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Intake Manifold Running Design - Ask Marlan

Tech Q + A

I would also add other performance features such as stainless steel valves, screw-in rocker studs, hardened pushrods, and guideplates. However, I have two concerns: My first is that by going to aftermarket heads with larger intake runners I will gain upper-rpm power, but will lose valuable low-rpm power. My second concern is cost; I cannot afford a switch like this without a decent gain in performance.
Gary Conner
Cody, WY

Aluminum heads do indeed have better heat-dissipation characteristics in comparison to cast-iron heads; this means that given a cast-iron head and an aluminum head that are otherwise identical except for casting material, the iron head will make more power. This is contrary to popular belief. Radiated heat is simply lost potential energy; the more heat you can retain in the combustion chamber, the higher the power potential-assuming the engine does not get into detonation. To make up for the power loss, engine builders take advantage of the aluminum heads' superior heat rejection characteristics to run at least a full point more compression.

As you surmised, most gains from typical aftermarket aluminum heads occur over 3,000 rpm. But with only 3.23:1 gears and a real-world 7.9:1 compression ratio, you can't afford to give up any low-end. Assuming your current heads have 76cc chambers, the typical 64cc-chamber aftermarket heads will raise compression about one point to around 8.9:1-still nothing to write home about.

One solution is GM's L98 Corvette-style aluminum cylinder head. The Vette heads' 58cc chambers will raise the compression ratio to around 9.4:1 (assuming you retain the current GM 0.028-inch compressed-thickness head gasket, PN 10105117), and their moderate 163cc intake runner volume won't screw up the low-end performance. PN 12556463 gives you a complete head assembly with heavy-duty springs, 1.94-/1.50-inch valves, and screw-in rocker-arm studs. The head is designed to use the late-model guided rockers which are already present on the Goodwrench engine. By shopping around you should be able to pick up a pair of these heads for around $800; figure on another $50 for center-bolt valve covers if you don't already have them.

Typically, a stock Goodwrench 350 engine makes around 230 hp and 330 lb-ft of torque. The biggest initial bang for the buck is a good performance exhaust system-1 5/8-inch headers, dual 2 1/2-inch pipes, and aftermarket performance mufflers. Your letter doesn't mention any exhaust mods, and if you don't have a system like this, all the other mods are wasted. Combine the good exhaust system with the Edelbrock Performer cam and intake and you'll push around 290 hp and 360 lb-ft. Out of the box, the Vette's heads are worth at least 30 more hp, so figure they'll bump the power up to around 320 hp. The aluminum heads will also shave about 35 pounds off the nose of the car. Is that cost-effective? Not in my book when for under $500 you can have an adjustable 75-175hp nitrous oxide power boost.

Vette aluminum heads have no heat riser or internal EGR provisions. Intake port entry remains in the stock location. The D-shaped exhaust ports are raised 0.100 inch, but the exhaust header attaching bolt holes remain in the stock location; exhaust manifolds and aftermarket headers may require modification to fit.

Passing Gas
Can octane be increased in racing gasoline in a way that might damage your engine (such as higher alcohol content)? If the octane can be increased with potentially engine-damaging ingredients, is there any way to tell which racing gasoline would be safe to use? Or is this racing gasoline simply a more refined version of a typical gasoline? I like using the racing gas but I don't want to jeopardize my engine.
Ted Kavadas
Highland Park, IL

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