Hurst Billet Plus Shifter for ZF6 Equipped C4

This week I got to do some actual hot rodding on my ’95 Corvette. Back in the late 80’s when I was an undersexed, pimple-faced high schooler, an absolute revelation was delivered upon me in the pages of Hot Rod magazine. This magnificent beast was eventually sold to the general public as the 1989 ZR-1 Corvette. Secondary to its breathtaking Lotus designed 32 valve, dual overhead cam re-invention of the legendary smallblock 350 was a dual overdrive six speed manual transmission built by Zahnradfabrik Friedrichshafen of west Germany. This particularly exotic piece of gearhead porn wasn’t necessarily accessible to those of limited means such as myself, but GM in it’s infinite wisdom also put this killer gearbox in many of it’s LT-1 equipped Vettes. 20 years later I happened to own one secondhand.

So after getting all the major gremlins of a 150,000 mile used car sorted out, I finally had scraped together a small bit of spare cash to work with and decided it would be a good time to improve the shifter in The Green Monster. After a few thousand miles of spirited driving, I’d grown a bit disenchanted with the factory shifter. Several hours of googlage later, I came across the Hurst 3915084 billet plus shifter that promised to cure all my shifting woes.

First and foremost of my gripes with the factory shifter was the vagueness caused by the rubber dampened shaft coupler. See exhibit A.

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The shifter to the top of the frame being the stock GM unit, please note the large bulge where the primary shaft links to the lower shaft (the piece with the aluminum thing on it). Below is the Hurst shifter, but let’s talk about the factory shifter for the moment. That aforementioned large bulge, as I discovered after clamping the lower shaft in a vise, is a coupler section filled with vulcanized rubber. This “coupler” seems to be the main cause of my frustration – the vague shift pattern. I wouldn’t be able to thoroughly articulate my difficulties with the factory shifter until I put a few miles on the solid shaft Hurst shifter, at which time it became immediately obvious why the rubber coupler appears to be the work of a lunatic.

Secondary to my commentary on the rubber coupler, note the differences in length between the two. This has been my other major “gripe” with the factory shifter. Since the ZF is a fairly massive gearbox primarily chosen for use in 3/4 ton diesel pickups, GM engineers thought a large primary lever would be a good idea so as to reduce the amount of effort required for gear changes. The side-effect of this decision is that gear changes in the cramped quarters of the Corvette resembled elbow and shoulder range-of-motion exercises that I’ve become all too familiar with in my 40’s. Those large motions don’t contribute much to the driving experience, whether you’re having a relaxed cruise on a winding road, or are discovering the limits of your tire’s grip on said winding road (my personal favorite).

So a few mouse clicks and $208 later (see http://www.hurstshiftersonline.com) a shifter and knob arrived on my doorstep begging me to install them.

Step 1 of the installation manual: remove the exhaust system from cats back. As it turns out the exhaust on this project took more time than the shifter installation, by an order of magnitude…

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A few hours and several dozen cursewords later the exhaust system was lying in the middle of my garage, separate from the car. For those considering this endeavor the advice I will give you is to unhook the muffler hangers from the aluminum rear bumper subframe BEFORE you start to work the slip-fit joint behind the passenger side cat apart. Also, in order to get that joint to clear the tunnel under the passenger seat you’ll need to rotate the entire exhaust system. So set your jack stands as far forward and outboard as possible. I used a soft rubber-faced deadblow hammer on the bend directly behind the slip joint.

Once the exhaust system is out of the way you can cut the stay-ties holding the shift boots on, and if you did as well as I did have loads of oily crud in your eyes. Thank god the previous owner didn’t fix that oil leak around the water pump drive shaft – the difference between normal underbody crud and oily underbody crud is that oily crud sticks in your hair like chewing gum. An hour later the new shifter was installed and functioning.

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This wouldn’t be the end of this “joyful” experience however. After 20 years of thermal cycling the catalytic converter studs had all but turned into glass. My trusty plasma cutter was the only tool that would remove the nuts holding the driver’s side exhaust flange to the cat. About six hours of drilling was required to make the necessary accommodations for new stainless steel bolts to connect the primary pipe to the driver’s side cat. If you look carefully at the picture of the cat flange you may notice what I did – sometime during the life of the car some spectacular specimen of moron took it upon himself to partially remove the ceramic catalyst media from only the driver’s side cat. The only reason I can find for why they spared the passenger’s side cat their wrath is it’s the side equipped with a secondary oxygen sensor. While I don’t necessarily care one way or another about the ecological impact of my habitual burning of hydrocarbons, the goddamned cats are almost 6″ in diameter, and flow about the same as a piece of 3″ open pipe. Or at least they did until some tool broke the honeycomb media and left pieces of it piled on each other inside the shell. Anyway, after spending two hours per stud drilling out the old hardware, I was almost ready to put her back together.

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But since I had the exhaust off I thought I’d perform another modification I read about on a Corvette owner’s forum – drilling and tapping the differential case for a drain plug.

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This was the smoothest task I would tackle on this job, but not the cleanest. Man do I love the smell of differential oil! The suggestion I took was to drill a 5/16″ hole 1/2″ forward of the case flange, and 1/2″ left of the center bolt. I chose to use the next size smaller drill, then a countersink, a step drill, and then a tapered reamer to get the proper hole shape for a 1/8-27 NPT tap. Now differential oil changes will be a breeze, as long as I don’t repeat the stroke of brilliance I had after installing the plug when I dropped the allen key pack in the coffee can used to catch the old diff oil. Ugh…

So after filling the diff with a little over a quart of Mobil-1 Synthetic differential lube, I can finally put everything together. Burning the midnight (diff) oil, around 1:00 AM I managed to get the car ready for some gear-banging the next day.

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After a few hours of sleep I managed to find an excuse the next day to take a short jaunt down my closest winding road to test the shifting while at the same time burning off the cutting oil and WD-40 that had contaminated the exhaust system.

My initial impression was that gear engagement was a little notchy, but of more importance, I came to understand the best part of getting rid of the rubber coupler. As I was learning the new shifter I naturally missed a few 4th to 5th transitions, and with the coupler no longer in the picture I was immediately aware of where I was and what I did thanks to the greatly increased tactile feedback. The coupler caused a vague, dodgy feel when you missed shifts that did nothing to help you learn. From what I gathered reading Bill Boudreau’s site http://www.zfdoc.com, the coupler was incorporated to help reduce vibration. But in my opinion, even with a solid resin shift knob in place of the stock padded leather shift handle, I can’t honestly say I feel any more vibration. After experiencing the solid shifter I guess the best way to describe the stock shifter is that it seemed to buffer the driver from the extreme mechanical reality he/she was controlling. This to me seems counterintuitive in a sport touring car – at least one without a Cadillac nameplate. Isn’t the whole point to experience the machine and be one with it? If I wanted soft and plush I’d have bought a puppy. So my advice to ZF equipped C4 owners who are sports touring purists, hot rodders, or anyone who believes cars with only two foot pedals are for pussies, is to snatch up a Hurst solid billet shifter for your ride before they stop making them!

Upon returning from my test drive I decided to unscrew the shift knob by three turns, in effect lengthening the lever and hopefully increasing leverage slightly, as I discovered the shift effort required by the changes the shortened shifter introduced coupled with my further shortening of the shift lever by cutting 1/2″ off the threaded end were quite significant. In fact, after more driving I think I’ll go a couple more turns out. Another contributing factor may be attributed to the difference between where your hand centers on the factory shift knob that this shifter was designed for, and the 2-1/8″ ball (Hurst 1630155) that I opted for. While I find no fault with the factory knob, mine was getting pretty ragged and I personally find a round shift ball more ergonomically suited to interactive driving. Also, no-one who looks in will mistake my car for a wimp-accommodating slushbox car. And did I mention it has the Hurst logo on the sides? 🙂

All things aside, the notchy feel and increased effort are handily eclipsed by the comfortably short arm movements and lightning fast 1st to 2nd speed shifts afforded by the shortened shifter. Most welcome is an overall sense of precision that was completely absent with the factory shifter. It will take some time to adjust to the new shifter, but I can tell after only a few short drives that the improvement was worth every penny and skinned knuckle.

Check back next week when I’ll be installing an oversize 52mm / 720cfm throttle body.

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