Q-Jet

[Tim Fortier]

"Want to increase a 650cfm Quadrajet to a 750cfm? Simply grind some material from the secondary stop lever. If your secondary air valve does not open perpendicular to the air horn, then the airflow is limited. Grind this tang to make sure the air valve fully opens. There were no internal differences between a 650cfm and a 750cfm Quadrajet other than the factory setting that allows the air valve to open all the way on the 750 model."

I am sure someone has done this but is it a noticeable difference or should I swap heads, intake manifold and buy a 750?

 

[shanghaismith]

"Want to increase a 650cfm Quadrajet to a 750cfm? Simply grind some material from the secondary stop lever. If your secondary air valve does not open perpendicular to the air horn, then the airflow is limited. Grind this tang to make sure the air valve fully opens. There were no internal differences between a 650cfm and a 750cfm Quadrajet other than the factory setting that allows the air valve to open all the way on the 750 model."

I am sure someone has done this but is it a noticeable difference or should I swap heads, intake manifold and buy a 750?

Here is a calculator to determine how much air your engine pumps at any given rpm. If your engine will not be moving over 650 cfm then a 750 cfm carb will make no difference in performance.
http://www.gtsparkplugs.com/CarbCFMCalc.html

 

[autonaut]

Afaik there's only 2 sizes quads. 750 and 800

 

[pontiacgp]

 

[L92 OLDS]

I am sure someone has done this but is it a noticeable difference or should I swap heads, intake manifold and buy a 750?

Your 442 already has an CCC (computer command control) 750 cfm Q jet. It's a good carb and an Olds 307 will not need anything bigger.

 

[olds307 and 403]

Anubis, you know these cars better than me but weren't the HO carbs 800 cfm? Either way they are plenty big but the secondary air flap may need opened up more. Pick up Cliff Ruggles book it shows where to set your air door opening. Also find out what secondary rods are in it. Doing this, a good tune up and the exhaust in the other thread will help a lot.

 

[Tim Fortier]

Here is a calculator to determine how much air your engine pumps at any given rpm. If your engine will not be moving over 650 cfm then a 750 cfm carb will make no difference in performance.
http://www.gtsparkplugs.com/CarbCFMCalc.html

Thanks for this. This will be a big help in the future for more. I do plan on a second GBody. 90% sure it'll be motor swapped. As I am not a pro with Carburators this will again be a useful tool.

Afaik there's only 2 sizes quads. 750 and 800

Everything I had read said that this stock was a Q-Jet 650 with the ability to go up to 750. But thank you your post and following posts has shown me otherwise.

Your 442 already has an CCC (computer command control) 750 cfm Q jet. It's a good carb and an Olds 307 will not need anything bigger.

Thanks Pontiac the video shed more light on this subject. And Anubis thank you this makes sense for the car. How ever the suggested "tip" is to ensure that it opens fully. Again I am assuming at this point. Or is it a moot point?

 

[pontiacgp]

The secondaries open only as much as the engine needs. If you open the secondaries more than what the engine needs you will lean it out and bog.

 

[MrSony]

Exactly what pontiacgp said. Qjets are variable CFM carbs which means they can only flow a couple hundred but when you get on it or you tune the carb you can get the hole 750/800. They only flow what cfm the engine required at any given amount of throttle. That's what makes these carbs so good and economical. It might've said in stock configuration your qjet is setup to flow 650cfm, but ALL qjets were 750cfm for small blocks and 800 cfm for big blocks, generally speaking.

 

[Tim Fortier]

Alright thanks guys! There was no real debate on the sites I looked at. It was a this is what you do thing. But this really puts some truth to it!