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Ask Anything - April 2014

Weak Vette

Rick Allemandi; Watsonville, CA: I have an '81 Corvette. I bought it really cheap from a woman I work with. It seems her husband was arrested with a another woman in the car, and she hasn't let him drive it in several years. Anyway, I picked it up really cheap. I've put on some wider tires, upgraded to KYB shocks, lowered it a couple of inches, installed new brakes, braided brake lines, and new calipers. I've also added some Flowmaster mufflers, a performance catalytic converter, and poly bushings all around.

I love the fastback body style, and I love the way it handles. The problem is the car can barely get out of its own way. It has the stock 350 and 4V carb. The car has an onboard computer that controls the carb and the timing. It is a California car, so I'm stuck with the emissions equipment. I can't find any smog-legal headers, and I don't think a cam swap would allow me to pass the smog test. I thought about installing headers and welding in the smog tubes, but I've been told that is illegal. What can I do to make this car move like it should?

Jeff Smith: Unfortunately, 1981 was second to the worst year for street small-block engines in the Corvette. That L-81 350 made a measly 190 hp. Imagine, a Corvette with only 190 hp. Sadly, back in 1975, the base 350 made only 165 hp—the weakest of the late-model Corvette engines and a low point in the Corvette history. The biggest problem with the L-81 engine, besides the low-rise intake, lame camshaft, and restrictive catalytic converters, is the compression—or more accurately, the lack of compression. When brand-new, the L-81 squeezed with what Chevy claimed was an 8.2:1 compression ratio. My guess is that if you measured all the variables, it's likely to be closer to 8.0:1 compression. Let's just call this lamentable. My first suggestion would be to replace the iron 76cc chamber heads with a set of aluminum 64cc chamber heads. This would immediately pump the compression up exactly one point—so if your current compression ratio is 8.0:1, a 64cc chamber will take it to 9.0:1. Theory states that this is worth roughly 4 percent power, which if the engine was making 190 hp, the compression only bumps that to roughly 198—call it 200. But the added flow improvement of, say, an Edelbrock Performer RPM emissions-legal head would also pump the power up perhaps another 20 hp at least. I would suggest adding a Performer EGR manifold that would also help power, but then the cork in the system is, as you have recognized, a lack of a decent header. The other restrictive part of your exhaust is the catalytic converter. I'm not 100 percent certain, but it's likely that your car may retain the original catalytic converter, which in 1981 could be a pellet-type converter—which is highly restrictive. The first thing I would change before any of the inlet modifications would be to a California-legal monolithic-style catalytic converter. This will at least help minimize the exhaust restriction and improve performance. The problem of headers is indeed significant because, as you pointed out, California severely limits modifications to the exhaust upstream of the catalytic converter to parts with a Bureau of Automotive Repair (BAR) executive order (EO) number. This means that the component has been tested and proven not to increase emissions. This precludes you from legally adding headers. At the very worst, you could install them and then every two years install the stock exhaust on the engine to pass the smog test. The major gain in performance from headers would certainly be worth the hassle.

Other changes you could make would be to add an emissions-compatible camshaft to accompany the heads and intake change. For example, Crane makes a hydraulic flat tappet, emissions-legal camshaft with an EO for your engine with 204/214 degrees of duration and 0.423/0.446-inch lift that would certainly contribute to an increase in power. Add a good set of rocker arms, and you have a package that could easily make 300 to 320 hp at the flywheel, which is nearly a 70 percent increase in power. All these changes could be accomplished with the stock short-block in the car. The biggest hassle would be swapping the cam, but the rest would be relatively easy. This isn't the only way to go, but it's probably the easiest. The Edelbrock heads (PN 60909, $1,369 a pair, Summit Racing) aren't cheap but are sold as direct-replacement heads and are therefore legal. The intake will be an Edelbrock Performer EGR (PN 3701, $189.97, Summit Racing) that will accommodate the EGR valve necessary to make your package emissions legal. The Crane cam and lifter package is another affordable swap (Crane PN 114122, $190, Summit Racing), but you will also need gaskets, and you should also purchase the proper break-in oil to ensure that the cam doesn't go flat. Lucas sells a high-quality break-in oil, as does Joe Gibbs Driven. You're looking at investing around $2,000 with all these parts plus gaskets, oil, RTV, new coolant, and perhaps a set of good head bolts from ARP (PN 134-3601, $80.24, Summit Racing). That's assuming you do the work yourself. You should also include a new timing gear and chain set (Crane PN 11975-1, $118.60, Summit Racing). One advantage is that you can install these parts a little at a time, but the best way to do it is to install everything at once so that you only have to invest in one set of gaskets. Otherwise, if you install the intake first and later the cam, you will end up buying two or three intake gasket sets, not to mention the additional effort.

We also didn't recommend changing the carburetor because that original Q-jet is a decent fuel mixer. But you should also consider having it rebuilt because those early Q-jets used a plastic material called nitrophyl for the float that will absorb fuel and become heavy as it ages. This raises the float level and makes the engine run poorly. These carbs also suffered leaks that would drain fuel into the intake manifold. All of these things can be repaired (and they may likely already have been performed). If so, then merely tuning the air/fuel ratio with secondary metering rods and adjusting the air valve door on the carburetor can also make a world of difference even on the stock engine. If you need a good place to take your Q-jet, Sean Murphy Induction can easily handle this for you in the Los Angeles area. If you live in northern California, Ole's Auto Parts in San Bruno has an excellent reputation for tuning these older cars to run on today's reformulated gasoline. In fact, tuning the carburetor and ignition curve should be up at the top of the list (but after the catalytic converter change) of things you could try before diving inside the engine with major upgrades. Along with the carb tune would be a good set of plugs, wires, and an HEI cap, and rotor replacement is another wise idea. Pay attention to the mechanical advance mechanism on the HEI distributor. Those early HEI's were notorious for seizing up the mechanical advance mechanism because of corrosion. If the area underneath the rotor has a brown, rusty color, that's a good indication the mechanical advance mechanism is frozen. Free it up with lube and making sure the weights move freely will also really help the engine run better.

More Info

Crane Cams; 866/388-5120; www.CraneCams.com
Edelbrock; 310/781-2222; www.Edelbrock.com
Joe Gibbs Driven; 866/611-1820; www.JoeGibbsDriven.com
Lucas Oil Products; 800/342-2512; www.LucasOil.com
Ole's Auto Parts; 650/589-7377; www.OlesCarb.com
Sean Murphy Induction; 714/843-9169; www.SMICarburetor.com


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