While it may look demure, this iron-headed AMC kicked out an honest 791 hp on E85 fuel and 13 psi of boost. While it may look demure, this iron-headed AMC kicked out an honest 791 hp on E85 fuel and 13 psi of boost. You gotta start somewhere. This 401 started life in a Jeep. The short-block was in good shape, although the original heads were cracked. You gotta start somewhere. This 401 started life in a Jeep. The short-block was in good sh Welcome to the latest AMC engine rodeo. In past years, we’ve showed you tunnel-rams and nitroused small-blocks, so the next logical step was to stuff a blower on the venerable American motor. Old-school provenance has always been to default to the classic 6-71 supercharger projecting carburetors up through the hood. But in the 21st century, you can get much more out of a small centrifugal that you might be able to keep buttoned up under a bulging hood. The distinctive supercharger whine is still a bit difficult to disguise, but with a near 800 hp on tap, maybe it isn’t really important if the rest of the world knows you’ve got power under the hood! Our pals at The Supercharger Store got a call from customer Glen Helmes, who wanted to build a centrifugally urged 401 AMC. The Supercharger Store’s Bob Woods enlisted the help of Larry Peto at Larry’s Engine & Marine to build the engine and then adapt a ProCharger D1-SC supercharger to the front of the 401. With that, we had the makings of a serious street engine. Of course, power plotting always entails trying to squeeze the most from any combination, so it didn’t take long for the guys at the Supercharger Store to suggest testing E85 along with pump gas and water injection. From previous testing, we knew that E85’s added octane and cooling abilities would probably be worth some additional power. Early on in the planning stages, the jungle drums clued us in on this esoteric effort, and we arrived at Larry’s Tucson, Arizona, shop just in time to catch the engine breathing hard on the dyno and spinning up some astonishing numbers. Our 401 was treated to a 0.030-inch overbore, using a set of 4.195-inch Diamond forged pistons, ARP fasteners, a Cometic MLS head gasket, Clevite bearings, and a simple Comp Cams hydraulic flat-tappet cam. Our 401 was treated to a 0.030-inch overbore, using a set of 4.195-inch Diamond forged pis While no supercharged AMC buildup can truthfully be called a budget effort, the plan was still to avoid breaking the bank on the way to horsepower-hero status. So Larry’s decided to retain the stock crank and rods as well as the original iron 401 heads. The rotating assembly wasn’t difficult to prepare with the addition of a set of ARP rod bolts and careful machining of the crank. The top-end part of the plan was a different story, eventually requiring three sets of iron heads to find two heads that weren’t cracked. With experience and hindsight as our guide, a future venture into AMC engine building would be far better executed by spending the money for a set of Edelbrock aluminum heads. The new aluminum castings would have been both more durable and lighter than the original iron stuff. They might also have been ultimately less expensive, as Peto had to spend considerable effort porting the exhaust side to make these production heads work. Let Larry’s experience be your guide. Left: With a nod to the all-important budget, we chose a flat-tappet hydraulic Comp cam and matching timing set to lift the valves. This necessitated using Comp’s Break-In oil to ensure the cam and lifters survived the first 10 minutes of engine time. Left: With a nod to the all-important budget, we chose a flat-tappet hydraulic Comp cam an Clouds of black cast-iron dust aside, the buildup of the 401 followed a fairly predictable path to power. Extra effort is required on the lubrication system, as AMC engines are known for being vulnerable to distributor-gear failure and are possibly weak in delivering sufficient oil to the rear mains. Larry dealt with all these issues, which clearly contributed to the engine surviving the rigors of near-800hp, supercharged runs. Looking back, it would probably have been a good idea to add a set of aftermarket connecting rods, too. Follow along as we detail both the buildup and the dyno test of the American way to horsepower happiness. Flow Chart Valve Lift Intake Exhaust 0.100 71 55 0.200 128 106 0.300 189 146 0.400 235 169 0.500 238 180 0.600 242 180 0.700 247 N/A Cam Specs Camshaft AB5445/5200H-14 Duration (Adv.) Duration at 0.050 Lift (inches) Lobe Separation Intake 273 230 0.523 114 Exhaust 290 236 0.523 N/A Diamond custom-made a set of 4.195-inch dished pistons that were fitted with Total Seal’s 1⁄16-, 1⁄16-, 3⁄16-inch ring set featuring a gapless top ring and a Napier oil-control second ring. Larry’s also used Total Seal’s special dry-cylinder wall coating to ensure the rings seated properly during the first few critical minutes of engine operation. Diamond custom-made a set of 4.195-inch dished pistons that were fitted with Total Seal’s Larry’s did all the machine work, including align-honing the block after fitting the two-bolt main caps with ARP studs. The stock rods were also reconditioned with ARP rod bolts and carefully pressed onto the Diamond pistons. Larry’s did all the machine work, including align-honing the block after fitting the two-b Larry’s also dumped the stock 401 valves in favor of a set of SI stainless-steel 2.083 intake and 1.688 exhaust valves and some simple bowl work. Larry’s also dumped the stock 401 valves in favor of a set of SI stainless-steel 2.083 int As you can see, the majority of the porting effort was reserved for the exhaust side. This produced a 74 percent exhaust-to-intake flow relationship at 0.500-inch valve lift, which helps the engine push the exhaust out at peak engine speeds. As you can see, the majority of the porting effort was reserved for the exhaust side. This AMC engines are well known for troublesome lubrication issues, so Peto added this -8 AN oil fitting that bypasses the stock lifter galley to ensure sufficient oil flow to the rear two main bearings. AMC engines are well known for troublesome lubrication issues, so Peto added this -8 AN oi Once the stock iron castings were ported, Larry’s added the valves and new Comp springs, retainers, and keepers along with a set of Comp roller rockers. Once the stock iron castings were ported, Larry’s added the valves and new Comp springs, r 1 | 2 | » | View Full Article By Jeff Smith Enjoyed this Post? Subscribe to our RSS Feed, or use your favorite social media to recommend us to friends and colleagues!