Current GM Q-jet Rules

[monte385]

Originally Posted by oldskool

While we're on the subject, I've got more questions.

(4) Haven't seen any pics of the Stock & SS cars running an air cleaner. So, aprox how much power is lost by using a good air cleaner set-up ? I assume there have been lots of dyno and track tests for this, thru the years.

FWIW I dynoed a street engine with a Qjet & with an air cleaner it lost 20-30 hp. We tried several a/c bases & filters & never could get the power up to no a/c pulls. This was a mild 421 that made 530 hp.

[Travis Miller]

Time for a history lesson.

A long time ago in teardown the Q-Jets had the top removed to see if all the plastic pieces were still there. Later we were told not to remove the top, only look at appearance and then measure the venturi and throttle bore. Research showed there was a change in shape of the Q-Jet in 1975. It was ruled that 1965 -1974 looked similar except for fuel line inlet and were called early design. Side inlet could not be swapped with straight inlet and visa-versa. 1975 and newer style had to remain that way including fuel inlet location.

Then came the polished secondary controversy. After a famous carb builder started selling Q-Jets with completely polished secondaries, we were instructed to look for any sign of casting flash removal or even the slightest scratch. Because of that ruling, a lot of Q-Jets failed teardown leading up to the famous "carburetor toss" at the Gatornationals. When a well known S/S racer's Q-Jet failed teardown, he walked over to the ditch behind the tech trailer and threw his high dollar carburetor in the creek. It was soon retrieved by another S/S racer's crewman who wadded into the green slime and picked it up.

Eventually the early vs late ruling was lifted and any year Q-Jet was allowed. So was the fuel inlet location lifted. Only correct venturi and throttle bore sizes for engine/year remain in effect. As with all make carburetors, no flash removal or polishing is allowed anywhere. The 2017 NHRA rulebook in Stock spells out what is allowed and what is not allowed.

[Travis Miller]

There is one style Pontiac Q-Jet that has the bullet shaped cluster in the primary venturi with no outer ring. It is only allowed to be used on the SD Pontiac engine which it came on. We have a few racers who still bring us that style Q-Jet to be looked at. Unless they are racing a SD Pontiac, it is not allowed.

[oldskool]

Originally Posted by Travis Miller

There is one style Pontiac Q-Jet that has the bullet shaped cluster in the primary venturi with no outer ring. It is only allowed to be used on the SD Pontiac engine which it came on. We have a few racers who still bring us that style Q-Jet to be looked at. Unless they are racing a SD Pontiac, it is not allowed.

I think you are referring to the 455HO engine carb. Some of them came without the outer ring, in '71, I believe. A few were also used on some other engines, besides the 455HO, including a 400. These were discontinued when it was discovered that they were not very street friendly. This is just info I've read. Have never had, or even seen one of these carbs, up close.

http://www.classracer.com/classforum...22&postcount=1

The '73-'74 SD455 engines came with an 800cfm Q-jet, which had a slashed vent tube, and the larger venturi primaries with double booster rings.

These articles contain info on both carbs.

http://www.cliffshighperformance.com...history_2.html

http://www.hotrod.com/articles/hppp-...et-carburetor/

https://www.facebook.com/pg/PontiacT...11336215645409

https://assets.hemmings.com/story_im...00-0.jpg?rev=2

That slashed vent brings up another question, for me. Are slashed vents allowed, on carbs that did not come with a slashed vent ?

[danoone]

Originally Posted by Travis Miller

Time for a history lesson.

A long time ago in teardown the Q-Jets had the top removed to see if all the plastic pieces were still there. Later we were told not to remove the top, only look at appearance and then measure the venturi and throttle bore. Research showed there was a change in shape of the Q-Jet in 1975. It was ruled that 1965 -1974 looked similar except for fuel line inlet and were called early design. Side inlet could not be swapped with straight inlet and visa-versa. 1975 and newer style had to remain that way including fuel inlet location.

Then came the polished secondary controversy. After a famous carb builder started selling Q-Jets with completely polished secondaries, we were instructed to look for any sign of casting flash removal or even the slightest scratch. Because of that ruling, a lot of Q-Jets failed teardown leading up to the famous "carburetor toss" at the Gatornationals. When a well known S/S racer's Q-Jet failed teardown, he walked over to the ditch behind the tech trailer and threw his high dollar carburetor in the creek. It was soon retrieved by another S/S racer's crewman who wadded into the green slime and picked it up.

Eventually the early vs late ruling was lifted and any year Q-Jet was allowed. So was the fuel inlet location lifted. Only correct venturi and throttle bore sizes for engine/year remain in effect. As with all make carburetors, no flash removal or polishing is allowed anywhere. The 2017 NHRA rulebook in Stock spells out what is allowed and what is not allowed.

The man you are referring to told me that story last year. He said there was casting flash the factory removed. He was selling new GM replacement carbs that he did his magic on. Joe is a good guy . Had several fun conversations with him getting the title for one of his old cars. And eventually they allowed those carbs also