Jag Diff Ratios and Resources

[LoBelly]

There is large pre-amble to this post which will come with pictures when I can take and edit them - but for now...

I've got a toploader behind my little windsor (ratios unknown) going through to a Jag Rear.

As with the gearbox, I don't know what the final ratio is for the diff but I believe that it is pretty tall - 2.?? Apparently the engine is doing about c.1800rpm+ @ c.100kph (maybe up to 2000rpm - speedo has mysteriously died and so speed is from a nearby observer)

I imagine that to some degree the tall gearing will be overcome by the power/weight ratio (-thoughts anyone) however I guess I should investigate the lower ratios available and where (pref. S/E Melb) might I get:

-lower gear ratio sets
-LSD or locker (or whatever) packs

and finally some idea of price from those who've been down the road and is it worth the pain.

thankyou in anticipation

LoBelly

[Excaliber]

4 or 5 speed trans? If 5th is overdrive, that makes a HUGE difference in cruise rpm. Your 'cruise' rpm sounds SO low either your rear end ratio is REALLY REALLY high, or your using 5th OD.

Almost all trans are setup for 1 to 1 ratio (direct drive) in 4th gear. In '4th', therefore, engine rpm is directly related to tire size and rear end ratio. Typical numbers for 4th might be 2500 to 2800 rpm at 60 MPH with a reasonably high rear gear (say 3:30 or 3:00). In the low 2000 rpm range (or lower), yeah I'd say you have a rear ratio in the '2.5 2.7' range. Wow, thats a really high rear gear ratio, normally something that high would be used with an automatic trans, not a stick.

Over drive changes everything, it's a new 'ball game' all around.

...ball park food for thought...

[LoBelly]

...should have mentioned

its your standard old (venerable) 4 speed toploader, the rims are 15x7 with some light truck tyres on the rear (225 x tall) at present trying to get some ground clearance.

LoBelly

[Yetiman]

What Jaguar did your rear end come out of ? Typically they have a 3.54 in them (Thats what was in mine from a 62' XKE, and in my 75' XJS and 83 XJ6)'

In my Cobra it has been changed out for a 3.07 as the car had traction issues with the 3.54 and turned too much rpm on the freeway.

I am running 295/50/15 tires and a Doug Nash five speed. The trans has a 3.27 first gear and a 1:1 fifth gear (5 speed, no overdrive).

That combo works pretty well. It's able to hook up pretty good in first, and turns about 22-2500 rpm at 60ish MPH.

You must also consider your engine configuration. Is it a built motor that turns big rpm and breathes well (while lacking some low end power) or maybe it has a strong torque curve but looses it's breath at 5k rpm.

It will play a role in your gear choices as well.

My engine has OK low rpm power, strong mid rpm power and pulls hard to 6700 rpm. I can lug it around a little around town, but when it's time to make things happen I need to get the tach flying.

[Excaliber]

Top Loader, 4th is 1 to 1, direct all right.

Now, if you can figure out rpm at 60 mph AND know how tall your tire is you can 'nail' the rear end ratio. I'm guessing your tires are around 25 to 26" tall.

Yetiman is right on the money about cam selection. Ideally the cam profile is selected to work with the rear gear for the type of driving you want to do. Cruising, 1/4 mile, road race, max mpg, that sort of thing. Once you get the cam 'dialed' then you could get the right heads, intake carb, etc. So it all works together.

Well, lets just start with the rear gear ratio and see what we got.

[Bryan Wilson]

Michael,
Do you have a little plate like this under a diff housing bolt if so it is your diff ratio.

My ratio is 3.31:1
Cheers,
Bryan

[LoBelly]

Thanks Chaps

should probably have also said that the car is still 'in build'

the diff unit came with the everything else and its origins (apart from being Jag) are a bit of a mystery.

I realise its all a bit of a guess and until I've been round/down/up the track a couple I times I wont be changing anything out and will calculate the actual ratio when I get a chance.

of more immeadiate concern however is where can I get 'em (the diff bits) especially around SE Melb.

ta

LoBelly

[ZOERA-SC7XX]

Pull the cover and the gears should have the # of teeth stamped on the outer edge...do the math from there.

[Dwight]

Bryan, Please explain the tag. I have a friend with a Jag rear end and his tag reads the same as the one you posted. What tell me the ratio? Dwight

[Gav]

LoBelly
Your toploader will have a 1:1 ratio in top unless it is some strange prototype.
Jag Spares in Adelaide used to drag a v12 roadster and had loads of ratios for jag rears. I don't have their phone but they may be a good starting place.
Gav

[MetaJacko]

Originally Posted by Dwight

Bryan, Please explain the tag. I have a friend with a Jag rear end and his tag reads the same as the one you posted. What tell me the ratio? Dwight

Hi

Tag says
43
--
13

IE divide 43 by 13 to give 3.3076923076923076923076923076923 (approx )
or 3.31 so thats a 3.31:1 ratio (Just like mine )

[Plums]

That tag is the number of teeth on the crown wheel and pinion (43 divided by 13 = 3.31).

An easy way to find out your ratio is:

Jack up the car and turn the tailshaft. Count how many times you have to turn the tailshaft to turn the wheels exactly one rotation. This will give you a rough idea.....

[pneall]

LoBelly,

Try Mike Roddy 95559847 or VA Spiteri 98994851 both in Melbourne. They know all there is to know about Jaguar diffs.....then there is the guy who does fully alloy casing with 9 "centres and you can have any ration you like...at a price

Phil

[Excaliber]

3:31 is a VERY common Jag rear gear ratio, not at all surprising and a good overall choice. If it's from an XJS it will (most likely) be a limited slip type rear, again an excellent choice. XJS rears are preffered over the XJ6 sedan type rear because the XJS is a little bigger, a little stronger.

We may be done here!

I WOULD have some concern when running a typcial 3:31 rear gear with a CLOSE RATIO Top Loader because 1st gear is a higher ratio than found in the WIDE RATIO Top Loader. Making the overall 1st gear ratio possibly to high for a small block or a 'hilly' area. 4th remains the same in either trans.

Me? BIG BLOCK, 3:31, CLOSE RATIO Top Loader, love the combo!

[AC302]

Michael,
When I bought my car it had a 302 boss motor 4 speed top loader and a 3.3 Jag diff.
100k was 2500 in top which was a bit much. I have now put a 2.99 LSD in and 100k is 1900RPM.
THe 2.99 only came out in a few model but are available. An XJS HE is where mine came from and from memory it was around the $800.00 mark.

Jeremy

[JLC800]

Toploader no OD Toploader no OD Toploader no OD
26" Tires 26" Tires 26" Tires
3.54 Final 2.88 Final 2.88 Final
3200 RPM 2600 RPM 2000 RPM
70 MPH 70 MPH 55 MPH
Made the change from 3.54's to 2.88's and love it.
The engine is making the torque so it loves it also.
Jim

[Excaliber]

AC302, I had an XJS HE, V-12 of course, limited slip. Man I LOVED that car. I've seen to many replace the Jag V-12 with a Chev V-8, heresy and blasphemy I tell you, they should be drawn and quartered.

[AC302]

I was brought up on V12 Jags...My father had one of the first XJ12's in the country and had several of them over a long period of time, finishing with an XJS.
At one stage my mother had an XJ6 and Dad the 12. My older brothers tell me the 6 kept up until about 80 mph then the 12 took over.....Mum and Dad when away for the week end and left the keys at home.
That was to much temptation for an 18 and 20 boy at home alone.

[stephen low]

So my build diff with 46/13 - 3.54 should be a little quicker off the mark but does anyone have experience with this ratio and a 5 speed tremec box? If so what revs can I expect at 100 kph?

Additionally there is another tag on my diff with "BP-L" stamped on it, can anybody offer an explanation for this tag and the code?

Cheers

[Barry Nelson]

Stephen, that is the ratio I have in my car at 100 kph, rpm's are 1750
I'm using 275/50 15 tyres
Regards