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Old 12-23-2010, 05:31 PM
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Carburator help please

Guys my 79 malibu bracket car has a 383sbc, 10.9 compression,protopline 220 runner heads, solid roller cam 260 266 @ 50 580 lift 112 lobe sep,all crower valve train, team G intake, Holley 750cfm vac secondarys,1 7/8 headers 3in exhaust, powerglide, 4000 stall, ford 9in, 486 gears. Cars gutted weighs 3200 with me in it. Car runs excellent 60ft 1.59 11.70s 113mph

Ok my question is how much if any horsepower am i giving up with the vac secondary carb verses a double pumper? Ive been givin a bunch of flack at the track that not only is the carb to small its a vac secondary. Thanks for any input.

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Old 12-23-2010, 05:35 PM
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i dont have an answer for you, but the whole time i was reading the second half your post i was thinking -
a vac. secondaries carb. on a race car?
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Old 12-23-2010, 05:43 PM
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The CFM your using is fine. As far as the vacuum versus double pumper? I'm partial to the DP carbs and that's all I run even on my street rod. Some people in here like vacuum secondaries but I prefer the mechanical secondaries opening when you smash the pedal to the ground. The DP will open and squirt fuel in the secondaries at about half throttle and continues to squirt fuel as you depress the pedal. The vacuum works on engine vacuum and opens by use of a diaphram and springs. Vacuum secondaries can be adjusted with springs kits, etc to work as well, but if your purpose is racing or brackets, I'd definitely be looking at the DP for sure. As far as horsepower? It shouldn't make a difference. Faster ET's? I'm sure of it...
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Old 12-23-2010, 06:24 PM
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Quote:
Originally Posted by matts37chev
i dont have an answer for you, but the whole time i was reading the second half your post i was thinking -
a vac. secondaries carb. on a race car?
Its just a low budget car and i had the vac secondary carb lying around. Ya that boggles some people it is a drag only car.
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Old 12-23-2010, 07:07 PM
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Generally, if the vehicle weighs over 3600lbs, a vacuum secondary carb will perform better, under 3600lbs, the mechanical secondary carb will work better. Bottom line, if you know how to tune it, you can make both carbs before equally.
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Old 12-23-2010, 07:33 PM
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Quote:
Originally Posted by bracketracer
Its just a low budget car and i had the vac secondary carb lying around. Ya that boggles some people it is a drag only car.
i understand a budget and working with what you got.
and now we are trying to see if its the next best place to spend $$ ?
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Old 12-23-2010, 08:18 PM
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Quote:
Originally Posted by FmrStrtracer
Bottom line, if you know how to tune it, you can make both carbs perform equally.
This is what I think also, if you can properly tune the carb there will be no difference at the track for identical size carbs.

All I use are Vacuum secondary's, a lot of folks do the "monkey see, monkey do" and toss them aside and get a DP, and thats where I get the Vacuums cheap.

You could ideally use a larger carb fore best performance, something like an 850 HP or 1000HP for peak power (avoid the 950, it is a 850 baseplate with a 750 size main body venturi and doesn't really equal 950, no one knows why Holley calls it a 950).You could add the Proform or Holley HP style main body, that takes the carb you have to the 780-800 cfm range. Thin the throttle shafts and switch to button head allen screws in the butterflys and you can get it to 820cfm. Thats my standard operation on the 750's I start with - turn them into 800-820 cfm.

Have you done any secondary spring tuning, pump tuning, etc on the carb you have??
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Old 12-23-2010, 09:09 PM
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Really the only thing ive done is jetting, larger squiters, got the yellow spring in secondarys. It really performs very well, i too have only used vac secondary carbs. Fifteen years of bracketracing this way lol. How much efficentcy you think this combo may be like 90%? Ive done some carb figures and it seems like 850cfm may be better. Shift at 6500rpm goes threw the traps at 6800rpm.
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Old 12-23-2010, 10:12 PM
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What pointed me to your needing a bigger carb is the ET and low MPH. We run a similar combo - '67 El Camino, 383, 11-1 comp, ProTopline 235cc heads, 2.08/1.6" valves, 268/276 dur@.050 .592/.595" lift 108 lobe seperation solid roller, Hurricane intake, "820"cfm Vacuum Holley, 1-3/4" headers, 3" exhaust, Glide w/Brake, 4100 stall, 5.13 gear, 30x13.5 tire(315/60R15 MT Drag Radial) 3400 with driver, 1.52 60', 11.17 @ 118 mph, 10.60 @ 123mph on 125 hp shot.

We have tried an 825 Demon DP, it isn't any better, next spring we are going to try the 975 sleeves and calibration in the Demon.

You might also try a 1.6 rocker on the intake side, it may show a decent improvement. Your MPH seems flat to me in comparison to the 60'.

My previous combo all steel stock '72 Nova, 406 , 10.8-1 on pump 93 octane, Dart Sportsman 200cc ported to 208cc, 2.05/1.6" valves, 260-266 dur@.050, .580/.595" lift(w/1.6 rockers) 108 seperation Crower solid flat tappet, Vic Jr intake, 1" spacer, vintage 950 Holley 3 barrel Vacuum secondary(List #3916-AAS), 1-3/4 headers, 3" collector, 3200 stall footbrake, 4.56 gear, 27x9 Dirt track IMCA Dot legal tire, 3600lbs w/driver 1.69 60',11.50's at 116 mph. Cross at 7200rpm

The old 950 3 barrel vacuum carb was 3 tenths and 3 mph better than either a tricked up 780 vacuum or a 800 DP, I had both on it for testing. The 800 DP was exactly the same as my polished, choke horn removed, thinned throttle shaft, button head throttle plate screwed, 50cc pumped "trick" 780 vacuum, it wasn't any better. It really woke up with that big ole honkin 950 3 barrel(basically an 855 vacuum secondary with the divider between the rear barrels cast "removed", one big oval rear blade). This was after everyone(racing buds) told me it would be too big. Ha Ha, joke ended up being on them Now everyone runs bigger carbs than they used to in the old day(1980's and 90's).

I got the idea to run the old 950 3 barrel from a Hot Rod Magazine article on a Joe Sherman built 406 SBC '69 Nova called "Parameter Perfect", 10 second ET's for $3500(in 1983).
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Old 12-23-2010, 10:32 PM
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Your rite i do think the mph is a little slow, thanks for your input i want to try a larger carb it just seems like its running out a little early its just not pulling that hard at 1,000 ft. with that cam and heads it should pull hard all the way threw. Heck that thing 1/8 mile 7.36 at 95mph
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Old 12-23-2010, 11:49 PM
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Using the NHRA correction of 1.57 for 1/8 to 1/4 looks like you should be capable of 11.55 1/4 mile from that 7.36 in 1/8 mile. Looks like if you picked up a couple mph you'd be right there
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Old 12-24-2010, 02:41 AM
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If you're running brackets, you need consistency over anything else. If you're able to hit your dial in w/the vacuum secondary carb, then you're golden.

Heads up I will always prefer to use a properly set up DP- especially on a fairly aggressively geared, stalled and cammed car such as yours.

I'd be interested to see what Tech @ BG had to say in this case.
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Old 12-24-2010, 07:47 AM
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Thanks guys, thats the thing cobalt this car is very consistent. Great bracket car i run footbrake class no et break.
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