Big-Block Chevy Engine Build - 707 HP For $6,720

We also tried a simple, polymer, open-plenum carb spacer that ended up being worth 6 hp at the peak. This worked to improve the plenum volume and also created more room between the bottom of the carburetor and the floor of the manifold to allow the air and fuel to turn the corner into the ports.

We also tried a simple, polymer, open-plenum carb spacer that ended up being worth 6 hp at

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

More Power
Here's what our dyno test reinforced from what we already knew about our 496-inch Rat. The big deal is clearly the induction system. We've seen plenty of 500-plus cubic inch Rats run to know that a 1,050-cfm carb would be beneficial. Our budget also forced us to use a stock pan-so there's a few horsepower in a good oil pan and tray assembly. It would also be nice to run thinner piston rings and forged pistons and maybe a point more compression. With all these ideas, it's possible with the parts and more tuning we could make 730 hp. If you think that's worthwhile, let us know and we'll put the whip to our phat Rat.

By Jeff Smith

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