American Motors Corporation put V-8s in lots of vehicles, from Matador police cars to Gremlins. The company's V-8s ran the gamut, including the 290, 304, 343, 360, 390 and 401; however, Jeep folk are mostly concerned with the 304, 360 and 401 versions because, from 1970 through 1991, that's what came in your V-8 Jeep.
The Jp Magazine staff has been wanting to build up an AMC 360 or 401 (don't waste your time building a 304) for quite some time, and we still may get around to it. But for now, we're going to lean on the Hot Rod magazine boys on the 10th floor for some tips and tricks on building up an AMC V-8. Although the AMC 390 Hot Rod built isn't a 401 and it's on the hotter side of what an off-roader would build, the buildup offers lots of good tech any AMC-ophile will benefit from. When Jp finally gets off its lazy butt and builds its own 401, you can bet we'll employ some of these tricks, but with a little less compression and slightly different cam specs.
- Christian Hazel
Rotating Assembly
Both 390 and 401 engines have stock 4.165-inch cylinder bores, but at 3.574 inches, the 390's stroke is 0.106 inch shorter than the 401's 3.68-inch stroke. All AMC cranks have 2.747-inch-od main journals, but rod-journal sizes differ. The 290 through 360 cranks are usually nodular iron and have 2.09-inch rod journals, while 390-401 cranks are made of forged steel and have 2.248-inch journals. Properly prepared, the externally balanced, forged-steel stock cranks are plenty durable and can handle just about anything you can throw at them. This engine's 390 crank was turned 0.020-under on the mains and 0.010-under on the rods. Federal-Mogul trimetal 34-groove bearings are used to ensure adequate oiling. The crank was also balanced with Mallory heavy metal to permit use of a neutral-balanced, SFI-certified ATI balancer, as well as a Centerforce neutral-balanced 40-pound flywheel drilled to accept a diaphragm clutch (AMCs came stock with Long-style clutches).
Brandes says AMC engines are not compression-sensitive, and he has run stock cast-iron heads at 11.25:1 without spark-knock on premium pump gas. Aluminum rejects heat better, and Indy revised the backside of the chambers for better quench -- so Brandes saw no problem running over 11.5:1 with the proper pistons, chamber shape and camshaft. Hypereutectic pistons were just fine for this dual-purpose street/strip engine, because in Brandes' view, their heat-rejection characteristics at low-rpm are better than with forgings, which helps increase a high-compression engine's detonation resistance. However, off-the-shelf piston availability was a problem when this engine went together.
Rather than depend strictly...
Rather than depend strictly on the strength of the standard oil system's hydrodynamic oil wedge, Brandes had the crank cross-drilled (as indicated by the red wires inserted through the oil holes) to provide constant oiling through all 360 degrees of crank rotation.
The 390 and 401 engines have...
The 390 and 401 engines have forged rods (most other AMC engines use cast rods). Due to the change in deck height from '69 to '70 models, there are two different length rods: '68-'69 390 rods (left) have 5.790-inch center-to-center lengths, while the '70 design (right) is 5.858 inches long. AMC factory-forged-rod failures are rare; just rebuild the big-end and remove any excess flash.
Indy closed up the rear-chamber...
Indy closed up the rear-chamber wall between the valves to yield superior quench. The KB piston (right, compared to the old Speed-Pro replacement forging, left) has a step that keeps the mixture out of the chamber's dead area, while the large flat dish area uses the whole chamber for a good burn. The result is a tumble-and-swirl-promoting 0.030-inch quench.
About the only readily available, mass-produced high-performance piston was a step-dish KB Pistons 401 hypereutectic casting, which has a shorter compression height (CH) than a proper 390 piston. But installing the 401 pistons with a 390 crank would normally place the pistons way down in the holes at TDC, causing a drastic loss of compression and quench. Brandes' multiprong solution: 1) Mill the existing 401 pistons (PN KB392) approximately 0.035 inch, reducing the 1.51-inch compression height and -17.3cc dish volume to 1.485 inches and 8 cc's, respectively. 2) Heavily mill the late tall-deck block deck to yield a 0.015-inch piston-above-deck height. 3) Install a 0.040-inch-offset bushing in the late 5.858-inch-long rods' piston-pin ends to obtain a final 5.898-inch center-to-center rod length. The bushing doesn't excessively thin the rod's small end because the KB pistons are designed to use smaller 0.927-inch-od small-block-Chevy piston pins. By the time you read this, KB should have a dedicated long-rod 390 piston designed for use with either a bushed long factory rod, or (if the crank's rod journals are machined to 2.100 inches) commonly available aftermarket 5.85-inch-long Chevy rods.
AMC 390 V-8 Blueprint Specs
Specifications not listed are the same as stock. Except as noted, all dimensions are in linear inches or fractions thereof. For cam and head specs, see the corresponding sidebars.
Actual Displacement: 397.1 ci (0.040-over stock)
Bore x Stroke: 4.205 x 3.574
Static Compression Ratio: 11.79:1
Cranking Compression: 150-155 psi
Idle Vacuum: 10-12 in-hg
Fuel: 93-octane unleaded pump gas
Rod Center-to-Center: 5.898 (0.040 long)
Rod/Stroke Ratio: 1.65:1
Piston Compression Height: 1.485
Piston Deck Height: -0.015 (piston above deck)
Piston Volume: -8 cc dish (after modification)
Head Gasket Bore: 4.27
Head Gasket Compressed Thickness: 0.046
Head Gasket Compressed Volume: 10.8 cc's
Combustion-Chamber Volume: 60 cc's
Main-Bearing Clearance: 0.00225
Rod-Bearing Clearance: 0.0021-0.0023
Rod Side Clearance: 0.015-0.018
Crankshaft Thrust: 0.007
Piston-to-Bore: 0.0018-0.0021
Piston Ring Endgap: 0.021-0.024, 1st; 0.025-0.027, 2nd
Piston-to-Head: 0.030
Piston-to-Valve: 0.110 intake/0.125 exhaust
Valvespring (Seat): Comp Cams PN 927-16, 195 lb/in at 1.950-inch installed height, 543 lb/in at 1.250 inches
Pushrods: Comp PN 4948-16, 8.55 long x 5/16 od
Spark plugs (for dyno): Autolite AR3933, gapped at 0.035-0.040
Ignition Advance: 14-degree base, 30 degrees total
Main-Bearing-Cap Studs: ARP PN 135-5601, 12-inch od, 105 lb-ft*
Rod Bolts: ARP, 3/8 inch, 50 lb-ft*
Head Bolts: ARP, 3/8 bolt, 35 lb-ft*; 1/2-inch bolt, 95 lb-ft*
*With ARP moly-based assembly lube
NewKB AMC Pistons
P.N.
| Engine
| CH
| Volume
|
KB354
| 401
| 1.51 inch
| -30.5cc dish
|
KB392
| 401
| 1.51 inch
| -17.3cc dish
|
KB394
| 390
| 1.56 inch
| -28.0cc dish
|