In 1985 GM released one of its last major updates to the first-generation small-block Chevy, the Tuned Port Injection (TPI) system. It was the General's first foray into individual-runner and port-fuel-injection technology and a major step in the gradual climb out of the horsepower Dark Ages for the Corvette.
The TPI intake lived in the hearts of C4 Corvettes for seven years before the "Gen II" LT1 small-block replaced it in 1992. During that span, the Corvette and its mountainous low-rpm torque were king. The L98's 18-inch intake runners were tuned for producing maximum power at lower rpm, yielding as much as 250 hp and a whopping 340 lb-ft in the '90 and '91 models. But while the tuned runners produced gobs of power down low, their length and somewhat narrow diameter ended up starving the engine of air at around 4,500 rpm.
Compared with the broad powerband in today's LS3-powered Corvettes, the narrow power range of even a highly modified TPI engine-and the frequent shifts that result-can greatly reduce your chances of winning a close race, despite the abundance of available torque.
Over the years, there have been numerous efforts to improve upon the stock TPI intake. These include larger intake bases and runners, short-runner intake manifolds (similar to the LT1 intake) and, of course, the venerable John Lingenfelter-designed ACCEL SuperRam (above right). Compared with the stock TPI intake, it features a larger plenum and throttle body along with slightly shorter (approximately 15-inch) runners. The result is an intake that is still tuned to make lots of torque down low but can also breathe and make power beyond 4,500 rpm.
But much to the dismay of many a torque-loving Corvette owner, the SuperRam is no longer in production, and good copies are becoming more expensive and difficult to find. Additionally, the SuperRam was developed in a time when free-breathing aftermarket cylinders were not widely available. Lingenfelter created an excellent manifold for use with stock ported heads (which flow around 280 cfm), but with the cylinder-head technology of today delivering an easy 300 cfm of flow, a need arose for a manifold designed with these capabilities in mind. Edlbrock's Pro-Flo XT looks to have the potential to fill this niche.
The Pro-Flo XT (above and top left) was designed from the ground up, utilizing computational fluid dynamics and CAD technology. It features a 90mm inlet for a front-mounted LSX-style throttle body on a 190ci central intake plenum. The plenum feeds eight individual 5.5-inch-long intake runners, which taper down approximately 30 percent to the intake port. Edelbrock says its tests have shown that the Pro-Flo XT beats the company's previous single-plane-style Pro-Flo 2 EFI manifold across the board, picking up as much as 15-20 hp on some combinations.
It's no coincidence that this tunnel-ram-style intake looks very similar to the custom sheetmetal manifolds popular with serious racers. The medium-length runners are tuned to make power across the board and into the 6,500-rpm range, and their tapered design is intended to ram air into the combustion chambers for maximum power. The Pro-Flo XT is 9.05 inches high at the tallest point-the throttle body flange-making it taller than the SuperRam. With the low hoods of Corvettes, and the C4 in particular, this could be a major roadblock. We set out to determine what it would take to make this intake fit a fourth-gen Vette, and how it would perform against the mighty SuperRam.
The engine featured here belongs to an '86 Corvette and sports a 383ci displacement, ported Dart Pro1 200cc cylinder heads, a Lingenfelter 219 single-pattern hydraulic roller camshaft (219-/219-degree duration and 0.560-/0.560-inch lift with 1.6 rockers), and Lingenfelter 1 3/4-inch long-tube headers dumping into a stock diameter 3-inch Y-pipe and a Magnaflow 2.5-inch exhaust. The car is street driven but spends a great deal of its time on the autocross course, where low-rpm torque is very important. Since the current output of 430 rwtq already overwhelms the car's 315/35-17 Kumho Victoracer race tires, we weren't too concerned about potentially sacrificing a small amount of low-end thrust in exchange for a wider powerband.
This particular SuperRam has stock runners (above) and plenum but features a heavily massaged base and a larger-than-stock 52mm throttle body. By contrast, the Pro-Flo XT will be bolted on with no porting or port-matching. While the casting is quite good, there's still plenty of material to clean up in the runners if one were so inclined. Port matching the runner exit to the appropriate gasket size would yield an approximately 20 percent increase in cross-sectional area and would increase airflow as well.
We chose an Edelbrock 90mm throttle body to match the Pro-Flo XT's 90mm inlet. It's also worth noting that our test engine was running a custom dyno tune that was optimized for the SuperRam combination. We didn't have a chance to retune the PCM for the new intake before testing, so you can assume there's more left in this combination. Our dyno session was carried out at Duke's Auto in Umatilla, Oregon. You may have heard of the owner, Duke Langley, as he campaigns an '89 Corvette nationally in SCCA Solo racing. Langley strapped the Vette onto his Dynacom dynamometer to record our before and after runs.
We removed the SuperRam intake using a 1/4-inch gear wrench and a long Allen ball-head driver on the runner-to-base bolts. (Replacing the original Torx bolts with Allen heads is also a great idea.) To ease installation, we had previously slotted the small bolts connecting the runners to the plenum. If you ever plan on installing or uninstalling a SuperRam, these tricks are a must.
After removing the SuperRam, we went to work on installing the Pro Flo XT. Compared with installing the five-piece ACCEL intake, the Pro Flo XT is a breeze. However, as the manifold is not designed as a direct replacement for the C4 Corvette, special care has to be taken on certain parts of the swap.
While we utilized braided lines, we could have saved a good deal of money using either flared hard lines or push-on style hoses. If you do decide to go the braided route, you'll save yourself a lot of time and several bloody fingers by picking up a Koul Tool assembly tool like the one shown here.
We also installed a stand-alone -6 AN bypass EFI fuel-pressure regulator to manage fuel delivery. In hindsight, it would have been better to use a smaller EFI regulator or a remote-mount unit, as we had to do a very slight amount of grinding on the corner of the Pro-Flo XT (above) to prevent interference. While we didn't utilize the tapped NPT hole in that corner, it would have worked well for routing a vacuum line straight to the brake booster.
The injectors are LS1 25-lb/hr (at 43.5 psi) units and require the use of mounting tabs (included with the kit) to locate the fuel rail at the correct height and angle. Our setup, with fuel feeding both rails at the front of the manifold, just barely fits.
We had previously installed an MSD small-cap HEI distributor (for '87-'92 GM F-bodies) to make timing changes with the SuperRam easier. If you're not already running a small-cap HEI unit, you'll need to get one to use with the Pro-Flo XT.
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.
Aside from a few minor modifications, installing a Pro-Flo XT manifold on a C4 is a refreshingly straightforward job, especially when compared with the notoriously tedious SuperRam install. With regard to price, a used SuperRam can be had for $800-$1,200. Provided it seals well, you can reuse your fuel rails and injectors, purchase a larger throttle body for $250 to $350, and end up with a total cost of $1,050 to $1,550. The Pro-Flo XT and all ancillary hardware, meanwhile, will run between $1,200 and $1,400 brand-new, depending on how you route your fuel lines and whether you have to buy a new distributor.
Based on our experience, it's clear that the Pro-Flo XT represents an exciting new option for any TPI-Corvette owner hoping to maximize the power of the L98. It looks mean and makes great power over a broad rpm range, even before it's been cleaned up and port-matched. In the case of our '86 C4, the Pro-Flo XT has found a permanent home.
Special thanks to Duke Langley, Scott Rewinkel, Robert Carosino, and Reagan Carosino.