Single 4bbl vs. Dual Quad vs. Tri-Power Shoot Out

Looks vs. Power - Who Would Win?

We would like to thank a Garagistry staffer for his help developing this article. We'll begin with his personal story...
"After years of "inspecting" the engine bays of almost every stock or modified Classic at dozens of car shows, I got real tired of seeing the same thing. If there were 200 cars, 198 of them had an Edelbrock dual-plane intake, single Holley 4bbl and a "cake pan" air filter."

"Yea, it's a simple and proven set-up, but it's about as interesting as a cow chewing her cud. I made the choice to do something different." 

Going Against All Logic
"Logic said I was an idiot to be considering anything besides the tried and true single 4bbl, but I refused to even consider it at that time. I began reviewing all available multi-carb set-ups for my BBC (Big Block Chevrolet)."

"Alternate choices would be a dual quad set-up or a high-rise factory Chevy tri-power originally used on 427/435 '67 Corvette engines. At 511 CI, the first test was would both be compatible?"

"At 6,500 RPM, a 511 CI engine would need a minimum of 865 CFM (@ 90% efficiency) and 961 CFM (@ 100% efficiency). Research delivered the following details regarding the factory Tri-Power set-up":

"The L72 (427 CI) had one Holley four-barrel flowing 780 cfm, and the L71 (427 CI) had three Holley two-barrel carburetors capable of feeding the engine at 1,000 cfm.  Along with flowing more air, theTri-power provided a more efficient flow to the cylinders, all of which resulted in 10 more horsepower. However, that 435 rating was not the peak horsepower that the L71 actually produced. It most likely made the same 460 SAE gross hp that LJ2 was to be rated. "

"Therefore the Tri-Power should work. It still had a narrow cushion of about 100 CFM. The dual quad set-ups all had greater capability. There were three other issues; cost, appearance and power output. After a tremendous level of research, I finally found real dyno numbers regarding the dual quad set-ups. They showed +/- 50 HP and about 30 lb-ft (torque) loss as compared to the most modern single 4bbl set-ups. Second issue was the design prevented the use of an HEI distributor. These were a deal killer. No dual quads."

"The next question was what would choosing a Corvette tri-power set-up do to to performance? As noted above, it was unlikely the L71 delivered significantly depreciated performance. But, were there any contemporary examples of use on a larger displacement engine. If so what were the results?"

"Proof came in the form of a nearly identical engine destined for a '67 Corvette. I was able to acquire a back issue of the magazine that featured the article. It included the names and contact information for the engine builder and carburetor specialist who tweaked the Holley's to work on a larger displacement engine. After conversations with both, it was determined I would have no issues using the 45 year old set-up, nor would I suffer measurable loss of power, if any. An additional point I found exciting was their depiction of the tri-power in use."

"It responds in a linear fashion, a lot like the sensation of a well designed turbo. No sudden jolt of power. It just pulls and pulls with a smooth and steady output. It's also probably the closest you can get to a modern FI in terms of fuel efficiency and performance using carburetors."
cast date was 11.17.66 and the carbs were tagged as assembled in '68
The "Kraken" on the dyno stand
"The last issue was cost. Sourcing the full set-up reflected a cost of somewhere between $4,500 and $5,500 plus all of the additional parts (linkage, fuel lines, etc.). Luck showed up in the form of an ad for an OEM intake and  three NIB (new in box) Holley's for $1,800 shipped. I spent a bit more to have the intake restored by Jerry McNeish, likely the highest qualified person in the country to do that. It came back looking as if it were brand new."

 Dyno Day
"Fast forward a few months, my engine was assembled and ready for testing. My objectives were simple. I wanted a "highly streetable" set-up with a broad power curve. I was not disappointed."
Click to ENLARGE

Are These Results Common?
It depends on many factors in addition to the choice of single vs. multiple carbs. Compression ratios, camshaft profile, cylinder heads and even header tube size. But we did come across this video where all three designs were dyno tested.  Before you click the play button, what set up do you think will take top honors?

1 comment:

Td Motion said...

This new Aluminum Jeep Performer Cylinder Head is a bolt-on performance upgrade for 1987-2006 Jeeps with 4.0L inline six cylinder engine. Works with 258 I-6 Too. Half the weight of a Factory Iron Head. Delivers broad power & Torque!!