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?

[Alan Roehrich]

I don't know about aluminum float bowls, but a real tech guy will bust you for 4 corner idle circuits on carbs that did not have them. Far as I know, billet metering blocks are not legal. Other than capacity, I can't see where float bowls are a big issue, you really can't save more than a couple of ounces of weight, and since you can run practically any fuel supply system, capacity can't be a real concern. Are 4 corner idle circuits a real performance advantage? Probably not, provided you are a really good tuner, especially since hardly anyone leaves from an idle.

Eventually, the issue becomes "How many changes are we going to allow?", and "How far from stock and original are we going to stray?" Stock Eliminator has gotten pretty radical and pretty far from the original intent. The line has to be drawn somewhere if Stock is to retain a real identity of its own.

[Crew Chief]

As I understand it the reason for the rule change a few years ago was to allow cars with Q-Jets to use either straight in or side inlet for the fuel line. The rule also allowed the use of early or late style Q-Jets on any year.

You better ask tech before you start changing anything on a Holley. And be sure you do have a correct Holley in your trailer if you try pushing the issue at a race without asking.

[Rory McNeil]

Also, I wouldn`t be suprised if this "non OE look" deal was to allow QJet * Carter AFB/AVS racers to use the new Edelbrock carbs.

[Aubrey N Bruneau]

Undoubtedly, Rory...
When I was in Mission, the tech guys DID have me remove one of my Edelbrock "AFB" carbs, and took measurements, etc.