Stk/SS carbs

[Bill Harris]

This forum is getting a little boring so I figure I'd start an argument.

I made an interesting observation in the NHRA rule book in the carb section. In every rule book I have up to and including 2005 the carb rule included the statement: "Replacement carburetors permitted provided they are the same model, type, throttle bore, venturi size and external appearance (e.g. fuel inlet location, smog versus non-smog differences."

This is a very useful rule for us who run old heaps since it is damn near impossible to find the OEM carbs with the numbers listed in the engine specs. For example, it lets me run an easily obtained Holley 3310-1 780 carb instead of the nearly impossible to find OEM 780 Holley.

Starting in 2006 and to date, the same rule exists, but without the "... and external appearance (e.g. fuel inlet location, smog versus non-smog differences)" part of the rule. So starting in 2006, the requirement that the replacement carb look the same as the OEM was dropped from the rule. This happened a few years ago but I don't recall any discussion about it.

First question would be, why the change? It's unlikely that NHRA changed it on a whim. Ya think someone quietly got a rule loosened up that was deemed beneficial? What engine combos would benefit from such a change?

Secondly, what are the ramifications of the change? Say I want to run a combo that originally used a 600 CFM Holley like an 1850. That would be a model 4160 (with the rear metering plate) and have 1-9/16" throttle bores, primary venturi of 1-1/4" and secondary venturi of 1-5/16" (that's "1563x1563/1250x1313" in NHRA spec sheet speak). The OEM carb has a single fuel inlet and side pivot float bowls.

With the external appearance rule change, is there any reason why I couldn't replace the side pivot bowls with center pivot "cathedral" bowls and use a dual-feed fuel line set-up with the Holley 34-2 conversion kit?

Similarly, there was a bunch of griping on the forum a while back about someone running secondary metering blocks on a Holley 4150 that had adjustable idle circuits, aka "four-corner idle". Why wouldn't that be legal under these rules? What about the new aluminum fuel bowls that Holley (and others) now have for the 4150's and 4160's? Are they also legal? (the ones with the fuel level sight glass are really handy). How about billet metering blocks?

I'm not too sure that many of these external things are a performance advantage on a Holley. Almost certainly billet metering blocks could be an advantage. Maybe the dual-feed on the 600 if it were running lean on the secondaries. But who cares if a carb has four-corner idle circuits? What difference does it make if the bowls are zinc or aluminum? Are there variations on some Carter or Rochester carbs that make one version better than another but have the same venturi and throttle sizes?

NHRA was pulling carbs off stockers at Bristol this year and measuring them. I don't recall anyone getting tossed, but I wonder if you had a four corner idle and aluminum bowls on a 4150 Holley if it would pass muster?