Video: The Difference Between Dual Quads & Hilborn 8-Stack On A Hemi

The gang at Prestige Motorsports in North Carolina have been featured in these pages before, and they’re back with an interesting comparison on a dual-quad versus individual-runner intake design on a sweet 572 stroker Hemi engine.

The engine is based around an Indy aluminum engine block and a brand-new set of Edelbrock Victor Jr. CNC-machined cylinder heads with 2.320-inch intake and 1.940-inch exhaust valves. Inside, there is a 4340 forged steel crankshaft and connecting rods to go along with a set of custom JE pistons and piston rings. The engine has been set at a street-friendly 10.5:1 compression ratio. To control the valvetrain, they’ve also selected a custom-grind camshaft built to their specifications, along with roller lifters from Comp Cams.

The engine in as-delivered form.

The engine in as-delivered form, complete with March Performance serpentine drive system, Milodon oil pan and MSD Pro-Billet distributor.

Initially, the engine was set up with a pair of Edelbrock carburetors and one of Indy’s Mod Man dual-quad intake manifolds, and for the second half of the testing, they installed one of Hilborn Injection‘s 8-Stack EFI units complete with Holley HP engine management system.

The comparison of the two different styles of induction was done for research purposes, according to engine builder Lloyd McCleary, Sr.

“We were trying to learn more about how the combination works, as this was our first time with the Hilborn injection on the Hemi. We wanted to baseline the engine first with a setup we were familiar with, and then put the injection on top to confirm its performance,” he explains.

Dual quads on the left, Hilborn's 8-Stack on the right. From the very beginning of the test, the Hilborn system is up 31.3 hp at 4,700 rpm along with 35.1 lb-ft torque. It's interesting to note that the injection peaks at 5,700 rpm with 703.1 hp, while the dual quads peak at 5,900 rpm with 693.1 hp. The torque gains under the curve with the 8-Stack are immense and should provide a much more street-friendly powerplant.

The Results


Stacks rule, don’t you agree?

Their research found that the Hilborn Individual Runner EFI system was capable of achieving higher performance than the dual-quad Edelbrock carburetor/Indy Mod Man manifold package on this particular engine combination.

“We found a bunch more torque with the individual runner setup, and it confirmed what we were looking for. We designed the engine with the dual quad intake knowing that was going to be the restriction – the engine has a better base than it does induction. The camshaft was designed around that; our data was telling us that the engine would make around 700 horsepower and make good torque with it. Adding the individual runner manifold showed us that’s where the restriction was,” says McCleary.

Dual quads on the left, Hilborn stacks on the right. Which one would you rather have?

With this big cubic-inch combination, a hotter camshaft could be used to improve performance even more and take advantage of the better induction system, especially in a race application.

“This camshaft used a mild lobe; my next approach would be to pick a more aggressive lobe; you could definitely take this engine to another power level with a better camshaft and the stack injection,” says McCleary.

It takes a lot of air to fill those voluminous intake ports in the Edelbrock cylinder heads.

It takes a lot of air to fill those voluminous intake ports in the Edelbrock cylinder heads.

As a full-service engine builder, Prestige Motorsports strives to provide hardware that not only meets the customer’s expectation in terms of horsepower, but actually performs once installed and isn’t a bucking, snorting, non-friendly package.

“We try to design an engine so that when the customer sticks it into the car, they are going to feel what they want in the seat of their pants. We go to extensive lengths to ensure the engine performs well and that the customer is impressed with it,” McCleary says.

One thing that’s performed with every engine that goes onto the Prestige dynamometer is an acceleration test to help them learn how each particular piece will perform in the real world once it’s delivered to the customer.

“It definitely wants to go with the IR manifold on it – there’s no lack of throttle. Every engine that comes through here we do a simulated throttle test where I’ll load the engine down to about 2,200 rpm and then let it go to make sure it will take it. That gives us a good feel for what will happen when the customer mashes the throttle from that RPM range. This engine was very impressive,” says McCleary.

Digging Deeper Into The IR Improvement

Hilborn’s EFI-R Specialist, Andy Starr, explains why and where the individual runner system perform so much better on this particular engine, when compared to the dual-quad carburetor design.

“It’s the torque that gives the secret away since torque is the function of cylinder fill, period. The IR manifold segregates the intake track, eliminating reversion pulses from opposing cylinders that negatively affect the laminar flow of the working fluid. When we have more laminar flow, air speed picks up resulting in dramatic increase in cylinder fill manifested by the large torque numbers,” says Starr.

“The low speed torque is significant in that it highlights the inefficiencies of the common plenum due to the reversion pulses. These pulses are more effective at disrupting the working fluid at lower rpm than at higher rpm. You’ll see the power and torque numbers start to come together at the higher rpm indicating that the reversion pulses are having less of an effect. The smooth graph is a direct correlation to just how much happier the engine is,” he explains.

Closeups of the Hilborn system with the stacks removed.

He also took the time to assemble the graph below showing where each induction system’s strengths and weaknesses exist, then shared with us how camshaft profile affected this particular engine. Keep in mind that there is nothing wrong with using a dual-quad carburetor design; this test just highlights some of the differences in induction capabilities between the two systems.

“We know that cam duration and overlap are critical in carbureted and common plenum EFI applications. This is due to a direct result in the low vacuum signal which ultimately affects fuel flow (carb) or fuel tuning (EFI). So what happens to cam selection when we remove these issues? We can go with a much more aggressive cam to produce even more power without hurting low speed drivability. But wouldn’t a larger cam hurt the torque? Remember, torque is a function of cylinder fill with the correct percentage of fuel,” Starr says.

“With a carb there is very little booster signal with big cams and therefore a loss of fuel flow. But with an IR design, we can provide the cylinder fill for an increase in torque since the big cam is allowing us more time to create that fill. Obviously there is a point of diminishing return in low speed torque increase, but the power goes up across the board. My rule of thumb is to pick the largest carburetor cam you would be comfortable with and then add 10 degrees to the @.050-duration number.”



About the author

Jason Reiss

Jason draws upon nearly 15 years of experience in the automotive publishing industry. Collaborating with many of the industry's movers and shakers assists him in the creation of compelling technical articles and high-quality race coverage.
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