OK, so after a lot of thinking about how to increase performance with the factory built V12, I have decided on running with an economical solution which will give me much more acceleration, 20% more torque to the wheels and no mucking around with transmission swaps which would devalue the car in my opinion.

The XJS V12 was designed for high speed, therefore all of the torque is reduced and exchanged for high speed at the top end.

In most countries this is not required and maybe more options should have been available when ordering the cars from new.


Here's what I am doing to overcome the issue...

1: Swapping the factory 2.88 Power Lok diff with a factory XJS 3.54 Power Lok diff from a 3.6 Cylinder vehicle.
2: Increasing the rear tyre size to 235-75R15 which increases the rear overall rolling diameter by exactly 2 inches. This makes them taller and higher geared. (Front tyres are staying at factory size of 215-70R15)

I have the parts on the way and will let you all know how she performs.


The only issue for me, is that the rear tyres will make the car look like a muscle car, which I am still on the fence about, however can be overcome by installing whitewall tyres which give an illusion of a standard wheel.

I have Photoshopped a car with both options to show what it will look like with standard and whitewall tyres

If this works without too much issues in the wheel arch, I will have much better acceleration and only increase the RPM by 235 @ 60mph.

At 60mph, it will only be running at 2465 RPM which is more than acceptable for me. At 70mph, she will only be doing 2880rpm which is still good.

By doing this, I will have increased power to the wheels WITHOUT having to install a 5-speed manual or overdrive transmission.

I think it is a great compromise and should be a win-win!!!


I will keep you posted on whether the theory is good and if the wheels don't cause me too much grief!

Photo examples:

 

So you’re lowering the diff gearing and increases the wheel size to almost counter the diff ratio change ? End result as I see it is just much larger rear tyres with a slightly lower gearing. Or am I missing something ?

 

You are missing something.

The lower end gears will accelerate much quicker and be more of a noticeable change than the top gear.

The power will be delivered much earlier as it is in a higher rpm range during 1st and 2nd gear, therefore most of the change is off the mark.

Depending on how she feels, I can always drop the tyre size until I get the right combo.

I am trying to counter the top end just a little to make it bearable with the lower rear with a 1:1 transmission gear.

Cheers
Craig

 

I understand Leo's point, so what will be the net gearing change? if it is not net lower, none of your desired effects will happen.

 

It will be net lower. Approx 310rpm per gear lower which will increase torque at the wheels and increase acceleration

Do you not agree on this?

i am confident I will see an acceptable change in acceleration and a compromise of slightly higher rpm in top gear.

without changing the tyres, the rpm will be slightly too high and will annoy me (3000 rpm at 65mph)

 

The diff ratio difference is 2.88/3,54 = 0,814

Tire size difference according to tiresize.com is 7.4%

Combined ratio change is 2.88/3.54 x 1.074 = 0,874. Your RPM in any gear at same speed will thereby increase with a factor 1/0,874 = 1.144 = 14.4% increase. That will make a difference, no doubt, but the effect will be the same in all 3 gears

BR Leo

 

Something my Father who built, drove, and raced Sprint Cars, taught me was the "Gear Ratio" is whats important. What's at the end of the axle,(tire size) is somewhat irrelevant. In other words it will have less effect, on the acceleration, and feel then the gearing change. I changed the gear in my 71 Mach1 Mustang from a 3.00 ratio to a 3.25 ratio, after I had back off ice, and hit dry pavement with new snow tires, and broke the spider gears. I went to the salvage yard, and all they had was the 3.25 ratio. I installed it, and the difference was quite substantial. It was a definite kick in the pants. I was young and dumb back then, and thats how dad explained it to me. That was a simple 1/4 turn increase in ratio. Crackerbuzz said he's going from a 2.88 to a 3.54 ratio thats a gain of .66, that's over 1/2 half turn added to the engine, for every revolution of the drive shaft. Which will be felt very easily, in the seat of the pants. I have pit crewed on my buddy's Late model Dirt track car, and we used the tires size, to fine tune the gear ratio, not to change it, if that makes sense. The tire size will effect it some, but not near what the "science" says it should. Science can't explain everything, that is unless you're a rocket scientist. LOL!

Jack

 

jacobra, that’s exactly what im doing with the tyres... fine tuning. I can play with sizes until its where I want it. Keep in mind a 2 inch tyre change is probably a little more than fine tuning!

In reality a 3.31 ratio diff would be bang on the money but getting very hard to find especially in a powerlok configuration.

if anyone has one sitting around I would be interested!

Craig

 

my approach was a little different,, modded V12 cams ,more duration loses some low end torque!
so i went with a 700R4 4 speed trans, 1st is very low , and 4th is overdrive ,, for the rear gear i use 3.73 ratio powerlok with Corvette clutches!

1st gear launch is wild ,almost out of control,(i like a car that is exciting to drive), BUT most important is final gear ratio in 4th is 2.60 to 1!

gives me the best of both worlds ,acceleration and top speed /MPG!

my rear tyres are 275/35/18, and seem just about perfect, for my setup, been 26yrs now ,all is OK !
quick pic of 1st gear!

 

I changed my diff to 3.58 from an XJ40 V12 and 3000rpm at 100km/h was not pleasant. I now have a 4L60e with OD and with the stock V12 it is far better off the mark and about 2100rpm at 100km/h for relaxed cruising.

Have you considered swapping the trans for a 4L60?

 

Ronbros...

If I had the money and time, I would probably do something similar.

I bet that is a heap of fun having the best of both worlds. Your tyres are a small diameter too which would help with your launch even more!

For the time being I think my plan should suffice and keep me happy for now.

 

Yes I have and if my plan is not quite as expected, I will look at that option also.

I would prefer not to swap the tranny if at all possible, but willing to do so if needed.

My tyre trick may just tweak it enough but we shall see!

Cheers

 

Hi Leo, Yes I definitely agree that its the same in all gears however will be much more noticeable in first & second as it is with any car.

 

Here's a 3.31 for sale.

https://sfbay.craigslist.org/sby/pts...865793472.html

You may want to snag it before I do

 

Thanks ECB, I will check this out.

Update... After checking rpm and diff calculators, and working out finances for USD/NZD and freight to the bottom of the earth, (NZ), It is not financially viable for me.

I Picked up my 3.54 PL for $450.00 NZD ($300USD) so was a good deal.

i will stick with my plan and tweak with tyre sizes which will get very damn close to what a 3.31 would produce with standard tyres.

ECB, Go buy it! Sounds like good rebuilt unit too.

Cheers
Craig



Cheers

 

FWIW I have a 3.31 LSD in my car (Series III V12) with the old TH400 transmission. I love it.

The acceleration is naturally much better compared to the original 2.88 ratio.

We've all become very accustomed to modern cars with 4-5-6 speed transmission with overdrive ratios. Engine RPM at highway speed with this combination is a bit busy at highway/freeway speeds, yes, but the engine is at a sweet spot, just beginning to boil a bit. The throttle response is very satisfying.

I was worried about excessive RPM but it really isn't so bad and, after all, Jaguar sold the Series II V12s with a 3.31 diff and nobody complained.

I seldom take long road trips these days so the trade-off is no problem for me. There was a time when 600 mile cross-country weekend jaunts were a regular thing for me. If that were still the case I might feel differently

Cheers
DD

 

Doug, yes I remembered you said you were running a 3.31

I am hoping to get pretty close with the 3.54 and tyres

In fact with the larger tyres it is almost identical. (Top gear 1:1)

2900 RPM
26.8 inch tyres
3.31 ratio diff
69.9 mph

2900 RPM
28.8 inch tyres
3.54 ratio diff
70.2 mph

Cheers
Craig

 

Out of personal experience, a larger tire size or better said larger wheel circumference will result in a slower acceleration again...

I used to go from a 215/45R18 summer wheel to a 195/65R15 winter wheel and would notice a HUGE change in acceleration between both. The 15" being at ~1.99m circumference where as the 18" was at ~2.05m circumference. The car would feel a lot more lively on smaller wheels.

It may just be easier to fit a size or two smaller tire walls and gain more that way - with a speedo correction to adjust for the difference in relative wheel speed. The effort of removing the diff and going through all the hassles of alignments and co would really be annoying if it doesn't work out...

 

Regardless of ratios and acceleration, those tires are waaaay too big for my tastes. Like large pizzas delivered in a medium box. Where will these go when you turn or go deep into the suspension?

 

Given your original insight that the car can't legally be driven at Autobahn speeds, and that most of your driving is going to be from 0 to 60mph, why muck about with the tires at all?

Either you want it significantly more lively, or you want it quiet at speed. As long as you stick to the Turbo 400, I think you'll need to pick your poison.

My guess is that if you run oversize tires, your gut will tell you that it doesn't look right and feels too boy racer for a Jaguar.