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NHRA issues new carburetor tolerances

07/17/01

The NHRA Tech Department has announced new venturi tolerances for carburetors used in Stock, Super Stock, and Super Stock GT classes.

"We have always allowed a tolerance from published dimensions in these classes," said National Tech Director Danny Gracia. "These are cast pieces, not machined, so there is some inconsistency from part to part, even in the phenolic plastic Carter Thermoquads. However, we recently ran across one or two examples in teardown that simply pushed the envelope beyond any reasonable limits. So, we've spent the last several months reviewing teardown cards, all the way back to 1996. By documenting the venturi diameters measured in our teardowns we've established the following criteria."

Rochester, Carter and Edelbrock Quadrajets:+ / - .015-inch
All other carbs:+ / - .005-inch

When asked why the difference for the Quadrajet, Gracia said, "The original Quadrajet castings are very poor quality. We've seen .010- or .011-inch variance on these venturis since day one. Conversely, virtually any other carburetor model we look at is rarely more than .002 or .003 out from manufacturers' specs. It did not make sense to hold the Quadrajet to .005-inch, when we know such a tolerance will obsolete many carburetors that have been in use for years. Likewise, it would not be right to allow .015 on carburetors that have historically been much closer to spec."

Gracia added, "These tolerances are NOT an invitation to modify a carburetor to bring the venturis out to maximum. Any modification to a carburetor in these classes is strictly forbidden, and we will continue to inspect carburetors for signs of tampering."

The new tolerances are effective Monday, Aug. 27, 2001.



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