Junkyard Hemi, Part 4: We test a stock intake against the Drag Pack single plane intake with a carburetor. Guess which one makes more power.
So far in the build of this neglected 5.7L Hemi, we got the engine running to establish a baseline of 376 hp and 414 lb-ft. Next, we changed the cam and springs, and on Westech’s Superflow engine dyno, we saw power jump to 434 hp and 440 lb-ft. After that, we experimented with different header designs, settling on a set of Hooker 1 7/8-inch-long tube headers with 3-inch collectors, which bumped the power up to 442 hp at 6,200 rpm and just slightly reduced torque output to 437 lb-ft at 4,500 rpm.
Now, we turn our attention to the induction side. The intake and exhaust are often thought of as independent entities, but the reality is they not only have much in common, but they are also interdependent.
As we saw in the header testing, the primary tube length played a major role in shaping the torque curve. Differing primary tube lengths and diameters influence the scavenging effect of the exhaust pressure waves to not only evacuate exhaust gases from the combustion chamber, but to also help draw charge air from the induction system. Naturally, this occurs during the overlap phase, where the intake and exhaust valves are open.
On the induction side, things like runner length, diameter, and taper play a major part in torque production. Much like the primary length in the headers, longer runners help promote torque production, especially at lower engine speeds. The important thing to remember is that intake manifolds are rpm specific, meaning they are designed to be efficient over a specific rpm range. The shorter the runners, the higher the tuned engine speed. The longer the runners, the lower the optimum engine speed.
See all 8 photos
To illustrate this point, we decided to compare an OE Magnum / 300C intake against the radically different Mopar Performance Drag Pack intake. You couldn’t get much different than the stock, port fuel injected manifold with its long runners, and the tall, carbureted single plane designed for drag racing. As the results show, the Drag Pack intake was obviously designed for a much more radical combination than our mild 5.7L.
See all 8 photos
To review the specs of the engine. This is an early Gen III Hemi that came out of a Ram pickup. We ditched the MDS cam and lifters, and installed Comp’s HRT Stage 2 cam with 220 / 230 degrees of duration at 0.050 inch tappet lift and 0.596 / 0.582 valve lift with a 113 degree lobe separation angle.
See all 8 photos
For all the EFI testing we have done so far, we’ve used Holley’s HP management system and MSD ignition coils.
See all 8 photos
For this round of testing, we ran the engine with Hooker’s 1 7/8-inch, long-tube headers. With the Magnum intake installed, the Hemi produced 442 hp at 6,100 rpm and 437 lb-ft of torque at 4,500 rpm. This long-runner, composite intake is our first choice for a street application, despite the fact that the aluminum SRT8 intake does offer slightly more peak power, though at the cost of low-speed torque.
See all 8 photos
With our EFI baseline done, it was time for some carburetion, so we removed the Magnum intake and prepared to drop on the Drag Pack manifold. It required adaptors between it and our early Gen III heads.
See all 8 photos
Because the intake has no provisions for vacuum ports, we installed this -inch, open spacer equipped with fittings for our map sensor.
See all 8 photos
Topped with a Holley 650 cfm Ultra XP carburetor, the engine made 443 hp at 6,200 rpm and 409 lb-ft of torque at 5,300 rpm. For those of you thinking we went too small with the carburetor, think again. We tried a larger 750 cfm carburetor, and the result was no extra peak power, and a loss in power below 5,000 rpm.
Conclusion
In all, the carbureted combo offered 1 extra horsepower, but the torque curve took a big hit compared to the Magnum intake. We knew the Drag Pack was not the right choice for this mild combination, but the test did illustrate the dramatic change in the power curve offered by alterations to the induction system. Believe it or not, intake manifolds really are application specific. The Drag-Pack intake was obviously designed for a much more radical combination. The problem wasn’t so much a bad design, as it was a misapplication. The Drag Pack intake was obviously not a good choice for our mild, cammed 5.7L, but would certainly be better suited to a wilder combination with increased displacement, compression, and cam timing. We’ve seen naturally aspirated combinations exceed 700 hp with the Drag Pack intake, so we know it works, just don’t try it on your Ram truck engine!
Sources
Comp Cams
(901) 795-2400
Compcams.com
Holley/Hooker/MSD
(866) 464-6553
www.holley.com
See all 8 photos
Source: Read Full Article