History
& Genesis

Two rivals, common parts

The story of the PRV begins in 1966, when Peugeot and Renault signed an agreement to design common mechanical components. From this collaboration was born La Compagnie Française de Mécanique — more commonly La Française de Mécanique — founded in 1969 at Douvrin, near Lens, in the north of France.

It is from the name Douvrin that the PRV engines would often be designated — not forgetting that the plant also produced a family of inline four-cylinder engines, found for example on the Citroën CX and Peugeot 505.

But the original ambition reached far higher. As early as 1966, Renault planned a large French-style luxury saloon — Project H — designed to dethrone the Citroën DS. To power it, Peugeot and Renault jointly developed a 90° V8 of 3,550 cc fed by two twin-barrel carburettors.

Deemed too costly for too uncertain a market, Project H — the saloon — was abandoned as early as July 1967; a single running example survives today, kept by Renault Classic. Its 90° V8, however, would not be buried with it: the fate of that unusual architecture would be settled a few years later.

The V8 that became a V6

In 1971, Volvo joined Peugeot and Renault to create the PRV group, each of the three marques acquiring equal shares. The Douvrin programme was designed as “modular”: V6s and V8s built on the same line with a maximum of common parts, on a set schedule — the V6 for 1974, the V8 for 1976 — the whole family structured around the 90° architecture inherited from Project H.

But the 1973 oil crisis decided otherwise: such a thirsty engine became undesirable in an uncertain Europe, and the V8 version was suspended — it would never see the light of day. The V6 arrived alone, as planned: the V8 minus two cylinders — a quarter of its displacement — the first PRV, the 2,664 cc of the first generation.

From the aborted V8, the V6 kept the 90° architecture — an unusual choice.

The norm was a 60° vee, which makes the engine more compact. This 90° angle partly explains the engine's early bad reputation: with non-offset crankpins, firing intervals alternated between 90° and 150° — the first PRV V6 did not run evenly. Fixing it was no mystery, but the cure — an offset-crankpin crankshaft — required forged steel and far more complex machining; for economy's sake the trio passed on it at launch, and the production crankshaft was even cast rather than forged, as custom dictated.

Douvrin starts, the world follows

Development was accelerated by Renault's need for an engine for its new flagship, the R30. At Douvrin, the production lines began to be assembled in summer 1973 and everything was operational by January 1974. The first PRV engine officially fitted under a bonnet did so in a Volvo 264, in October 1974.

Then everything moved fast: before the end of 1975, at least five different PRV V6 models were on sale. Business going well, some V6s crossed the Atlantic, even nesting under the bonnet of exotic machines such as the DMC DeLorean.

The second generation

At the end of 1985, the Renault 25 inaugurated the first turbocharged V6 — the famous R25 V6 Turbo. With it appeared the second generation of the PRV V6, defined by the adoption of an offset-crankpin crankshaft that finally made it run far more smoothly, as one is entitled to expect from a V6.

This new generation also generalised electronic injection, consigning carburettors and mechanical injection systems to the old boxes. The magic turbo quickly equipped the flagship versions of several marques, notably at Alpine and Venturi.

The 3.0-litre comes of age

The V6 now undergoes its deepest overhaul since the arrival of the turbo. Displacement is taken to 2963 cc and the block gains a balance shaft, which absorbs the residual vibration of a 90-degree V6, along with hydraulic tappets in place of the rockers — an end to valve-clearance adjustment. Less spectacular than a turbocharger, these are the refinements of an engine that has stopped proving itself and is now being perfected.

Two ways of gaining power

The same year, the PRV takes two divergent routes to more power. At Renault, the 25 V6 Turbo goes from 182 to 205 bhp — and behind those 23 horsepower lies anything but a chip: a deep re-engineering entrusted to the BEREX design office, reworking the block, the oil pump, the bearing reinforcements, the compression ratio, the turbocharger and the engine management, right down to the catalytic converter. At PSA the answer is the opposite: no turbo at all, but four valves per cylinder.

In 1990 the first 24-valve cylinder heads arrived on the 3.0-litre block — at Citroën with the XM and at Peugeot with the 605. Renault would adopt this technology only much later, staying faithful to the turbo route or to more sensible 12-valve versions.

Four valves per cylinder mean, first of all, far better breathing: the naturally-aspirated 3.0-litre (PSA code ZPJ4) climbs to 200 bhp. But its real singularity lies in its intake — the ACAV, for Admission à Caractéristiques Acoustiques Variables (variable-acoustic intake). Instead of a fixed manifold, a plenum (a “lung”) and butterfly valves change the effective length of the runners with engine speed: below about 5200 rpm the long runners exploit the resonance of the intake pressure waves to swell torque; higher up, the geometry opens to free top-end power. Enough to marry flexibility and reach — nearly 80% of the torque on tap from low revs — on a naturally-aspirated V6.

Which was perhaps no bad thing, as in their early days the 24-valve versions suffered from numerous camshaft problems and chaotic fine-tuning.

The Alpine A610

In March 1991, at the Geneva show, Alpine unveils the A610. Where the GTA used the 2.5-litre turbo Z7U, the newcomer moves to the 3.0-litre block: the single-turbo Z7X 744 with electronic injection, 250 bhp — the most powerful of all the factory Alpines with a PRV engine, and the only supercharged version of the 3.0-litre in the Renault range.

It is on this engine that the exhaust valves are filled with sodium: the metal melts as the engine runs and carries heat away from the valve head towards the stem, where it can be dissipated. A solution reserved for the most violent thermal stress — the one imposed by a turbocharger on a 3.0-litre.

Euro 1 and the catalytic converter

At the turn of the 1990s, the tightening of emissions rules — culminating in the Euro 1 standard, mandatory for every new car from 1993 — made the catalytic converter compulsory. For the PRV V6 the bill was sometimes steep: choked by the depollution gear, several engines lost power. The Alpine GTA V6 Turbo dropped from 200 to 185 bhp on the French market, and the Renault 25's naturally-aspirated injected V6 fell from 160 to 153 bhp once catalysed.

But the catalytic converter did not wait for 1993: from the mid-1980s several markets already demanded it. Switzerland mandated it in 1986, followed by Sweden and Austria, while Germany encouraged it through tax breaks — all made possible by the spread of unleaded petrol, since lead destroys the catalyst. To keep selling in those countries, manufacturers had to offer catalysed, less powerful versions years before the European deadline: the catalysed Alpine GTA V6 Turbo appeared as early as 1987 for Switzerland, before being imposed on the French market in 1990 — three years ahead of Euro 1.

The catalytic converter forced another, less visible revolution under the bonnet: that of the ECU. A three-way catalyst only does its job around a very narrow air-fuel ratio (the famous λ = 1) — meaning the mixture has to be dosed permanently within a few percent. Earlier calculators, built around fixed analogue maps, could not deliver this precision. The PRV therefore moved over to EPROM-based ECUs: the engine map is now stored on an erasable, reprogrammable memory chip, refined run after run on the test bench and tailored to each variant. The PRV crossed over from the era of carburetion to that of digital engine management.

A paradox: while the Alpine lost power, the Renault 25 V6 Turbo gained it — pushed to 205 bhp with its catalyst.

For depollution did not always mean decline: re-tuned at the same time the catalyst was fitted, the saloon's turbo version actually moved up. But for many enthusiasts it was the power cuts on the flagship versions that left a mark — which is why catalysed examples are still carefully told apart from the rest today.

The divorce, and the end

On Volvo's side, despite the divorce pronounced with Renault in 1992, the PRV V6 continued to be fitted in the marque's saloons until 1997. After a total production of 970,315 units, it was on 15 June 1998 that the last PRV V6 left the production lines, giving way to a new generation of more modern engines.