Current GM Q-jet Rules

oldskool · 02-26-2017, 06:30 PM

Please excuse this Dummy question. But, I just wanna get this straight in my mind.

In the Class Racer Info guide, I see that for many engines there are a couple of numbers listed for legal factory carbs, and either a 1901 or 1903 Edelbrock Q-jet, depending on whether the engine came with what we call a 750 or an 800 Q-jet. It appears that they are separated by their max allowable venturi size. So, for this post I'll just refer to 'em as a "big" or "small" carb.

So here are my questions:

(1) Have these rules been changed, or relaxed, to allow ANY small carb, for any GM engine that came with a small carb, and ANY big carb for any engine that came with a big carb ? Again, I realize that no carb can have a larger venturi size than what is listed as legal, for the engine.

(2) If the answer to #1 is no, then exactly which replacement Q-jets are legal ?

For example: Can you use a different number carb, from your engine brand, but not from another GM brand ?(such as a side inlet Chevy carb instead of a BOP front inlet carb)

And for the big carb engines, can you use any big E-carb, or just the 1903 ?

http://www.carburetion.com/Rochester.asp

J.R. Haddad · 02-26-2017, 06:43 PM

Casting number of the carb no longer matters, measurements do. J.R.

oldskool · 02-26-2017, 06:52 PM

Quote:

Originally Posted by J.R. Haddad

Casting number of the carb no longer matters, measurements do. J.R.

OK, thanks !

That brings up more questions. Since this info is not mentioned in the Class Racer Info guide, exactly when did this new rule go into effect ?

Is the rule only mentioned in an official NHRA rule book, or is there another source I can find online, if I know the correct link ?

Mark Yacavone · 02-26-2017, 07:15 PM

Quote:

Originally Posted by oldskool

OK, thanks !

That brings up more questions. Since this info is not mentioned in the Class Racer Info guide, exactly when did this new rule go into effect ?

Is the rule only mentioned in an official NHRA rule book, or is there another source I can find online, if I know the correct link ?

It's not a new rule, by any stretch. Been this way for 20 years...Probably more

oldskool · 02-26-2017, 07:32 PM

Quote:

Originally Posted by Mark Yacavone

It's not a new rule, by any stretch. Been this way for 20 years...Probably more

OK, just going by what I read in the Class Racer Info guide. If I'm reading it correctly, there is no mention of the fact that any Q-jet that is within the legal venturi size is OK.

So, another question: Does an Edelbrock Q-jet have any advantage over a GM Q-jet ? Or, can the good Q-jet builders get the same power potential from either, just as easily ?

J.R. Haddad · 02-26-2017, 08:43 PM

I think that would be a question for the good quadjet builders. J.R.

Dwight Southerland · 02-26-2017, 08:53 PM

Quote:

Originally Posted by oldskool

OK, just going by what I read in the Class Racer Info guide. If I'm reading it correctly, there is no mention of the fact that any Q-jet that is within the legal venturi size is OK.

The tech sheet info on Class Racer Info is obtained from the NHRA Tech Sheets. Neither do the NHRA Tech Sheets or rule book specifically describe the situation you are questioning. There is a statement under the Carburetor paragraph of the Stock Cars section of the rule book that states "Replacement carburetors permitted provided they are same model, type,throttle bore, and venturi size." The provision for using Qjet carbs apparently falls under that statement.

older racer · 02-26-2017, 08:58 PM

hello, question? does the inlet have to be the same location? on any carbs? just curious. thanks

Mark Yacavone · 02-26-2017, 09:08 PM

Quote:

Originally Posted by older racer

hello, question? does the inlet have to be the same location? on any carbs? just curious. thanks

No

oldskool · 02-26-2017, 09:52 PM

"Replacement carburetors permitted provided they are same model, type,throttle bore, and venturi size."

OK. I reckin the words "same model, type" just mean they have to be a Q-jet type carb, and not necessarily a Chevy Model, or Buick model, or whatever car brand you race, or one specific Edelbrock number.

I suppose it only matters how the tech guys interpret the rule. It makes sense to me, since I don't see any big advantage to a particular Q-jet. But then there are lots of very similar parts, from different year cars, that would not provide an advantage, but are not legal.

Don't know if more of these similar, but different year parts will become legal, as parts for the older cars become more scarce, or not. ?

02-26-2017, 06:43 PM	#2
J.R. Haddad Senior Member Join Date: Jun 2012 Posts: 784 Likes: 935 Liked 1,464 Times in 307 Posts	Re: Current GM Q-jet Rules Casting number of the carb no longer matters, measurements do. J.R.

02-26-2017, 08:43 PM	#6
J.R. Haddad Senior Member Join Date: Jun 2012 Posts: 784 Likes: 935 Liked 1,464 Times in 307 Posts	Re: Current GM Q-jet Rules I think that would be a question for the good quadjet builders. J.R.

02-26-2017, 08:58 PM	#8
older racer Member Join Date: Aug 2010 Posts: 212 Likes: 13 Liked 66 Times in 26 Posts	Re: Current GM Q-jet Rules hello, question? does the inlet have to be the same location? on any carbs? just curious. thanks

02-26-2017, 09:52 PM	#10
oldskool VIP Member Join Date: Dec 2014 Location: The Land of Bayous, Boudin & Crawfish Posts: 1,668 Likes: 223 Liked 747 Times in 362 Posts	Re: Current GM Q-jet Rules "Replacement carburetors permitted provided they are same model, type,throttle bore, and venturi size." OK. I reckin the words "same model, type" just mean they have to be a Q-jet type carb, and not necessarily a Chevy Model, or Buick model, or whatever car brand you race, or one specific Edelbrock number. I suppose it only matters how the tech guys interpret the rule. It makes sense to me, since I don't see any big advantage to a particular Q-jet. But then there are lots of very similar parts, from different year cars, that would not provide an advantage, but are not legal. Don't know if more of these similar, but different year parts will become legal, as parts for the older cars become more scarce, or not. ?

02-26-2017, 06:30 PM	#1
oldskool VIP Member Join Date: Dec 2014 Location: The Land of Bayous, Boudin & Crawfish Posts: 1,668 Likes: 223 Liked 747 Times in 362 Posts	Current GM Q-jet Rules Please excuse this Dummy question. But, I just wanna get this straight in my mind. In the Class Racer Info guide, I see that for many engines there are a couple of numbers listed for legal factory carbs, and either a 1901 or 1903 Edelbrock Q-jet, depending on whether the engine came with what we call a 750 or an 800 Q-jet. It appears that they are separated by their max allowable venturi size. So, for this post I'll just refer to 'em as a "big" or "small" carb. So here are my questions: (1) Have these rules been changed, or relaxed, to allow ANY small carb, for any GM engine that came with a small carb, and ANY big carb for any engine that came with a big carb ? Again, I realize that no carb can have a larger venturi size than what is listed as legal, for the engine. (2) If the answer to #1 is no, then exactly which replacement Q-jets are legal ? For example: Can you use a different number carb, from your engine brand, but not from another GM brand ?(such as a side inlet Chevy carb instead of a BOP front inlet carb) And for the big carb engines, can you use any big E-carb, or just the 1903 ? http://www.carburetion.com/Rochester.asp