Hi all. I recently became the owner of a 1986 Jaguar XJS V12. I purchased it to use the front end under my 1977 Chevrolet C10 pickup that I'm building and as I've not decided on an engine yet I wonder about the potential of the HE V12. I haven't delved too deep into it yet but what I do know is it hasn't run in years, has 89k miles on it, was parked up because the fuel pump failed, and amazingly still cranks! Given what the engine is to make only 265hp makes me ask what is holding it back? It has a large bore, short stroke, and high compression. Are the valves too small? the cams too small? the heads flow poorly? the engine management underdone? the injectors? the throttle body? Where is it being restricted? My thought is to put the engine in front of an overdrive toploader. I'll need to build a wiring harness for the engine. Is it worth using the factory engine management (I know I'll have to get it modified for the manual transmission) or building a MegaSquirt system for it? I know that's been done. What can I expect by way of power, torque, and drivability with a MegaSquirt system, larger throttle bodies (and injectors if necessary), and long tube headers? Do you need to change the cams in these engines? (I come from the world of Chevy engines were little more than an exhaust and and a cam wakes an engine up) What is this engine capable of with just a few simple modifications?

 

define simple

 

The engine has some capacity... perhaps 450HP, but nothing inexpensive and off the shelf like a Chevy.

Those of us who seek HP in the V12 do so for originality, the sound, the torque, coolness factor, etc.

The HE heads have a shrouded exhaust valve pocket and that's a major factor in preventing breathing. It's good for lean burn, fuel mileage, and smoothness, but not so much for high HP.

That being said, 450HP + has come out of spending $$$ on the HE version. Flat heads have even more HP potential. The unobtainable race versions of the flat-heads (TWR & Group 44) could potentially produce 850HP or more. None of this mentions the other expensive changes to the engine that goes along with that HP.

If you do put the V12 in, it'll be cool, beautiful sounding, heavy, and have lots of torque though.

 

Further to FG's points, on a stock 5.3 litre HE engine with only bolt-on mods the maximum you can get is about 320 BHP doing as follows:

• Large throttle body, modded ECU and high flow air filters with larger airbox intakes. AJ6 Engineering in the UK make the kit
• Free-er flowing exhaust (AJ6 again)
• Rig up some ram-type intakes to the airboxes helps a higher revs quite a bit
• Advance the ignition a touch
• Remove all emissions stuff. There is no gain to speak of from ditching the OEM ignition and ECU

The later 6 litre HE is a touch more powerful, but not much if any, more than a decent 5.3. Individual engines do vary; the harder they are worked the better they go. On a flat-head (pre HE engine) you can get substantially more power because the heads and valves flow better; but at the expanse of far worse fuel economy. After that, far bigger BHP numbers can be gained by raising the capacity with bigger pistons and liners on the flat head. All the parts are made in the UK, but you are getting into big expense.

 

OK, I am heading down this road now and luckily with the help of a mate who has built more high HP V12 Jaguar engines than I've had hot dinners. The last 6.7L HE he built made just over 400hp and 600ft/lb, it twisted the 1/2 shafts on the dyno.

Biggest restrictions in the HE are
The pocketed exhaust valve restricts the size of the inlet valve.
The exhaust port is tight.
The spark plug is not in the centre of the bore.

That being said my 6.7L HE will make around 400hp and north of 550ft/lb. What the V12 does have is a wide torque band, especially in the later Marelli ignition cars.

Intake manifolds (mainly the plenum) are a restriction.

MegaSquirt will definitely help with tuning.

 

If you don't want to answer I understand....but what will you be spending, all-in ?

Cheers
DD

 

 

Don't forget there are 2 versions as well, the USA emissions trim was 262 hp, and European market cars were 299 hp. The main difference was compression, the European cars were 12.5:1, North American cars were 11.5:1 compression.

You have to consider it in historical context too, I believe at the time the HE came out (1982) it was the most powerful production car you could buy. A Ford 5.8 at the time put out 132 hp. Obviously things have moved on, but it was a remarkable achievement at the time for a company with so few engineering resources.

 

lets remember back 20yrs before Jag HE , american engines were putting out over 400HP++.HP

1963.

 

Not in normal production cars, though.

 

Great days

But don't forget that those were rated under the old 'gross horsepower' system.

Cheers
DD

 

Not at all !

Many manufacturers had 400-450hp V8s on their option lists. Getting a rip-snortin' engine like that in your Impala, Galaxie, Satellite, Corvette, etc was just a matter of marking the right box on the the order sheet.

In the grand scheme of things only a fraction of total production was so equipped, as some of those engine options were expensive and sometimes not pleasant to live with: lumpy camshafts, very poor fuel economy, noisy, and so forth. And expensive to insure. For example, of about 500,000 1970 Chevelles built only about 4500 were ordered with the 450hp engine.

Cheers
DD

 

Thanks for the correction Doug. What sort of DIN BHP did they give?

 

What's holding it back.... The heads. The valve arrangement is not idea and the exhaust port is too long. The long exhaust port keeps too much heat in the head and as HP builds so does the heat transferred from the exhaust port to the head. For those with the funds there are coatings that can be used to insulate the port but they dont last and are for frequently rebuilt racing engines.

In any case...I think you need to consider HP/weight/price. This is where most V12's fail. No matter who makes them they are heavy and expensive to modify. I've presently own many V12's so I know from experience.

At about 800Lbs dressed and filled the V12 produces 300HP with bolt ons. Not great. Conversely...modern "American Iron" which is mostly Aluminum these days can produce 400HP on a modest budget while weighing in at hundreds of pounds less.

Furthermore when people say the... "V12" has a wide torque band" what they are really saying is that the heads are so poorly designed that the engine peaks early and torque doesn't climb with RPM as it could if the heads would flow better.

For a vehicle to "feel" fast or exciting... torque must accelerate along with RPM. If it doesnt the "fun factor" is not there. There is an mathematical reason for this... As vehicle speed increases so does all form of drag, to overcome this, torque must increase with RPM to maintain the delta in acceleration. This is why a 3.0L Ferrari V8 that revs and climbs the HP ladder to 8000RPM "feels" more fun and exciting than a "torque monster" that is flat after 3500RPM.

Same goes for the SBC world. Many people have had fond experiences with long legged 327's that "rev to the moon", cheap and fun to build and drive but it's the high dollar big blocks with light weight everything that ultimately win races.

Building a high HP V12 is a emotional and more romantic experience for those with lots of time. And for some that's the attraction as it is for me with my AM v12/T56 450HP project in the werks.

"American Aluminum" is the rational choice for value and all out performance. And for those who want a total package, there is nothing that makes more HP/dollar with perfect smoothness than a Turbo 6.0L LS.
- 600HP / 20 MPG and perfect manners as a starting point.

 

About half the gross number.

 

.

so true but you could go to any local car dealer, go over the option list with sales person , pick out what you want, and 2/4 weeks the car was there for pick up!

and those 450hp Chevelles would outrun ANY Jaguar made in that era, set up properly even today!

 

Has anyone here explored options for using more efficient injectors? If I recall there was a series (year / whatever) of Mustang who's injectors fit the XJS perfectly with multiple smaller spray apertures as opposed to the V12's turkey baster ones.

 

the most that can be expected from replacing injectors spray pattern is just maybe 2%.

hardly worth the effort, it only makes a finer spray, still the same amount of fuel volume!

if making a Jag V12 more power it would have been done already!

i use HONDA Denso injectors in my V12, runs great , with adjustable ECU, that could be one of the best mods you can do!

 

Is the adjustable ecu part relevant? I probably have to buy or refurb injectors on my car. Had planned to send the ECU to AJ6 Engineering one day but not right at the moment.

 

VAN , engine tuning is by far the best kept knowledge of a HI-PERFORMANCE engine,ALL of them,all brands!