The Six-Second Advantage

Also from Issue 197

  • Michael Mauer on the 991
  • 1950 356 cabriolet
  • 1984 911 Carrera Targa
  • Falken's change of pace
  • 1994 911 RS America
  • 1978 928: A lovely old shed
  • Project 914 3.6 — Part 18.5
  • Slave and master cylinder
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The goal from the outset was 500 hp, and Prueninger’s team evaluated every part of the 3.8-liter GT3 RS flat six to get there. With the decision to increase displacement came the option of increasing the compression ratio. “We played with ratios as high as 13.0:1,” ad­mits Preu­ninger. “How­ever, we encountered some detonation at this level, so we settled on 12.6:1, up from the 3.8’s 12.2:1.”

Bore remains unchanged at 102.4 mm, so the displacement bump is down to an increase in stroke from 76.4 to 80.4 mm. This was achieved by using the full-race GT3 RSR’s crankshaft and shortened titanium connecting rods married to the RS 3.8’s Mahle pistons. The rods are strengthened to withstand the greater angular thrust velocity created by their 2.1-mm length reduction to 127.9 mm.

“A long stroke is never good for high revs, and we had to maintain the engine’s ability to rev and not go flat when at 8000 rpm,” explains Prueninger, who says the camshafts and valve timing were altered in addition to much more. “We revised the design of the whole intake system, from the point where air enters the in­takes all the way to the combustion chambers.

“During this review, we found that the 3.8 air-filter system worked up to 475 horsepower and then ran into a brick wall,” he continues. “We even looked at a special airbox we were testing for the GT3 R Hybrid race car and found we could gain 10- to 12-percent air volume from this alone. In the end, we went for a larger-volume, hand-laid carbon-fiber airbox housing a pair of high-flow conical air filters.

The next bottleneck the team ran into was the intake plenum system, which could not use all the air that was now being directed to it. “We invested in a new lightweight, cast-alloy plenum and intake-runner system with thinner walls, which also saved 400 grams,” explains Preuninger. “Nor­mally you gain power and revs with shorter intake runners, but you also lose torque. In this instance, using ten millimeter shorter intake runners with a five millimeter increase in diameter, we managed to find a gain in flow velocity. However, anything outside that ideal ratio decreases airflow, resulting in a loss of power.”

The RS 3.8’s exhaust system proved to have excessive back pressure when bench-tested with the larger motor. “The catalytic converter is normally the biggest bottleneck in a production exhaust system,” says Preuninger. “It is always a balancing act between efficiency and emissions, and we reached our target by reducing the cell count from 400 to 300 and using a new special coating to help meet the cold-start emissions benchmark.” The new exhaust system, which uses a titanium rear silencer, is made by Ricardo’s German branch.

The extra torque and high-revving capability of the 4.0-liter motor put more stress on many associated components, so the chain tensioners were beefed up and the camshaft adjuster is now fitted with surface-hardened 14.9-grade (DIN) bolts to cope with the higher loadings. Due to the added stress on the clutch, the pressure plate is now mated to its backing components using high-strength steel bolts rather than rivets.

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