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“Pore ol’ Chicken Farmers Gone Racing…”

During the time NASCAR allowed big-block Chevrolet engines in what was then called Grand National racing, I got a phone call from Jr. Johnson.  To those not familiar with Jr. and his exploits in this category of racing, suffice to say he was among the earlier pioneers in the sport, first as a driver and then as a highly successful car/team owner with Cup championships numbering among his many accomplishments.  But before we get into why Jr. called, let’s set the stage for the concerns he expressed.

From an intake manifold design perspective, there is a problem approaching equal-length runners when a single 4V carburetor is used.  The “inboard” runners in a V8 configuration (4, 6, 3 and 5) become shorter than the “outboard” runners (2, 8, 1 and 7).  As a consequence, the shorter runners tune to a higher rpm than the longer runners.  In the day, conventional wisdom was that a design that contemplated making all runners the same length (or approaching that concept) was preferred.  As a result, we chose to “rotate” the carburetor roughly thirty-five degrees away from a straight-ahead and more standard position.  By so doing, the four longer runners were shortened slightly and the four shorter ones became a bit longer.  However, this change in carburetor position created some problems with cylinder-to-cylinder air/fuel mixture distribution, and that brings us back to why Jr. called me.

Long story short, he was sorting out big-block engine combinations with versions of Chevrolet’s aluminum cylinder heads that traditionally had four “good” intake ports and four “bad” intake ports.  Based on the directional path of how a single 4V manifold’s interior runners linked to their corresponding head ports, the flow path was continuous.  However the outboard ports (1, 7, 2 and 8) dumped into head ports that turned away from the manifold runners, creating an abrupt change in the flow path created as inlet flow passed from manifold into the cylinder heads. 

It turned out that these engines would be running in a very narrow span of engine speed, roughly w.o.t. and from 7200-7400 rpm, and as a combination of cylinder head inadequacies and layout of the intake manifold, cylinder-to-cylinder mixture distribution was creating some long-term parts durability issues.  None of the engine dyno tests performed in the manifold’s development addressed durability concerns, only ways to optimize power.  So as a result, I didn’t have any immediate information to share with Jr. about how to solve the problem.  What followed was a trip to his shop in Ronda, North Carolina, and I’ve previously shared some of the exploits on that visit that had nothing to do with the manifold.

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