Carb Size Street Car - Advantage of QF 780 cfm over 735 cfm?

[topside]

Google "Z28 carburetor heat shield". Notice some are Holley/Carter square bore, but there are also Spreadbore-compatible heat shields.
Also pay attention to the cutouts for linkage - the Jeg's part might need modification.

 

[68 Sport Satellite]

Google "Z28 carburetor heat shield". Notice some are Holley/Carter square bore, but there are also Spreadbore-compatible heat shields.
Also pay attention to the cutouts for linkage - the Jeg's part might need modification.

Thanks. I guess you mean this one? Has good reviews. Some mods possibly required as you said.
OER 3969835 OER Holley Carburetor Heat Shields | Summit Racing

 

[topside]

Yup. Prevents heat-soak in the float bowls. Any mods on the GM-spec shields depends on your throttle linkage setup. Easy either way.
On some cars, I also use "fire sleeve" on the underhood fuel lines, which can be bought in either a silver or orange color, probably other colors as well. Cheap on Ebay, but buy it from a race shop, not some Chinese knockoff. On race cars where i run 1/2" hardline, I'll do all the hardline under the car too, as much for protection as insulation.

 

[68 Sport Satellite]

Yup. Prevents heat-soak in the float bowls. Any mods on the GM-spec shields depends on your throttle linkage setup. Easy either way.
On some cars, I also use "fire sleeve" on the underhood fuel lines, which can be bought in either a silver or orange color, probably other colors as well. Cheap on Ebay, but buy it from a race shop, not some Chinese knockoff. On race cars where i run 1/2" hardline, I'll do all the hardline under the car too, as much for protection as insulation.

Yep, we think alike. I've got the fire sleeve insulation on all under hood fuel lines. It helps a lot.

 

[BSB67]

Advice on which small venturi 950 DP you would try? Quickfuel or Proform or something else? I'm just recently learning about smal l venturi size for increased air flow speed, but I don't yet know what venturi size would be considered small.

1.375". Any make. Tune as needed.

Annular booster would be a consideration too.

Can you borrow a good/known carb.

 

[Geoff 2]

I would definitely try a divided spacer [ two hole ] in place of an open spacer.
For reasons known only to Edel, the 440 RPM intake has the divider between the planes notched. The Perf intake does NOT have the notch.
About 85-90% of the Perf & RPM intakes in the Edel catalog do not have notched dividers.

 

[Geoff 2]

...pushed wrong key before I was finished. I assume the notching provides a little more top end hp, but not sure why only a few have it. What the notching does do is interferes/compromises with the divided nature of the manifold.
An open spacer &/or notched divider might give more top end but might also lose some bottom end. Swings & roundabouts.
So I would definitely try a divided spacer & see if the engine 'likes' it. This sort of spacer can add a little top end because of the increase in plenum volume, but with little or no penalty in bottom end because the divided nature of the intake is retained.

 

[Rich H.]

Hey guys,
let me preface by saying I don't have much carburetor experience and am trying to make an educated guess with recommendations from others. I know every engine / car combo is unique and lots of variables come into play.

I've been running a Quick Fuel Vacuum Secondary SS series 735 CFM carb on my mild cam 451 since new (currently 6500 miles on the motor). The carb performs pretty well with no big issues and I had it jetted and tuned at a dyno shop. The car ran a best 1/4 mile ETA out of 10 track runs of 12.80 sec. Rear wheel numbers are 391 HP / 523 lb-ft torque. Not bad, but would a carb swap make it any better? I know the online calculators often show a smaller carb than what real world results are and I've often wondered if I'm leaving power on the table and have read up quite a bit on using 750 vs 850 carbs. I don't need maximum dyno numbers, but what I am looking for is a noticeable power increase on the street and a better 1/4 mile time would be nice!

The way I understand it, the Quick Fuel Holley type 750 carbs include the 735 and 780 Vac Secondary SS carbs offered. If I don't care about fuel economy, for street driving and a once a year track visit, is there any noticeable driving advantage of a 780 or 850 cfm Quick Fuel Vacuum Secondary carb over my current 735 cfm for my combo?

I've had numerous mopar friends advise me to get away from the Vacuum Secondary carbs and switch to Mechanical for better responsiveness. However, my car is an Auto Trans 727 with 2200 stall converter so I'm hesitant to go with mechanical secondaries. One thing I came across in a separate thread that many don't seem to talk about is the intake manifold effect on vacuum vs mechanical secondaries. I don't know enough on that to comment.

Here's my car info:
451 ci
727 Auto trans
Lunati Voodoo 60302 Cam (.475" lift, 220/224 at .050")
3.55 gears
28" tall rear tires
10.2:1 compression
Eddy RPM Heads
Eddy RPM dual plane intake w/1/2" phenolic spacer
Schumacher Tri-Y ceramic coated headers
2.5" exhaust w/H-pipe/Dynomax Ultraflow Welded (straight through) mufflers

Thanks in advance.

I believe you should pick up a little with a
double pumper (aka mech secondary). I would be looking in the 800+cfm range.


Old combo from the '90s:
8.8:1 1973 440/stock 1973 pistons (hand honed/re-ring)
346 iron heads milled .060, 8.8:1, MP 484 cam, hooker competition headers, 3" short exhaust. Started with 3000 stall, 750 vacuum secondary on a street dominator, first pass [email protected] ish, ran 12:80s later that day. Switched to a standard 850 DP, 12.70s similar conditions, but did not like how it drove on the street. Tried an annular 850 DP, same ET but drove much nicer on the street, more responsive. Better converter, 12:60s. Tried an RPM intake, still 12:60s but improved streetability/more responsive. Finally went on a cold temp day (low 50s), removed the belt driven fan. 12:50s. coolant temp never got over 180 degrees, it was that cold outside. 106 mph was a pass with good traction, 108 was a pass with wheelspin, it never varied any more.

Good luck with your decision

 

[INTMD8]

I would test high rpm manifold vacuum if I was trying to determine this. (or if worth pursuing)

Ideally would like to see .5 in/hg.

If 1-1.5 or more in/hg I would think there is some power on the table with more cfm.