicles (Approval) Regulations 2001"

My car was manufactured and type approved in 1989 so that legislation hardly applies.

As a case in point "the noise measured at 0.5
metres from the exhaust outlet
at 45 degrees to the axis of the
outlet pipe in a horizonatal
plane, shall not exceed
101db(A)."

Good luck with that! My inlet noise alone is 110dB(A)!!

The relevant one for us Wedgers is the 1980 statute: http://www.legislation.gov.uk/uksi/1980/1182/pdfs/...

This does not mention the speedo, nor the exhaust.

Edited by adam quantrill on Tuesday 4th December 23:04

ChimpOnGas

8,266 posts

115 months

Wednesday 5th December
quotequote all
adam quantrill said:
My "new" diff has been in now for a few months and is much "longer", resulting in lower revs when cruising at a given speed.

I've found that sometimes 5th just isn't good enough for acceleration and have to "drop a cog" or even two for some fun. Max revs in thirs is now around 90 I calculate.

On the other hand my mpg has improved by about 20%, which saves some juice for a bit of fun now and again, at no extra cost!
My Chimaera is geared at 15mph/1,000rpm in third so at 6,000rpm that's 6 x 15 = 90mph or exactly the same as you Adam.

The full gearing in my BW T5 gearbox and 3.77 diff equipped Chimaera looks like this:

1st - 5mph/1,000rpm - 31.25mph at 6,250rpm
2nd - 10mph/1,000rpm - 62.5mph at 6,250rpm
3rd - 15mph/1,000rpm - 93.75mph at 6,2500rpm
4th - 20mph/1,000rpm - 125mph at 6,250rpm
5th - 25mph/1,000rpm - 156.25mph at 6,250rpm

TVR quoted the top speed for my Chimaera at 153mph which was the same as them saying the engine can only reach 6,120rpm while pushing the car through the air around the 150mph region, I can confirm this is exactly what happens in my own 4.0 litre Chimaera which is rolling road proven to make 250hp at 5,600rpm and 260ft/lbs at 4,600rpm on LPG.

Given sufficient distance the car will continue to slowly accelerate past the point of peak power (but holding it) as the revs climb beyond 5,600rpm, so from 140mph my Chimaera will slowly continue to increase road speed to a sat nav confirmed 153mph, after this wind resistance overcomes or at least meets the available engine power.

I looked at fitting a longer diff but studying the gearing (box in all gears/diff ratio) and plotting this gearing against engine behavior and where peak torque arrives (4,600rpm) and then laying that over real world driving and overtaking situations it became obvious TVR had already done the same calculations and got them absolutely spot on. A taller diff may give me a cruising fuel economy advantage but it also clearly stands the very real risk of spoiling the car's in-gear real world performance and rewarding driving characteristics on the road, it would also negatively impact on the car's top speed.

Still keen to improve the fuel economy of my Chimaera while keeping the perfect gearing, real world overtaking performance and ideal spread of ratios I took a different path to reducing my fuel bills. By burning LPG a fuel that is nearly half the cost of petrol I immediately moved my average petrol fuel economy from a painful 23mpg to a pocket friendly 45mpg (petrol cost equivalent).

Using a clever engine management system and contrary to all the myths that surround LPG I was also able to actually improve the car's drivability, smoothness and engine refinement while increasing engine horsepower and torque by some 7% over the original Lucas 14CUX & distributor petrol only set up.

My 1996 Chimaera retained it's ideal gearing, yet became nicer to drive and faster than when it left Bristol Avenue some 22 years ago.

Best of all not only is the car faster and smoother it's also now almost half the cost to fuel, for all these reasons I've proved what I worked out on paper before I started all this was correct... IE keeping the gearing the same but converting the car to burn cheap as chips low carbon clean burning LPG is a far superior solution to improving fuel economy that fitting a taller diff.

And its worked out so well everything I spent converting the car has long since been covered in fuel savings, these days I'm still gunning past mostly everything I care to overtake but doing so at a pocket friendly £0.11p a mile wink

Sometimes to achieve one's goals doing the sums before you commit to an idea, then thinking outside the box.... becomes the best way to achieve success tongue out




adam quantrill

Original Poster:

10,547 posts

178 months

Wednesday 5th December
quotequote all
In my case I started with 4l but that irresponsible chap Dave Haughin slapped on a supercharger without changing the gearing.

So I have more torques but les revs (as the blower is now dictating the rev limit).

The gearing was a bit on the low side for the package, it probably works a lot better now.

LLantrisant

255 posts

95 months

Wednesday 5th December
quotequote all
for keeping the acceleration and in the same time reducing revs on motorway or other longer drives with higher speed you should have better installed a 5 speed gearbox.

but I can tell you why you haven't done that. costs more and needs more skills or workshop capacities because you need to change gearbox gearbox mount clutch prop shaft....

now you have installed the best anti-tuning device available on the market


magpies

3,803 posts

118 months

Wednesday 5th December
quotequote all
"The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed. A vehicle meeting these requirements would not be able to travel at a greater speed than that shown on the speedometer and a driver could not, therefore, inadvertently exceed speed restrictions. Her Majesty's Government have no plans to introduce instrument tests."

If the Road Vehicles (Construction and Use) Regulations 1986 act allows use of speedometers that meet this EU regulation, UK specific regulations can't be any tighter.

Interesting that it is illegal to be able to travel at a speed greater than the maximum that the speedometer can display. I'd never thought of this as a possible 'get out of jail free' before, but can see that the lawmakers did!

RCK974X

2,510 posts

85 months

Wednesday 5th December
quotequote all
Just if anyone is interested, or considering any swaps etc.

I can explain how to calculate road speed vs RPM for ANY combination of tyres and gearboxes, you just need the diff ratio, tyre size, and gbox ratios.

It's actually quite easy to do. Just ask.

Max road speed is complicated (as above) because it depends where the engine is on its power curve....and drag factors and so on.
You can do a theoretical maximum (i.e. max revs in highest gear) but there's no guarantee car/engine will actually get there.

Speedo calibration

For old stuff, as far as I know all Ford speedos are calibrated at 1000 turns per mile, and BL/Smiths actually have the turns per mile printed on the face around the edge.

Newer stuff is of course electronic, so it varies by manufacturer.

Edited by RCK974X on Wednesday 5th December 18:56

magpies

3,803 posts

118 months

Wednesday 5th December
quotequote all
I had to change the speedo in my S when putting a cossy v6 / MT75 / 4.27 diff (lsd) and 17" wheels, so fitted an electronic programmable speedo and found a 'marked mile; to calibrate. Using SatNav and roadside electronic signs the speedo seems to be less than 1mph reading fast up to 70ish leptons


since then I have taken those out and about to fit a 3.0 Jag engine and box and fit either a 3.64 or a 3.35 lsd

SEvans

1,118 posts

203 months

Thursday 6th December
quotequote all
If anyone does need to re-calibrate their speedo this is useful.
https://speedycables.com/speedometer-calibration/
Did this on the Tasmin when I changed the engine, Gearbox and diff. Speedy cables did a great job and it's absolutely spot on compared to my satnav.
Cheers Steve

adam quantrill

Original Poster:

10,547 posts

178 months

Thursday 6th December
quotequote all
magpies said:
"The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39.....
So - when was the amendment made? The original 1986 statute states:

"Speedometers

35.—(1) Save as provided in paragraphs (2) and (3), every motor vehicle shall be fitted with a speedometer which, if the vehicle is first used on or after 1st April 1984, shall be capable of indicating speed in both miles per hour and kilometres per hour, either simultaneously or, by the operation of a switch, separately.

(2) Paragraph (1) does not apply to—

(a)a vehicle having a maximum speed not exceeding 25 mph;

(b)a vehicle which it is at all times unlawful to drive at more than 25 mph;

(c)an agricultural motor vehicle which is not driven at more than 20 mph;

(d)a motor cycle first used before 1st April 1984 the engine of which has a cylinder capacity not exceeding 100 cc;

(e)an invalid carriage first used before 1st April 1984;

(f)a works truck first used before 1st April 1984;

(g)a vehicle first used before 1st October 1937; or

(h)a vehicle equipped with recording equipment marked with a marking designated as an approval mark by regulation 5 of the Approval Marks Regulations and shown at item 3 in Schedule 4 to those Regulations (whether or not the vehicle is required to be equipped with that equipment) and which, as regards the visual indications given by that equipment of the speed of the vehicle, complies with the requirements relating to the said indications and installations specified in the Community Recording Equipment Regulation.

(3) Instead of complying with paragraph (1) a vehicle may comply with Community Directive 75/443 or with ECE Regulation 39. "

Stil no mention of the "under-reading" rule.

magpies

3,803 posts

118 months

Thursday 6th December
quotequote all
you could ask on the https://www.pistonheads.com/gassing/forum/speed, plod and the law pagethumbup

Andrew Gray

4,764 posts

85 months

Friday 7th December
quotequote all
adam quantrill said:
My "new" diff has been in now for a few months and is much "longer", resulting in lower revs when cruising at a given speed.

I've found that sometimes 5th just isn't good enough for acceleration and have to "drop a cog" or even two for some fun. Max revs in thirs is now around 90 I calculate.

On the other hand my mpg has improved by about 20%, which saves some juice for a bit of fun now and again, at no extra cost!
Adam not on a Wedge but on my V8 Tuscan with a 5 speed Box rather than the usual 4 speed.
Its an unusual choice for the car and was on it when i got it.
I was going to switch however once i got using the car i found it is fast enough off the mark to scare me if i push her and great for cruising at 130mph on the autoban its at 5400 RPM with it red lining at 7500 its certainly an option i will be keeping and still great fun on track.


adam quantrill

Original Poster:

10,547 posts

178 months

Friday 7th December
quotequote all
magpies said:
you could ask on the https://www.pistonheads.com/gassing/forum/speed, plod and the law pagethumbup
I have, and so far I haven't got any sense there, either ;^)

TV8

2,614 posts

111 months

Friday 7th December
quotequote all
I changed the diff on my Stag to a longer one and it was one of the best modifications I did. Hold the gears longer if you are driving in a spirited way, plus these cars, relative to their weight, already have a very good torque ratio.

I think that Derek from the Chimaera section put a longer diff on his car and he is more focussed on accelleration than 99.99% of TVR owners