Please, Don’t Put a Chevy In It…

Bryan D
Postmodern Motoring
4 min readMay 27, 2019


Series III XJ6

It’s about time we stopped viewing the Jaguar XJ6 as a good looking but troublesome car best powered by a Chevy small block. Enough time has elapsed that remaining survivors ought to be appreciated for the fine machines they are, engine and all. Granted, in the past it might have made sense to replace Jag’s exotic twin cam six and even more exotic V12 with a cheap, reliable Chevy engine. But at that time, they were simply used cars and the engine swap was a way to keep them on the road. In 2019 Series I,II and III XJ’s have reached the age where originality should take precedence over a quick fix. The saloon’s svelte looks, luxurious trimmings and almost preternatural balance of ride and handling made it as earth shattering as the Lexus LS400 when it was introduced. No small part of that winning character came from its engine, which is one of the all time greats.

3.4 Litre XK Engine, as fitted to the XK120

The XK inline six debuted in the scrumptious XK120 sports car at the 1948 London Motor Show. It would go on to win LeMans several times and was tough enough to power everything from tanks to royal limousines. At the time it was introduced, L-head engines were still quite common; that an upstart little Coventry firm would offer an engine with twin overhead camshafts spinning in an aluminum cylinder head with hemispherical combustion chambers was something to write home about. In original 3.4 litre form the engine was good for 160 bhp at 5100 rpm and could propel the XK120 to 120 mph, making it the fastest production car in the world. Even the planning phases of the XK engine are significant. Coventry was England’s industrial heartland and a target for the Luftwaffe’s bombers. Sir William Lyons, Jaguar’s founder and his engineers would stay up all night on firewatch to protect the factory and discuss plans for postwar cars. It was at these tense meetings that the parameters for the XK engine were decided upon. They did their job well, for the engine would remain in production in various displacements, from 2.4 to 4.2 litres, until 1992.

Fuel injected 4.2 litre XK engine, in a Series III XJ6

The notion that a Chevy engine swap will render a Jag bulletproof is somewhat spurious. The XK engine is not inherently unreliable. It does, however, demand stricter care and feeding than your average American cast iron V8, and it suffered in the hands of owners who treated it like a domestic car. The Jag six has an iron block and an aluminum head, if factory coolant flush protocols aren’t adhered to an electrolytic reaction can occur between the two metals leading to clogged coolant passages, overheating and a damaged cylinder head; an expensive repair on the sophisticated engine. Further, to meet US emissions regulations carburetors were run lean and other emissions modifications contributed to higher engine temperatures, exacerbating any overheating condition. Later, fuel injected versions have fewer problems, but coolant change intervals are still all important. The Borg-Warner automatics fitted to the cars are robust, though it must be admitted don’t shift as smoothly as a Hydramatic. Simply maintaining the car by the book will go a long way towards insuring reliability and longevity without having to replace the engine. The same applies to the V12, though it is constructed entirely of aluminum.

Elegant Series III XJ6 at Malaise Daze LA 2019
Series III XJ6 Vanden Plas
Luxurious Vanden Plas Interior

An XJ is a true thoroughbred, and that applies to what’s under the bonnet as much as its handsome body. A DeLorean would probably be more practical with motive power other than the PRV6, but people tend to leave them as they are, why are they so obsessed with Chevy-ising Jaguars ? My opinion is that it’s a kind of reverse snobbery born of a desire to take the Jag and its owner down a peg. I was driving a 2005 Jaguar XJ8 and someone asked me if I’d put a Chevy 350 in it. I explained that it would be retrograde to replace that car’s alloy, 32 valve engine with something from the ‘50’s, but my interlocutor persisted. Old stereotypes die hard. In today’s world a classic XJ6 or XJ12 should be a treasured possession to be motored in graciously on high days and holy days. You wouldn’t ask The Queen to run the Boston Marathon ? So why would you ask an old Jag to be a Chevy truck ? You wouldn’t treat a Maserati that way. Even with a small block it’s still a semi exotic car with complex suspension and electrical systems. So why not keep your Jag as it left the factory ? In the long run it’s the right thing to do. Remember, the car is only original once.