Need an "off the line" performance improvement
 

1987 XJ-S V12, 5.3L. Cold air inlets have been installed, but no other real performance improvements.

The situation, I live in the city that has the most roundabouts in the US, and the local govt. is building more. So, since most of my driving is done in town, I don't really need a 50 to 70 MPH performance improvement for highway passing, but a 0 to 20 MPH improving for launching into a roundabout...or face sitting and sitting waiting for an opening.

Palm mentions several performance mods, the one which is attractive to me is the torque link...basically tying the engine to the left side of the subframe. It's attractive because it seems the simplest to accomplish, but what Palm doesn't mention is how much "gain" one can expect from this modification.

So...looking for the performance gain I'm seeking, is a torque link worth doing?

Thanks,

John

Hi JCR

Have you considered Starting off in 2nd gear, as that will give you all the acceleration that you can handle and more

Well...actually...no, I have not. But I'll give it a try!

Thanks,

John

Hi John

I've heard its recommended that if you start off in 2nd, its probably best to change up to 3rd before you hit 100mph or whatever your Speed Limit is

I don't think the GM Turbo 400 in the 87 XJS can start off in 2nd gear. As a matter of fact, as far as I know, the GM turbo 400 can be pulled into first gear, but will still shift to second at a certain speed, regardless of gear selector location.
To get a better low end acceleration, a lower rear gear is needed. The 87 XJS most likely has a 2.88:1. A 3.56:1 would give the car better off the line acceleration at the expense of top end speed. The Jag V12 just isn't a "stump puller" as we say here in the south.

The amount of gain is none! it might stop the engine rocking a bit, but that doesn't increase the torque, which is what you want.

To get the low end performance I would change the axle ratio. You will have 2.88 now, options are 3.08, 3.31 or 3.54. 3.54 is probably the most common, you can get that from a later XJS with a 4 speed auto.

Starting in second will reduce the acceleration over starting in first, not increase it. That being said, I don't believe it is possible to start in first with the TH400. The later 6.0 cars with the 4L80E do start in second unless in sport mode, and then they start in first.

It doesn't matter if you select first or second, it still starts in first if you floor it and then goes up to second when you hit about 63mph if you keep your foot in it. Second is good for 97 mph as I recall (vaguely).
I've been caught out a couple of times when tootling along at about 40, floored it and it has changed down into first for a couple of seconds before shifting up to second; almost rear ended a couple of vehicles that way.

Hi Jagboi

As far as I know with the Shifter in 2nd, it does start off in 1st Gear but changes up into 2nd before you can 'Blink' and if you let it can accelerate like a Jet being fired off the deck with a steam catapult

I can tell you from experience lower diff ratio plus 4L60e (700R4) smartens up off the mark performance significantly.

I have 3.58 diff and 4L60e with a stock engine (CAI and 2.5" exhaust from the end of the stock down pipes) and off the mark performance improved significantly. Plus with the tall 4th lower rpm at cruising speed.

To get best acceleration, you want the lowest gear possible. The problem is always getting the mass into motion. So it doesn't help getting off the line in a higher gear.

If you can't get a lower gear, then you want a shorter rear diff ratio. Like that you achieve a lower gearing and increase the acceleration but that then goes against economy and top end speed...

But you also want to keep it to change gear at the most powerful area. Also you want to pull away at a higher rpm to be in the sweet spot. A higher stall speed torque convertor will be a good start. But then you'll be sitting there with your left foot on the brake and right foot on the accelerator pedal at the same time, keeping the revs high and then possibly get away with a wheel chirp and lose acceleration again (spinning wheels don't grip so no traction so no acceleration).

I think, as Jagboi has stated, a shorter ratio (3.54) is a better choice in the US. Especially since fuel is cheap! ;)

I went to a 2500 rpm stall converter on my XJS and was pleased with the improved acceleration. But I never did as you describe. I just mashed the pedal and....whoooosh! Away I went! The higher stall speed allows the engine to rev into a more powerful range more quickly


I went from the 2.88 to a 3.31 in my present V12 and am very happy. Significantly better acceleration and response in all RPM ranges, including off-the-line jump.

Cheers
DD

A lot depends on how much money you want to spend when compared to what you will gain

Changing the Gear Ratios for 'Nipping out at Roundabouts' seems Overkill to me

If I plant the pedal from a standstill it will nearly always spin up the tyres.
So I don't, tyres are expensive.

Absolutely, Steve. If I floor it my car will take off like a scalded cat, tyres scrabbling for grip. Takeoff from a standstill at full chat is fantastic. Maybe the lower BHP of the USA cars, and all their catalytic stuff tones them down a bit?

Yeah, maybe.
I tend to wait until I am rolling until I ladle on the beans; she is 29 years old after all which is like 130 in dog years or something.

Mine is the same as Greg's.

2.88, 1985 HE, and a stomp at the lights will always break it loose.

Mine has the "large" throttle discs, 16CU, updated TPS, NO emissions junk, and slightly "open" exhaust (not audible in the cabin), timed by the "drive time" method, and Efans.

It all makes a difference as a package.

There's a difference of about 35 horsepower. That's a significant amount; more than enough to feel.

Plus, a lot depends on what you're accustomed to driving. Nowadays many modern and very ordinary family cars are very quick and responsive. I'm sure a lot of this comes down to less weight and modern 4-5-6 speed transmissions. In city driving...which is what we're talking about here.... it isn't so much about full throttle power. It's more about the response and acceleration you get at something less than full throttle.

Or, if you're accustomed to USA V8s the V12 can seem a bit lazy in initial, off-the-line power. The V8s typically develop their power at lower RPM ranges than the V12.

Cheers
DD

The biggest difference though is in torque... The "full power non catted" V12 puts out significantly more torque than the US models.

My 89 UK model without cats has by papers 295 bhp (so about 302 ps). It has 433 nm @ 3000 rpm. For those non-metrics 319 lb ft.

The same year US model with cats has by papers 264 bhp (so about 268 ps). It has 377 nm @ 2750 rpm. Non-metric 278 lb ft.

So the emissions gubbins and cats and extra air injection with a slight compression drop result in 56 nm less torque (noticable!) and 31 hp (noticable) less power...

Just for comparission...

Very informative answers...thanks!

So let's say that I go to a 3.54, is this something that is "doable" at home, considering that I can't raise the car up all that much?

If not, roughly what would a job like this end up costing? Never had anything done like this before.

I'm not all that concerned with top end speed. Max legal speeds around here are 80 MPH.

Is there a preferred source for hunting down a 3.54, or just start Goggling?

Thanks!

John