history lesson by pablo deferrari

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the PRV engine (image: wikipedia)
The Renault Alpine 310A had it...so did the Citroën XM, Delorean DMC-12, Dodge Monaco, Eagle Premier, Lancia Thema, Peugeot 504, and the Volvo 760 GLE too. Porsche even tried it...
You see, they were considering their engine options for the 924's successor after considerable debate in the boardroom. Since the normally aspirated, VW LT van engined baby Porsche disappointed lots of enthusiasts, they wondered if the new car should possess something with a few more pistons.

Should it have an eight cylinder like the 928? Or maybe a six in a Vee configuration? Well, I'm sure they shoe-horned in the 928's mill for shits and giggles, but then they sobered up just a wee bit and tried out this ever so slightly more sensible 90° V6 already tested and proven by their French-speaking neighbors...the Euro PRV V6.
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twin turbo PRV (image: oddfireV6)
In 1966, Peugeot and Renault got together to manufacture an engine with common components in an effort to save a little cash and optimize efficiency amongst the French auto industry. They formed a subsidiary called La Française de Mécanique and built a plant in 1969 Douvrin, a town in northern France and the name these engines would be given later on.

Then in 1971, Volvo decided to join in on the fun since they too wanted in on the cost savings on future engine creations that could be used on multiple platforms. So this triad formed the public limited company known as "PRV" (Peugeot, Renault, and Volvo).

They planned on making a V8 at first, but with the energy crisis still looming over their heads, and the government ready to tax the hell out of anyone making an engine greater than 2.8 liters, they scrapped that idea and looked into lobbing off two cylinders to create a more compact and fuel efficient 2.7 liter V6.
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the first design utilized a "common crank pin" design used from 1974 up until 1984. here, the journal circled in red is shared by two sets of rods/pistons, hence the name "common." (image: mopar)
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the second generation utilized a much sweeter "split crank pin" design circled in red. notice how the journal is offset or split. (image: buick)
Now pay attention here because this is where you'll understand the principles of this V6 engine design.

What makes the PRV V6 unique is that the angle of the Vee is 90°. One of the benefits being that it affords a lower hood line. This is all well and good for the V8 since it's not only the most universally used configuration, it affords a more natural firing order. Put another way, the V8 has four sets of crank pins, or journals if you like, where each one has two sets of rods/pistons. What this means is that each piston fires every 90° for smoother operation because the crank pins are all 90° from each other.

The common practice with early V6s was to cut off the first pair of the 8's cylinders but keep the 90° angle. A V6 doesn't like that angle because the firing sequence becomes unbalanced with this angle using a common crank pin design crankshaft. So unlike the V8 that has four sets of common crank pins with every piston firing every 90° (90° x 4 crank pins = 360°), the V6 with the same exact configuration has to have each of the 6 pistons joined in 3 sets of common crank pins fire every 120° (120° x 3 crank pins = 360°) since the crank pins are 120° from each other.
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the Volvo 264 was the first car to have the newly designed PRV mill. (image: wikipedia)
So a V6 with this configuration must have what's called "uneven ignition timing," or "odd firing" as it's known. In this case, the cylinders fire at 90° and 150° intervals and this uneven firing results in rough-running engines with undesirable harmonic vibrations at certain engine speeds.

This is exactly why Porsche, when they tested this engine, deemed it too coarse and rough running...and it lacked power. They'd have to sort of re-invent the wheel to make this thing right. Remember, this all took place around 1979 during the development of the 944.

Besides, as Michael Cotton points out in his book Porsche 924 944 968, Professor Helmuth Bott was digging the experimental 924 Turbo engine equipped with twin balance shafts. He claimed it was the way to go since they had higher capacities in mind down the road, especially one that could be developed in three or four stages much like the 911 engine.

He also didn't want Porsche to have three totally different engine designs. The 911 and 930 counted as one, and the 928 as a second; so the logic was to use as many components from the 928, like the head, valve train, and bore sizes leaving them to develop the bottom end. This mentality fit their tight budget much better it seems.
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the Eagle Premier had the PRV...(image: wikipedia)
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as did the Lancia Thema...(image: wikipedia)
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the Citroën XM...(image: wikipedia)
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the mighty Renault 310A...(image: wikipedia)
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yep, the Delorean DMC-12...(image: wikipedia)
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the popular Peugeot 504...(image: wikipedia)
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the insipid Dodge Monaco...(image: wikipedia)
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and lastly, the Volvo 760GLE. (image: wikipedia)
But one has to wonder if PRV nailed it right the first time, like they did in 1984 with the more refined version, if Porsche would've bitten then. 

Unlike the first generation lump, this one had a split crank pin design with the journals offset 15° in opposing directions to achieve the even firing 120° ignition pattern...this yielded the smoothness the first gen engine needed in the first place. And although the design of a split crank pin seemed a bit weaker than a straight one, better metallurgical techniques quelled that fear.

These engines, both first and second generation, got lots of traction as they were widely used in an impressive amount of cars from October of '74 up until June of '98, when it was finally laid to rest. Aside from being continually refined, there were turbo-charged versions, larger displacement versions, changes in head and valve train design, and it even made a cameo in Le Mans proving just how versatile this mill was. In the end, 970,315 units were made...not a bad run.

The important thing to consider is that Porsche made a sound decision in passing the PRV up. They caught enough flak with the 924 using the LT short block, and when enthusiasts got wind of that same block being sold to American Motors to power their Gremlin, the shit hit the fan.

Imagine, for a moment, if Porsche repeated history and pulled the same stunt with the 944 not only utilizing non-Porsche PRV steam, but also using the same engine found in nearly every other French car, a few American jalopies, and Sweden's favorite sedan. Shiver me timbers...

highball!

 


Comments

christian
06/26/2014 09:47

Hi, Pablo !

The PRV was never mounted on 504 sedans, but only on coupés and cabriolets.

Reply
Leo
06/26/2014 14:44

Interesting piece Pablo! I did not know this story.
The pictured Lancia Thema actually has an engine based on a Ferrari 308 engine, but with a cross plane crank instead of the flat plane crank. And apparently made by Ducati. Bu that's another story... These Thema's have the yellow pinstripe and small badge at the radiator grille.

Reply
09/03/2014 09:28

Does that engine have any theory. The engine design very ethical. How to progress and maintain it.

Reply
Ben
11/10/2014 03:11

People generally believe that the V6 was decided to be produced only due to the petrol crisis but, in fact, PRV had to be originally a family of modular V8 (3552cc) and V6 (2664cc) engines with a 444cc per cylinder basis with the standard bore and stroke of peugeot 404/504 4 in line engine.

About a dozen of V8 'prototypes' were build, an exemplar is still possible to be seen at Douvrin's plant (I have some pictures from each side of it with a K-jetronic, alternator, power-steering pump and a genuine volvo oil filter).

VOLVO's 260 serie (262, bertone's 262c, 264, 265) were the first car to receive the V6 in 1974.

VOLVO left PRV group to develop (wi!th... Porsche! Yes!) his own new modular engine family with the 6 in line 24valves (B6304 3000cc) introduced in the 960GLE in 1990, the 5 in line 20 valves (B5254 2500cc) introduced in the 850GLT in 1992 and later the 4 in line in the S/V40 "made in Holland" as the Mitsubishi Carisma.

Nowadays, De Lorean still propose an update of the V6 with the split crankshaft and other features.

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