Cylinder Heads
Currently, Indy offers two different basic AMC heads: the Street-Replacement 401-SR head and the full-race 401-1 with huge valves and raised intake and exhaust ports. For this street-driven engine, Brandes chose to go with the milder 401-SR design to enhance midrange torque output. Unlike the full-race version, the SR's intake-port entrance, exhaust-port exit and valve spacing mirror the original AMC design, simplifying installation in a street car. However, the SR head's 2.10 intake/1.650 exhaust valves are longer than stock, making them more compatible with stout aftermarket valvesprings.
As delivered from Indy with a mild bowl-port and intake gasket-match, the SR casting considerably outflows the AMC stock heads over 0.400-inch lift -- even the '70-and-later design with its improved dog-leg exhaust ports. Brandes turned the heads over to his main "tumble-and-swirl" man, Ron Sharp at Advanced Airflow. Sharp is mum about the exact details, but he does reveal that the as-delivered Indy three-angle valve job was upped to a five-angle on the intake to help atomize incoming fuel. The exhaust-side valve job is completely radiused with no discreet angles off the seat.
Sharp also reworked the intake-side short-turn radius, then opened up the cross-sectional area around the pushrods. He touched up the chamber and reworked the exhaust-port roof. All this results in significant flow improvements across the entire lift range.
Intake:
Stock AMC
Indy 401-SR The intake floor...
Indy 401-SR
The intake floor on the Indy 401-SR head is smoothly radiused in its approach to the valve bowl. It has generous 235cc intake runners -- 69 cc's larger than typical high-perf AMC heads.
Exhaust:
Stock AMC
Indy 401-SR The exhaust side...
Indy 401-SR
The exhaust side has the most "sweeping" changes. Note the Indy 401-SR head's smooth and consistent cross section. As-delivered volume on the enlarged port is 105 cc's, yet it still exits in the stock location.
When this engine was built,...
When this engine was built, Indy heads were the only aftermarket AMC castings available--but for 2004 there'll also be Edelbrock AMC heads with 2.02 intake/1.60 exhaust valves, 58cc combustion chambers and adjustable stud-mounted rocker arms. Intake and exhaust ports remain in the stock AMC location.
The Indy head's thicker outer...
The Indy head's thicker outer flange area can cause interference with some header sets, which may need to be ground for clearance. The engine was dyno'd with a set of ceramic-coated Hooker headers (PN 7104-1HKR, 1 5/8-inch primaries x 3-inch collectors).
Camshaft and Valvetrain
One of AMC's advantages is its large 0.904-inch-od lifter foot, the same as a Chrysler. Most GM and Ford stock flat-tappet lifters measure only 0.842 and 0.875 inch, respectively. The larger lifter diameter results in a greater foot-to-cam-lobe contact area with reduced unit loading. This allows grinding really aggressive flat-tappet lobe profiles that approach roller-cam performance without the expense of a billet roller.
Brandes worked with Comp Cams to come up with a custom, "real AMC" mechanical flat-tappet grind that takes advantage of the big foot, matches the flow characteristics of the Indy cylinder heads and complements the engine's high static compression ratio.
Camshaft Specs
Manufacturer: Comp Cams
Part No.: 10-000-5 (custom)
Grind No.: A8 6585/7118 S 108.0
Type: Mechanical flat-tappet
Valve Lift (1.6:1 rockers): 0.621/0.624 inch
Duration (at 0.050): 255/262 degrees
Intake Opens/Closes (at 0.050): 19 degrees BTDC/55 degrees ABDC
Exhaust Opens/Closes (at 0.050): 59 degrees BBDC/23 degrees ATDC
Intake Centerline: 108 degrees
LDA: 108 degrees
Valve Lash (Hot): 0.018/0.020 inch