Directing Intake Air/Fuel Charges…

The introduction of Edelbrock’s original single-plane “Tarantula” intake manifold was the historical benchmark for this type of design, both in racing and street applications.  It also introduced a way to reduce the rather circuitous paths air/fuel charges followed in prior two-plane manifolds.  Early on, as pointed out in numerous technical pieces of high performance and racing journalism, such flow path directional changes can have a material and negative impact on the objective of trying to deliver the same air/fuel charge ratio to each of an engine’s cylinders,  just ahead of combustion.  Early emissions testing while using two-plane Edelbrock manifolds and measuring carbon monoxide (CO) existing each exhaust port of a small-block Chevrolet V8 showed air/fuel ratio differences (during the burn) as much as 4-5 ratios!  Aside from how this affected net exhaust emissions, it underscored how power was being lost to inefficient combustion.

Then along came the Tarantula.  When we returned to the emissions lab with this single-plane design, the range of CO variations was dramatically reduced and power gains resulted, not just because of higher net air flow enabled by the design but for reasons that included more uniform cylinder-to-cylinder air/fuel ratios.

Another feature of the single-plane concept related to how carburetor size played a role in cylinder-to-cylinder mixture distribution.  As it turned out, the smaller the total flow capacity (cfm rating), the greater the air/fuel charge plowed into the plenum floor, especially at higher rpm where the manifold really liked to work.  During such conditions, the physical impact of air/fuel mixtures caused fuel to be taken out of suspension, right before the time such charges entered the manifold’s runners.  This led directly to further variations in cylinder-to-cylinder distribution inequalities, re-liquefied fuel (un-suspended) randomly being applied to equally random runners.  Of course, spacing smaller carburetors higher with respect to the plenum floor with spacers tended to help, but this was still a band-aid solution.  Plus, when four-hole spacers were used, the net effect was air/fuel charges leaving the base of the spacer as if from the base of the carburetor itself, thus negating much of the reason for a spacer in the first place.  Open spacers worked better.

So-called cross-ram intake manifolds tended to be worse than the smaller single-plane 4V manifolds like the original Tarantula (and copies that followed very quickly after its introduction).  There were many instances, during cold-starts of an engine, that lean mixture back-fire could blow out the side of a cross-ram’s plenum chamber or split a chamber lid.  Fire in the manifold was not infrequent, following such explosions.  Not fun.

OK, so how were all these distribution problems addressed?  Actually, in a variety of ways.  Edelbrock’s approach was to apply methods intended to direct un-suspended fuel to where it needed to go or, stated another way, help it back into suspension before passing into a manifold’s runners.  But we had to learn about any other conditions that could exacerbate the suspension problem.