By adding the exhaust closing and intake opening points on these two cams, you can establish overlap at the 0.006-inch checking point. The "overlap" number indicates only one more degree of overlap between these two cams despite the 10 degrees of additional exhaust duration. All of these points tend to increase top-end power. All specs in this chart are at 0.006-inch tappet lift, which is also advertised duration.
274H/286 XE 278/296 BASELINE NITROUS CAM
DIFFERENCE (DEGREES)Intake Opening 31 BTDC 28 BTDC 3
later Intake Closing 63 ABDC 67 ABDC 4 later
Opening 77 BBDC 99 BBDC 22 earlierExhaust Closing 29
ATDC 33 ATDC 4 later
Overlap 60 degrees
61 degrees 1 degreeIntake Centerline 106 degrees 109 degrees
3 degrees retarded
Our test mule was the Ford 466 detailed in the Feb. '06 issue using 9.6:1 compression, Edelbrock Performer RPM aluminum heads and a Performer RPM Air Gap intake, a Holley 750 carb with an HP body, and 2-inch headers. For this test, we installed the XE 274H Comp hydraulic cam and broke it in on the dyno using a can of GM Engine Oil Supplement oil additive to ensure a smooth wear pattern between the cam and lifters. After several pulls to ensure optimized jetting and timing, Jim Grubbs Motorsports' Andy Hairfield ran the engine several more times to establish a normally aspirated baseline. This is when we noticed manifold vacuum of as much as 1.3 inches at peak power, which indicated the carb was undersized for the application. We also were concerned about using a dual-plane intake manifold with the nitrous, so it was at this point that we swapped on an Edelbrock Victor 460 single-plane intake and a Holley 950-cfm Ultra HP carburetor. The motor responded by trading a little torque for about 10 more peak horsepower, and with that we were ready for our nitrous test. These baseline runs also included the Nitrous Pro-Flow 34-inch nitrous plate under the carburetor.
For the nitrous testing, we chose to run a Nitrous Pro-Flow system similar to the plate system used in our nitrous-plate test a few issues back. In order to test the theory of a nitrous cam, we decided to hit our big Ford with a 175hp shot of nitrous. Nitrous Pro-Flow recommends a 73 nitrous jet combined with a 73 fuel jet with 7.5 psi of fuel pressure for this combination. Pro-Flow also recommends pulling 3 degrees of timing per 50 hp, so we backed the original total from 35 degrees down to 25 to be safe. With this tune-up in place, our procedure was to start the test at 3,000 rpm, hit the nitrous at 3,500 rpm, and then pull it through to 6,000 rpm. Two pulls would be performed with a full bottle at 950 psi to generate back-to-back runs. After a few tries, we realized it was hard for the dyno to harness the massive torque created by the nitrous along with the twist the 466 was already making. So we changed our procedure to loading the engine, hitting the nitrous, and then capturing data from 4,000 rpm and up.
This is the Holley 950HP Ultra carb eith its billit metering blocks and billit baseplate.
Once we had tested the baseline Comp XE 274H cam, JGM's dyno operator Andy Hairfield helpe