Jensen SP 1971-73

The Jensen SP (for "Six Pack") has the distinction of being the most powerful car ever to have been made by Jensen Motors. Indeed, at the time of its release in August 1971, the SP was one of the most powerful road cars anywhere in the world. Its specification seemed to turn back the hands of time because in 1971 Mopar engines had been reduced in compression ratio and power output to the point where they were mere shadows of their former selves.

Paradoxically, at the very time Jensens started manufacturing the SP, it was illegal for Chrysler to put the Six-Pack engine into a new car in the United States. Six-Packs were listed briefly in the Mopar catalogs for 1972 but were deleted immediately after the engine failed its emissions test. Jensens had bought up their stock of engines at the end of the production run so, by the time the SP was in production, it was no longer possible to market the car in the States.

The appearance of the SP coincided with the demise of the FF. When Kjell Qvale took over at Jensens in 1970, he decided against pouring any more money into the four-wheel drive project, preferring to divert his resources towards the main goal of mass-producing a sports car for the American market. He decided that the FF would go but he agreed that work on what would become the new flagship for Jensen Motors should continue.

This work had begun in 1970 when a production Interceptor II was fitted with one of the 440ci Six-Pack engines. The chassis used for the trial, number 123/3917, was also fitted with dial-up speed control which was tested as a possible production fitting. After the SP development work was finished, the chassis was lengthened and used for development work on the F-type, the intended but abortive Interceptor replacement. In this guise, the car was nicknamed "Big Bertha". In an unusual move following the collapse of Jensen Motors in 1976, Qvale allowed Chief Engineer Brian Spicer to keep the car in 1976, in recognition of all the good work he had put into this and other projects over the years.

On its release, the SP gained the exclusive distinguishing features of the blue bonnet badge and blue wheel medallions used on the FF (the Interceptor III had red ones as before). The new model also gained a blue SP badge for the radiator grille. A louvered bonnet to aid heat dissipation was fitted, with similar bonnets becoming a no-cost option on later examples of the RHD Interceptor III. The true SP bonnet, not to be confused with the louvered bonnet of the regular Interceptors, has four long rows of louvres which stretch almost the length of the bonnet panel (those for Interceptor IIIs have much shorter rows). Most SPs were further distinguished from their humbler brethren by having vinyl roofs, usually black, although many Interceptor IIIs received a vinyl roof as well.

By 1971, the G-series 383ci engine used in Interceptors, running a compression ratio of 8.7:1, put out about 300 bhp. By comparison, the high performance G and H-series 440 SP engines, running 10.3:1 pistons, were specified at 385 bhp at 4700 rpm. There was a downside, however, and it lay in the decreased reliability and everyday driveability of the car. It took a combination of skilled tuning and regular hard use to keep an SP running well, and many SP owners did not take kindly to their new cars when they began belching smoke, misfiring, stopping dead or refusing to start at all. Across the Atlantic, in the similarly equipped Road Runners and Chargers of the era, these problems were not as prevalent. But in the cramped and overheated engine bays of the Jensens, all sorts of gremlins seemed to breed and fester.

Driving the SP was quite a different experience from piloting the normal Interceptor and Jensens would caution prospective buyers to try both before committing to an SP purchase. Although it was a fine and potent design, the SP needed skilled handling and perhaps some compromise to get the best from it.

Under normal driving conditions, only the centre carburettor was in operation, supplying enough fuel for high-speed cruising and good acceleration. However, the three carburettors were not identical. The centre unit was rated at an airflow of 300 cubic feet per minute while the secondaries were larger 500cfm units. They were vacuum-controlled, opening only as the airflow through the centre unit reached a certain preset level. Additionally, they would only open to the same throttle position as the primary carburettor, so all three could be brought into operation without necessarily flooring the accelerator.

The transition was set to occur "late and slow" with the opening of the secondaries determined by engine speed and their rate of opening set to give a smooth transition. In practice, when the whole Six-Pack did come into operation, the car could be propelled forward from high speed with breathtaking acceleration. The tricky bit was that the car might continue to accelerate even after the driver had lifted off the accelerator — the secondaries could cut in at speed on an uphill run as the primary carburettor began drawing more air, leading to a rather dramatic hillclimb!

Once the driver knew what to expect, however, and once he had mastered the driving technique, the SP could provide exhilarating and almost unmatched performance, and with quite reasonable fuel economy if driven gently. Certainly, the SPs could be more economical than standard four-barrel Interceptors; 16–18 mpg could be obtained with careful use.

Magnificent when working properly — and utter pigs when not — the triple carb engines tended to fall foul of back-yard tinkerers and owners who didn't understand the nature of the beast. The complication of the system made it prone to breakdown and more than a few SPs were converted to Thermoquad carburation by fed-up owners and indeed by the Jensen factory, which offered just such a conversion to those who couldn't otherwise live with the cars. The engines, with their high-compression pistons and camshafts, remained untouched which meant that the cars remained potent machines indeed. In Britain, 5-star fuel had to be used in all SP engines although in later years, when 4-star became the top rating, other adjustments had to be made.

The good news today is that, regardless of what was done to an SP in the past, you can still get all the bits from Mopar Performance to convert it back to the original specification. If you intend going this route, you will need to keep an eye on the fuel situation in your part of the world. The reduction in octane rating and the removal of leaded petrol from most points of sale these days makes it imperative to bear in mind the consequences when rebuilding one of these engines. SPs are rare motor cars and they deserve to be treated right!

During the SP's model life (September 1971 to July 1973), 231 examples were built. One more was completed in October 1973 to special order, making a total of 232. Most were built in RHD, the few LHDs made being sold mainly in Europe. Strangely enough, the last car to be built was ordered to US specifications in all respects, even though the engine was not US-legal. It was even assigned a unique chassis code to reflect this. Why this was so, I have no idea. Ultimately, the car was sold in Britain and I have not heard that it survives.

The chassis codes applicable to all the SPs are as follows:

Herein lies a point of possible confusion. The term “J-series”, as applied to Jensen cars, is usually taken to mean a car bearing a 1973-series Chrysler engine. Coupled with the engine update, J-series Interceptors also featured a range of detail improvements to the body fittings and trim. These body changes naturally carried through to the 138-series SPs, which were described as "J-series" cars. However, it is important to realize that no J-series SPs were fitted with J-series engines, for one simple reason — no such engines existed. Because Chrysler had stopped making SP engines with the H-series model year, all of the so-called “J-series” SPs carried H-series engines. As a further point, included among the 132-series LHD cars is a single European "J-series model" for which a separate type indicator was not assigned.

Whether there was ever an intention to sell SPs in the States is not clear. This may have been the plan in 1970 when developmental work on the car began, but certainly by 1971 it was clear that emissions legislation changes were going to make any such sales impossible from 1972 onwards. As a result, the SP remained essentially a British car, a fact which is borne out by the sales figures:

As flagship vehicles, SPs came loaded with all the goodies so there were really no options. Standard inclusions, even before they became standard on the RHD Interceptor IIIs, were stereo 8-track players (Voxson or Learjet), Sundym glass, air-conditioning and Town & Country air horns. A vinyl roof was almost de rigeur on SPs, but not mandatory — you could have it deleted if you preferred the plain look. In fact, of the 232 SPs made, 185 had a Black vinyl roof, another 15 had it in Tan and two were given a White top. A further three SPs received the Duotone paint option in which the roof was painted a contrasting color to the body.

SPs received a good mix of paint schemes including all of the standard colours then on offer, as well as quite a few specials. The most common paint was Tangerine but there were lots of other colours. In fact, the paint table looks like this:

As a point of interest, the White cars tended to develop unsightly stains around the louvres in the bonnet, upsetting owners and puzzling the factory until it was found that the heat of the engine was causing the rubber hoses to emit a vapour which stained the bonnet as it exited through the louvres. If you look in your parts manual, you'll see a special shroud for the top radiator hose, retrofitted to White SPs (and later to White and Yellow Interceptors with louvered bonnets). White rubber radiator cowl flaps, rather than the standard black ones, were also added to Interceptors. Other cars won't have had them.

Trim on the SP model was more or less standard but there were a couple of oddball leathers: 

Given the small production run and the wide variety of paint, roof and trim colours, there are many uniquely specified SPs. Aside from being rare as a model variant, they also offer an opportunity to find a real one-off. No less than 51 of the SPs are unique in terms of their paint, roof and trim combination. At the other end of the rarity spectrum, the most common combination is Tangerine paint with a Black vinyl roof and Black trim. There were 28 cars done in this combination.

I have relatively few fates for SPs as a whole. Of the 232 produced, I have tracked only about 130 at some point since they left the factory, a mere 60 of them in the last five years. Still, I'm sure there must be more lying around somewhere. I'm always interested to hear from anyone who can cast light on the fates of Jensen cars of all kinds. Contact me at tfmuch@bigpond.com.

The first edition of this article was prepared in 1992 for publication in the Australian Jensen club magazine. It was revised for republication in other club magazines around the world during the 1990s. This text is an update prepared in line with information available after 2000.

©1992–2020 Richard Calver

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