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780hp AMC 401 Build - American Horsepower!

Posted in How To: Engine on October 16, 2012
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Welcome to the latest AMC v8 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 AMC 401. 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 AMC 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.

While no supercharged AMC 401 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 v8 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.

While it may look demure, this iron-headed AMC kicked out an honest 791 hp on E85 fuel and 13 psi of boost.

Clouds of black cast-iron dust aside, the buildup of the AMC V8 engine followed a fairly predictable path to power. Extra effort is required on the lubrication system, as the AMC v8 engine is 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 of the AMC 401 and the dyno test of the American way to horsepower happiness.

AMC V8 Engine 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

PhotosView Slideshow

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.

PhotosView Slideshow

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

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.

Dyno Numbers
RPM TQ1 HP1 TQ2 HP2 Boost (psi)
3,600 540 370 571 391 5.1
3,800 558 404 598 432 5.3
4,000 585 445 630 480 5.9
4,200 609 487 663 530 6.6
4,400 635 532 685 574 7.3
4,600 662 580 692 606 7.6
4,800 662 605 696 636 8.6
5,000 674 642 715 681 9.4
5,200 675 669 714 707 10.1
5,400 684 703 715 735 10.8
5,600 686 731 719 766 11.7
5,800 670 740 701 774 12.1
6,000 653 746 692 791 13.3
Peak 661 723 715 791 13.3 psi
Average* 637.9 588.7 676.2 623.3 8.7 psi

*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.

Parts List
Description PN Source Price
Diamond 4.195 piston Custom Diamond Pistons Call
Total Seal rings MS7560 65 Summit Racing $376.99
Total Seal cylinder wall and assm. lube QP1 Total Seal 25.00
Comp camshaft Custom Comp Cams Call
Comp hydraulic lifters 867-16 Summit Racing 94.95
Comp valvesprings 926-16 Summit Racing 86.95
Comp retainers 740-16 Summit Racing 52.95
Comp valve locks 614-16 Summit Racing 22.95
Comp guide plates 4835-8 Summit Racing 71.99
Comp aluminum rocker arms 1044-16 Summit Racing 314.95
Comp rocker studs 4512-16 Summit Racing 55.95
Comp roller timing set 3118 Summit Racing 64.95
Comp Break-in, 10w30 (12 quarts) 1590-12 Summit Racing 59.95
Edelbrock Torker intake 2930 Summit Racing 249.95
Larry’s Stage II porting Call Larry’s Engine Call
MSD Distributor 8519 Summit Racing 266.95
MSD 6AL 6421 Summit Racing 251.95
MSD plug wires 31189 Summit Racing 82.95
Canton oil pan 15-554 Summit Racing 349.95
Canton pickup 15-555 Summit Racing 52.95
Supercharger kit D1-SCAMC Supercharger Store 6,990.00
Edelbrock valve covers 4431 Summit Racing 46.95
Rockett Racing E85 fuel E-85-112 Dealer network 6.00/ gal.

Accessible Technologies, Inc. (ProCharger); Lenexa, KS; 913/338-2886;
Comp Cams; Memphis, TN; 800/999-0853;
Competition Carburetion, LLC; Sun Valley, NV; 775/331-5609;
Diamond Racing; Clinton Township, MI; 596/792-6620;
Edelbrock; Torrance, CA; 310/781-2222;
Larry’s Engine & Marine; Tucson, AZ; 520/623-5373;
MSD Performance; El Paso, TX; 915/855-7123;
Rockett Brand Racing Fuel; Yorba Linda, CA; 714/694-1286;
The Supercharger Store; Huachuca City, AZ; 520/456-9706;
Total Seal Piston Rings; Phoenix, AZ; 623/587-7400;

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