words by pablo deferrari

the heart of what would become Porsche's savior in the 924 is this EA831 motor
it's very easy for those choosing to wade in the shallow pool of 924 history to suggest its engine as merely that of the lowly VW LT series vans, all well and good if yours is a decision to establish such a fact to those less interested. I, on the other hand, prefer the deep end of it because with it lies the dangers of the unknown fathoms, the unverified facts, and of course the risk of getting entangled in its murkiness. 
the venerable VW LT31
the EA831 taking residence between the front seats of a VW LT van
an unobstructed view of the beast
this workhorse of a lump has its history not with VW, nor Audi, but Daimler Benz. in 1958 Daimler acquired 88% of Auto Union at the behest of a German industrialist named Friedrich Flick who was looking for a strong suitor to help support the company while it was in the midst of establishing itself after the ruins of post-war Germany.

after reformulating itself as Auto Union GmbH on September 3, 1949, DKW remained as the sole survivor of the original merging which included Horch, Wanderer, and Audi. DKW went from producing affordable and reliable vans and motorbikes immediately following the reformation to a new compact front-wheel drive car in 1953 called the "Meisterklasse" (Master Class) two cylinder two-stroke F89 which essentially became their first passenger post-war car.

eventually, their persistence in producing two-stroke engines to power their cars led to a slump in sales, something Daimler needed to fix in order for their newly owned subsidiary to remain solvent. their solution? Mercedes-Benz decided to inject new life into the brand by designing a new four-cylinder, four-stroke engine for what would be the "new" DKW F102. the shedding of DKW's two-cycled skin called for a resurrection of the Audi name to showcase its new four-cycle dermis that would be known as the Audi F103.
DKW's first post-war passenger car, the F89
end of the two-stroke legacy culminated with the DKW F102 in 1966
as the skirts rose ever higher, so did technology and prestige with the Audi F103
the f103's engine designed by Daimler ranging from 1,5 to 1,8 liters
it must be woven in that by 1964, VW decided to bugger all and acquire 50% of Auto Union taking over the Ingolstadt plant and assuming all trademark rights of Auto Union leading Mercedes-Benz to abandon the brand altogether. woeful bottom lines with the brand along with heavy investment with their own new models underscored their abandonment and by '65, VW took total control of Auto Union. spinning the historical film reel a bit quicker for the sake of the plot, by 1969 VW merged Auto Union with NSU Motorenwerke creating Audi NSU Auto Union, this combining of expertise blossomed a generous range of models prompting the slogan "Vorsprung durch Technik," or " Advancement through Technology" by 1971; pari passu with a marriage of such engineering prowess.

this somewhat condensed chronology is necessary in order to understand how this engine would go on to define three very important models borne of this parent company; the Audi 100, VW's LT series, and Porsche's 924—one other model bears mention, but its existence is one many would like to wish away into the annals of lapses in judgement, the ridiculously unfashionable AMC Gremlin.
such was the F103's engine a genius in execution that it gave Popular Mechanics cue to give this Mitteldruckmotor very good press in its November 1967 issue. the exotic German term used meant nothing more than it being a "medium pressure motor" with a lofty 11.2:1 compression ratio that placed it squarely in the middle of a super-high compression diesel engine and a run of the mill lower compression petrol engine. designed as a "slant" motor, meaning its axis assumed a more pronounced angle toward the passenger side, the signature of its engineering was in having the combustion chamber set inside the piston crown leaving the bottom of the head to be completely shaved flat save for the slight recess of the valves and a spark plug hole; this design is known as the Heron head.

I must digress here for a bit and explain this so-called Heron design. named after it's inventor, Samuel Heron an engineer in the Royal Aircraft Establishment, this ingenious design allowed for turbulence to be created as the piston reached TDC thereby compressing the fuel/air mixture more intensely which reduced nitrogen oxide emission by some 30% more than the conventional "combustion-chamber-in-head" design could offer. aside from this, there was also a reduction in tooling costs should it be decided to change compression ratios since only the pistons with a revised "dish" in the crown needed to be produced and swapped out as opposed to machining the entire head. conversely, a flat-bottomed designed head afforded more precise tolerances during its machining.

although similar to its block design c. 1965, the next evolution of this lump would find itself in the legendary Audi 100, a car designed secretly by Audi Vehicle Development Director Ludwig Kraus in his spare time to protect the Auto Union heritage threatened under VW rule. putting out 99hp in 1,8 guise under the bonnet of Audi's C1 first generation 100LS until its production ended in 1976, a variation of this engine bored out to 1,9 liters with an output of 113hp was used in the Audi 100 Coupé.

Audi F103 engine in restoration

the car that helped solidified Audi's reputation for luxury and avant garde engineering, the legendary and very popular 100 C1
engineering would be a trifle boring without the presence of the fairer sex.
the 1,8 Audi 100 C1 inline four in situ
the second generation Audi 100 C2 (1976-1982) had either a 1,6 inline four, or the 115hp 2 liter EA831 offered from 1976-1978.
an Audi 100 C2 with its EA831 lump comfortably tucked in
VW failed Porsche when it chose to walk away from what would become the ultimate VW sport car, but not all was lost as we all know since Zuffenhausen went on to develop the stillborn EA425 into the progenitor of Porsche's future water cooled models, the 924.

many stars aligned the moment decisions were made on both camps. Porsche would have complete freedom to romp through VW's parts bins to assemble their entry level car and VW would reap the benefits of keeping its industrial empire from crumbling under the hostility of the period's financial crisis specifically in keeping the NSU plant from closing its doors forever in 1975.

the 924, aside from being infused with Porsche's engineering pedigree, would become the beneficiary of Audi's new 2 liter four cylinder block it was producing for their second generation Audi 100 C2 (1976-1982) and VW LT vans (1975-1982). given the name EA831, this engine would do away with its predescessor's pushrod valve actuation in favor of a more economical overhead cam design driven by a toothed rubber belt. this design reigns superior since the engine can be spun at higher RPMs increasing power output for a given torque, inlet/exhaust ports are optimised in their design by the omission of intrusive pushrods allowing for a more fundamental "cross flow" design which increases the engine's ability to exchange induction and exhaust gases, overall valve train mass and complexity diminishes, and production is simplified. carried over from the previous design was the combustion chamber cast in the piston crowns (mentioned earlier).
aside from pulling Porsche away from the precipice of bankruptcy, it saved a plant from closure and secured jobs
Porsche designed 924 head
the EA831 as it sat in the 924
for history's sake, the goddamned AMC version of the EA831 must be seen.
the stroke of the EA831 followed tradition and was kept at 84.4mm, the bore however was punched out to 86.5mm bringing the total volume to 1984cc³, bore centers remained 95mm and because very little space was left between the bores for water passages, the cylinders were siamesed. further down, the pistons aside from having dished crowns also had valve recesses in case of belt breakage, the conrod's center to center dimensions remained at 144mm, the sizes of the five main bearings were enlarged from 60mm to 65mm allowing for the use of cast crankshafts in some applications, for Porsche's version however a stronger forged crank was utilised. the crescent-type oil pump was affixed to the nose of the crank, a new design that was to be used not only on subsequent VW engines but also on the upcoming 944/968 series. covering the bowels of the block was a beautiful, deeply finned cast oil pan to aid in oil cooling. the whole package  leaned 40° to the right towards the passenger side allowing for a lower hood line.

care for more technical prose? ok then...

moving up to the head is where we begin to witness Porsche's engineering. the cam is held in by five bearing caps, the flat-topped cup-type tappets are similar to those used in the 908 and 917 engine, the inlet valves are 40mm for the Euro version and 38mm for the US market while the exhaust remains the same for both continents at 33mm and were seated on Rotocaps to rotate the valves prolonging their life—all valves are closed by a double coil spring design. rounding out the head, the spark plugs are on the exhaust side following Porsche tradition, the distributor is driven by the camshaft at the rear end, and fuel injector bosses were cast on the inlet side.

"one could sleep soundly knowing that scrap yards across this great land will have a dearth of Gremlins waiting for their engines to be pried out."

when all was said and done, the engine in Euro guise out out 125hp at 5800 RPM and 122 lb-ft at 3500 RPM DIN while gases were compressed at 9.3:1. nothing to snicker at when you compare Ingolstadt's efforts against the 2 liter 911 engine of 1965 vintage put out 130hp at 6100 RPM and 129 lb-ft of torque at 4200 RPM while the 1972 914's 1971cc lump wheezed out a slightly anemic 100hp at 5000 RPM and 116 lb-ft at 3200 RPM.

the LT28-31 got EA831 model CH with 75hp, Audi's Typ 43 100 (C2) model WF WA netted 113hp in carburetted form, and the silly Gremlin, the pinnacle of American automotive kitsch would only muster 80hp on a good day...quite sad when you come to the understanding that AMC apparently bought licensing rights and tooling to manufacture this engine themselves in Richmond, Indiana for a mere...wait for it...USD$60 million; in the future, if ever there were a shortage of EA831 blocks, one could sleep soundly knowing that scrap yards across this great land will have a dearth of Gremlins waiting for their engines to be pried out.

production of this engine at VW's Salzgitter plant is said to have ended around 1982. word has it that this left Porsche in a predicament necessitating them to use their newly minted but de-tuned 2,5 liter four used on the 944. clearly there must have been a 3 year surplus of these engines as production of the 2 liter 924 ceased in 1985 which remained basically unchanged since its inception in 1976.
the end certainly justified the means
it's as far as I care to take you. any further beyond the end of the jetty presents a danger of getting lost in a sea of immense technical proportions the likes of which begin to cloud the point of this discussion—the origins of this engine. 

it becomes quite clear how easy it is for one to begin the EA831's history with the LT van and the Audi 100, because that is of course when it drew first breath. but much the same as one who praises Mozart's work without a clear understanding of just how he arrived at such greatness misses out on the very foundation that made him so. 


thanks to my man Jim Doerr of 928 Classics who's always ready to talk such intense history with me-


12/21/2014 21:21

Citing Aaron Severson from ateupwithmotor.com:
"Porsche's original deal with Volkswagen obligated it to use that engine in the first 100,000 924s."
This is in agreement with the sales numbers, and an explanation why there were was a "surplus": there was no upper cap.

Alex Roy
01/24/2015 03:35

I love these technical articles. You just won't find detailed information like this anywhere else on the internet in such an easy to read format and complete with pictures too! Keep it up!


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