Volume II, Issue 9, Page 2

Shootout at the Edelbrock corral...

Here’s the scene.  Edelbrock dyno cell, dated proximate mid-1970s.  Engine: One of the first 9,000 rpm small-block, drag race engines…that lived run after run.  Owner: Bobby Thompson of NHRA Modified Production class renown.  Induction system: Prototype “Pro Ram” intake manifold, later to become the design of choice for mouse-block Pro Stock engines. 

Dyno rooms are frequently scenes of the unexpected.  It can be the untimely expiration of a high-winding race engine (well, these are all untimely). Or it can be failure to meet certain power level expectations which, in reality, can also be untimely. At other times, it can be the result of the totally unexpected, all by itself. During my nineteen years of designing, building, testing, and laboring through the rigors of birthing induction systems at Edelbrock, few dyno experiences eclipse the two about to be described.  Classic doesn’t even come close. 

At that time, producing a prototype intake manifold had not even approached the “rapid modeling” or “stereo lithographic” techniques of today.  This was particularly the case if a new design was sufficiently different from a previous version that did not lend itself to modification to the newer bullet.  In such cases, either by modification of port cores or patterns, the “updated” casting could be produced in aluminum by foundry processes.  If, however, the design was truly a new departure from its predecessor, it was difficult to fabricate something that could be subjected to “live” dyno or in-vehicle testing beyond the air-flow bench.  So we Edelbrock R&D types hit upon an idea that produced a solution to this problem, although it came with its own potential problems.

The process was simple, or so it seemed. Once a particular manifold passage (runner) was designed, a wooden “plug” or port shape was dimensionally hand-crafted.  Around this shape we placed small lengths of resin-impregnated fiberglass.  Once cured, we’d split this outer layer of ‘glass, remove the wooden plug, seal the seam and have formed a functional passage. You can picture the objective. By assembling all the designed runners and placing them in a bedding plate that formed the base of a V-type intake manifold and capping the grouping with an aluminum plate that accepted a carburetor, we produced a “fiberglass” manifold that could be evaluated on the flow bench and ultimately run, either on a dyno engine or in a vehicle. It wasn’t until later in the evolution of this prototyping process that we discovered the benefit of a “back-fire” valve placed at the rear of the plenum. That’s another story, entirely.

One of the first subjects for the fiberglass method was Edelbrock’s second iteration of the Tunnel Ram. Equipped with flow passage concepts evolved from NASA research and from runner design that required the use of shapes heretofore not found in such manifolds, the design sported the first-known “V-bottom” plenum, which was intended to help equalize runner roof and floor lengths and aid fuel suspension in the transition from plenum to runners.  Even though plenum volume was significantly reduced from comparable designs of the era, there was still sufficient capacity to prevent the proper absorption of a sudden increase in manifold pressure of damaging proportions. You’d call it a back-fire.
We were in the process of warming up Thompson’s little bullet with a partial load running at a steady speed of about 3,000 rpm.  Usual stuff.  The cell’s fuel delivery system consists of a tank, line-connected to the engine’s carburetors and fed by a regulated air supply that maintains pressure at about 7psi. As you face the dyno console, peering through a large pane of safety glass, there are two entrances to the enclosed cell; one from the right and the other from the left. As a point of reference, there are large CO2 fire extinguishers parked just outside each of the cell’s two doors. 

Murray Jensen, Edelbrock’s carburetion and engine dyno specialist extraordinaire, entered the cell with timing light in hand. Envision him reaching around to the back of the engine with one hand (to adjust the magneto) while aiming the light at the crank damper with the other. In a sense, he’s embracing the manifold. I’m at the console throttle. There are two Edelbrock employee bystanders watching the process, both anticipating some benchmark power levels from this little screamer. As Murray adjusted the timing, and for reasons that to this day I cannot fathom, the engine barfs back into the fiberglass manifold. Hell breaks loose.

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