If you've been following our C4 project from the outset, you know that the past few years have seen the car make depressingly little progress toward its goal of achieving quarter-mile parity with its late-model kin. There are a number of reasons for this, not least a self-imposed mandate to minimize our theoretical cash outlay along the way. The results of our ministrations have therefore been decidedly mixed, with some highly touted "upgrades" performing well below expectations and other, oft-overlooked tweaks proving surprisingly effective.
Having all but exhausted the usual slate of bolt-on performance hardware-cold-air kit, headers and exhaust, higher-ratio rocker arms, gears, and the like-we somewhat grudgingly decided to focus this month's installment another item whose efficacy is the subject of much debate in speed shops and on enthusiast Internet forums: an aftermarket throttle body.
While the stock 48mm TB is generally regarded as adequate for output levels of up to 400 horsepower (indeed, this was the primary reason we had resisted installing one up till now), we've recently seen several dyno tests in which 52mm units made 5-8 extra horses on stock-to-lightly modified engines. Would our 80,000-mile LT1 be similarly invigorated, or would we once again be left in the delicate position of making excuses for an underachieving part?
At a street price of around...
At a street price of around $225 ($200 for the satin-finish version), Professional Products' polished 52mm LT1 throttle body offers top-tier looks and assembly quality for a comparatively modest sum. Ours came with fresh bolts and gaskets, along with two different throttle linkages to fit both the Corvette- and F-body-style LT1 engines.
To find out, we contacted Professional Products, the Hawthorne, California-based outfit perhaps best known to late-model-Corvette fans for its wild-looking Hurricane LS intake manifold. PP currently offers one of the aftermarket's broadest selections of induction and fuel-delivery hardware for Vettes of all vintages, along with distributors, engine dampers, fasteners, filters, and more. The company provided us with one of its polished 52mm LT1 throttle bodies (Part # 69700), which came packaged with fresh bolts, gaskets, and detailed installation instructions.
Although flush from a mostly successful coolant-temperature-sensor swap performed just days earlier (note to self: always drain antifreeze from engine before working on cooling system), your author decided to leave this installation to the pros at AntiVenom in nearby Seffner, Florida. That said, anyone with a modicum of mechanical ability and a decent set of hand tools should be able to replicate the job in a garage or driveway in around an hour. Follow along now as we cover the highlights of the swap, after which we'll wrap things up with post-install dyno results and driving impressions.
A peek down the gullet of...
A peek down the gullet of the new throttle body offers a good look at the convex area between the throttle bores. Like those bolt-in "airfoils" that were popular in the L98 days, this feature helps smooth the flow of air into the intake manifold.
If you're performing this...
If you're performing this installation yourself, begin by removing the intake tract ahead of the throttle body. It should come out as one piece, freeing up plenty of room for you to work. Remember to unhook the Air-Injection Reactor (AIR) and OptiSpark-vent tubes.
Disconnect the throttle linkage...
Disconnect the throttle linkage (top), followed by the harnesses for the Idle Air Control (IAC) and Throttle-Position Sensor (TPS).
If the TB's coolant lines...
If the TB's coolant lines are still connected (we bypassed ours in a previous issue), you'll need to unhook them as well.
Next, remove the four bolts...
Next, remove the four bolts that secure the throttle body to the intake manifold.
Even with the bolts out, the...
Even with the bolts out, the throttle body will most likely remain affixed to the intake. A sharp rap with your fist should break it loose.
Set the old throttle body...
Set the old throttle body aside, then use a razor blade to scrape away any gasket material that remains on the intake's mating surface.
Take care to prevent debris...
Take care to prevent debris from falling into the intake bores.
Place the old throttle body...
Place the old throttle body on a bench or table, and remove the bolts that attach the IAC manifold.
Remove the manifold and set...
Remove the manifold and set it aside.
Next, unbolt and remove the...
Next, unbolt and remove the TPS. Set it aside, along with the round rubber insulator.
Now you're ready to transfer...
Now you're ready to transfer the IAC manifold to the new throttle body. (Be sure to use the fresh gasket that came with the kit.) With that done, install the TPS and insulator.
The finished product should...
The finished product should look something like this.
To install the new throttle...
To install the new throttle body on the car, simply reverse the teardown process. Before reattaching the air-intake tract, have a friend sit in the car and, with the engine off, fully depress the gas pedal. Verify that the throttle blades open completely, as shown above.
Reinstall the intake tract,...
Reinstall the intake tract, reattaching the AIR and Opti-vent tubes along the way.
You may need to use a flathead...
You may need to use a flathead screwdriver to gently stretch the intake bellows over the larger-than-stock throttle-body opening. Tighten the clamp securing the bellows to the throttle body, then fire up the car and check for air leaks. The engine may idle a bit roughly for a few moments, as the ECM adjusts to the additional airflow.
Here's a look at the engine...
Here's a look at the engine with the new throttle body installed. Too bad the rest of the top-end hardware doesn't look this nice.
Having verified that our installation...
Having verified that our installation was leak free, AntiVenom honcho Greg Lovell strapped the car to the shop's in-house chassis dyno and made a pair of pulls. While the larger throttle body didn't add to our '96 coupe's peak output, it did boost horsepower by as much as 6 between 5,000 and 6,000 rpm. A post-install test drive also revealed throttle tip-in to be noticeably sharper, giving the car a more responsive feel in everyday driving. Perhaps most important, the extra airflow capacity offered by the 52mm throttle bores should help optimize the performance of the head-and-cam kit we have planned for the near future. More on that in an upcoming issue.