It's easy to get carried away with horsepower plans. The "more is better” approach to horsepower and torque is as hard to ignore as the promise of stylish new shoes to a Hollywood starlet. But we're reading Lindsay Lohan's new book 10 Steps to Avoiding Temptation and practicing on our Blue Collar 454. So far, we've refrained from throwing expensive alloy heads and other big-ticket items at our cast-iron creation, but the lure is incredibly tempting—how does Lindsay do it? Back in the Oct. '12 issue, John McGann stuffed a budget set of Sealed Power 12.5:1 compression pistons in the iron Rat that, along with a simple Comp mechanical flat-tappet camshaft and the Blue Collar 454 (close friends call him BC), stumped out a reasonable 550 hp on E85. The more we looked at those numbers, the more the devil angel said we could do better. We yanked the heads and took them to Jim Grubbs Motorsports for a personal massage session, along with a bigger E85 carburetor, and uncovered a large pile of torque and nearly 40 more horsepower. We're not near done yet, although the jungle drums speak of a tunnel-ram and nitrous that we can neither confirm nor deny. In the entertainment business, that's called a hook, and we not-so-subtly planted it. Stay tuned, as we have major plans for BC—assuming we fall off the Lohan wagon.
The first step with the iron, oval-port heads was to have Jim Grubbs Motorsports install n
Cylinder Head Testing
For mild street and budget bracket big-block Chevys, the factory iron 049 heads are outstanding; if they were cast in aluminum, they would be ideal. After BC did so well with bigger valves, we thought more attention to the valve job might add a little power. We took the heads to Jim Grubbs Motorsports (JGM) and established a flow-bench baseline. Big-block Chevy heads feature what are referred to as good and bad intake ports. The good intake ports direct the inlet charge toward the center of the cylinder, while the other four unfortunately point into the cylinder wall. This directly affects airflow. We included all our modification efforts on all ports, but we did all of our testing on the same good port on the flow bench.
The late John Lingenfelter once told me that as little as a 1-degree change in valve angle will affect airflow. While we'd love to do the definitive test on valve and seat angles, the electrical load for all that flow-bench testing would probably cause a three-state brownout, so we settled instead for a test of varying back-cut angles. A back cut is a shallow angle cut on the inboard or back side of the standard 45-degree valve-seat angle. Over the years, we've relied on the standard 30-degree back cut, but this time we augmented our testing with 28-, 32-, and 35-degree angles. As our results reveal, these angles redistributed the flow gains but didn't improve upon the classic 30-degree effort. Note that the 30-degree cut helps the flow at valve lifts below 0.600 inch, while above this lift, flow measurably decreases. Since our Blue Collar 454 Comp Cams flat-tappet XS290S camshaft achieves a net lift of only 0.582 inch, we planned to emphasize flow improvements in the valve-lift range between 0.200 and 0.500 inch. We also added a set of Manley Racemaster stainless valves from Summit Racing to which we also added 30-degree back cuts. This was mainly to take advantage of the smaller, 11⁄32-inch stem diameter to shave some valvetrain weight. These valves are not too expensive, but you could opt for 3⁄8-inch stem Street Flo valves that are even more affordable. The flow numbers in the accompanying chart are intake cfm using JGM's SuperFlow 600 flow bench at a 28-inch test depression using a 4.310-inch bore adapter. The numbers in parenthesis are the improvements against the test 1 baseline.
After Jim Grubbs installed new, smaller 11⁄32-inch guides for the new Manley valves, Ryan
It's important to point out why we chose to emphasize the mid-lift flow improvements. We can start with the fact that the valve will be at the mid-lift position twice in its lift curve as opposed to only once for peak valve lift. But equally as important is that when the intake valve is on the closing side of the induction cycle, the piston is rising and pressure is increasing in the cylinder. A valve job with strong flow velocity (higher cfm flow equates to greater velocity) at lower lift will be able to overcome the pressure in the cylinder and do a better job of filling the cylinder, which will improve power across the engine's entire powerband, but especially at lower engine speeds, where the longer-duration cam is a detriment to torque. In our experience, higher peak valve-lift flow numbers generally only improve peak horsepower but not the overall curve. A head with a stronger average flow curve will tend to make more power across the entire rpm band. This is exactly what we saw with the modifications to the stock Rat head.
The main reason for the Manley valves was to take advantage of the thinner 11⁄32-inch stem
We also tried very light pocket port work on both the intake and exhaust, but the unfeelin
We evaluated each intake back-cut change on JGM’s Superflow 600 flow bench, using the same