C4 Upgrade Air Management to Improved Fuel Delivery - Got Air?

25 After having our friendly localmachinist open up the ports in the plenumto match those in our new, larger runners,there was still further machiningnecessary. We had him relieve the innersurfaces of the various bolt bosses inorder to eliminate all possible restrictionsto intake air flow.

26 We also machined out the bridgesin the front of the plenum in theinterest of reducing restrictions atthat location in the air-intake system.You can see that the bridge inthe left port has been machined away,with the right side not yet done.While we believe this will enhanceperformance, we think removal ofthese bridges may have resulted in aslight induction system whistle duringcold engine operation.

27 Here you can see how much the portsin the plenum had to be enlarged to matchthose in the new runners.

28 Shown here is the vacuum fitting forthe power brake booster at the left rear ofthe plenum. We found a slight interferencebetween this fitting and the new runner(shown in the next photo) and had to grinda relief in the runner to clear this fittingand the vacuum line that connects to it.

29 Here you can see how close a fitthat vacuum line is to the runner; youcan also see the radius we had togrind out of the new runner. A smoothradius is essential, since any sharpedge or corner can create a stressriser that can serve as a startingpoint for a stress crack.

30 The scribe mark shows where wehad to grind to get the small runnerport to match the corresponding porton the plenum to feed the ninth injector.If you look closely, you'll alsonote casting marks and flash in theport to the right. We carefully groundout these irregularities from eachport in order to minimize restrictionsto smooth air flow.

31 Note the very tight fit of the ninthinjector to the valve cover, resulting fromthe configuration of the new runner intowhich the injector fits. We chose to grind aslight radius in the valve cover to ensureclearance.

32 We found these snap-on, ball-type hexand torx drivers to be especially helpful inreaching through odd angles in order toloosen the original Torx runner bolts andreplacement indented hex bolts.