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

The 90 Percent Rule
Mike Hamilton; Grimsby, Ontario, Canada: Of all the mags I'm writing to, you give the best tech answers-period. I've got a 350 Chevy in an S-15 truck with 1 1/2-inch headers, a 750-cfm Holley, a 300-degree-duration cam with 0.488-inch lift, a Weiand dual-plane intake, 1.94-inch heads, and 10.25:1 compression. I ran 13.2 at 100 mph with a 3,000-rpm stall converter and 4.11:1 gears. It ran 1.8 short (60-foot) times, jetted and tuned to get the max out of it. After a few years believing my 1.94 heads were the weak point, I wanted to go faster so I tried a set of 1970 441 castings with 2.02 and 1.60 valves. With some jetting, the truck only ran 1 mph faster!

Previously, I overcammed the motor to prop up the 1.94 heads. If the cam is too big I can understand the same e.t., but why not more mph? Surely with 5,800 rpm at the top end it would be breathing right. I was hoping for at least 25 more horsepower, a few tenths, and more mph. What do you think?

Jeff Smith: I think the truck is running exactly as it should, Mike-it's just your expectations are too high. There really isn't an appreciable airflow difference between those two sets of heads. While you might think larger valves would improve the flow, there's more to the story than just big valves. The real cork with small-block heads is actually on the exhaust side. You mentioned that you crutched the engine with a bigger cam. But with the restricted exhaust, adding intake duration won't help. You didn't mention the specific cam timing, but the trick is to add about 8 to 10 degrees more exhaust duration than intake-such as a 230/240-degree dual pattern camshaft. This will extend the potential horsepower peak slightly, but even that trick is really just a crutch. The truth is that production iron small-block Chevy exhaust ports are weak. If you can't evacuate the exhaust gas from the chamber at high rpm, there will be exhaust gas that will remain in the chamber for the next power stroke. This exhaust gas won't burn again, so it reduces the cylinder pressure and the peak horsepower suffers. With a more efficient exhaust port, you can gain that 25 hp you were looking for. There are two ways to accomplish that.

The first is the least expensive, but it requires some effort and skill. Remove and disassemble the heads and have your machine shop evaluate both the intake and exhaust valveguides. If the guides are loose, you must rebuild them. You should also consider new valves, although I wouldn't go over 2.02/1.60 inches. Now comes the fun part. The place where you can make the most gain for the least amount of time involves concentrating on improving exhaust port flow. The effort will require considerable restraint on your part. If you look closely at the exhaust valve seat in the head, you will see a small ridge just under the seat. This area is called the throat. The idea is to eliminate this ridge while still leaving a slight venturi effect. The best way to accomplish this is to use what cylinder head porters call the 90 percent rule. Using the diameter of the exhaust valve as a reference, multiply the diameter by 90 percent: 1.60 x 0.90 = 1.44 inches. This will be the maximum diameter of the venturi area directly below the exhaust valve seat. Maintaining the throat diameter at 1.44 inches or smaller will maximize the overall flow potential of your pocket porting. This is not as easy as it might sound because it will be very tempting to hog out the throat far beyond the 90 percent point.

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