Z28 Carb
Le Mans Master
Joined: Oct 2002
Posts: 7,098
Likes: 378
From: Las Vegas - Just stop perpetuating myths please.
Too much cfm is not problem but rather if it has enough signal a idle and low rpm to maintain good mixture through the booster/venturi. Thats the idea with the spreadbore carbs like the Qjet. Qjet used tiny primary venturies with double boosters but had large bore secoundaries for at least 750cfm and was used for both the 305"sb and 454"BB's.
There's a lot of engineering that goes into matching a carb to an application. Items most of us don't deal with much like idle circuits and air bleeds.
But how they got the 780cfm to work on a 302"sb i really don't know how they set it up. But i do know large overlap cams need more airflow to breath at idle while vacuum is very low.
Nowdays the newer carbs like Demon have much less restriction and while venturi size maybe the same they flow much more cfm than an original Holley of same size. Demons have modern boosters also that mix much better along with an adjustable idle air bleed. But BG Demons have suffered some quality conctrom as Summit drop them.
Well i hope this helped some w1 though it really didn't answer your question. How's that Stingray running now after the rebuild or was it just a top end - can't recall which?
cardo0
There's a lot of engineering that goes into matching a carb to an application. Items most of us don't deal with much like idle circuits and air bleeds.
But how they got the 780cfm to work on a 302"sb i really don't know how they set it up. But i do know large overlap cams need more airflow to breath at idle while vacuum is very low.
Nowdays the newer carbs like Demon have much less restriction and while venturi size maybe the same they flow much more cfm than an original Holley of same size. Demons have modern boosters also that mix much better along with an adjustable idle air bleed. But BG Demons have suffered some quality conctrom as Summit drop them.
Well i hope this helped some w1 though it really didn't answer your question. How's that Stingray running now after the rebuild or was it just a top end - can't recall which?
cardo0
Race Director
Joined: Jan 2000
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From: Corsicana, Tx
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Because the formulas are WAY conservative...it used a dual plane intake that likes extra CFM capacity.....and it was a vacuum secondary carb that was pretty forgiving.
750 cfm range carbs run very well on just about anything..mild to wild.
JIM
750 cfm range carbs run very well on just about anything..mild to wild.
JIM
Drifting
Joined: Feb 2003
Posts: 1,978
Likes: 2
From: San Antonio Texas
I think also for the Trans Am race series that it was built for. They knew it was going to be tricked up some, and reved high, where it could use more carb. Later they came out with a dual-carb "cross-ram" type setup.
Thread Starter
Melting Slicks
Joined: Sep 2003
Posts: 3,050
Likes: 2
From: Orange Ct
Thanks for the opinions. My car has a 327 and the previous owner had put a 780 on it. It was ok but the secondarys opened very little on an L79 configured engine (6000 rpm self imposed limit). A 600 CFM carb replaced it and was much nicer to drive on the street with crisper low rpm responce. The only real way to test is with the dyno but, on the street, it's all about feel not numbers.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by w1ctc
A 600 CFM carb replaced it and was much nicer to drive on the street with crisper low rpm responce. The only real way to test is with the dyno but, on the street, it's all about feel not numbers.
RACE ON!!!
Thread Starter
Melting Slicks
Joined: Sep 2003
Posts: 3,050
Likes: 2
From: Orange Ct
Too much cfm is not problem but rather if it has enough signal a idle and low rpm to maintain good mixture through the booster/venturi. Thats the idea with the spreadbore carbs like the Qjet. Qjet used tiny primary venturies with double boosters but had large bore secoundaries for at least 750cfm and was used for both the 305"sb and 454"BB's.
There's a lot of engineering that goes into matching a carb to an application. Items most of us don't deal with much like idle circuits and air bleeds.
But how they got the 780cfm to work on a 302"sb i really don't know how they set it up. But i do know large overlap cams need more airflow to breath at idle while vacuum is very low.
Nowdays the newer carbs like Demon have much less restriction and while venturi size maybe the same they flow much more cfm than an original Holley of same size. Demons have modern boosters also that mix much better along with an adjustable idle air bleed. But BG Demons have suffered some quality conctrom as Summit drop them.
Well i hope this helped some w1 though it really didn't answer your question. How's that Stingray running now after the rebuild or was it just a top end - can't recall which?
cardo0
There's a lot of engineering that goes into matching a carb to an application. Items most of us don't deal with much like idle circuits and air bleeds.
But how they got the 780cfm to work on a 302"sb i really don't know how they set it up. But i do know large overlap cams need more airflow to breath at idle while vacuum is very low.
Nowdays the newer carbs like Demon have much less restriction and while venturi size maybe the same they flow much more cfm than an original Holley of same size. Demons have modern boosters also that mix much better along with an adjustable idle air bleed. But BG Demons have suffered some quality conctrom as Summit drop them.
Well i hope this helped some w1 though it really didn't answer your question. How's that Stingray running now after the rebuild or was it just a top end - can't recall which?
cardo0
Bob
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Advanced
Joined: Aug 2006
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Most of that 780 cfm was probably required with the "off-road" cam and some head work, and the SCCA said the engines had to use "stock" carbs and other parts. As I recall, the next size smaller Holley back then was 585, as used on the 350hp/327 and entry level 427s.
Read an article a while back about Ford signing Bud Moore to build Mustangs that would compete with the Camaros. The first thing he did was buy a Chevy 302 engine to find out what he was up against. He said the thing was putting out more than 500 hp on his dyno. (That may have been the dual quad version.)
Read an article a while back about Ford signing Bud Moore to build Mustangs that would compete with the Camaros. The first thing he did was buy a Chevy 302 engine to find out what he was up against. He said the thing was putting out more than 500 hp on his dyno. (That may have been the dual quad version.)
