Other special pieces include a short water neck, such as the GM 660 we used or the Transdapt 9229 recommended by Edelbrock. In a C4 you'll also have to reduce the thermostat housing from 1 1/2 inches to fit your stock radiator hose of 1 1/4 inches.
To adapt the throttle body to connect to the mass airflow sensor, we used a 4-to-3-inch silicone reducer from a local speed shop, and then cut it with a razor blade to come off of the throttle body at an angle. We also used a 3-inch piece of aluminum tubing to route the new connections for the PCV draw and IAT sensors.
The stock TPI harness will need to be connected to the LSX-style IAC and TPS. FAST sells a plug-and-play harness specifically for this modification.
The factory throttle cable isn't long enough, so we opted for a Lokar replacement. We did make one small modification to increase hood-clearance, by removing the top of the Edelbrock throttle bracket as shown. While we don't have the cruise control hooked up at the moment, there are still holes on the bracket and connections on the linkage to reconnect it.
As we mentioned previously, one of the biggest challenges of this swap involves hood clearance. And despite our thorough pre-swap measurements, we remained concerned that the tall Pro-Flo XT might not fit under the C4's low-profile hood. Keep in mind that we're using a 90mm throttle body, which represented the largest diameter that would theoretically fit. You could also use a stock LS1 throttle body, which has a much smaller inlet.
We observed no clearance issues initially, but after traversing a few railroad crossings and speed bumps, we found a very slight interference at the corner of the throttle-body flange. Luckily, the 90mm throttle body has plenty of extra material that can be milled down for additional clearance.
On the dyno, we found that the larger, tapered runners of the Pro-Flo XT uncorked the engine to provide a gain of 20 rwhp, pushing the peak number to 384 at 5,250 rpm. Power now peaks 500 rpm higher than with the SuperRam. Peak torque, meanwhile, dropped 13 lb-ft, to a still-healthy 418 at 4,200 rpm. These numbers translate to approximately 468 hp and 509 lb-ft at the crank.
As you can see in the graph (above right), the Edelbrock intake's torque curve is wider and flatter than that of the SuperRam. Despite a drop in peak torque, the Pro-Flo XT performs better from 1,500 to 6,300 rpm, with increases in both average torque (382 vs. 368) and average hp (281 vs. 265). The new combination has also raised the Corvette's shift points by approximately 500 rpm, to 6,300. This boost in usable rpm results in more time spent accelerating before an upshift, which in turn improves acceleration by taking advantage of each taller gear's increased torque multiplication.
A simple camshaft swap to a grind with increased duration (remember, the current cam was designed specifically for use with the SuperRam) should result in even more power, and at a higher rpm. But for now, 384 hp with almost 420 lb-ft of torque at the rear wheels while shifting at 6,300 rpm is working well.
It's clear that the SuperRam manifold is still the king of torque. It makes big power off idle, and it can spin the engine much higher than a stock TPI intake. Driving an engine equipped with a SuperRam, however, reveals the unit's limitations. Peak output arrives quickly but falls off soon thereafter. An engine equipped with a Pro-Flo XT intake doesn't provide the same instant rise to peak power, but it also doesn't drop off as dramatically thereafter. The Pro-Flo XT hits hard (harder than the SuperRam, in fact, from 1,600 to 2,800 rpm) and keeps pulling until you back out of it.