Does anyone know how they measure the 0-60 time of an XJS V12.

Do they measure it when the Car is in Drive or when She's in 2nd Gear.

Would the 0-60 be quicker if the Car is floored while in 2nd Gear?

 

That's an interesting question.

To the best of my knowledge there are no government regulations that mandate how the performance of a car is to be measured. There are some standard measuring processes but I don't know how obligated a manufacturer is in using them.

There are required standards for emissions and fuel economy measurements. But 0-60 times? I don't think so.

We all know how Ferrari will custom tune their cars before allowing anyone to conduct performance test. They do everything possible to get the car to perform at its best. It would not surprise me to find that other manufacturers do the same thing to obtain the best possible numbers.

I doubt that most manufactures just put a car in "Drive" and mash the accelerator. To get the best 0-60 times an automatic transmission would have to be manually shifted to allow maximum RPM's between shifts. They might also use more optimum tires instead of the standard all-season radials that come stock.

 

OB,

If you put the car in "2" and floor the throttle from rest, it will take off in 1st gear anyway. For an optimal 0-60 time it's about trying to get the car to hold 1st gear for the optimal period. So with an early box that doesn't have a "Sport" option, I'm not sure whether it would matter if it was in "D" or "2".

Paul

 

as Paul says

select 2 and the car will start in first and auto shift to 2 when it wants but hold 2 all the way to the red line ,

there is more' performance' this way , and is good for getting off the line, or going up hills

BB

 

On an HE 5.3 with the GM400 box, the quickest way to 60 is to put the stick in "1" and leave it there. It will chnage up when the governor reaches a certain point. As Paul says, the lowest gear for longest is the fastest way. The governor (which is a centrifugal device a bit like Watt's steam engine self governor) can be lightened a bit so that delays the upchange if the 0 to 60 time is all important.

For the GM400 box, leaving it in D and flooring it may make the box change up earlier than 'stick in Ist', as the modulator, I think, may decide it needs to make it upchange depending nupon its setting, manifold vac signal etc. Also the kick down and throttle cable adjustment may mean the box is not getting as determined a 'floor it' signal as you hope!

Lightness is a very big factor, also. Removing the spare wheel, tools etc etc, not too much fuel, all have noticeable effects, as does even the slightest head wind.

Greg

 

Hi Greg Thanks

While I'm not too bothered about the 0-60 time (as such) its more that when I am driving along, at something like 30-50.

And then change down into 2nd, the acceleration is just frightening!

So I was wondering as to what gears they were using in order to clock the 0-60 time.

 

You're not wrong there BB!

When you change down into 2nd She takes off like a Rocket and doesn't want to stop accelerating.

 

Hi Paul

Thanks

I don't pretend to understand the way She changes gear but holding her in second, is an 'out of body experience!'

Just wondering if maybe there is a way of making the gearbox start of in 2nd and stay there, until you decide to change up into 3rd.

 

Maybe they clocked the 0-60 with a Car that was fitted with a Manual box.

Or put Her in first and floored it with an Auto.

 

I do not know the year of your Jag, however some of the older models actually take off in 2nd gear when put in "2". It was meant for winter ice and snow on the roads so you didn't spin the wheels so easy from a dead stop. If you have a "D, S, L" or a "D, 2, 1" your fastest 0 to 60 time will be to start in "1" watch the RPM tach carefully and shift to "2" as soon as you hit redline, it happens very quickly in first gear. You should hit 60 in second gear before redline. This will work with either type of auto transmission. In "D" your shift pattern is "1,2,3,4" fourth gear is an overdrive (some of the newer cars actually have 8 speed gears in "D") No matter what gear you use you should never drive above the redline on your tachometer.

 

There is a fair bit of technique, and things that you wouldn't want to do to your own car. You'd hold the brake, add lots of power until the car is overcoming the brakes and then release the brake and hold the revs to redline in each gear. Very hard on the clutches in the transmission.

Normally, the governor will upshift at 5000 rpm, and it doesn't matter what position the shifter is in. The governor will override the shift lever if the rev exceed 5000. You can change the weights of the governor to change the shift points, and Jaguar would have made a special one to get the last ounce of performance out of the car. For the purposes of getting a good 0-60 number, the engine and transmission has to stay together just the once!

 

My 1983 HE, with the TH400 transmission, will exceed 60 mph in first gear. At wide open throttle (WOT) it shifts from first to second at about 64 mph, at about 5800 rpm. I do not see how the 0-60 time would be any shorter in a different gear either second or manual first. I don't think the second position gives you a second gear start on this model. Ill have to check that out. Starting in second will make for a longer 0-60 time because the engine will take longer to get into the strong part of the power curve. Brake torqueing at a stop will help but I don't think there will be any significant wheelspin due to the limited slip rear end.

 

100% correct. On the GM400 as fitted by the factory, selecting 2nd on the stick starts in first like normal. First gear on the stick will get you past 60 and the box will on its own change into 2nd foot hard down all the while. Second gear on the stick will NEVER change up to 3rd. unless you move the stick to D, whatever the revs on the clock.

 

Agree, mine has a 1989 driveline and is for sure well above 100 kph (62 mph) before changing to second. Third gear comes in at around 175 kph (around 110 mph) as I remember it. I don't do that often...
All in D and on 215/70R15. I actually never tried to use the 1 or 2 on the gear selector to keep it from changing.

 

holding the brake and the gas at the same time isn't hard on the clutches in the transmission, the clutches are only engaged on the shift. it will cause extra heat in fluid threw the torque converter witch is a fluid coupler. it is also possible to damage the vanes in the torque converter doing this. think two fans facing each other one spins the other that is essentially what a torque converter is doing.

 

Jaguar recalibrated the TH400 at or near the time Marelli ignition was introduced...which was mid-1989 model year in the USA.

I can't remember the exact details (somewhere I have a TSB covering the changes) but I think a different valve body and governor were in the mix, with higher shift speeds being the end (and desired) result

Another consideration is that, by now, many of these transmissions have been rebuilt. It's somewhat common for transmission shops to routinely install a mild shift kit....which generally improves longevity (which is why they do it) but can also alter the shift points and other characteristics.

Cheers
DD

 

Which clutches are engaged changes during the shift, but if you are sitting stationary in D, the forward clutch is engaged. There are 3 clutch packs, 2 one way roller clutches and 2 bands in the TH400, and they apply and disengage at various operating modes, but there will be a clutchpack engaged if you are not in N or P.

Here is a teardown video where he walks us through how the TH 400 works:

 

i suppose i worded that wrong but the slip of the clutches is only on the shift sitting in drive the at a stop the slip is in the torque converter not the clutches.

the slip of the clutch is only on gear change to soften the shift once the gear is engaged the clutch is not supposed to slip. the slip as in what you would think of slipping the clutch in a manual transmission is done in the torque converter not in the clutches. so when you hold your foot on the gas and the brake the gear is already applied the clutches should not be slipping. the torque converter will be heating the fluid so yes you can still damage your transmission doing that it is also possible to damage the torque converter but it should not be wearing on the clutches.

 

Indeed. If the clutches or bands slip when holding gas and brake at a stop, then they would likely slip while you're accelerating hard too.... and if that's happening, you already have a failing transmission if it can't handle a brake launch.

That said, I feel this kind of launch isn't such a good idea for a 30+ year old car you think of as a family member.

 

I had a brain fart, of course the clutches need to be able to not slip during the application of power.

Can I blame in on being distracted because I had a bride to drive to her wedding on the weekend in my 1966 S Type, and the day before I had the brake master cylinder fail? Tough to drive the car with no brakes...