Hi there I have a 1988 Jag XJSC with the V-12 and wondered if there were some easy (not to expensive) modifications that I could do to boost the power of my new ride?
I am getting a shift kit installed and figure maybe to go the next step.
Thanks, Stephen

 

Welcome to the forum Stephen,

I've moved your question from General Tech Help to XJS forum. Members here with the same model will be able to help.

Please follow this link New Member Area - Intro a MUST - Jaguar Forums - Jaguar Enthusiasts Forum to the New Member Area - Intro a MUST forum and post some information about yourself and your vehicle for all members to see. In return you'll get a proper welcome and some useful advice about posting to the forum.

Graham

 

 

What budget do you have? Hundreds? Thousands?

AJ6 Engineering offers some performance mods. I can vouch for the Super-Enhanced ECU working as described. About $500 or so. Haven't tried the other offerings

aj6 engineering | For the Jaguar enthusiast

Low restriction exhaust and air intake systems help a bit. Not a lot.

As LongJohn alludes, the transmission and gearing really hold the car back....worsened by the lazy calibration of the transmission. So, as budget modifications go, you're on the right track with a shift kit.

On my V12 I replaced the 2.88 ratio differential with a 3.31 ratio. This really woke things up. The down-side is high engine RPM at highway speeds. I don't mind as I don't do much travelling these days. A 4L60 transmission, as mentioned, would solve that problem since it has an overdrive 4th gear.

First things first, though. Are you sure your engine is even operating up-to-snuff without mods? It's common for these cars to suffer inoperative vacuum and mechanical advance systems in the distributor, partially clogged injectors, mis-adjusted throttles, etc.

Cheers
DD

 

 

i use a rear end gear ratio of 3.73 LSD, and a 700R4 overdrive trans,(same as 4L60 use electronic), acceleration is very good , and when cruising in overdrive , my FINAL ratio is 2.60, very good combination!

i agree the thing with early Jag V12 cars was/is the gearing that limited acelleration performance ,contrary to most opinions, the 5.3L V12 does NOT have a lot of torque, so you make the torque at the TIRES thru gearing!
ron

 

4l60 or 700r4 would be a nice upgrade over a turbo 400 as mentioned much lower first gear as well as over drive. combine that with a slightly looser torque converter would really wake a v12 jag up off the line. this would probably be the best bang for your buck. combine that with a 3.54 rear from a 4.0L facelift convertible. it would certainly be a very noticeable improvement.

 

Having wasted...*cough*..enjoyed spending thousands on racing spec suspension and other goodies I've yet to spend the money on a 4spd or gear vendors 7spd auto conversion so I can't comment on that, yet.

But as for easy performance mods you can try my approach.

Cut the inside face of the intake boxes off and refit with a mesh or othet suitable 'face'

Buy K&N filters ignore stock paper

Delete all needless weight and vac line bs
- smog pump & pipes GONE
- Fuel Charcol Canister GONE

Fully gut Cats
Delete all secondary Cats / Mufflers etc (you want straight pipes until the very rear of the car)

Suggest you retain the final resonators under the trunk but see about adding a second exhaust tip to each to improve flow without sounding like a lawn mower.

 

just guessin, ALL those mods together may give maybe 5% more power!!

not enough(my opinion) for the work and money involved!

all the best cams, and pistons , and injectors, exhaust systems , air filters, ETC, might give 40%, and a ton of money!

some thing to think about, a good super/turbo system, at just 1 bar pressure,14.7 psi measured in the inlet manifold , would give roughly a 100% increase in power and TORQUE!
also with careful money spent be less cost than all the other mods!

ron

 

the problem with adding boost to a jaguar engine is you will need to rebuild the engine first. compression ratio is already on the high side for pump gas adding boost is not going to help that, you will need to rebuild the engine with lower compression pistons first. i originally was looking at adding a turbo to my 4.0L 6 but getting pistons to bring the compression ratio down was going to be expensive. and then you still have to take care of fuel management. trying to control fuel management with boost threw the jaguar ecm would be quite difficult, your best option would be to convert to megasquirt. your also going to need injectors that can flow the required amount of fuel. not quite as easy as just building a turbo manifold and oil lines ect. or just working out mounting a supercharger.

 

how much power do you want??
how much money do you want to spend??

nobody has ever made a Jag V12 make great power without spending lots of money, the engine just doesnt lend itself to modifications , not like USA V8s!

ron

 

yeah that's why i went small block chevy, if you wan't big horsepower its the way to go im conservatively pushing over 500 hp on mine now, you can check out my thread on my latest changes here https://www.jaguarforums.com/forum/x...intake-209285/

if your not looking for big gains but want to make the car get up and go a bit better 700r4 and some gears would really wake it up and feel much spunkyer there is probably a reasonable obtainable 30-50 horsepower to be had in your jag engine. but if your after more than that i would really really be looking at a engine swap.

 

Ha! that is because it was already engineered to sublime perfection, and 40 years of technological improvements can't even squeeze much more out of her majesty's 12 cylinders.

I've been heading down this road...more power and exponential costs. I may just stop when I get to a 4spd and extruded manifolds.

The 4spd is the same from the XJ6 right? I have one (xj6) sitting in a barn I'd like better gears if it's relatively cheap as in under $3000.

 

EZ, of course get rid of the antique Jag ECU(altho maybe the Japanese Denso might be capable?).

but no matter what boost you make, a M/W50 kit would be required,fully adjustable Fuel/ignition! some Megasqirt have available in them, i kinda like a seperate kit tho!
with hi compression detonation can be controlled with a proper tuneup,fact is all modified performance engines key, is the tuning ,tune ,tune!

and i dont have any problems with lumping a Jaguar, BUT V8s dont come with 12 cylinders, its about talking the walk


when people see 12 cylinders script on the side!
ron

 

if i remember right the compression ratio on the jag engines is in the 10.5-11:1 range. you can only squeeze a certain volume of air/fuel into a certain amount of space without the fuel auto igniting working within the limits of commonly available pump gas. no matter how you tune it its not reasonable to run that compression ratio on the street with 15lbs of boost. you could probably get a decent amount of boost in it on e85. but that would require a even more complex fuel management system. how the engine is tuned is critical on any performance engines but the more you push the limit the smaller your safe window gets. i think running boost on a jag engine without first lowering the compression ratio would not really be reasonable on a street car. a track only car on 118 octane would be a bit different story.

best bang for the buck would probably be to spray it with a wet nitrous kit as far as horsepower goes.

a 4l60-700r4 swap and rear gears would be the best bang for the buck that doesn't have to be re-filled.

mine was a 6cyl from the factory so i gained 2 by lumping it ;-), i gotta admit i would feel worse about lumping a v12 car.

 

I have to agree.

Personally, I can't get excited (and certainly don't have the pockets) to do any more than that with mine....and I'm an unashamed fan of the V12 ! For real power gains, though, it just isn't a practical starting point for average guys like me.

Actually, I'll go a step further and say that if I wanted tons more power I'd likely take the easy route and simply buy a car that gives the level of power/performance I'm after. There are plenty on the market...including later model Jags....for reasonable money.

As an aside I should add that a 30-50 hp increase is not insignificant. That's more than enough to really feel.

Cheers
DD

Cheers
DD

 

I can vouch for the 4L60 (700R4) and lower diff.

I have a 4L60e and 3.58 gears behind a stock 5.3L and the difference was significant and the final drive in 4th is 2.50 giving relaxed cruising.

Even this swap was not inexpensive I think it cost me $4000 total and I bought the 3.58 LSD for $80, most of the cost was in the transmission and TCU. Plus there is no kit available here in Aus so I had to fabricate all the mounting.

 

40-50hp will most defiantly be a noticeable difference.

i can honestly say that i didn't buy my jag with the intention of modifying it to the extent that i have, i was never really happy with the power and acceleration the jag offered its more of a long distance at high speed cruiser than an acceleration car. but there were other things i really liked about the car, i liked the styling the suspension works well, tracks high speed corners really well, basically i really like the car and the chassis and uniqueness of the car but was never satisfied with the power. something about having to hang your head in shame if a kid pulls up next to you in there moms Honda at a stoplight and revs his engine cause you know that he would probably beat you off the line when your in a jaguar just doesn't sit right...lol

then i started having lots of engine management related problems with the car making it unreliable, and i already had a lot of chevy v8 parts laying around including a good core engine. so i decided to swap it. but i felt like if your going to swap it there is no sense in doing it unless your going to go all out. so i set out some goals for the car, i wanted to be able to drive it anywhere at any time i wanted to build the engine to be fairly well mannered, but make enough power to be in the relm of super car fast. (mid to low 11's in the 1/4 @ 120-130mph) then i figured out what my horsepower goal would need to be to reach that figured id need at least 500-hp or more to get into the 11's. looked at what i had laying around already and figured i could build a supercharged engine that would meet those goals rather budget friendly. but knowing that there was costs in building the engine and swapping the car ect that would eat up the majority of the budget, so i built the engine and put it in the car n/a and ran it that way for a couple years. while i sorted out any bugs with the swap and just now finally added the boost.

i don't have a ton of money in my car but iv got a few cents in it

bought the car for $2500
spent about another $800 bucks on paint and body (did the work myself)
spent about $1000 maintaining and fixing issues that would pop up with the original drive train
about $1500 in building the chevy long block (had a fair bit of parts already) and did the labor myself
about another $1000 getting the engine in the car odds and ends ect.
another $250 on a junkyard 700r4
then another $2300 getting another 700r4 rated for 650hp built after i blew up the first 700r4
then the hei ignition wasn't up to the task even n/a so upgraded to msd igntion for $400
then another $2000 for the super charger setup
probably pretty easily another $1000 in other things im not thinking of at the moment

of course this is spread out over the probably 5 years iv had the car.

its more money than the car is worth but i like it don't have any plans of ever selling it, and its still not that much money for a car that preforms at that level and there is a certain level of satisfaction on building something a bit different rather than just buying a car.

its also not the first car iv ever done lots of modifications to and it wont be the last but its defiantly my favorite.

 

EZ ,have you read post#6 on this thread, i did those mods back in 1994??

trans from 1992 Chevy Van 2500, shift calibrations are much better for performance,last yr for cable operated TV HD units. 10" converter light weight rotation!



rear gears from Jeep 3.73,, custom built LSD ,Dana 44, with Corvette clutch discs!

this not my 1st rodeo thank you.
ron

 

EZ is that a GM Blower a 4-71 model diesel ?
ron