I was holding up an Edelbrock Performer intake manifold, about to bolt it on to my car's small-block, when I noticed the runner design: It has four short runners and four long runners. That is typical of a dual plane intake manifold made to give the engine a broader range of power. I was just thinking (a dangerous thing, most people tell me), "Why isn't the rest of the engine designed this way?" If power under the curve is everything, why not design the whole engine like a dual-plane intake manifold? Why not have valve sizes, cam specs, and header primary tubes all different sizes to optimize their respective function? For example, a stout street engine would start with a 9.5:1 Chevy 383 and a Performer RPM intake feeding some Dart heads with both 2.02- and 1.94-inch intake valves (depending, of course, on which runner led them there). The cam would be a custom-ground roller with 210-degrees duration on the torque side and 235-degrees duration on the horsepower side (both measured at 0.050-inch tappet lift) with 0.500-inch valve-lift on both sides. The headers would also have two different-size primary tubes-1 3/4 and 1 5/8 inches, respectively.
How would this engine run? Would it have the broadest, flattest powerband, or would it rip itself into several large pieces? It's about time to rebuild my engine and I am considering some crazy ideas!
Rick Bradshaw Jr.
The legendary Jim McFarland built an engine like this back in the early '80s. The first version used two different intake and exhaust runner sizes, port volumes, valve diameters, and cam lobes. The two configurations alternated in a regular sequence corresponding to every other cylinder in the firing order so the engine would still run smoothly. Essentially, the result was an engine that behaved like two V-4s with two different, pronounced torque peaks-but unfortunately, there was a large dip between each peak. To compensate, "Phase 2" varied pipe, runner, and cam timing even more, changing the combination for each individual cylinder, not just every other cylinder. Cam timing changes eventually evolved beyond lobe-to-lobe lift and duration-per-cylinder variations to also include lobe centerline and intake/exhaust lobe displacement angle variations as well. It was tedious, cut-and-try work (back then tuners didn't have today's computer modeling and simulation techniques available), but eventually McFarland wound up with (as he puts it) "a torque curve as flat as a billiard table." The engine had great off-idle torque and "it stayed there all the way through the top-end." The first "proof of concept" engine found its way into a street-driven Camaro. Later McFarland built this type of engine for one of Junior Johnson's short-oval track stock cars where it "kicked ass." Some successful short-oval track and road-racers may be building engines using these techniques today, enhanced even further by modern computer modeling; but if they are, no one's talking! Lunati reputedly ground the special cams for McFarland.
Lunati Cams, Inc.
4770 Lamar Ave.
Memphis, TN 38118-0021
Shedding Some Light
I would like to know how much benefit there is in switching from cast-iron cylinder heads to aluminum heads on a small-block Chevy in general, and in my particular case as well. The car in question is a '69 Chevelle with a TH350 trans and 3.23:1 rearend. My car's engine is a Goodwrench 350 with the stock heads and an Edelbrock Performer intake manifold, cam, and 600-cfm carb. It has an advertised 8.0:1 compression ratio with the standard heads.
What gain in power could I expect by switching to aftermarket aluminum heads? I know I could gain more power through smaller combustion chambers (to provide a higher mechanical compression ratio), bigger valves, better heat dissipation, and improved port shape and flow characteristics
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.
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.
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.
Highland Park, IL
An oxygenate (oxygen-bearing compound) like alcohol can dangerously lean out an engine when used in excess in a fuel-metering system not calibrated for such use. However, engines specifically designed and calibrated for alcohol make more power than an equivalent gasoline-fueled engine...so to prevent cheating, sanctioning bodies test for the presence of alcohol compounds in gas. Reputable race gas manufacturers-including Phillips, Sunoco, Trick, and Tosco (the manufacturer of 76 Racing Gasoline, formerly Unocal)-are well aware of this, and produce their high-octane fuels without resorting to doctoring the blend with alcohol. Unlike backyard bathtub blenders the major companies also publish complete specs on the racing gas so you know what's in it.
Modern high-quality racing gas is manufactured using selectively picked, high-octane hydrocarbons as the base stock. These include toluene, isooctane, butane, and pentane, among others. Off-road-only racing gas also receives a dose of good ol' tetraethyl lead to further raise the octane. While tetraethyl lead-bearing fuels aren't legal today for street cars, in addition to its 110, 114, and 118 (R+M/2) octane leaded 76 Competition Racing Gasoline, Tosco also produces 100 octane unleaded race gas that meets all state and federal regulations for street-legal CARB Phase II and EPA reformulated gasoline (RFG). While quite expensive in undiluted form, blending 100-octane unleaded with standard 92-octane gas may raise the octane enough to support engines presently pinging on normal street gas. For example, a 1:1 blend of 100 racing unleaded and 92-octane 76 Super unleaded yields a 96 (R+M/2) octane fuel. Detailed blending tables are available from 76 Lubricants Company.
Unleaded race gas does contain MTBE, an ether-bearing oxygenate. Unlike alcohol, MTBE has no tendency to separate out from the rest of the gas mix. EPA regs also limit max street-legal gas content to 15-percent MTBE, so at worst you would have to fatten up the carb two to three jet sizes from its optimum calibration with Tosco's 76 Competition 110 "NASCAR" leaded race gas (closed-loop electronic engine management systems automatically self-compensate).
In fact, any change in gasoline brand and/or octane may require carburetor recalibration. The governing factor is the gasoline's specific gravity (SG). If moving from a higher SG gas to a lower SG gas, you need to richen the mixture by going to larger jets. If moving from a lower SG gas to a higher SG gas, lean the mixture. Tim Wusz at 76 Racing Gasoline recommends leaning or richening the mixture by one jet size for every 0.010-inch change in SG (assuming the carb was correctly jetted for the old gas). Reputable race gas suppliers publish their fuel's SG.
Note that the presence of any oxygenate in the fuel will cause the fuel to fail the sanctioning body's fuel-check. You have the paradox of street-legal and environmentally-friendly gas that's illegal for most competition use, while toxic lead-bearing off-road gas is OK for racing but prescribed on the street. In other words, everyday unleaded street gas as well as the special unleaded race gas isn't considered "gasoline" at present by NHRA or NASCAR! Unleaded gas is legal in AMA and SCCA competition, however.
76 Lubricants Co
P.O. Box 7600
Brea, CA 92822-7600