780 American Horsepower...with A Little Help From A Blower And E85

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.

It’s amazing what a little bit of boost does for a 40-year-old motor when you seal it up nice and tight. Larry’s didn’t baseline this engine normally aspirated, but we can assume that this engine could easily make at a minimum of 1 hp per cubic inch, which would put the power at around 400 to 420 hp. With that as a base, looking at the pump-gas test, the blower package was worth a solid 330 hp, which is just shy of a 80 percent power increase. That’s fantastic power considering the limitations of pump gas. The water injection certainly helps, especially with the near-12 psi of boost. The accepted rule is that beyond 8 psi of boost, most engines really need an intercooler.

One other way to gain both reduced outlet temperatures and added octane is with that magic alcohol E85 fuel. To ensure the correct 85 percent blend of alcohol with a safe octane rating, we poured in a few gallons of Rockett Racing brand race E85 rated at 112 octane. With no changes to ignition timing or blower pulleys, the discharge temperature dropped at the lower boost levels from 117 degrees F to 92 degrees, but at peak rpm, the differential had shrunk to roughly 10 degrees. Nevertheless, peak power increased by 40 hp.

As a final test, we used a Total Seal leakdown tester on three cylinders, and all three read between 2 and 3 percent leakdown. So, clearly, those Total Seal rings are doing their job. End

As a final test, we used a Total Seal leakdown tester on three cylinders, and all three re

According to Peto, this is not unusual, as he’s seen greater increases—and this is E85 against an engine with water injection. The pump-gasoline discharge temperatures would have been much higher without the water injection. We were shocked by the 791 hp peak number at a mere 6,000 rpm. That’s damn strong for any street engine—especially considering the simplicity of adding the supercharger to the engine. Big power with a simple bolt-on is the ultimate in instant-power gratification.

*Test 1 was the engine with max boost of 13.9 psi and pump gas with a 50/50 mix of water and methanol.

*Test 2 was with E85 and no other changes. Max boost was 13.3 psi.

Sources