The Lucas Petrol Injection System

In 1968 Triumph introduced the TR-5. The TR-5 retained the bodywork and the four-wheel independent suspension of the TR-4A, but replaced the 105 horsepower carbureted 2.2 liter 4 cylinder engine with a 150 horsepower fuel injected 2.5 liter inline 6 cylinder engine. While the TR-5 and its fuel injected successor the TR-6 PI got rave reviews in the rest of the world, here in North America we had to be content with the carbureted TR-250 and TR-6 which, while smoother, put out approximately the same power as their 4 cylinder predecessors.

The TR-5 used the Lucas MK II mechanical petrol injection system, a distinction it shares with a couple of larger Triumph 2500 cars, most TR-6s sold outside of North America, and the front engined Maserati 3500GTI. No other production car used this system, which was originally developed for a military application. And no, the Maserati did not use a Triumph engine. It used a 250 horsepower 3.7 liter alloy straight 6 with dual overhead cams.

The injection system was calibrated for its application. While TR-5s and early TR-6 cars produced 150 horsepower, later cars produced only 125 horsepower due to a milder camshaft and different manifolds.

The Lucas Petrol Injection system fig. 1 consists of the intake manifolds, fuel injectors and plumbing, fuel metering distributor and control unit, a high pressure fuel pump with filter and pressure regulator, and throttle and enrichment (choke) controls.

The Lucas PI intake manifolds fig. 2 house a throttle butterfly and an injector for each cylinder. All six butterflies are connected to the throttle cable through a complex linkage which allows each pair of cylinders (there are three separate manifold castings) to have its idle stop and travel adjusted independently. The linkage also provides a master idle speed adjustment screw and a fast idle adjust tied to the cold start cable. The adjustments make it possible to synchronize the airflow to all six cylinders throughout the throttle range.

It should surprise no one that the Prince of Darkness chose the one electrical part of the system to cause the most problems. The Lucas electric fuel pump used in the system is notorious for overheating and has been known to vapor lock in warm weather. The standard solution is to replace the Lucas pump with a more reliable Bosch unit, which gets mounted in the left rear wheel well.

High pressure (105psi) fuel from the pump is fed into the metering distributor fig. 3, which sits between the ignition distributor and the camshaft. A cylindrical rotor inside the metering distributor is driven off the camshaft and has ‘valve holes’ in it. When one of these holes lines up with one of the six injector pipe ports it allows a short high pressure pulse to open the appropriate spring loaded injector fig. 4 and spray fuel down the appropriate intake port.

In the center of the metering distributor’s rotor is a metering shuttle fig. 5 which is driven back and forth by the fuel pulses. The distance the shuttle travels determines the amount of fuel that gets injected. The metering control unit fig. 6 used manifold vacuum to control the position of the moving stop. A set of adjustable control springs, a wide-open-throttle mixture control, and an adjustable sliding cam they call a datum track allows the fuel mixture curve to be adjusted to taste. In a modern engine this unit would be replaced by a MAP sensor (manifold absolute pressure), a small computer, and electrically operated injectors.

Instead of a choke cable the PI equipped car has an enrichment control cable which serves a similar function during cold starts. Pulling the cable out opens the throttle butterflies slightly and also pulls a lever on the metering unit to temporarily provide a rich mixture.

What’s it like to drive? A TR-6 with triple Webers and the appropriate engine modifications will probably produce about the same amount of horsepower, but the PI system has a wider power band. Contemporary magazine road tests (your mileage may vary) gave the PI car a 0-60 time of about 8 seconds, compared to the North American car’s 11. Additional performance modifications can shave at least a second off of that time.

You can convert a carbureted TR-250 or TR-6 to run the Lucas PI system. You will need to find the parts first. PI components are no longer available new, and while replacement parts are available from suppliers in the U.K., most of the parts are supplied on what they call a ‘strict’ exchange basis- they want the old core back for reconditioning. The injection system on my PI TR-6 originally came from a German market parts car. Recent discussions on the internet triumph mailing list http://www.team.net/sol suggest that salvage yards in Australia may be a good source for complete used setups since Triumph sold a lot of their 2500 PI sedans down under. If you do find a source for the parts, buy extras! The extra cost of a set of spare injectors is easy to justify if you factor in the cost of an emergency FedEx or DHL shipment from England.

To take full advantage of the PI system you will need to obtain a cylinder head from a post 1972 TR-6, mill it to increase the compression ration to 9.5:1, and change to a PI camshaft or equivalent. The intake port spacing is different on the early heads and the PI manifolds will not fit. The compression ration of the last US market cars was down to 7.75:1, and the camshafts used in North American cars were milder than those used on PI cars. You might also want to modify your fuel tank or replace it with a baffled tank; if you don’t, the fuel pump will starve during hard cornering if there is less than about ¼ tank.

There has been some discussion in English magazines that the injection system is not compatible with unleaded fuel, and some major U.K. parts suppliers offer ‘unleaded compatible’ metering units at considerable cost. To date the Triumph 2000 Register has seen no evidence that unleaded fuel causes problems. The system in my car has run close to 60,000 miles on unleaded fuel and has experienced only normal wear. My valve seats were toast before my recent rebuild but the injection system was fine.

While a few states such as California have a visual inspection of emissions equipment that make it difficult to register a PI car in their jurisdiction, PI cars actually run quite cleanly despite the absence of emissions tweaked Stromberg carburetors. My car has passed emissions testing in both Pennsylvania and Illinois without difficulty.

The TR-6 workshop manual published by Robert Bentley has a chapter devoted to maintaining the PI system. The Triumph 2000 Register offers reprints of the Lucas Petrol Injection Mk II manual for only £5, which describes the calibration procedure. Their web site also contains valuable tips on running a PI car http://www.kvaleberg.com/t2000.html. The dealer service manual is out of print but the same information is covered in the other two manuals.

By Eric Quackenbush

Copyright © 2000 Illinois Sports Owners Association