Rat Buster Intake Manifold - Dual Holleys For The Hemi
Rat Buster Intake Manifold Breaks New Ground
From the May, 2005 issue of Mopar Muscle
By Steve Magnante
Photography by Steve Magnante
There's great news for builders, drivers, and racers of 426-style Hemi engines. Stage V Engineering has just introduced the Rat Buster single-plane aluminum intake manifold. Unlike many of the latest aftermarket offerings for the 426-style elephant, the Rat Buster is a dual four-barrel design. We don't know about you, but single four-barrel carburetor manifolds, even those boasting a big Holley Dominator, look lost between the broad-shouldered Hemi's rocker covers, not so with the Rat Buster.
But the classic dual-quad look of the Street Hemi is only part of the Rat Buster story. Because its designers added 2 inches to the distance between the carburetors, the Rat Buster finally allows Hemi owners to explore and enjoy the performance of Holley carburetors without resorting to a cross-ram or tunnel-ram configuration. That's right, you can finally put Holleys on your Hemi, and it all fits underhood.
Street Hemi Versus Rat Buster...
Street Hemi Versus Rat Buster Manifold
From below, the dual-plane architecture of the vintage Street Hemi intake (left) contrasts sharply with the Rat Buster's single-plane configuration. The rectangular section spanning the two x-forms contains a passage to equalize manifold vacuum between the separate plenums beneath each carburetor.
Best of all, with prices of original '66-'71 Chrysler Street Hemi intake manifolds approaching the stratosphere, the Rat Buster sells for $495. Pretty low bucks, considering it's a modern street/strip design, and it opens a whole new world of carburetion possibilities, while maintaining the classic dual-quad vibe we've come to associate with the mighty 426 and its even larger, stroker-equipped offspring.
To see what the new Rat Buster has to offer, we stuck one on a 520ci Stage V Hemi conversion motor, added a set of short-bowl-equipped Holley 750 vacuum secondary carburetors, and picked up 22 hp over a Vanke-modified '67 Street Hemi intake manifold and dual 750 AFB-style carburetors on the chassis dyno. Comparison testing on an eighth-mile dragstrip of the same motor in a 3,400 pound Dart saw the Rat Buster and Holleys increase trap speed by better than 4 mph and reduce the e.t. by nearly two-tenths of a second. Remember, that's on an eighth-mile track. On a full quarter, these improvements would nearly double. All the while, the Dart's docile street manners and 11-mpg fuel economy remained on par with the Street Hemi/AFB baseline induction combo. These are good times for Hemi fanatics, and the new Stage V Rat Buster makes them even better.
At 19 pounds bare, the new...
At 19 pounds bare, the new Rat Buster is four pounds lighter than our Vanke-modified Street Hemi intake. The Vanke modification is named after Super Stock drag racing legend Arlen "Akron" Vanke and involves milling away the dividing walls and cylindrical passages that separate the left and right banks on a stock Hemi. This vastly increases the volume of fuel available to each piston during the intake stroke. The huge open plenum chambers of the Rat Buster do the same job, but with even greater efficiency.
The Rat Buster (left) is nonheated...
The Rat Buster (left) is nonheated and does away with the stock Street Hemi's exhaust heat transfer tubes. Elimination of hot spots beneath the rear carburetor increases intake charge density and adds power. Guys who buy Rat Busters aren't sticklers for originality and are likely to run headers rather than expensive iron manifolds. They want to go fast.
Rat Buster plenums empty directly...
Rat Buster plenums empty directly into individual runners for maximum breathing. Despite a free-flowing design, the manifold is only .100 inch taller than a Street Hemi manifold and fits beneath stock hoods.
The Rat Buster accepts original...
The Rat Buster accepts original or reproduction MP Street Hemi oil splash shields (PN P4529431) to further reduce intake charge temperature. We attempted to transfer the ten stock attaching bolts, but they were .020 inch too long and bottomed out in the .730 inch deep bolt holes, so we ground .250 inch off each bolt to correct the problem. Be sure to use a high-strength, thread-locking compound to prevent loose fasteners from entering the lifter valley.
Installing the Rat Buster...
Installing the Rat Buster on our '67 Dart test car was a simple matter of removing the Street Hemi induction (after the conclusion of baseline chassis dyno and dragstrip testing) and replacing it with the new goodies. We like to squeeze a thin bead of Fel-Pro yellow gasket adhesive (PN YEL5) to stick the gaskets to the heads.
While MP intake manifold gaskets...
While MP intake manifold gaskets (PN P4120086) are acceptable, Stage V Engineering offers these .060-inch thick paper composite gaskets that offer wider sealing surfaces for less chance of leaks. The application of a thin film of white grease allows for easy, no-stick manifold removal, so the gaskets can be reused several times. A thick bead of RTV silicone seals the ends of the manifold to the block. Just trim the excess with a knife after it cures.
Art Carr Saves The Day
We must extend a hearty thanks to drag racing Torqueflite pioneer Art Carr's California Performance Transmission for reviving the Dart's ailing 727. Midway through testing of the Street Hemi induction, it developed slippage on the 2-3 upshift that threatened to invalidate further data. Art's crew installed new bands and clutches so we could continue.
| ||Street Hemi||Rat Buster|
Like the Street Hemi manifold,...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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,...
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,...
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...
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...
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.