C4 ZR-1 Upgrades - The Purple Project, Part 2

C4 Adventures: On The Road And Under The Hood

Hib Halverson Apr 1, 2000 0 Comment(s)
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At this point, you could say this project is struggling. If you read Part 1, you know I blew up the motor in our ZR-1 project car during its first dynamometer test. Nothing like a jinx right out of the box, eh? This wasn't just any ordinary engine failure, either. When I blow motors, I make sure they're really expensive ones. A ZR-1's LT5 costs about 25 grand, new, and probably a third of that to overhaul.

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But, we're comin' back strong. This month, our Purple Project C4 gets an engine overhaul. Then, we'll move on with a brake upgrade and some transmission work.

Getting Back On The Road
The intention of Part One's test was to verify our C4 project's advertised performance. Testing was on a Dynojet, inertia-type chassis dynamometer, which is about as abusive as accelerating on a slight downgrade. The engine failed at 6,400 rpm, well below its 7,072-rpm rev limit. Considering the test's lack of severity along with the performance, reliability, and durability standards to which General Motors stated the LT5 engine was built, it should not have failed.

Fortunately, when this happened back in '96, the car was still under warranty. Jim Van Dorn, whom I'd met via the "ZR1 Net" electronic mail list, owns Automasters, one of the few Corvette service shops in the western U.S. doing LT5 overhauls. A few Chevrolet dealers in southern California sublet Corvette warranty work to Automasters. Can you see where we're going, here?

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One of those dealers, Mac McGruder Chevrolet in Palm Springs, was reasonably close to the dyno test venue, so I had the car flatbedded there. I never told them a magazine article was underway. To them, I was just another customer. The people at McGruder processed the warranty claim and the following week the car was moved to Automasters' shop in nearby Palm Desert for the overhaul. A week or so went by and Jim Van Dorn telephoned. "We got it out. Come down and we'll pull it apart."

I arrived the following Saturday to find Van Dorn and one of his service techs, Joe Delara, starting the tear down. By lunch, the top of the motor was spread out on the bench. It didn't look good at all. The upper structure of the right cylinder head was fractured in numerous locations around the #4 cylinder's intake valves. The primary intake was stuck in its guide and the intake camshaft and the #4 primary intake valve lifter, locks, retainer, and valve spring were destroyed. The #4 piston top was damaged. Sitting in a little cup on the bench was a pile of mangled debris that came out of the oil pan and the top of the head.

"Because we couldn't find all of what's left of the valve gear for number four," Jim said in his office after lunch, "it's hard to be sure what caused the failure. It's interesting that it blew up at such a low rpm. If it was a split lock or a retainer that broke, even down at 6,400, the results will be what you saw in your motor. You're lucky it didn't suck that intake and get torn up even more."

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That afternoon, as I watched Automasters disassemble the bottom end, I was dismayed to find: 1) the rear main bearing was very worn, considering the 27,000 miles on the engine at the time of failure, and 2) the rubber sleeve that seals the oil pickup tube in the block had been improperly installed at the factory. It is possible the sleeve had not been sealing at high rpm when there was a significant pressure differential between the oil pump intake and the oil pan. That might have caused air to enter the oiling system through the improperly installed seal. Aerated oil could have caused the seriously worn bearings. Even if the engine hadn't blown up, maybe an overhaul was a good thing.

For a couple of reasons, it took a few months to get the engine back in the car. "Take all the time you need, guys," I told the folks at McGruder Chevy. I wanted no cut corners due to an impatient customer. Also, Chevrolet and I had an exchange of views on what parts were to be replaced during the overhaul. That took a week or so to resolve. Chevy felt it should only replace the #4 piston and liner. My view was, since the engine had near 30,000 miles on it, the right way to repair it and have it running at its optimum after the job was done was to replace all eight piston/liner sets. Eventually, Chevrolet agreed. In the end, the major parts replacements were all eight pistons and liners, a new right cylinder head, its intake camshaft and valve gear, a set of bearings, and an oil pump. By the middle of the winter, the car was back on the road. The people at McGruder Chevrolet and Automasters did a good job.

I changed the oil filter at 500 miles after the overhaul and cut it apart to inspect for any debris. I saw nothing. I also drained the 10W30, petroleum-based oil run during the break-in period and replaced it with Red Line 10W40. Then I re-installed the K&N Air Filter, the Flowmaster Force II cat-back, and the DRM ECM calibration, all of which were reviewed in Part One of this series and which were on the car before I put it back to stock for the first dyno test.

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Eventually, I'd come back to my powertrain enhancement work, but for now, I was sticking with only those limited bolt-ons.

Changing Brakes
About six months after the engine overhaul, I wanted an improvement in braking performance but, when I went looking for aftermarket brake upgrades, I liked the hardware but couldn't hack the price. Nevertheless, high-end brake upgrades, such as Baer Racing's Baer Claws, are popular, as are the kits Doug Rippie Motorsports or Exotic Muscle sell that adapt C5 front brakes to a C4.

Corvettes built between 1988 and 1994 with the Z51, Z52, Z07 or ZR1 RPOs have larger front brake rotors, along with specific calipers and mounting brackets to fit around those rotors. The RPO J55 "big" brakes were made standard equipment for the last two years of C4 production. In addition, the '96 Collector Editions (RPO Z15) and Grand Sports (RPO Z16) had a specific front caliper, different from other J55 units. The casting structure was different, it had "Corvette" embossed onto its outboard side, and it was powdercoated gloss black.

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Amongst techies there's confusion about the design of these calipers. GM said the '96s were more rigid. Under heavy braking, caliper flex was supposedly reduced and pedal feel improved. In recent years, I've heard, from other sources in GM, there was, in fact, little or no difference in the stiffness between the regular J55 and Collector Edition/Grand Sport front calipers.

At this point, we don't know what to think, other than the GS fronts look cool, so I ordered a set (PN 12528689, left, and 12528690, right) for our Purple Project from my usual source for GM parts: Clippinger Chevrolet in Covina, California. The rear calipers for Z15/Z16 share the design of other '88-96 rears and, we ordered a set (PN 12528692, left and 12528693, right) for their black powdercoating. All the brake rotors for Collector Editions and Grand Sports were treated with a gray, anticorrosive coating, so we got a set of those (PNs 10257631 left front, 10257632 right front, 10257721 rear), too. Someone upgrading from a set of '88-94 base front brakes will also need caliper mounting brackets and brake pads. Anyone with an '88-94 base car who drives even moderately hard ought to consider this upgrade, because the base brakes are barely adequate and suffer from brake disc warpage problems even in normal street driving. The upgrade can be done for less than we spent if standard J55 calipers and rotors are used rather than the Grand Sport pieces. The price can drop even lower if good used calipers are overhauled and installed on used brackets.

Using my Lincoln floor jack, the Jack-A-Vette tool (both discussed in Part 1) and a set of jackstands, I got the car up in the air, then stripped it of its original calipers, brake hoses, brake rotors and wheel bearings. One of the four bearings was worn outside of the runout specification listed in the Service Manual, so I replaced all four with new parts (PN 20-65, front and 20-24TR, rear) from L&S Automotive Products Company. While on the subject of factory manuals, they are strongly advised for DIY service work beyond oil and spark plug changes and are a requirement for diagnostic work with the car's various computer systems.

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I took the Grand Sport brake discs to Global West Suspension Systems and had spiral grooves milled into the rotors' friction surfaces. Part of the problem that causes brakes to "fade" occurs when the brake pad gets really hot and emits gasses. When this happens, a boundary layer of hot gas is trapped between the surface of the brake disc and the brake pad. This interferes with the friction necessary for braking action. What you feel is the car's deceleration not increasing with pedal pressure.

One anti-fade strategy is to give this gas a place to go. Racers first did this to vented brake discs by drilling holes through the disc's friction surface into the vent passages. The gasses would be carried away by the cooling air flowing through the vents. In recent years, knowing it's a hedge against brake fade, enthusiasts have taken to cross-drilling production rotors. This can be a dangerous for a car that sees a lot of heavy braking. While many racing brake rotors are robust enough to tolerate the loss in structural integrity and heat sinking that comes with cross-drilling, some production discs are not and this may be true of some C4 units. In this respect, the J55 discs are marginal and the '84-94 base rotors are unacceptable. It is our opinion that drilling these rotors may lead to cracking and, in some extreme cases, fracturing.

The best thing to do, if you want to reduce the potential for brake fade due to out-gassing, is have them grooved. This procedure requires a machine shop experienced in brake rotor modification and a NC or CNC mill. Global West has both.

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Back at the shop, I installed the brake hardware. With a couple of exceptions, I followed the instructions in the Service Manual. The book calls for new caliper mounting bracket bolts each time one removes the calipers, but I see that as an unnecessary expense as long as the bolts and their bolt holes are in good condition. I carefully cleaned the bolt threads and the threads in each front and rear knuckle, applied Valco Cincinnati Thread Locker to the bolts, and installed the parts.

Caution! The 1995 and 1996 Service Manuals incorrectly list the rear caliper mounting bracket bolt torque. For all '88-96 rear brakes, the correct figure is 70 ft-lbs. When it is misprinted, the front bolt torque is listed for the rear bolts. If the rear bolts have been tightened to that incorrect value, damage to the rear knuckles will occur. When changing the rear brakes on a '95 or '96, carefully inspect the caliper mounting bolts and the bolt holes in the knuckle. If the bolt hole threads are damaged due to overtorque, the knuckle must either be replaced or have its threads repaired with a TimeSert kit, which is discussed later in this article. Also, in that case, new rear caliper bracket bolts are required.

I decided that until I actually take our purple project car to a race track, I'd use the OE brake pads, so on went all four "loaded" calipers right out of the GM boxes. Further improvement in pedal feel comes if the production brake hoses are replaced with braided, stainless steel covered, Teflon brake hoses. Doug Rippie markets braided brake hoses for C4s, so I installed a set (PN10-105).

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Next, I flushed the system. In the case of a '95 ZR-1, a pressure bleeder must be used because the ABS controller's brake fluid prime pipe is not pressurized when the brake pedal is stroked during a "manual" bleeding session. I use a Mac Tools bleeder (PN BBT2), but there are a wide variety of similar devices on the market.

There are three different ABS brake pressure modulators used on '86-96 C4s. All are located in the well behind the driver seat. The '86-91 brake pressure modulator is not easily bled. The '92-94 base and '92-95 ZR-1 BPM is bled via a fitting on its side. When doing this, take care to avoid spilling brake fluid in the well. The '94-96 non-ZR-1 "Electronic Brake and Traction Control Module" (EBTCM) can only be bled using a scan tester, such as the Vetronix TECH1A or Mastertech, both of which activate the unit's "autobleed" function.

The rest of the bleeding procedure is pretty conventional and can be done with a pressure bleeder or manually. Interestingly, I use Ford brake fluid (PN C6AZ-19542-AB). Back in the early '90s, I was an observer at a Corvette development test session at a race track near Grattan, Michigan. As I watched the development engineers work on the cars, I noticed a cache of Ford brake fluid in their tool chest. Intrigued with its presence, I inquired. I was told that Corvette development had tested a number of brake fluids, including GM's own brand, and found the Ford to be best by a slight margin. When doing racetrack testing, which taxes brake fluid to the maximum, they wanted the safety margin provided by the Ford fluid.

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The other reasons I like Ford fluid for street high-performance applications are: 1) value-the product performs well and has a reasonable cost, and 2) a low-level of hygroscopicity (moisture absorption) typical of factory-fill brake fluids that works well in an application where the frequent fluid changes associated with race cars are not an option.

Whatever fluid you pick, do not use silicone-based brake fluid in any '86 or later Corvette. Silicone fluids are not compatible with the antilock braking systems (ABS) on those cars.

The last step in our brake thrash was installation of a Mid America Designs front air dam with integral brake cooling ducts (PN 18721). This replaces the stock air dam and accepts ducts that run to the inside of the brake discs and direct cooling air at each disc's cooling vents. It takes several hours to install using common tools. We did run into one problem-the cable ties that come with the kit. They are a weak design, and we broke all but two trying to tighten them as we tied down the cooling ducts to suspension parts. We needed a more robust tie, like the type intended for electrical work, and we found a "variety pack" of cable ties at Costco Wholesale. When installing the kit, put the ducts in place first, starting from the outlet next to the inside of each brake rotor. Work forward, attaching the cooling duct as shown in the photo. Put the Mid America air dam outboard sections in last and connect the ducts to them. While Mid America supplies an entire air damn, we saved the new centersection for when our OE center finally wears out.

For race track use, the stock C4 front brakes are marginal in their ability to deal with heat. I figure the combination of grooved rotors, Ford fluid, the better brake cooling from Mid America's air dam, and, when the time comes, a more aggressive brake pad, would give us enough additional brake performance to be able to run occasional time trial events with the car and not have to worry too much about brake fade.

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At some time in the future, if our race track adventures become more frequent and our budget gets larger, then I'll consider an aftermarket brake conversion.

Leaky Gearbox
Another six months went by and one day, I noticed a little spot of oil on the floor. After a week or so, this became a bigger spot below the car's ZF S6-40 six-speed manual transmission. I got underneath the trans with a bright light. The transmission was leaking at the part-line between the extension housing and the rear case.

The Service Manual provided the torque specification for the extension housing bolts. The first bolt I put the torque wrench on was the lowest one, and it turned but didn't tighten. Obviously, the threads in the case were stripped.

The ZF was used in all stick-shift Corvettes from 1989-96. Generally, it's a robust design, but there are a couple of caution areas. First, the synchronizer assemblies were designed for low shift effort in spite of the massive gears necessary to meet GM durability schedules with a 450 ('89-early-'93) or a 400 (late-'93-96) lb-ft input torque rating. As a result, the synchros, especially the multi-cone units on the lower gears, are not tolerant of power shifting. This makes the ZF a poor choice for hard-core drag racing. If you power shift a ZF, expect short synchronizer life.

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Secondly, based on my experience and that of other Corvetters I've been in contact with using Internet mail lists such as VetteNet, the ZF seems to have occasional trouble with extension housing bolt hole threads in the rear transmission case getting stripped. It's hard to imagine those threads stripped themselves, so I suspect ZF had a problem with overtightening of those bolts during assembly. The C4 powertrain consists of the engine, transmission, torque beam and the differential bolted together as a rigid unit. Because the powertrain mounts to the car at the front, with the engine mounts, and at the rear, with the differential mounts, torque loads are transmitted from the torque beam to the transmission through its extension housing. If the lower bolt is in partially damaged threads (due to overtightening), the fluctuating loads from drive axle torque may eventually pull the bolt out of the hole.

DIY maintenance of the ZF is not easy. Parts availability is restricted by a peculiar legal agreement between General Motors and ZF Industries that makes most of the S6-40's parts unavailable in North America. Whatever ZF and GM were thinking about when they made that agreement in the mid-'80s, it clearly was not the interests of Corvette customers. For major repairs, in most cases, practical choices open to most '89-96 owners are either a new transmission, an overhaul at the ZF Industries Service Center in Indiana, or one of several independent rebuilders around the country. Both choices are very expensive. Certainly, forcing a new gearbox or an overhaul on someone needing only new synchros is a bit of a rip.

Minor repairs, such as gaskets, output shaft seals, the reverse light switch, skip-shift solenoid, replacement extension housings, and front bearing retainers are serviced by GM Service Part Operations (SPO).

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My leaking gearbox was going to require repair of the lower extension housing bolt hole threads in the transmission's rear case. In a situation like this, some C4 owners would have either an extension housing replacement or a transmission overhaul forced upon them by an ignorant or greedy repair facility. We found a simple and inexpensive way to fix this problem, and the repair can be done with the trans in the car for minimal cost and several hours work.

To start, the gearbox must be drained. Some DIY's are baffled by the metric socket-head drain plug. The easiest solution is a Volkswagen transaxle plug tool, which also fits the ZF. Next, the driveshaft and torque beam must be removed. I found the easiest way to get the torque beam out, since the extension housing had to come off, anyway, was to remove the bolts and take the housing and the beam out together. For more specifics, read the manual.

A durable fix for most any stripped bolt hole is the "Time-Sert" Thread Repair system sold by Time Fastener Company. A Time-Sert is a thread insert that is locked in place during installation. Because it's not a coil-type thread repair, it is a permanent and more durable way to fix a damaged bolt hole. I ordered a Time-Sert kit for M8x1.25 threads.

The kit contains half a dozen or so inserts and the tools necessary to install them. First you drill the damaged hole out to a larger size. Then, you use the counterbore tool to cut a recess at the top of the hole. Next use the kit's tap to cut the threads for the insert, then blow the chips out of the hole with shop air. Lastly, screw the insert onto a special insert driver, lubricate its threads, then install the insert. As the upper part of the insert bottoms in the counterbore, the driver expands the bottom of the insert, locking it in place.

With the ZF's offending bolt hole Time-Serted and a new rear seal (PN 10140538), the next step was to put the extension housing and the torque beam back in place. The 1995 Corvette Service Manual requires that a caulking product be used between the torque beam and its mounting surfaces on the back of the transmission and the front of the rear axle assembly. The caulking is necessary to eliminate the noise (usually a creaking sound) made by very small movements of the torque beam on those surfaces as the car's powertrain flexes due to torque load.

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The caulking kit GM dealers sell for this purpose is expensive and often needs to be special ordered. A suitable replacement is urethane caulking of the type used in home improvement work to seal doors, windows, and siding. I found some of this stuff at Home Depot and the cost was a fraction of the GM caulking kit. With the beam in place and caulked, the last step was to re-install the driveshaft.

I refilled the transmission, this time with Red Line Light Shockproof lubricant. Red Line's Shockproof gear lubricants are perhaps the ultimate choice for lubrication of gearboxes and rear axles in high-performance applications. The Light is best for the ZF and other manual transmissions that use low-viscosity lubricants such as engine oil or automatic transmission fluid. I'd previously used Red Line MTL, which got me a slight improvement in shift quality over the factory-fill, Havoline 10W30 engine oil. Going to the Light Shockproof gained another noticeable measure of shift effort improvement.

I drove the car for several weeks then put it up on stands again for a careful re-inspection of the transmission for leaks. None were found.

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This Month's Finishing Touches
With good performance from Red Line products to date, I changed the rear axle oil to Heavy Shockproof Gear Lubricant. I also changed to Red Line's Synthetic Power Steering Fluid.

During the really hot parts of the year, C4 owners have air conditioning to keep them cool. What about spring and early summer, when you may want a breeze from the outside rather than cold air blowing out the HVAC? Coupe drivers can always unbolt and take off the roof, but to me, that's a pain in the butt. All I want is a little cool air, not a lot of sun and the top taking up all the room in the back.

The solution is Eckler's Ventmaster kit (PN 30728). This device fits '84-96 Corvettes and allows you to operate the car with the rear hatch cracked about an inch-and-a-half. The effect on interior ventilation is amazing. Interior temperature can be lowered by as much as 10 degrees. No modifications are required to install this device. When you are ready for a little more airflow in the interior, simply pop the hatch, slip on the Ventmaster's hatch light and lock striker extensions, then close the hatch. What a cool little doodad! In the spring and summer, I use the Ventmaster all the time. I can't imagine a C4 coupe owner without one.

Modifications to set the car's look apart from others is always an objective of any magazine project. We have several appearance changes in-store for the Purple Project, but for now, we've only got the Grand Sport brakes and the front air dam. The last appearance change we made for Part Two was a Mid America European Tail Lamp Conversion Kit (PN 17870). Mid-America offers these kits for all C4s and C5s.

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C4s exported from North America have been equipped with special taillights having separate red brake lights and separate amber directional signal lights. This was done to meet motor vehicle standards in countries that require that type of lighting.

Mid America buys the export taillight assemblies from GM, then manufactures wiring harnesses that will adapt them to North American cars. The installation of the Lamp Conversion kit requires only basic electrical system skills and common tools. The kit comes with all necessary hardware. The installation is fairly easy on '91-up base Coupes and Convertibles. On ZR-1s and other C4s with the center-high-mount stop-light (CHMSL) on the halo bar just above the hatch, the job is a bit time consuming. You need to disassemble the interior trim on the right side below the hatch to access the CHMSL wiring harness and make a small wiring change.

The installation requires the removal of the license plate and the reverse lights to access the area between the taillamp positions and the fuel tank. While we had the reverse lights off, we replaced the stock bulbs with Mid-America's 50-watt halogen Back-Up Bulbs (PN 17136). This little extra increases safety and driving ease with its brighter reverse lighting.

Well, that's about it for this installment of Vette's Purple Project. Next time, we are going to get to the real meat of a project car: engine performance modifications.


How To
C4 Adventures: On The Road And Under The Hood
Hib Halverson Apr 1, 2000


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