Rat Buster Intake Manifold - Dual Holleys For The Hemi

Like the Street Hemi manifold, the Rat Buster bolts to the heads using sixteen 1/4-inch coarse thread fasteners. The exception is while the Street Hemi uses a combination of nine 2 1/2-inch bolts and seven 3-inch bolts, the Rat Buster is designed to use only the long, 3-inch bolts. We got our bolts and hardened AN washers from Specialty Fasteners.

Specialty Fasteners also supplied eight 5/16x 1 1/2 coarse-fine combination carburetor studs, washers, and hex nuts. A few of the carburetor stud holes break through into the plenum area and must be sealed against vacuum leaks with plumbers tape or paste (as shown). A 3/8-inch pipe plug must also be used to seal the vacuum port at the rear of the manifold. Yes, the Rat Buster is compatible with power brakes, but our Dart test car has manual discs.

Here's a look at the stock Street Hemi throttle cable bracket (a Year One reproduction, PN A0801). Because of the Rat Buster's revised carburetor placement, this bracket places the end of the throttle cable too close to the carburetor throttle lever. A rework is mandatory.

For carburetion, we specified a pair of vacuum secondary 4160 series Holley 750s (PN 0-3310C) to feed the hungry 520-cube Hemi and to compete fairly with the duo of 750-cfm AFB-type carburetors used for baseline tests. Holley 3310s come standard with cathedral-type, center-hung float bowls and must be converted to side-hung, short bowl status to fit the 8.75-inch distance between air cleaner studs designed into the Rat Buster manifold. This photo shows a box stock 4160 on the left, a 4160 after conversion to short bowls, and the parts we replaced in the foreground. The conversion parts needed are: (2) short primary float bowls (PN 134-101), (2) short secondary float bowls (PN 134-105), (4) side-hung brass floats (PN 116-4), and (2) fuel transfer tube kits (PN 26-115). An alternative would be to source a pair of OE short bowl Holleys from a Ford 427 8-V application or switch to a set of side-hung (short bowl) Holley 600s (PN 0-1850). To see the specific step-by-step details of our carburetor conversion process, visit www.moparmusclemagazine.com.

The only snag we encountered after mounting the reworked 750s was interference between the transfer tubes and a portion of the throttle lever that serves as the transmission kick-down lever on Ford installations. On the rear carburetor, we simply bent the lever outward slightly to achieve the needed transfer tube clearance. Things were a little more complicated at the front carburetor as the superfluous kick-down lever also interfered with our hand-made 5/16-inch fuel line. Trimming the offending lever with a hacksaw blade solved the issue.

From left to right, this comparison shows the stock Hemi bracket, an evolutionary tin mockup bracket used to test progress and the final bracket design, which relocates the throttle cable hold-down clamp 2 inches rearward, so the Dart's existing throttle cable could be retained. We made the bracket ourselves in an hour from an .100-inch-thick steel plate, which was hammer-formed on a bench vise, trimmed with a hack saw, then drilled to fit the fasteners. Our Dart's full-manual California Performance Transmission Torqueflite doesn't require a kick-down rod or the bellcrank pivot pin, so these features were not transferred to the new bracket.

Ready for testing, the completed Rat Buster/Holley installation gives the Hemi a whole new look. Our handmade 5/16-inch fuel lines tap into the stock brass Hemi T-fitting and feed the primary bowls via dual 90-degree Holley banjo fittings (PN 26-25) and short lengths of rubber hose. The universal throttle linkage kit is from Mr. Gasket (PN 3830), the throttle return springs are from Pep Boys, and the reproduction Street Hemi return spring bracket is from Year One (PN RB1012).

Testing, Testing
For the chassis dyno portion of our testing, we went to Joe Jill's Superior Automotive where Don Spaccarotella hammered the throttle. With the Vanke-modified Street Hemi intake and dual 750 AFB-style carburetors, the Dart made 501.4 hp at 6,250 rpm and was still showing 446.3 lb-ft of torque at 5,820 rpm.

To dispel some of the myths about Holley tuning difficulty, we enlisted the help of 18-year-old Steve Benoit. If he can do it, so can you. Right out of the box, the stock Holleys delivered 484.8 hp and 427.4 lb-ft, down significantly from the baseline test. A second pull revealed visual confirmation that the stock "black" secondary diaphragm springs were too stiff to be overcome by the available vacuum signal and only allowed one-quarter opening of the secondaries.

Replacing the springs in both carburetors with the shortest, lightest "yellow" springs in the Holley secondary diaphragm spring kit (PN 20-13) allowed full opening of the secondaries and resulted in 517.9 hp and 445.2 lb-ft. But we wanted more.

Thanks to the dyno's O2 sensor, we saw the air/fuel ratio at full power was a bit rich at 11.2:1. To introduce more air at full throttle without disturbing the favorable box stock jetting (No. 70 jets in primary metering blocks, No. 21 secondary metering plates, equal to No. 75 jets), the secondary side high-speed air bleeds of both carburetors were enlarged from 0.025 to 0.038 using a pin drill. The primary side high-speed air bleeds, as well as the low-speed air bleeds that admit extra air for idle and low speed operation of both carburetors, were not altered. This simple modification netted the best readings of the day: 523.7 hp and 453.5 lb-ft and also leaned the air/fuel ratio down to 12.5:1, ideal for maximum power. By drilling the high-speed air bleeds, more air was admitted to the secondary side of each carburetor at WOT. An alternate method would be to remove the carburetors and change to secondary metering plates with smaller orifices to reduce the volume of gasoline flow. But this solution could also impact part-throttle engine performance, possibly causing a lean surge in freeway cruising situations. Though drilling the high-speed air bleeds is a permanent modification and must be done in small increments, it can be a worthwhile means of manipulating the A/F ratio at WOT without disturbing factory calibrations elsewhere in the carburetor's theatre of operation.

Armed with the Rat Buster, dual Holleys, and an extra 22.3 hp, we returned to the Irwindale Dragstrip where the Dart's best e.t. with the factory-style Street Hemi induction was a 7.185 at 97.39 mph. With the updated induction, the new time slip read 6.993 at 101.62, an improvement of nearly two-tenths and over 4 mph. Multiplying the eighth-mile e.t. data by 1.57 approximates quarter-mile data and shows the Rat Buster is responsible for transforming this 11.28-second Dart into a 10.97-second stormer.

The brawny Holleys do a great job of filling the space between the rocker covers. Paint the manifold a Hemi Orange color, and it could pass for factory equipment. ours is raw, but we'll soon be shaking the rattle can.

Finding the right air cleaner has always been a major stumbling block for Hemi owners, and the extra 2 inches of space between the Rat Buster's Holleys rules out adapting factory Street Hemi bases and lids. For now, we're stuck with these rinky-dink 6 1/2x2 1/2 filters, though Stage V Engineering says they're working on a low-profile air cleaner that accepts Fram reproduction oval Street Hemi filter elements and has factory vibes. Though we conducted all before/after testing minus the seemingly puny filters, we were surprised to discover they only reduced peak power output by around 15 hp on both manifolds.