Dual Quad Carburetors Testing - Twin Win, Or Double Trouble? Part 1
The results of our single-four and dual-quad dyno tests on a hot 331-inch, small-block will prove surprising to some
From the June, 2012 issue of Modified Mustangs & Fords
By Eric English
Photography by Eric English
For a long time now, we've been aware that some of the prime speed pieces of the '60s performance scene are largely missing from the current landscape. We're speaking of multiple carburetors, specifically, the dual four-barrels that were popular on everything from factory muscle cars to modified street brawlers for at least a couple of decades. As well, we'll note that when sanctioning bodies allowed, two-fours were the induction of choice for the racing set--think such disparate venues as NASCAR, Trans-Am, and NHRA drag racing. If multiple carbs were the hot ticket back in the day, why no longer?
While we still see fully restored machines with original eight-barrel induction, the modern enthusiast crowd seems to have largely moved on. Those who have done so in belief that a good single-four makes better power, may have been helped in their opinions by magazine articles in the last decade that purport the same. We can recall several tech stories in various titles that involved hot street V-8s of 350 or fewer cubic inches which, in dual-quad dress, were all blessed with at least 1,000 cfm of airflow. The results for the two-fours were 10-15 horsepower shy of a modern four-barrel setup, with the numbers always couched in a "well, it's still respectable power, and the good looks justify the penalty" attitude. Frankly, we here at MM&F find that a pretty lame perspective, yet are less than convinced the results need to be this way.
Surely the single four-barrel is a straightforward and cost effective choice, so if bang for the buck or simplicity are your priorities, it's hard to argue against it. Yet in today's market, many buyers are looking for performance and something more. It may be visual appeal, historic connection, improved driveability, improved horsepower, or a combination of all. So, do dual quads fit into this equation? They certainly do for the first two elements, but is it really true that the fabled induction will slow down a healthy street motor? If so, there's justified reason for their relative disappearance. However, with the magazine tests referred to above in mind, we're struck by the ton of cfm that 1,000 or more represents on a traditional sized small-block. Can you say over-carbureted? What about using smaller carbs on these smaller cube motors? Yes indeed!
We recently had the opportunity to test two sets of small Holley four-barrels on a healthy small-block Ford, and believe the results are enlightening. One of our parts sources is Carl's Ford Parts in Beloit, Ohio, which specializes in factory-style, multiple carb setups for a variety of vintage Ford engines. Owner Carl Binius was on board with our desire to dig into the subject, and was one of many with whom we discussed the pros and cons of multiples with. We involved racers, product reps, and speed merchants of differing backgrounds, and found advocates and pessimists alike.
But before we dig into the nitty gritty of testing dual quads, we first need to set the table by baselining a couple of popular four-barrel intakes for comparison. With this in mind, we identified two predominant schools of thought in the current '60s car hobby, one being those who are playing with real factory hot rods, or building something to accurately emulate them. Think G.T. 350. Such enthusiasts likely want to stick with the original high performance Cobra intake, or one of several others which are strikingly similar or period correct. The second group of enthusiasts is building cars without concern for originality or historical perspective, rather they're looking more toward ultimate performance, a modern appearance, and/or the best bang for the buck. In the world of current street/strip oriented single-four intakes, Edelbrock's Performer RPM has a rep for being one of the best. With this in mind, we set out to test both sides of the single-four coin by testing with a genuine S2MS Cobra aluminum high-rise from a '66 G.T. 350, followed by a Performer RPM. Holley supplied one of its new aluminum 670-cfm Street Avengers (PN 83670) as a constant for both intakes, and we were impressed by its attributes--light weight (5 pound savings over conventional Holleys), four-corner-idle circuitry/secondary metering block, quick change secondary diaphragm lid, and sparkling finish.
The results of our single-4...
The results of our single-4 and dual-quad dyno tests on a hot 331-inch, small-block will prove surprising to some. Here are two of the six combinations we evaluated--the Cobra intake with Holley 670 Street Avenger, and the Blue Thunder two-4 with Holley 465s.
1 There was nothing wrong...
1 There was nothing wrong with the engine in Randy Dunphy's '65 Falcon at the start of our efforts, but it did lack visual pop, and proved a great candidate for evaluating the potential of dual quads on a small-block Ford. We found the 331-incher running an Edelbrock Performer and older 650-cfm carb, a combo we didn't bother to test for two reasons--1. The focus of the project wasn't to test a bunch of single-four intakes, and 2. The Performer is one of those 20-year-old manifolds that isn't old enough to be cool, or new enough to be state of the art.
2 Our two-4s analysis needed...
2 Our two-4s analysis needed to start with something to compare to, so we decided to baseline with two favorite single-4 intakes-a Cobra-lettered, high-rise original to early GT350s, and a modern Edelbrock Performer RPM. Holley's 670cfm Street Avenger was used on both.
3 We found the Holley 670...
3 We found the Holley 670 (PN 83670) to work great right out of the box, particularly on the Cobra intake. It's clearly an easy route to making good power, and the aluminum construction is an appreciated weight savings. Compared to the pair of traditional Holleys we'll be running as a pair in later tests, this single aluminum carb saves roughly 15 pounds.
4 Impetus for this project...
4 Impetus for this project stemmed from some less-than-stellar dual-quad magazine tests in the last 10 years, along with the memory of a long ago tech story in Hot Rod. The magazine's July and October '68 issues tested a variety of intakes and carbs on Ford's then-new 302. The results indicated dual-quads could be made to run better than a single-four on a pretty modest small-block. Would we find the same?
5 After making several runs...
5 After making several runs with the Cobra/Holley 670 combination, our best rear-wheel numbers were 243.9 hp at 6,000 rpm, and 252.5 lb-ft at 3,750. Factoring items mentioned in the main text, we figure this 331-inch engine was making 330-340 horses at the flywheel.
At home in a '65 Falcon owned by Randy Dunphy, our test engine is a 331-inch Windsor that is similar to what powers thousands of hot Fords across the land. It's fitted with GT40X aluminum heads, roller rockers, Tri-Y headers, and a nice Comp Xtreme Energy hydraulic roller with 224/230 degrees duration at 0.050, and 0.513-inch lift. Up top sat a 650-cfm carb on an Edelbrock Performer intake, a combo chosen simply because it was sitting around when the engine went together. Dunphy had been considering ideas for increasing the visual appeal and performance of his engine, but wasn't willing to commit the budget to a couple of currently popular options--EFI or supercharging. We pitched our ideas about dual quads to Dunphy, and he readily agreed to be involved in our testing. Commonly used cfm formulas call for around 600 cfm for a moderate street 331 such as this one, so the 670 put us just a bit on the aggressive side of the ballpark.
6 It's been more than 45...
6 It's been more than 45 years since the Cobra intake was designed, while the Edelbrock Performer RPM is current generation thinking. It's interesting to note just one of the differences here; that being how the upper plenum (left primary and secondary for both intakes) feeds different cylinders, the Cobra feeding numbers 2, 3, 5, and 8, the Edelbrock feeding 1, 4, 6, and 7. The upper plenum on a dual-plane generally has a nicer shot at the intake port of the cylinder head due to its elevation.
Our testing took place at Blood Enterprises' shop in Auburn, Washington, using a Mustang MD250 chassis dyno. The shop has extensive experience tuning everything from stock restorations to vintage road race and drag cars, so we knew we'd be in good hands. We made a decision to test all intakes with the same 34 degrees of timing Dunphy has found to work well with this motor. Likewise we tested all carbs sans air cleaners, not wanting restrictions there to influence our results. Virginia Classic Mustang supplied a variety of OEM style hardware such as intake water necks, carb studs, and nuts, and once we got everything tightened down, the original Cobra intake/Holley 670 combination performed well. Pulling strongly to our self imposed 6,000 rpm redline, the Holley was surprisingly spot-on out of the box, its No. 65/No. 68 jetting contributing to a near perfect 12.5:1 air fuel ratio (AFR) as read by Blood's wideband O2 sensor. After a couple of runs, dyno operator and technician James Leahy opted to quicken the vacuum secondaries by switching from a plain diaphragm spring to a purple one. Peak horsepower was 243.9 at 6,000 rpm, with 252.5 lb-ft at 3,750.
Keep in mind that a Mustang dyno typically reads in the range of 20 percent lower than the oft-quoted Dynojet, plus the reciprocating loss of the manual transmission drivetrain is along the lines of 15 percent. Factor these into the mix, and Dunphy's 331 looks to be putting out 330-340 flywheel horses--not bad for a tractable street SBF. The fact is, the most important element of dyno tuning isn't ultimate numbers, rather its repeatability and comparison of changes. We certainly got that here.
Swapping for the Edelbrock Performer RPM was quick thanks to Dunphy's mechanic Freddy Jonsson. Throughout our testing, Jonsson spun the wrenches and had carbs and intakes on and off more times that we care to count--thanks a million Freddy! With the improved airflow dynamics of the Performer RPM, we weren't surprised to see initial dyno pulls run a bit leaner than with the Cobra. The 13:1 AFR at peak power wasn't bad, but the engine was only in the realm of an extra 5 hp and 5 lb-ft, and we felt it vital to try and equalize the air/fuel ratios as much as possible. To this end, Leahy fattened up the secondary jets three sizes to No. 71s, and let it fly. The results were much more as anticipated--with A/F ratio near 12.5:1, the Performer RPM showed it's superiority over the vintage Cobra design. In the end, peak numbers were 256.6 hp at 6,000 rpm and 262.1 lb-ft of torque at 4000, an improvement of 12.7 hp and 9.6 lb-ft at the peaks. The numbers were better for the Edelbrock from 2,200 rpm and upwards, thus a very usable gain overall. Modern technology prevailed, and likely would have looked even better on an engine with 6,500rpm capability.
With solid numbers for our four-barrel intakes in hand, we look forward to a dual quad smackdown in the concluding segment of this story. The tuning capability of Blood Enterprises' dyno with a wideband oxygen sensor proved valuable to this point, but will become vital as we move forward--experimenting with four combinations of two-fours. We'll be testing a currently available Blue Thunder intake, and an obsolete Ford piece that was used by 1967 Trans-Am teams and offered through catalogs for a number of years. Each will be tested with two sets of carburetors in an attempt to determine the potential of dual quads. We'll evaluate the 465-cfm Holleys, which Carl's Ford Parts typically packages in its small-block two-fours setup, but we also took the opportunity to test the even smaller Holley 390s thanks to Holley coming through for us with the necessary carbs. We'll stick our neck out a bit here and predict good things for the dual quads compared to a similar vintage four-barrel, but how will they do when stacked up against modern single-four technology? Which intake proved its worth? Which set of Holleys ended up being the better performers? Tune in next time to find out.
7a Shown here are the ports...
7a Shown here are the ports on the '66 Cobra intake (below) and Edelbrock Performer RPM (top).
7b We were surprised to find...
7b We were surprised to find the port size to be very close--within 0.002-inch.
8 With the Holley 670 in...
8 With the Holley 670 in the same configuration as when run on the Cobra intake, we found the air/fuel ratio to be leaner with the Edelbrock Performer RPM--13:1 rather than 12.5:1. Here, the secondary jets get bumped up three sizes to No. 71s, to help hit the 12.5:1 number. Power and torque improved with the richer mixture, and the benefit of dyno tuning was obvious. Peak horsepower and torque from this combination was up 12.7 hp and 9.6 lb-ft over the Cobra.
9 Ford- and Shelby-lettered aluminum four-barrel intakes were sold over the counter, and are kissing cousins to the Cobra-lettered G.T. 350 intake in our test. Even Edelbrock's F4B had a strong familial look, and we suspect very similar performance. Such intakes are available on the used market today, and are popular period pieces, with this Shelby-lettered intake from a vintage Shelby parts catalog advertised to be worth 22-26 additional horsepower when teamed with a 600-cfm Holley. It isn't explained as to whether this gain is based on an original two-barrel or four-barrel configuration.
10 Our upcoming dual quad tests will use the currently available Blue Thunder intake (bottom), and an obsolete Ford/Shelby intake (top) said to be considerably better. We'll have an opportunity to evaluate 780 cfm (two 390s) and 930 cfm (two 465s) on each manifold, both of which would be commonly viewed as "too much" carburetion for a hot street 331-inch engine. Do traditional cfm formulas hold true when delivered via dual quads? We aim to find out.
Dyno Comparision Chart