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Discussion Starter · #1 ·
I am thinking about upgrading the carb on my 347 this winter. I have been very happy with the vacuum secondary 650 Speed Demon that is on there but I think i could make more power on my combo with a larger carb.

What would you guys choose? 750? Vacuum or double pumper? I don't need a choke here in GA.

See sig for combo.
I have been shifting at ~6200-6300. The cam is a Comp. XE274H hydraulic flat tappet 230/236 @ .050"
Headers are 1 5/8" Hooker longtubes

I will switch out the cam to a larger roller someday but for now I want to try out to see if I can pick up some with a simple carb change. I am inclined to just try out a Speed Demon 750. Would it be worth it to go for a Mighty Demon or Holley 750 HP series or equivalent?
 

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A 700DP would work well with the cam you currently have. I was running a similar setup for a while and ran a 700DP on it, then whenever I went with the larger solid roller cam I switched to a 750HP main body.

Too bad with the C4 you have to keep the gearing like it is... otherwise it would be very advantageous to gear it to cross at around 6400-6500.

<font size=-1>[ This Message was edited by: gork1rogues on 11/15/06 1:42am ]</font>
 

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I wouldn't hesitate to go with a 750dp... I'm running one on my little 289 and picked up a couple tenths over my 650dp. Part-throttle driveability didn't degrade as far as I can tell either.
 

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I'd beg borrow or steal a 750 from a buddy or something to see what you thought before I shelled out big $ for a new carb. Especially if yours is working well. Somebody has to have a 750 laying around.
 

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Discussion Starter · #5 ·
If I knew someone around that had a 750 I could try then I would sure do it. That is a great idea. I may just try to find a used one, try it out, and sell the one that doesn't perform as well.

Most people say to go to a double pumper. Will that help over the vacuum secondaries? I've read many places to stick with a vacuum secondary carb for an auto and mechanical secondaries for a stick. Is that old saying intended more for daily drivers?
 

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Go with a holley 750 DP. You will love it. By the way I use a 650 Dp on my 302 so a 347 will like the extra carb.
 

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750 Double Pumper all the way......When you get it set up it will make better power all the way around....Jesse
 

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I agree with the herd on this one.

Double pumpers are better on anything other than a true daily driver.

Bigger carbs are better - I can improve the driveability on a big carb with tuning - I cant make a little carb flow more air. All the common formulae for carb sizing you see are way too conservative - and wrong. Carb airflow is rated at 1.5" of vacuum below the carb - meaning that at the rated flow the carb has become a restriction.
 

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FWIW, I'm running the same cam in a 357W w/ stepped long-tubes and an RPM AirGap intake (much heavier car) with a Bigs-built 750DP. Even from a dead stop with a much-too-tight converter, this thing screams! No dead spots, no bogs.... just balls-to-the-wall.

The only adjustment the carb required was to crack open the secondary butterflies to bring the secondary idle mixture screws back into play (and I squared it with Jesse @ Bigs before I touched it).

I will also add that my combo is only pulling 8.5-9.0" Hg @ 925 RPM idle in park. Timing is currently at 14* BTDC with another 20* advance built into the distributor. The cam was already installed when I got the car, and I've not double checked it myself, so I'm not sure if it was installed straight up, advanced or retarded. Judging by the vacuum, I seriously doubt it's advanced. With the carb, cam, timing and headers it'll jump to 6300+ without breaking a sweat.

Give Jesse @ Bigs a call. Just have all your facts in order and he'll fix you up right.


Kevin
 

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Call and talk to these guys http://www.4secondsflat.com/fuelsystems.html I'm running a 800cfm mighty demon carb with annular boosters on a 347. If you buy from these guys you'll pay a little more, but before they send it out the door they take it apart, and get things pretty close for your engine. Plus they support what they sell.
 

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If you are running a manual transmission then you may get more performance from a carb up to 760 CFM with vacuum secondaries. If you are running an automatic transmission it will not handle as large a carburetor unless you have a high (3000 rpm) stall converter and a lower rear gear. When choosing a double pumper Holley has a chart that asks what the lowest RPM is that you will be opening all four barrels up. If it is low (like from a stop-light) then you need a smaller carb. If you are sure that you will never open the carb up until there is sufficient demand for the secondaries then you can go bigger on the carb.

The Holley Selection guide shows that a 350 cu" engine turning 6000 rpm can use 600 CFM IF it has 100% volumetric efficiency. To use 700 CFM you need to turn a 350 cu" engine to 7000 rpm.

In the selection guide for a 351 cu" Ford "street and strip" they recommend a list #4118. That is a 725 CFM vacuum secondary carb. It is not an emission rated carb and they warn not to use it on the street where local emission laws apply. For racing use only they recommend a list #4777 which is a 650CFM double pumper.
Those are the carbs that Holley recommends for a '66 Mustang with a 351W. Note that the 4777 will not hook up to an automatic transmission so you will need to adapt a kick-down linkage kit if you are running an automatic.
Paul
 
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