Why was the Z28 302 SB supplied with a 780 cfm Holley carb when all the formulas and conventional wisdom say its too large?

 

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

 

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

 

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.

 

The 780cfm carb runs good (on a 302) at 8000 RPMs.

 

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.

 

Dyno numbers are hard to interpret for what you are looking for and need. I rarely say this, but in YOUR case, S.O.T.P. feel is a better indicator for your street use. Your smaller carb has the faster air flow at lower speeds that you need for sharp, crisp, throttle response. The 780 may have provided a higher peak horse power number, but since you don't drive, constantly at 6000 RPMs and above, that is if little consequence. You have the right carb, now.

RACE ON!!!

 

Well I put the Edelbrock heads on and the Magnaflow system with an H pipe. Worked out to 10.25 CR. After chasing my tail around with a hesitation problem, it turned out that 180 thermostat was needed. With no cross over, the manifold stayed too cool in anything less than 80 degree weather with a 160. Guage says 200-210 but IR gun says 180 and it runs like a clock. Timing back up to optimum (pinged bad with the iron heads). So now I have a new rear suspention with pastic spring and rear sway bar, rebuilt trany, new clutch, spreader bar and new shocks all around. I'm happy till the next bug bites. Thanks for the interst Gordo.
Bob

 

do u have a spreadbore manifold?
4165-6210 double pumper is a big improvement over a vac. 600

 

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.)