I started modifying this car in '94 with gradual increases in boost until reaching 13psi and the addition of a Stewart Warner 30in/hg-15psi boost gauge. I hooked a DVM to the O2 sensor signal to monitor the mixture and to be sure the stock fuel system could handle the increased demands. One winter later the exhaust system rusted out so I ran a pipe from the end of the stock downpipe to the passenger side that had a single ANSA resonated tip just ahead of the rear wheel. I also discovered and removed the intake muffler that consumes the entire hollow space inside the passenger fender well. This modification alone improved throttle response in an almost supernatural way. At this time I went ahead and ran the boost up to 15psi. This set up ran well and sounded good for a while. In that state of tune the car would out run almost everything but high end V-8s and could cruise at 130mph, which is redline at 6500rpm in top gear, at 2/3 throttle opening. Not bad considering the investment.
Some time after, though, disaster struck. On a return trip on the interstate where I had been running about 75mph and about 5psi boost I began to notice that I had to use more 'pedal' to maintain the same speed and I seemed to be consuming fuel at an accelerated rate. When I came to a stop at the end of the exit ramp I knew something was wrong. The engine would only idle on three cylinders. Upon completion of a compression test the next day it was clear that the #3 cylinder had no compression at all. Removing the heads revealed nothing about what had gone wrong. Once the piston was removed , though, it became all too clear. The accompanying photo shows the catastrophic destruction of the rings and lands. Note that the ring that meted away was the second compression ring and not the top ring.
It was not immediately apparent why this had occurred. Increased boost levels were sure to have aggravated the problem, but only slightly, as it turned out. Detonation was also suspected, but the evidence did not support that unequivocally. The injectors were sent to RC Engineering in Torence CA for a cleaning and balance. They returned with the report that the #3 injector was pretty bad off when they received it. It is my conclusion that the poor spray pattern and decreased capacity caused the #3 cylinder to run lean, but not necessarily to the point of detonation, as there was no detectable evidence during driving or on the piston crown upon inspection. This, in turn, caused the cylinder temp to increase and heat the rings to the point of gap bridging. The factory spec for the ring gap is .0079-.0138. The lower end of this tolerance is probably a bit too tight.
At this point it was time for a new piston. The one that showed up at the local dealer looked nothing like the original and weighed 18 grams less than the other three. It appeared to be a cast piston while the originals appeared to be forged. Since this piston superseded the original part number and since using this piston would have resulted in balance problems too great to deal with at the machine shop, the search was on for aftermarket replacements. JE Pistons of Huntington Beach CA came up with a set of forged alloy pistons for the now 92.5 mm bore size with a reverse dome set to 8.0:1 CR in lieu of the stock 7.7:1 ratio. These pistons actually cost less than the dealer items. I should note here that if the #3 injector had not gone bad then I would definately be running stock pistons today as I believe that they are more than up to the task. After I received and deburred the pistons they were sent to HPC in Manchester CT for a ceramic coating on the crowns and an oil attracting film on the skirts. I also had the exhaust valves ceramic coated while I was at it. I used a moly faced top ring(1.5mm), cast iron second ring(1.5mm), and standard expander type oil control (4mm). I had JE move the ring package down on the pistons a little to get them away from the heat of combustion. The machine work on the cylinders was performed by Marco Machine of Gray TN using a 6in thick torque plate I had made by Master Modelcraft of Bristol TN.
The heads are stock castings that were purchased a few years ago. The originals were destroyed by overheating. The ports were cleaned up quite a bit and hand sanded to a satin finish, as they were fairly rough as cast. There was considerable excess material in the bowl area on the intake side and a major occlusion in the intake tract caused by the intrusion of one of the hydraulic lash adjuster bore bosses protruding into the port from the other side. If you stick your finger in the port from the manifold opening you can feel it right away. I removed quite a bit of material here. I did not grind it down to nothing for fear of finding the steel cast-in lash adjuster bore sleeve, but I put a real slow outside radius on it. The exhaust side only required a little clean up. The combustion chambers were polished to a mirror finish using jewelers rouge and the sparkplugs were unshrouded slightly.
The engine refused to fire when reinstalled in the car. Close inspection revealed injectors that would not open when energized. A call to RC Engineering confirmed that they were 'stuck'. The oil that had been applied to them after cleaning at RC had coagulated to seize them closed. A quick trip to CA and back and they were as good as new. This time the engine fired almost immediately and sounded good. After warming for a few minutes I eased it into reverse only to find that the car would not move. No matter what gear was selected, the car would not budge. After three different clutch disc installations I decided to open the trans. Everything looked to be in order until I got to the centre differential. It seems that the tow home had overheated and melted the centre housing. (You see, while this motor was down, a spare was run in its place. That motor snapped the left timing belt and the car coasted down in gear with no oil supply[no belt] so the bearings were wiped out. This resulted in a tow and an extended hibernation.) Now the search was on for a trans. Imagine my surprise and elation when I found an exact replacement locally for $450. Imagine my horror and suspicion when I pulled the dip stick out to find that the length of it was missing from the handle. A little luck and a telescopic magnet fished it out from the drain opening.
With all of this back together it was time to try it again. Viola!! It's alive!! After a break-in of 1000-1500 miles I'll switch back to Mobil 1 synthetic oil. (Back in the early '80 when Reeves Callaway first started building his turbo Corvettes Mobil 1 helped save him from the heat related failures that had plagued his cars.) I may even try a Slick 50 treatment. A friend of mine treated his Ford TurboCoupe and noticed a difference.
The car now has part of the new downpipe on it with no other form of exhaust. Needless to say it is painfully loud. I wanted to see the results of running an open exhaust. The power comes on very strong with the boost. So strong, in fact, that the new OEM clutch slips rather badly when the boost gauge swings around to the high pressure, BUT... I can still feel the sensation of running out of breath in the uppermost rpm. Some of this may be eliminated with the addition of the new header/crossover pipe, but I feel as though that the power will ultimately be limited by the minuscule size of the factory turbo. As you can see from the photo the turbine housing outlet is about 2 5/16in.
If you reach up in the housing right at the turbine wheel you get 1.89in. I expect that the limits of this turbocharger will be reached very soon. At any rate, the down pipe and back is 2.5in. This should provide more than enough flow capacity for quite a few horsepower. When I get it all bolted up I'll let you know the results.