More Horsepower for Less - 707 HP For $6,720

AirFlow
Before we assembled the engine, we flow-tested the as-cast AFR 305cc heads on JGM's SuperFlow 600 flow bench at 28 inches of test depression using a 4.280-inch bore diameter. We also used a handformed clay radius for the intake entry, and no flow tube was used on the exhaust. We also tested both the "good" and "bad" intake ports to give you both flow curves. The good intake ports aimed the intake flow toward the center of the cylinder while the bad intake ports aim the intake flow toward the cylinder wall. This is the main reason for the differences in flow.

We tested the AFR 305 heads on a slightly smaller bore size (4.280 inches) compared with our 4.310-inch-bore-diameter 496 Rat. Many published big-block flow tests use the much larger 4.500-inch bore size. This simple 0.220-inch bore-diameter increase is worth a few cfm in both intake and exhaust flow. So when comparing Rat head-flow numbers, pay close attention to the flow bench's test bore diameter. For example, AFR publishes a 367-cfm flow number at 0.600-inch lift when measured on a 4.600-inch bore, while our number, measured on a 4.280-inch bore, is 347 cfm. Keep in mind that max cam lift is calculated without the lash. This means our actual intake valve lift was really closer to 0.645 inch (0.660 - 0.012 = 0.648 inch), including assuming a few thousandths for valvetrain deflection.

By the numbers
AFR 305cc, as-cast, rectangle-port heads2.25/1.88-inch valvesCNC-machined combustion chamberCFM

Bolted to the Westech SuperFlow...        read full caption Bolted to the Westech SuperFlow 901 dyno, the 496 delivered some impressive numbers, but only after we ran through the requisite timing, jetting, and valve-lash tuning exercises. On pump gas, our motor was happiest at 34 degrees of total lead and made 701 hp with a lash change.

Lash caps fit between the...        read full caption Lash caps fit between the valve-stem tip and the rocker arm and were originally designed for titanium valves to increase their durability. Using lash caps requires special valve locks that create a locating groove for the lash cap. Our AFR heads came with lash-cap-ready valve locks.

Tightening the lash from 0.016...        read full caption Tightening the lash from 0.016 inch to 0.013 inch increased the lobe duration at 0.050 inch roughly 3 degrees. This cost a little power below peak torque but bumped peak horsepower by 5 and got us over the 700hp threshold.