331 carb size - Ford Mustang Forum
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post #1 of 8 (permalink) Old 1 Day Ago Thread Starter
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331 carb size

Time to trade in my vacuum for a double pumper.

I have a 331 DSS stroker Pro Bullet engine. 68 Mustang coupe. 3.44-1 Eaton Detroit TrueTrac 8" rear, T5 tranny. Edelbrock Performer RPM dual plane intake. Pertronix ignition, Trick Flow Twisted Wedge Aluminum heads.
Intake valve is 2.02, Exhaust is 1.60, 61cc/170cc, TFS-51402000 stage one cam, Intake duration 221 @ .050, Exhaust duration 225 @.050, Intake lift at valve .499, Exhaust lift at valve .510 w/1.6 hydraulic roller rocker setup. Compression is about 10.25, Mechanical fuel pump rated at 110 GPH, Cam lobe separation is 112. Tri Y header with Magna Flow mufflers and 2.5 pipes. Idles at 750 RPM with 15 inches vacuum. Currently I have a Holley 670 street Advenger Vacuum secondaries with 72/74 jets with yellow spring in vacuum housing. Car runs good and pulls strong. Put about 800 to 1000 miles a year with no track time. I want to replace current carb with a double pumper. Tom at DSS recommends a double pumper Quick Fuel 650 SS carbs which he sells. Called AED carbs twice and talked to two different people. One person recommends the AED Performance Ultra 650 HO (said 750 would work but likes 650 since I don't take car to the track) and the other person was for the 750 version. These two AED carbs are not custom builds. I also called ProSystems and talked to Patrick and he recommended a custom build 780 Pro Series 4150. Price difference is about $100 between them.

I'd like comments on carb sizes and comments on carbs that I mentioned above.

Is anyone running a similar setup as mine and what size carb are you running

Thanks for any and all info

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post #2 of 8 (permalink) Old 1 Day Ago
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Invest in an AFR gauge setup, use your old carb as a guide and see if it keeps the engine in a good A/F band. Then make an educated decision.


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post #3 of 8 (permalink) Old 1 Day Ago Thread Starter
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I have an AEM A/F gauge installed. A/F shows idle is 13.5 to 14. Steady increase from 1500 to 5000 RPM is around 12.5 to 13. 1500 to 5000 at WOT is about 11 to 11.5 while lighting up the tires. I feel the A/F numbers are good. The 670 is really OK. Vacuum secondary's are a little slow coming on, that's all. I just want the ole stang to be a little more fun to drive and I feel the double pumper will give me what I'm looking for. Just trying to decide which CFM carb to buy. Talking to different carb guys I get different opinions. Just want to see what people on this site say.
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post #4 of 8 (permalink) Old 1 Day Ago
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The best way is to buy one, try it out, if you don't like it, try a different one until you find the one you like. Your likes and dislikes will be unique to you and no one will have the perfect advice for you. Trial and error is the way to go for anything custom.
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post #5 of 8 (permalink) Old 15 Hours Ago
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A 750 with annular boosters would probably be a great choice. (yeah, I know, everyone's shocked. I'm suggesting a 'big carb!') Thing is, with carburetors, it's not just about how many CFM they are labeled with, but more about how well they can deliver a stable stream of fuel, and atomize it properly. Since you're dead-set on a double pumper (which actually is a kind of a lousy choice for a street only car by the way), best get one that can actually deliver peak performance with a very low pressure drop at WOT, and still get the fuel mixed properly.

Just make sure you shell out for the annulars. With the normal boosters, I think you'd be disappointed in basically every aspect of the carb *except* WOT.

If I were in your shoes, and just burning to get a different carburetor, I'd get this instead: Summit M-Series 750

It's vac secondaries, easier to tune, and would deliver better part throttle response and economy, hands down. Similar peak power, too. Or, just tune up your existing carb and figure out what the performance lag actually is. I'd start with having your dizzy's timing recurved by someone who knows what they are doing. Very rarely is a bog related to the secondaries not opening as they're supposed to. Double Pumpers are not known for fixing 'bogs', but more typically create them. Since they don't care what your engine actually wants, fuel-wise, they just drop the manifold pressure to 0, and rain fuel. When it comes to full throttle, the engine eventually figures out what to do, and goes, hopefully starting to draw fuel from the venturii before the squirters stop shooting. At part throttle though, a double-pumper is pretty challenged to come up with the right combination of fuel and air at any given time, and is usually a poor compromise of both.
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post #6 of 8 (permalink) Old 13 Hours Ago
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"Tom" is an idiotÖÖÖÖ run the basic CFM calcs for your engine.....500-650 cfm is plenty for your engine. BTW, the 2.02 intake valves, although they ae run on street vehicles, it has never been my 1st choice as at typical low speed/rpm operation they can induce unburn fuel droplets which over time does work it's way into the oil pan..... without consistent/frequent oil changes or running at freeway speeds consistently the potential of wiping out the crankshaft bearings is real..... had many a friend experience this....including one who owned a Cobra (kit)Ö. the double pumper provides absolutely no benefit in regards to tuning, drivability, etc.....with the exception of annular booster type carbs as Grim has very nicely explained!
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post #7 of 8 (permalink) Old 11 Hours Ago
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Have you had it dynoed before? If you haven't, I'd spend the money on some runs. I wouldn't throw a good carb away for something untested unless you know for sure it will work , a new carb will surely start the vehicle up but as you may know will need to be tweaked to get it where you want it to be. I'm pretty sure your 670 cfm still has room for improvement and here where some dyno runs will come handy. I run my 347 with a 600 cfm weber, I had the carb professionally rebuilt then it took a trip to Cali to be dynoed with my engine. Carb had metering rods swapped out and jets as well. Engine shop assured me I could see close to 450 hp with a 650 cfm but I'm happy with my 429..




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1965 Mustang Resto-Modded with a classic charm
347 ci stroker 10:1 CR ( street warrior engine) Weiand Stealth,mild roller camshaft, intake ported to AFR 185 cc ,CompCams GOLD 1.6 Roller rockers, Hedman ceramic shorties
AOD stage 1(450 HP), 3.80 gears, MG Trac Lok LSD, 15" shoes
1970 Mustang fastback BOSS 302 tribute in the works..
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post #8 of 8 (permalink) Old 8 Hours Ago
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A smaller carburetor covers a lot of tuning issues, because even though it might rob you of a few horses on the top end, you spend literally 99% of your time NOT at wide open throttle anyway, at peak RPM. A 'too big' carburetor with lazy flow through the primaries and boosters will feel sluggish and unresponsive, delivering a lean mix because there's simply not enough airflow to suck fuel out of the bowls.

Still, a 'just right' carburetor is always best. Your engine, given what it is, may be hitting something like 110-115% volumetric efficiency if it's set up right. If your redline is 6k, and you're pulling 100% VE, you'd need about 570 CFM. But with your heads and cam, you might be pulling a few more RPMs, and may be hitting higher VE numbers, as I mentioned. If that's the case, you could be needing closer to 700 CFM at peak. There just aren't many good carbs 'rated' at 700CFM (and knowledgeable people will admit, the numbers on carbs are really mostly just advertising, so people have a rough idea what they are getting).

Better to have a very free-flowing carb, if you're after maximum power. But along with that, the big open ports will do you no favors for fuel atomization. A downleg type booster would be horrible for the street use you're talking about, especially with a double-pumper. It'll run lean a lot of the time, and when it finally does start dribbling gas down the bore, it'll be big fat drops that do not burn well. So, bye bye crisp throttle response, and bye bye gas mileage. Annulars will offer a good compromise even in a big carb, because they respond even to weak vacuum signal with enthusiasm, and provide very good mixture quality.
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