Why audi I5 started as 2.1 liters - Audisport Historical Trivia

[QSHIPQ at aol.com]

Q-heads:
Been having discussions on Audis use of the I5 and it's origins.  Specifically, why 2.1 Liters.  The answer can be found going back to FIA Homogolation Rules (x many production cars had to be built to be a 'homogolated' competiton 'production' car) in the early A1 Rally cars.

When Audi introduced Quattro, they wanted to introduce the system and prove it in racing at the same time.  The theatre of war they chose was WRC rally.  After a full year of behind the scenes lobbying (competition factory efforts had to endorse the awd allowance, along with FIA), they got the variance to the rules allowing AWD in WRC competition.

Now comes the engine history.  Turbocharged cars were allowed by FIA, but they were displacement corrected against the normally aspirated cars.  This adjustment was known in FIA as 'equivelency'.  The number assigned to equivelency on Group4>GroupB competition turbo car: 1.4.  This was a key number to Audi AG in I5 construction and displacement.

Audi chose to compete in the sub 3liter class.  This meant that the maximum motor equivelency displacement had to be less than 3 liters.  2.1 * 1.4eq = 2.940 Liters corrected.  Sound really close to the max?  Hardly.  By the time Audi completed FIA Group B (evolution of Group 4 where audi started competing), they had tweeked bore and stroke right up to 2.133liters.  2.133 * 1.4eq = 2.9862 Liters corrected!  

Street cars originally got the 2.1Liter motor (basically the A1 A2 Rally motors and 016 quattro transmission).  Later production Audi motors got bore/stroke tweeked a bit to a maximum stock displacement of 2226cc.

There has been much speculation over the years on what would have Audi chosen for the I5 displacement had the FIA 1.4eq rule not been in effect.  After a lot of calculations on piston speed vs the Audisport cars, I've proposed over the years that 2.4Liters (overbore + 2.1 liter crank) would probably be the optimum for a race I5turbo.  That said, the torque and HP figures we see from 'big block' I5's (2.5 and 2.6L) are pretty impressive in street cars, but they really aren't conducive to high rpm racing.

The I5 has proven itself in racing and in street car development for almost 25 years now.  A virtually bulletproof motor, the abuse many have put it thru (not just audisport), sometimes defies logical thinking and accepted engine limitations.  Gotta like that!

Cheers

Scott Justusson
QSHIPQ Performance Tuning
Chicago IL
'91 v8
'84 RS2URQ Project