Why didn't Jaguar build a 8.0 liter V12 for the street?

I would like to see what the result would have been of an 8.0 liter DOHC engine for the Jaguar XJS. (just “dovetail” two 4.0 liter AJ16 engines (on the same crankshaft) at a 60-degree angle. Talk about fast, long-lived, and long-legged!
Hook this up to a GM ZR1 transmission (gearbox) with eight, or ten speeds with carbon ceramic brakes and a stiff suspension (half again as stiff as what came upon the original XJS with the V12).

Grand Touring to the Nth degree!

Bill White of Christchurch, NZ build an 8-liter version of the Jaguar V12. His objective, really, was to develop power at lower RPM.

In general the amount of power you can get out of an engine is limited by how much air you can flow through the intake valves. If you make the engine bigger but don’t revise the inlet tract, you end up developing the same power, just at a lower RPM. Conversely, improve the intake tract while leaving the displacement alone and the power will increase accordingly, usually by moving the HP peak farther up the graph. And, of course, superchargers and turbochargers work because they push more air through those intakes.

I too wish Jaguar had gone with a larger displacement motor, especially seeing that the V12 block could be opened up to 8 liters and beyond. They didn’t have the resources for such development at the time, and everyone was crying about energy conservation anyway, so they developed the H.E. instead.

This is precisely why the LS12 was created. The engine begins at 9.4 liters and generally develops 750 horsepower and 745 foot-pounds of torque. With an aluminum block, it can weigh less than the Jaguar 6.0-liter V12. Is it any wonder why a modern American engine can, within normal engineering constraints, drop right into an XJ6, XJ12, or XJS-12? These wouldn’t be overburdened as the LS12 (commercially being produced in the US of A) is only 8.5" longer than the 6.2 liter LS2?

A 90-degree V12!

20 char


I remember these engines. The 600 cubic inch engine is one of the smaller Falconer units. (9.832 liters).
These were robust and reliable (even 31 years ago). This would be a perfect fit under the bonnet of an XJS.
Hook it up to a GM 8-speed dual-clutch automatic transmission (gearbox) and arrange for 14" 6-caliper ventilated disc brakes in the front and 4-caliper ventilated steel disc brakes in the rear with a set of four magnetorheological shock absorbers at all four corners with a suspension that is at least 75% again as firm as the original suspension springs.
You can see James Bond driving a car that is this fast. (Top speed of about 220 mph).

I’m guessing the Falconer is based on a SBC while the LS12 is based on the LS?

Either way, if you’ve got that much room in the engine bay, you could go with a 12-banger or you could go with an LS V8 with a couple of turbos. I’m betting the latter would perform better. The 12 might have a sweeter sound, though. It’d be interesting to hear what a 90-degree V12 sounds like.

Jaguar didn’t build an 8 litre version of the v12 because there was no economic case for it. After 1973, there wasn’t even much of a case for the existing 5 litre engine.

kind regards

Looks like big block Chevrolet architecture.

This isn’t the year I graduated from High School and joined the Navy. Bluntly, we have enough petroleum to last 500 years (not counting plant-based gasoline(s) the Nazis were producing more than 70 years ago). The modern gasoline (Petrol) V12 has the ability to run on 6 cylinders at speeds between 55 and 70 mph. This can yield a nice 22 (US) mpg until you “put your foot into it”.
These can serve in both realms. You can use a 23.1 USG tank at 22.2 miles per gallon (510 miles per tankful of fuel) or you can run it at 12 miles per gallon (US), yielding 276 miles per tankful of premium fuel.
YMMV (and it usually does).


As far as the 90 degree v12, Ford did it first with the GT90 concept back in the 90s, and Jan Baker more recently with a pair of 302 blocks he cut and welded together:

Baker runs it with dual distributors and each bank has inline 6 timing. Looks like he is currently over 800 hp without doing anything particularly special with the build, and he thinks with a more modern system he can break 1,000 horses.

Here’s a YouTube video of it running:

The GT90 was bought by a private collector, but he posted a video of it running:

Gee, neither one of those sound particularly appealing.

This is precisely how the LS 12 runs. Like two perfectly mated in-line six-cylinder engines. This is what makes it run so well. No counterbalancing weights. Perfect revving to 5500 rpm with torque by the bowlful! Think about a six-cylinder that winds up so hard that a tall transmission (gearbox) can hustle an XJS or XJ6L so hard that it can effortlessly propel it to over 220 mph.
This could handily scare the hell out of a competitor in a Corvette C8.

Yeah, but I don’t particularly relish the idea of spending $30k+ on an engine.

Just augment that relish with some Sauerkraut. With proper care, you would discover that the LS12 could (and likely would) provide you with 400,000 miles, (or more) of yeoman service. This would last as long, or longer than any Toyota Camry on the road today.
Think about it.

I’d rather spend that money developing a cast resin version of the jag engine. The open deck design is ideal for it. Forged Composite would make for a 20%-30% lighter block. You could apply this to con-rods, pistons, and crank as well. Have the bearing surfaces be ceramic (and the piston cap). Imagine how much lighter that crank would be…

That’s a pipe dream I’m sure, but it’s just as realistic as me being able to drop the cash on that LS12 (and more fun for me).

If you could build the Jaguar engine for the same amount, why not? I wondered what a concept engine would be like with two 243.5 cubic inches (487 cubic inches) mated into one V12 Jaguar engine from two 4.0 Liter AJ16 engines joined by a single crankshaft. It could have a service life of 400,000+miles and still be serviceable. It would rev freely and produce gobs of torque without employing either a turbocharger or a supercharger. Simplicity equaling longevity… what a novel idea!

I’m curious about this notion of a 400,000 mile engine. If it’s 400,000 miles with no need to rebuild it in some fashion, I don’t see this in a street car other than rare one-off scenarios.

I don’t expect any LS motor, factory or aftermarket, to last 400,000 miles without needing some work on the mechanicals somewhere. The people who achieve these mileages have engines with no design weaknesses or faults that affect longevity, and usually drive them long distances at steady rates of speed (RPM.)

I doubt even a vintage Volvo or Mercedes motor goes 400,000 miles in predominant stop and go city driving. High longevity is obtained by getting on the freeway and sustaining low RPMs. Each revolution represents one cycle of engine wear. The more RPM’s, the more wear. That’s the main factor for engine life. Big commercial diesels last forever because they have low RPM limits and their drivers are trained to keep them in an ideal RPM range.


I agree. This is why most Lincoln Town Car owners have long-lived engines. They set them at a given low rpm and stay there. With good maintenance, engines are built to run for a long time. It isn’t like the old days (pre-1994) when 100,000 miles and the engine is worn out. A twelve-cylinder engine that is steadily run on interstate highways with reasonably frequent synthetic oil changes, air filter changes, and tune-ups can easily run 300,000+ miles. (I have seen a Lincoln Town Car Signature L run for 405,000 miles. It wasn’t perfect, but it was still running well).
I know that it can be routinely done.

Not really. Diesel engines last forever because diesel fuel is a lubricant.