General Service Advice and Diagnosis

Callaway Twin Turbo Corvette
General Motors RPO B2K

The purpose of this document is to aid in the diagnosis and repair of Callaway Twin Turbo Corvettes.

To begin, let’s look at what is different in the Callaway Twin Turbo Corvette, as compared to a standard, non-turbocharged Corvette and how this can affect drivability in general. Specific problems will be addressed later. The turbocharger system is the obvious answer, but here is all that it is: Two turbochargers that have air, exhaust gas, oil, and water flowing into them, and of course out of them, plus the wastegates. Let’s look at these one at a time.

Fresh air enters at the air filter, goes through the airflow sensor, and is split (by either the cast “Y” pipe in 1987 cars, or the “WonderBar” in later models) to go to the turbochargers. Ignoring what the turbos do for now, the air passes through the compressor side of the turbos and then goes up the compressor discharge pipes, through the intercoolers, the “Ram’s Horn” and into the throttle body. Now we are back into standard, non-turbo Corvette territory. How could all of this affect running? Air leaks or restrictions. Air leaks show up in the usual way: poor idle, low boost, or Code 33. Restrictions show up as poor high-speed performance, usually exhibiting unusually high vacuum, or a drastic decrease in boost and power at high RPM.

Exhaust gas is fed into the turbochargers from the exhaust manifolds. It goes through the turbine side of the turbos, the heavy cast iron side, through the turbine discharge, possibly the wastegate, and finally into the exhaust system. How does this affect running? Exhaust leaks, poor performance due to excessive back pressure, or overboosting due to wastegate malfunction. More on that later.

The turbochargers require, and are fed, engine oil either from a tee at the back of the intake manifold on 1987 cars, or an adapter fitting above the oil filter on later models. All that can go wrong here is turbo failure due to oil starvation (This is serious!) or leaks. The oil drains from the turbos into separate sump cans on the 1987 cars, or into a single can on later models. A scavenge pump then pumps this oil back into the engine oil pan. 1987 cars had a pump that was driven off the front of the power steering pump. This pump is obsolete and should be replaced! If you see one of these on a car, call the factory immediately. All cars should have a scavenge pump that is integrated with the accessory belt tensioner. This pump sucks the oil through a 1/2″ hose that is connected to the scavenge can(s) on the right side of the engine compartment to the top fitting on the pump. The pump discharges the oil out of the port facing the center of the engine, into another 1/2″ hose, where it is returned to the left side of the engine oil pan via a banjo fitting and a boss welded to the pan. How does this affect running? It doesn’t, but if the pump stops pumping, due to an air leak on the suction side, or a pump failure, oil will back up into the turbos filling both the exhaust and inlet tracks with oil. Tremendous clouds of smoke will be visible out the tailpipes, and in a matter of minutes the entire supply of engine oil will be ‘exhausted’ through the turbos.

The last topic is the wastegates. A wastegate is simply a valve operated by a pressure regulator that allows exhaust gases to bypass the turbochargers. Wastegate failure can cause overboost (most common failure; stuck closed), or no boost (stuck open). How does this affect running? No boost obviously means no power. Overboost (more than 50-52” Hg on the gauge) can lead to detonation, high-speed skips, blown head gaskets, or melted pistons. All drivers of this car should be made aware that boost is regulated to a maximum of 55” and that an overboost condition cannot be tolerated for more than a few seconds. Simply back off the throttle until the condition can be corrected.

The next thing to look at is the “MicroFueler” system. This system consists of the additional injectors mounted on the ““ram’s horn”” (the casting in front of the throttle body), a control module (ECU), a wiring harness, and a fuel hose connection from the fuel rail at the intake manifold. This system is required to provide additional fuel to the engine under boost conditions only. The injectors are driven by the MicroFueler ECU mounted on top of the kick panel over the driver’s knees. This ECU senses engine RPM and manifold pressure, and drives the injectors accordingly. With the exception of relying on the fuel pump for pressure and the tach connection on the distributor for engine rpm, it is 100% independent of the Corvette electronic engine management system. Failure of the MicroFueler system will cause a lean condition in boost that will, at a minimum, make the car feel down on power and, at worst, will cause engine misfire and/or detonation. Another possible failure mode of this system is to hold the MicroFueler injectors open constantly when the ignition key is in the run position. This will flood the engine and cause a no-start condition. If disconnecting the electrical connectors at the injectors enables the car to be started, then there is a problem with this system.

The eprom (or mem-cal) is reprogrammed by Callaway. The Callaway eprom gives the ESC more control over ignition timing, and more power enrichment. A faulty eprom will produce the same symptoms that a bad eprom causes in a standard non-turbo Corvette. NOTE: serious engine damage will occur if the Callaway eprom is replaced with a standard non-turbo eprom and the car is driven in any performance mode.

On 1987-1989 Twin Turbo Corvettes, a boost gauge is located in the left most central A/C duct. On 1990 and 1991 cars, the gauge is below the radio. The purpose of this gauge is to display manifold pressure. The gauge is calibrated in inches of mercury, and shows pressure and vacuum using the absolute scale. We do this so that you can refer to pressure or vacuum using the same scale. This is the scale that the pilots, IndyCar drivers, scientists and even weathermen use when referring to pressure, any pressure, regardless of being over or under atmospheric pressure. A gauge reading of less than approximately 30″ is indicating the engine running in a vacuum condition, which it does under most part throttle running. A gauge reading greater than 30″ means the engine is running in a boost condition, which happens when the engine is loaded and/or at wider throttle openings. No boost can be generated by revving the engine under no load, that is, with the car standing still. We recommend testing boost level by applying the throttle from cruise to full load repeatedly, with the car in 4th gear at 50 mph, and see if you can coax some positive manifold pressure. That would produce a reading over 30″ on the gauge (NOTE: Don’t allow more than 52″ [~10 psi] ever!) During testing, as always, make sure your car has fresh premium gasoline that is less than a year old with an octane rating of 91 RON or greater. If manifold pressure reading does not exceed 30″ during this test, the turbocharger wastegate actuator arms may be stuck partially open – a condition sometimes caused by sitting inoperative for an extended period. On 1987 models, you should be able to operate the wastegate actuator arms manually while on the car. On 1988-91 models, the wastegates must be removed from the car to inspect for a stuck actuator. In either case, if the arms do not move freely, we recommend using GM Heat Riser Lubricant – and only the GM Lubricant. It may be necessary to repeat daily applications over several consecutive days.

Generally, with the engine at idle, the gauge will read between 15 and 20 inches, and as the engine is placed under load, manifold pressure will swing over to the positive side of the gauge and reach a control pressure of 50″ to 52″ maximum (10.5 to 11 psi). The gauge will read 30″ (0 psi) when the engine is not running. If the gauge indicates more than 52″, please back off the throttle and diagnose. Too much manifold pressure will very quickly cause engine damage. A malfunction of the gauge itself will not cause any noticeable drivability problems. Almost always, however, a malfunction of the gauge is indicative of a problem elsewhere.

The last area to be looked at is the engine, including the cooling and oiling system.

The engine has been modified internally in order to make it able to withstand the additional stresses applied by its increased power output and to lower the compression to a level suitable for turbocharging. As far as engine operating problems, diagnostics are the same as for the standard non-turbo Corvette engine.

The radiator and thermostat have been upgraded in 1988 and 1989 Twin Turbo Corvettes to a custom four-row radiator and an external thermostat with bypass. This is to promote more stable coolant temperature. Again, problems are diagnosed as a standard non-turbo Corvette would be. There is no thermostat in the gooseneck, and one should not be put there unless the external thermostat and hoses are replaced with standard, non-turbo pieces. (Recent experience indicates that there is an advantage to the standard, non-turbo thermostat and radiator hoses as long as a small air bleed hole is punched or drilled in the thermostat flange.)

If a squeaking noise is detected at the accessory drive, the cause is usually the serpentine belt. The belt is exclusive to the Callaway Twin Turbo Corvette engine, a belt for a non-turbo engine will not fit properly. Use Callaway Part No. 210.98.0250 (Continental #4060887) for 1987 models or Callaway Part No. 210.98.0251 (Continental #4060919) for 1988 to 1991 models.

On Callaway Twin Turbo Corvettes, an engine oil cooler has been added in front of the radiator. With the exception of oil leaks, this will not affect drivability. It may, however, lengthen the warm-up time of the engine oil in cold ambient conditions. Engine life will be increased if the driver waits until engine oil temperature reaches at least 150°F before asking for performance.

As it is often necessary to remove various parts of the turbocharger system in order to service other engine components, the following includes directions on how to remove and replace various parts:

Intercoolers and Compressor Discharge Pipes

To remove the intercoolers, completely loosen the rear-most hose clamps at the blue Vulco connectors that connect the intercoolers to the “ram’s horn” (the hose clamp wants to be disconnected from itself at the screw clamp). Loosen the bottom-most hose clamps on the Vulcos with the heatshield wrappings that connect the intercoolers to the compressor discharge tubes. Lift firmly at the back of the intercooler until it comes off the discharge tube. Pull the front of the intercooler out of the “ram’s horn”.

To remove the compressor discharge pipes, it is necessary to jack the car up and support it on appropriate stands.

The left side compressor discharge pipe can be removed by loosening the upper hose clamp on the Vulco at the compressor discharge on the turbocharger. Remove the exhaust manifold bolt that holds the pipe bracket to the engine, and twist the pipe up and out.

The right side compressor discharge pipe is held to the compressor discharge on the turbocharger by two Allen capscrews through a flange which is accessed from the bottom. A ball-ended, or “Bondhus”, Allen wrench makes removal of these capscrews possible without disconnecting any water lines from the turbos. The Snap-On part number for a suitable tool is FABLM6. Mac Tools has a conventional 90 degree key that they sell as part number HKSM6. This tool will work better if the short end is cut off and it is used in conjunction with a 6mm 1/4″ drive socket. Simply remove the capscrews and washers and lift the tube up and out. Be sure and get the o-ring that is in the groove on the turbo flange. Unless it is damaged, it is okay to use this o-ring again.


To replace the right side compressor discharge pipe, simply reverse procedure. A little anti-seize on the bolts will make their removal easier next time.

On the left side compressor discharge pipe, make sure the tube flare is completely inserted into the Vulco before tightening the clamp. Start the bolt into the exhaust manifold and rotate the pipe towards the exhaust manifold before tightening the hose clamp at the bottom. Completely tighten the bolt into the exhaust manifold. It is important that the spark plug heat shield not be allowed to rotate and touch the spark plug wire or boot.

The procedure for replacing either intercooler is the same. Lubricate the inside of the Vulcos with silicone spray lubricant, or WD-40. Push the back of the intercooler down onto the compressor discharge pipe until the bottom of the intercooler is butted against the top of the compressor discharge pipe inside the Vulco. Push the front of the intercooler down over the Vulco on the “Ram’s Horn”. Lift the top edge of the Vulco up while pushing down on the front of the intercooler. Using a small screwdriver or a cotter pin removal tool gently lever the Vulco completely around the discharge of the intercooler. If any difficulty is encountered doing this, use more lubricant. The left side intercooler needs to be pushed down firmly while the hose clamp is being tightened in order to maintain its alignment with the “Ram’s Horn”.

The “Ram’s Horn”
To remove the “Ram’s Horn”, remove the intercoolers as outlined above. Disconnect the electrical connectors to the MicroFueler injectors. There is fuel pressure in the fuel line to the MicroFueler fuel rail. Support the rail with a 1/2″ wrench, and crack the banjo bolt to the fuel rail with a 17mm wrench. Wrap a rag around this connection while the fuel pressure bleeds off. After the fuel pressure bleeds off, disconnect the fuel hose from the fuel rail. Save the two copper washers, as they may be used again unless they are damaged. Remove the four Allen capscrews that hold the “Ram’s Horn” and throttle body to the plenum. Be careful not to scratch the “ram’s horn” with wrench, screw heads, or washers. (The Bondhus Allen wrench mentioned above is handy for this operation also.) Notice that there is a large o-ring inside the “Ram’s Horn”. Do not lose it.

Re-installation is the reverse procedure of removal. Just be sure that the throttle body gasket is properly positioned before installing the bolts. Tighten the bolts evenly.

Further questions should be directed to: