Want sick power from a basically stock engine? Try a 401. Using factoryheads on the portly 4.170-inch bore, you can build torque where itcounts and still make good power on top with a reasonable cam, goodvacuum, and off-the-shelf parts. In the case of the buildup we'reoutlining here, the payoff was 484 hp at a streetable 5,600 rpm andnearly 500 lb-ft of torque at 4,600, with more than 450 lb-ft from 2,500to the horsepower peak.
With the help of JT Payton at Payton's Performance, Holley MarketingManager Bill Tichner did all of that. His goal for the 401 was to use asmany stock parts as possible and create as much power as possible whilestill maintaining at least 13 inches of vacuum for an aftermarket fuelinjection system to come later. The performance key to this engine isheavy port work on the iron heads, attention to the oiling system, and acouple of other tricks we will outline in this story. This engine'sdestiny is to be stabbed into an AMX, and with a little shot of spray,lift the wheels at its favorite track.
The Block
The practical limit of the plebian AMC 401 is about 500 hp at 7,500 rpm.The main caps will begin to walk above that, so limit these engines tohealthy street mills if you want to keep the price down and use stockparts. Just because we're interested in this kind of thing, we foundthat with an available four-bolt main kit, you can go to 700 hp and9,500 rpm for the full-race Chihuahua. Whee. But this is a streetengine, so Tichner used a post-'70, tall-deck block bored 0.030-over for4.195 inches and used the stock two-bolt main caps with big-block Chevystuds from ARP.
We'd recommend using the stock bore if you can. Payton's says the highnickel content in these blocks makes them resistant to wear, and0.030-over is on the limit of the 401's block design. So if you don'thave to bore, hone it and move on.
The Rotating Assembly
Lots of goodness here. The stock rods and crank on all AMC 390 and 401engines are forged from the factory. To keep them working, Payton'sshot-peened the stock 5.85-inch rods to relieve stress and used apressed piston pin for street durability and low bucks. For full-tiltmills, Payton's uses a 6-inch Chevy rod and turns the rod-bearingjournal down to 2.100 inches from the stock 2.248 for reducedcylinder-wall loads and 7,500-plus-rpm capability. Taking no chances,they use ARP rod bolts as well.
The 401 crank has a 3.68-inch stroke and is more than manly enough withjust a polishing of the journals and chamfering of the oil holes tolive. It's worth mentioning that the '72-and-later 401 crank had aTorqueFlite 727 flange, so look for the casting number 812-0510 or youwill have to modify the flange to get it to work with a modernautomatic.
By Bill Tichner
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