Who remembers the late 80’s and early 90’s Callaway twin-turbo Corvettes?

Introduced in 1987, the Callaway B2K RPO was offered as a regular production option right from Chevrolet. Check the box and you received (among other things) a serious motor upgrade, that combined forged internals, lower compression, and (best of all) a pair of turbos pressurizing air through a pair of equally impressive air-to-air intercoolers.

The net results of all this insanity was to increase the power output of the corporate Tune Port Injected (TPI) L98 350 motor from 240 hp and 330 lb.-ft. of torque by a solid 100 horsepower. Running near 10+ psi of boost, the twin turbo Callaway option offered 345 hp and 465 lb.-ft. of torque. This elevated the Callaway Corvette to super car status, thanks to not only serious acceleration (compared to the standard L98-powered Vettes), but also a top speed near 180 mph. These power numbers jumped to 382 hp and an astounding 562 lb.-ft. of torque on later models.

Perhaps the only thing holding back the amazing potential of these cars was (what now must be considered rudimentary) the Miro-fueler used to add additional injectors to provide the fueling necessary for the extra power.

Despite this, the Callaway Vettes were viewed with great admiration by lesser ‘Vette owners—especially a young journalist with an iron-headed, first year 1985 TPI Corvette.

engine on stand with turbo system attached
How hard is it to make Callaway Corvette B2K power from a run-of-the-mill L98 TPI motor? (Image/Richard Holdener)

The Test Motor: A Stock(?) 1987 TPI 350

Not making Callaway Corvette money back then (or even now), I was left with the only option available to a guy who specializes in dyno testing. I would build my very own turbo L98 and run it on the dyno as my homage to the time spent drooling over the Callaway in the assorted automotive performance periodicals.

Rather than go whole hog on a build as Callaway did, with forged internals and (boost-friendly) low compression, I decided to try my luck with what we thought was a stock 1987 L98 motor. The mystery motor came from the boys at the (now-defunct) Chevy Race Shop, and while it looked all the world like an aluminum-headed L98, replete with the factory, long-runner, Tune Port Injection, we wondered about the cam specs since the motor made a touch more power than other L98s we had tested in the past.

In addition to what many consider the best-looking factory induction system ever offered by GM (the dual quad cross ram DZ302 notwithstanding), the L98 also introduced factory aluminum heads. Prior to the introduction of the aluminum heads in 1986 (converts only), GM never offered factory aluminum heads. The L98 heads were also big over-the-counter sellers back in the day, as the lightweight aluminum construction and small (58cc) combustion chambers made them a serious upgrade for the previous generation, iron, big (72cc) chamber (low compression) smog-headed small blocks.

To get thing started, we installed the mystery L98 up on the dyno and equipped with long-tube (1-3/4 inch) headers, collector extensions, and a Meziere electric water pump. Running an open (but stock) throttle body (no air intake system), long-tube headers (no cats or cat-back exhaust), and optimized air/fuel and timing (not the conservative factory tune), meant that our test method always produced higher power numbers than the factory rating.

It was not uncommon to see an extra 50 to 60 hp over the factory rating, but we were quite surprised when the dyno spit out peak numbers of 332 hp at 4,800 rpm and 394 lb.-ft. of torque at 4,000 rpm. The torque number didn’t seem too far off from what we expected (and have seen on others), but the elevated horsepower numbers indicated this L98 motor might be sporting something other than stock in the cam department. The idle quality and vacuum were smooth as stock, but the numbers were a solid indication of extra power.

In the end, we elected to not look the Chevy Race Shop gift horse in the mouth, and continued on with the experiment by adding some boost.

Building a Turbo TPI Motor

To make Callaway Corvette performance, we needed the most important part of the B2K option, namely boost. Sure, it would have been cool to run twin turbos (it rolls off the tongue much better), or even simulate the original Callaway configuration, but we figured the Behr intercooler cores might be hard to find or duplicate, and all we were really wanted from the L98 was to duplicate (or say we say exceed) Callaway performance.

We also had an ace up our sleeve in the form of a dedicated single turbo kit from (now defunct) HP Performance, designed specifically for the TPI Corvette. Using a single Holset turbo, the kit also included dedicated, tubular exhaust manifolds, Y-pipe and front-mounted, air-to-air intercooler. A front mount would be much more efficient than the engine-compartment mounted twin cores on the original Callaway, but it certainly didn’t offer the visual statement of the Callaway engine bay.

Unlike the original Callaway motors, that offered additional fuel under boost through additional injectors controlled by the Micro Fueler, we simply installed larger injectors in the L98 rails and dialed in the A/F and timing curves using the FAST XFI engine management system.

Running just 8.5 psi of boost on pump gas, the turbo L98 produced an even 500 hp at 4,800 rpm and 609 lb.-ft. of torque at 3,900 rpm. Note the shape of the curves (NA and boosted) remained the same, meaning you never had any shortage of tire-turning torque—in fact your only shortage would be the lack of traction!

Remember, not only was there considerably more boost (and horsepower) to be had from this turbo, but we would eventually modified this NA motor with AFR 195 heads, a bigger cam, and Stealth Ram intake that added another 100+ hp NA!

Since we never ran that combo with the turbo, we can only dream of the potential, but for now, I need to find a way to afford a REAL Callaway!

dyno chart for a v8 engine
Several things are obvious from this graph. The first thing is that turbos are awesome and add impressive power, even at relatively low boost. Case in point, the naturally aspirated L98 TPI motor produced 332 hp at 4,800 rpm and 394 lb.-ft. of torque at 4,000 rpm. We expect this type of curve from the long-runner TPI induction system, as they were turned for torque production. What we didn’t expect was to see this much power from a motor rated at 245 hp and 345 lb.-ft. of torque. Though our combo was run in “Optimized” trim, meaning it had long-tube headers, an electric water pump, no air intake, and with the tune dialed for maximum power (engine was also run colder than stock), we still suspect that this motor had something other than the production L98 cam (there we no markings). After adding the turbo, note the shape of the power curve remained the same. This occurred because the turbo offered a consistent boost level through the tested rev range. Running 8.5 psi, the turbo increased the power from 332 hp and 394 lb.-ft. to an even 500 hp (499.8 hp) and 609 lb.-ft. of torque. If you are looking for tire-turning torque, this turbo TPI has all you want! (Dyno Chart/Richard Holdener)
gm TPI Manifold on table
The turbo TPI test motor started out life as a 1987 aluminum-headed (Corvette) L98 350, equipped with the legendary (long-runner) Tuned Port Injection. (Image/Richard Holdener)
close up of throttle body on TPI Manifold
For our test, we ran the factory dual blade L98 throttle body. Potentially restrictive on higher-horsepower naturally aspirated builds, the stock throttle body was more than sufficient with the help of positive pressure. (Image/Richard Holdener)
fuel injectors arranged on a table
Because we would be dramatically increasing both peak power and—especially—torque, we replaced the factory injectors with these 42-pounders. (Image/Richard Holdener)
installing a camshaft into a v8 engine
We are not sure of the cam on this particular L98 TPI 350 motor. It was said to be a stock L98 cam, but this 350 made impressive power even in stock trim. (Image/Richard Holdener)
fast xfi engine management system module
We relied on a FAST XFI stand-alone management system to dial in the air fuel and timing curves on the NA and turbo L98 combos. (Image/Richard Holdener)
tpi engine on a run stand
Run on the dyno first in naturally aspirated trim, the tuned for torque L98 produced 332 hp at 4,800 rpm and 393 lb.-ft. of torque at 4,000 rpm. (Image/Richard Holdener)
close up of a turbocharger
After running the L98 Tune Port motor in naturally aspirated trim, we installed a single turbo kit. While the B2K option on the Callaway Corvette relied on smaller twin turbos, this larger Holset turbo offered more than enough flow to support our power needs. (Image/Richard Holdener)
custom turbo header for a small block chevy
In addition to the 70mm Holset turbo, the Corvette kit from HP Performance featured this dedicated tubular shorty header to route exhaust gas under the oil pan to the Y pipe. (Image/Richard Holdener)
turbo header installed on a small block chevy
The driver’s side of the turbo kit also featured a dedicated tubular (shorty) header that merged with the passenger side exhaust into a Y-pipe to feed the T4 turbo flange. (Image/Richard Holdener)
blow off valve installed
The boost pressure supplied by the turbo was controlled by a single, 60mm Tial waste gate. (Image/Richard Holdener)
large intercooler for a turbocharger
For our test, we ran a maximum of 8.6 psi (though more was certainly available). Keeping the IATs in check on the turbo TPI motor was this front-mounted, air-to-air intercooler. (Image/Richard Holdener)
turbocharged intercooled small block chevy on dyno
Run on the dyno in turbo form, the boosted L98 stayed true to form by offering massive torque (like the original Callaway) and a solid horsepower gain. The 1987 B2K Callaway twin turbo was rated at 345 hp and 465 lb.-ft. of torque (382 hp and 562 lb.-ft. later), but our combo pumped out an even 500 hp and 610 lb.-ft. of torque. (Image/Richard Holdener)
close up of afr logo on a cylinder head
Here is what keeps us up at night. Though we never ran the TPI in modified form with the turbo again, we did upgrade the NA L98, first with CNC-ported, AFR 195 heads. (Image/Richard Holdener)
close up of camshaft and lifter
We stepped up from the unknown cam in the L98 to a Comp XR276HR that offered a 502/510 lift split, a 224/230 degree duration split and 110 degree LSA. (Image/Richard Holdener)
small block chevy tpi on engine dyno
We ran this combination with a variety of different induction systems, carb and EFI, but can’t help wonder what our AFR-headed, mild-cammed, Stealth-Rammed 350 would do with boost now that it was making 100 more horsepower in naturally aspirated trim? (Image/Richard Holdener)

Share this Article

Richard Holdener is a technical editor with over 25 years of hands-on experience in the automotive industry. He's authored several books on performance engine building and written numerous articles for publications like Hot Rod, Car Craft, Super Chevy, Power & Performance, GM High Tech, and many others.