Single Carb or Dual Quads which is better?

[427 S/O]

IMO, 450 mechanical secondaries, with some modifications, are best. You control the secondaries and you learn real fast when to make her zoom...............

[mickmate]

I like Johns too (The man from Nantucket) they work and they look awesome. I have a dual quad set up for mine and can't wait to dial them in. The cars I know that have them have very little trouble with them.

[601HP]

After several years I changed from 1x4 to 2x4's (600 cfm vacuum secondaries). The power increase was surprising, but I expect the majority of that increase was due to the dual S&H air cleaners as Naumoff stated.

I did some minor tuning/tweaking that wasn't difficult. I found it interesting. The 2x4 setup is quite smooth for me.

The wow factor appeals to me. My wife misses the turkey pan.

David

[CobraDan]

[Power Surge]

I like the look of the 1x4 with the turkey pan, which is why I chose that for my car. It went a little against my grain of originality, since S/Cs came with 2x4s. But since S/Cs were leftover comp cars, and comp cars used 1x4, well that worked for me . I think the 1x4 with T-pan looks more period correct than the 2x4 does, but that's just my personal taste.

I have tuned plenty of cars with 2x4s, and as long as they are setup right, they run great. In particular, the throttle response from the dual 450 mechanical setup is unlike anything else.

[601HP]

I am a big fan of horsepower and torque, but that twin Paxton setup intimidates me. You'd have to drug me into a stupor to get me in that car as a passenger.

David

[patrickt]

Originally Posted by Power Surge

I like the look of the 1x4 with the turkey pan, which is why I chose that for my car.

Absolutely correct.

[Kzof]

I do, always have, and always will believe in one for its simplicity.

[rsb427]

I have dual 4 barrels 2 600 holleys on med rise intake healthy cam alum heads etc. wow factor is awesome! runs good i dont ever mess with them! have almost 2,000 miles on car no problems and when I stand on it look out! it gets it!! only thing I have noticed is with 10% ethenall in gas now (in oregon ) idle is not what it used to be have to run about 3 gallons of 110 octane mixed with 12 of super to run good but that has had an effect on alot of friends cars as well so its up to you but whenever i lift the hood i always get the attention!!!

[Excaliber]

I never thought about the Turkey pan being in the way of the carb, but now that I do, man what a hassle that would be! I've had a bunch of carbs on my ride and tinkered and tweaked them no end until I finally got a combination I liked. That T Pan would have drove me nuts while getting stuff dialed in.

[patrickt]

Originally Posted by Excaliber

I never thought about the Turkey pan being in the way of the carb, but now that I do, man what a hassle that would be! I've had a bunch of carbs on my ride and tinkered and tweaked them no end until I finally got a combination I liked. That T Pan would have drove me nuts while getting stuff dialed in.

I do my carb with a vacuum gauge. I believe it would be physically impossible for me to get my fat, stubby fingers around the vacuum fitting at the base of the carb without my T-pan being removable.

[SuperHart]

Not to get into a discussion about mechanical versus vacuum secondaries, but Shelby used vacuum secondary carbs for a good reason, the secondaries open as engine demand increases rather than being either closed or wide open with those extra pump shots of fuel to cover the flat spots which is a waste on the street. Of course, I talk about only Holley carbs. Sorry Edelbrock and Carter AFB lovers, those carbs just don't cut it on a Shelby Cobra. Using a pair of 600 cfm carbs on your 427 is still overkill. A 427 at 7000 RPM assuming 100% efficiency will only need 875 cfm of venturi area. I had a friend who used a pair of Holley #8007 390 cfm carbs on his 427, fiddled with the jets a little and loves them to death. Then again, he drives 100% on the street and never pushes the motor to the breaking point. In addition, when mounted on the intake and covered with the S&H air cleaners, they look identical to the 600s. One last point, if you choose to run dual Holleys, ALWAYS use the dual quad secondary diaphragm cover kit to equalize the secondary diaphragms.

[patrickt]

Originally Posted by SuperHart

Not to get into a discussion about mechanical versus vacuum secondaries...

There are many nay-sayers on this board, but the truth is that mechanical secondaries offer our cars no advantages whatsoever over a properly sized and tuned vacuum setup. When properly adjusted, vacuum secondaries get the nod every time.

[Excaliber]

I was running mechanical secondaries with a pair of 600 cfm 'center squirters', which are basically 'drag race' carbs, not even a power valve on them! I did like the 'instant kick' they provided when you mashed the gas (this assumes all the planet were lined up, the atmospheric conditions were perfect and Jupiter also aligned with Mars). When they were working man it was great (mpg was horrible, by the way). I swapped them out for vacuum secondaries, same size, smaller jets with a power valve. I DID feel the loss of 'kick' when you mashed the gas hard, BUT, 1/4 mile times were virtually the same (as measured to the hundredths of a second)! They were also more reliable, but then again, those center squirters were old and finicky on the best of days.

[Silverback51]

Originally Posted by Excaliber

I was running mechanical secondaries with a pair of 600 cfm 'center squirters', which are basically 'drag race' carbs, not even a power valve on them! I did like the 'instant kick' they provided when you mashed the gas (this assumes all the planet were lined up, the atmospheric conditions were perfect and Jupiter also aligned with Mars). When they were working man it was great (mpg was horrible, by the way). I swapped them out for vacuum secondaries, same size, smaller jets with a power valve. I DID feel the loss of 'kick' when you mashed the gas hard, BUT, 1/4 mile times were virtually the same (as measured to the hundredths of a second)! They were also more reliable, but then again, those center squirters were old and finicky on the best of days.

I have the 660 center squirters and love them. And personally I also prefer the mechanical secondaries. My foot controls when they open.

But as some have mentioned. They can be a pain in the butt to tune.

[patrickt]

Originally Posted by Excaliber

I swapped them out for vacuum secondaries, same size, smaller jets with a power valve. I DID feel the loss of 'kick' when you mashed the gas hard, BUT, 1/4 mile times were virtually the same...

The mechanicals were giving you a "bog and rally," which fooled you in to thinking you were accelerating faster than you really were. The vacuums were coming on evenly and efficiently -- the proof is always in the numbers, not in the seat of the pants.

[Excaliber]

"Bog and Rally", thats fairly accurate patrickt, describes it well. But every once in awhile (planet alignment) they would hit 'just right' and it was great!

[Michael C Henry]

I have inline dual fours on a Dove tunnel wedge intake manifold. I'm limited to the little trimmed down S&H aircleaners . I think that is my limiter. I need another hoodscoop to get taller elements up into the flow. I'm leaning towards a reversed scoop ,like cowl induction. If I had nothing ,I'd buy a single four intake manifold Like a new Edelbrock Victor set up for multi-port fuel injection and a large single four style air door. and a huge aircleaner.

[greg schroeder]

I guess there are assorted opinions. For me I sort of like the idea of best throttle response, linear throttle response, less weight, more power and getting it to stick to the street. If I did it again I'd definitely get another single Pro-Systems unit with single plane intake and Mickey Thompson ET street drag radials, 275/60 15s rear.

Pro-Systems carb without air cleaner
Pro-Systems carb with air cleaner
SAE corrected wheel power, 427W(small block), non-Superformance exhaust

[Ant]

Originally Posted by greg schroeder

I guess there are assorted opinions. For me I sort of like the idea of best throttle response, linear throttle response, less weight, more power and getting it to stick to the street. If I did it again I'd definitely get another single Pro-Systems unit with single plane intake and Mickey Thompson ET street drag radials, 275/60 15s rear.

Pro-Systems carb without air cleaner
Pro-Systems carb with air cleaner
SAE corrected wheel power, 427W(small block), non-Superformance exhaust

Greg,

I have a Pro-systems 950 road race carb much like yours on my 373, I think its actually a 750cfm but flows more, with over 600bhp I have only dynoed the engine to date, but I think the mechanical secondaries with the fuel pump squirting would give better throttle response than a vacuum secondary carb for road racing, sort of kick in the pants coming out on a turn. The dyno guy compared the response of my engine to a fuel injected engine for nice progressive power. I dont think I will be changing to vacuum secondaries, but I have raced on them and they still work good! Selecting the right carb and calibrating to suit the engine, correct size are important, and with the Pro-systems carb it has some clever attributes over a standard Holley, I havent had a carb out of the box ever run as nice as this one! Keith Craft has done a nice job selecting the parts for this custom built road/race engine.

I have seen some real serious US$3-4000 carbs for racing with clever emulsion blocks etc that cover more track situations, but they are on high tech Transam, and probably Nascar total race engines