Perhaps the first thing we'd...
Perhaps the first thing we'd do with a stock C4 TPI Corvette is open up the air cleaner assembly. SLP, for example, offers a much larger air filter kit for less than $200.
Power Program
Let's cut to the chase here. Our goal is to present a few performance-enhancing ideas you can apply to a typical C4 Corvette to unleash the beast and not spend a bunch of money along the way. Some may want to start slowly, using the existing engine and drivetrain, which is fine. Let's start with a typical TPI 350 with an automatic. The classic car crafting mods work with Vettes just like they do with Camaros, including an unrestricted inlet system, an adjustable fuel pressure regulator, headers, an exhaust system with less backpressure, and then perhaps a mild cam such as TPIS' ZZ9 or ZZ409 grinds. TPIS, for example, sells a set of long-tube headers for '84 to '91 cars for around $750 coated, and you'll need a Y-pipe and an after-cat exhaust for another $600--but then the exhaust is done. These prices are a little higher than for a '69 Camaro, but the headers are high-quality pieces that fit. All these basic bolt-ons will also work on the LT1 engines.
One intake modification that received a lot of attention back in the L98 days was the Lingenfelter SuperRam manifold. The problem with the stock TPI intake was the runners were too small and too long for a 350, since the manifold was originally designed for a 305. The longer runners make excellent torque but are all done by 4,800 rpm. Lingenfelter cut the runner length, added a large plenum, and was able to make power right up past 6,000 rpm. Accel still sells this emissions-legal manifold.
While factory aluminum heads...
While factory aluminum heads might look high performance, they're not nearly as good as even the Vortec iron heads. A much better approach for a C4 TPI engine is a set of aftermarket heads. AFR, Brodix Dart, Edelbrock, TFS, and many others make great heads that can make serious power.
Displacement is always an easy way to make more power. An affordable idea is to build a 383 or 408ci Gen I short-block and add a good set of aluminum heads, long-tube headers, and a SuperRam and it's possible to make 450 hp or more and still be emissions-compatible. The SuperRam makes gobs of torque (we've made as much as 550 lb-ft on a 420ci small-block), so the biggest problem is hooking all that power to the ground.
Drivetrain Upgrades
While more horsepower is obviously a major requirement, don't forget that the drivetrains in these early Vettes were somewhat anemic, especially the automatics. If you want to stick with an automatic, then a stronger 700-R4 should be in your plans. But if we were going to build a stout C4 to burn a few street jousters, nothing less than a durable five-speed would make the grade. Thankfully, Keisler offers a direct-fit kit to bolt several different manual transmissions in a C4. Our choice would be the Tremec TKO-600 box with its 2.87:1 First gear and 0.64:1 Overdrive. A stock rear axle ratio of 3.08 makes for a deep First gear ratio, while cruise rpm will be low in Over-drive. Of course, if that idea's too steep for your wallet, then you could always go retro with a Super T10 or Muncie.
Working our way to the rear, if your C4 candidate came with an automatic, then it also has the weaker, Dana 36 rearend assembly. You can swap in a complete Dana 44 with its stronger 8.5-inch rear gear if you can find a suitable donor. But know this: The market is well aware of what these are worth and they are not cheap--but it's the best way to get there.
Suspension and Brake Upgrades
Even the earliest '84 C4 suspension is superior to those in any '60s or '70s muscle car, so right out of the box the Corvette offers dramatic opportunities for a killer track car. But any good deal on a C4 will also include at least 100,000 miles of abuse. The best approach would be a simple upgrade of the existing ball joints and suspension bushings. Along with the suspension rebuild, we'd go with at least Bilstein replacement shocks, or you could upgrade to full-race double-adjustable shocks from QA1 or Koni, for example. Then there are the more exotic modifications that we can save for a later date, but all these simple mods will only improve handling for a relatively minor investment.