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Measure The Diameter of the Exhaust Throat - What's Your Problem?

Know that you can increase max lift airflow by exceeding this 90 percent diameter rule. If all you were interested in was peak flow cfm at maximum valve lift-more than 0.600-inch valve lift-then you could increase the throat diameter by more than 90 percent. The problem is that this move is guaranteed to hurt the flow at the lower valve lifts such as the 0.200- through the 0.500-inch range. Several years ago, we comparison-tested of a set of Patriot aluminum heads against a set of factory iron Vortec heads. The Patriots offered an outstanding peak valve lift flow number, which was achieved with a larger-than-90-percent throat diameter on the intake side. On the engine dyno, the Patriot heads did make slightly more horsepower than the Vortecs. The stock iron Vortec heads, however, made far more torque in the midrange. The point of this is that if you improve the overall flow in the 0.200- to 0.300- and 0.400-inch valve lift areas, especially on the exhaust side, the engine will make more torque and more peak horsepower. That's where you will pick up that 25 hp at the peak, plus the engine will make more torque across the entire rpm band and be much more fun to drive. It's a win-win deal.

The second way to gain the extra power is the easiest but hardest on your wallet. The small-block Chevy enjoys the largest selection of cylinder heads on the planet, so there's no lack of heads from which to choose. The aforementioned Vortec iron heads are very inexpensive and flow well, although they too can benefit from pocket porting the exhaust. Find a set of those heads and do the exhaust port work, and they will be far better than a set of 441 heads. Next in line might be the Edelbrock E-Street small-block head (PN 60739, $1,093 from Summit Racing). It is an affordable aluminum head with 70cc combustion chambers and excellent flow numbers. The beauty of this head is that it offers substantially better flow numbers over the typical 441 iron head with 185cc intake ports and 2.02/1.60-inch valves, not to mention the weight savings of aluminum. Considering that completely rebuilding and modifying a set of iron factory heads might easily cost $700 or more, the Edelbrock heads are a great deal at only another $400. This would be my choice rather than going through all that work to make an old iron head work-unless you prefer swimming upstream. It certainly builds character!

More Info

Torrance, CA

GM Performance Parts
Warren, MI

Summit Racing Equipment
Akron, OH
800/ 230-3030

Knock Control
Michael Latine, via I have an '87 Trans Am that still has the original carbureted 305 and a five-speed. I am working on replacing it with a 406ci small-block and a T-10 four-speed. I would like to keep the computer-controlled Quadrajet and the computer control of the spark and knock sensor.

Where can I find a chip? None of the chip companies services my year car anymore. Will my stock ECM still work because of the mild nature (low rpm) range of this motor?

The 0.030-over 406 has cast pistons with 9.3:1 compression, 64cc Vortec iron heads, an Edelbrock Performer manifold, and a cam with 230 degrees of duration at 0.050-inch lift, with 0.455-inch lift and a 114-degree lobe-separation angle. It also has 1 5/8-inch block hugger headers with 2 1/2-inch collectors and exhaust. Peak torque is expected to be 3,600 rpm and peak horsepower is 5,100. The T-10 is a close ratio, and the rearend gears will be 3.27:1 with a BorgWarner 9-bolt posi.

The basis for my engine is a buildup on a similar 406 in Chevy High Performance in the June '03 issue written by Scott Crouse. I worked for Jim Russell Racing Drivers School at Laguna Seca (Infineon Raceway) running the garage and also taught the driving school toward the end of that time. I love cars and your magazines and have been a car nut for 55 years now!

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