Step By Step
While we've been improving the performance of our '93 Pontiac Firebird project car, Thunderchicken, we've been constantly nagged by one recurring problem: The 6-speed's shift lever refuses to go into gear easily. In prior issues, we've covered several attempted fixes. All of them helped some, but none of them actually returned the shifting effort anywhere close to stock level.
The last time we covered this ground (see, "Here's The Beef!" January 2001) we replaced the clutch's hydraulic actuation assembly. This was done first because it was easier and less expensive than another clutch swap. We figured that as one of the few remaining pieces of OEM driveline hardware, it was one of the likely suspects. This did help some, enough in fact to allow Thunderchicken to be driven to an 11.94/117.5 quarter mile pass.
Nevertheless, it became painfully obvious that the problem was not solved when we drove down to Bristol, Tennessee for the GM High-Tech Performance Nationals last October. The shift lever often required excruciating levels of force to go into gear, and in one particular traffic jam the car sat interminably in the middle of an interstate highway while gear selection was attempted.
Fresh out of ideas, we were beginning to resign ourselves to the fact that it was all in our mind: all 6-speeds in high-powered LT1s must be this balky. Then we got a cold call from David Norton of Star Performance Engineered Clutches in Hueytown, Alabama. David's company specializes in LT1 and LS1 clutch assemblies for F-bodies. Street, drag race, road race--David has built clutches for all these applications. "I've followed Thunderchicken for quite a while and I know what your problem is," David told us. "You've got the same problem as many others who come to us. Try our clutch and you'll see the difference."
At first, we couldn't imagine that our problem could be with our Centerforce dual-friction clutch. Other than the high shift effort, it engaged smoothly and gripped fiercely. If all we needed was to change the clutch, we'd give it a try, but we warned David that this was a two-edged sword: If your clutch cures the problem, you're a hero. If it doesn't, you just threw yourself under the bus. David's confidence never wavered. Two days later we received our new Star clutch assembly. We had chosen to use the Stage III system which has a performance pressure plate and a 4-puck sprung disc with Kevlar pads ($499).
To handle the install, we decided to shun the normal suspects and try our luck with someone new to us: Strope Speed Shop in Washington, PA. Strope has been in business in the Pittsburgh area since 1971. Bill Strope, his wife Cheryl, sons Aaron and Bill Jr. and technician Roger Creech have certainly kept pace with the times--their six-bay shop is home to a Dynojet chassis dyno with data-logging capability and they are fully conversant in all types of electronic fuel injection for GM vehicles. An engine-building facility and full-service installation service is also available for whatever Strope's customers demand. When we arrived, we were impressed!
Once work on the clutch began, we also took the opportunity to swap our leaking 12-bolt cover with an aluminum piece from Drive Train Specialists. The cast unit features a built-in drain plug (a feature we sorely needed) and main cap supports for around $170. The slight whine we've always had in our KTRE 12-bolt rear has gotten louder in recent weeks and we wanted to inspect it. This was the perfect time to swap the covers and change the lube.
In the process of changing the rear lube and installing the new DTS cover in our KTRE 12-bolt, Aaron Strope discovered that all the gear whine we were experiencing was due to a failing pinion bearing. Ever since replacing our driveshaft with a new DTS chromoly driveshaft, we have suspected our continuing vibration problem was related at least partially to the rearend.
We're not sure, but the lame pinion bearing could've been caused by several things--an improperly set up bearing, runout in the pinion flange, or a cheap low-quality bearing. In any case, we expected more from a rear costing $2500. Until we replace the pinion bearing, it would be extremely unwise for us to do any kind of racing.
Besides the clutch and the rearend cover, we also discarded our old driveshaft safety loop in favor of a bolt-in one from Metco. The original loop was built from a universal Lakewood kit seven years ago in an age when no 4th-gen specific kits existed. It would've been fine if the driveshaft didn't bottom out on the top of the loop on rare occasions. TC still gets used as a family truckster and when the entire family's aboard with cargo (our recent trip to the Woodward Dream Cruise for instance) the banging can get serious. The Metco piece--which sells for around $100--is literally the easiest bolt-on we've ever seen. Remove the factory tunnel brace, bolt on the Metco cross brace, and hook the Metco loop over the driveshaft and exhaust--you're done in five minutes! According to Strope, they have yet to find an exhaust system that didn't work with the Metco unit.
Before beginning any of the work, we decided to quantify the change in shift effort--if any. In the past, we tried to informally gauge improvement without knowing for sure if any had taken place. In short, your mind can play tricks on you--that's one of the reasons seat-of-the-pants horsepower always seems to evaporate when it's dyno time! This time it would be different. We found the nearest Wal-Mart and headed straight to the sporting goods section where we procured the most accurate digital fishing scale we could find. A few Mylar tie-wraps later, we were measuring the raw effort (in pounds and ounces) it took to move the shift lever from neutral to second gear (second being the easiest to measure from the driver's seat). We checked the shift effort with and without the engine running and the numbers gave voice to our frustration. With the engine off, it only took 7 lbs. to move the lever, but with the engine running it took as much as 40 lbs. of force (of course we could only get it to read 36 lbs. in front of the camera).
After the clutch was installed, we rechecked our force effort with the digital scale. Again with the engine off, the scale registered 7 lbs. With the engine running, the most we could get the scale to read was 10 lbs. before second gear engaged! (On most tries, however, it read the same as with the engine turned off!) That's a reduction in effort of between 26 and 33 lbs.. That agrees with our subjective evaluation that shifting was far easier. We'll even say stock-like, but without a stock car it's hard to tell.
On the street, the engagement of the Star clutch was smooth--not at all jerky or chattering like Kevlar clutches we've owned in the past. (For more on this, see the accompanying photo captions.) We had expressed reservation about using Kevlar to Star's David Norton, citing our previous difficulty with the material in a 10-second Mustang using another brand of clutch with Kevlar. He told us we wouldn't even know it was Kevlar and we're happy to say he was 100 percent right. Our faith in the streetability of Kevlar has been renewed!
On the track is where the real test will come. Unfortunately, we first have to delve into the KTRE 12-bolt to replace the pinion bearing. (We'll probably replace all the bearings while we're at it.) Once we fix the rear we'll bolt on the slicks and see if our Star Stage III clutch holds up to 5000-rpm launches and 6000-rpm powershifts. As of this writing, we've thrown quite a few back-to-back powershifts on the street with radials. In spite of our intentional and repeated abuse, we've not detected any slip or smelled any telltale burning.
David Norton tells us that Kevlar is one of the best wearing materials for clutches and it is odorless for the most part. When equipped with Kevlar, Norton says our Stage III unit is good up to 500 rearwheel horsepower--and we're only at 420 rwhp. If we do encounter slippage, we can move up to the carbon or ceramic pads which have a higher horsepower rating.
From the looks of it, Star has pulled off one incredible miracle. When it comes to clutches, we're not easily impressed; until now nobody has satisfactorily addressed the requirements of the F-body's hydraulic pull-style system. Although several manufacturers have paused to pop out an LT1 product here or an LS1 product there, it's obvious from the result that they have spent only the bare minimum of research needed to bring such products to market. Star on the other hand seems to understand the engineering problems of this unique GM platform. They have not only responded with one great product, but an entire line of clutches for every horsepower level and application. For this we're thankful, so we're placing our "star" of approval on this terrific product!
Thunderchicken Driveline History
*Factory clutch and shift mechanism operates flawlessly until the stock clutch burns up.
*Stock clutch is replaced with a Centerforce dual-friction clutch. A KTRE 12-bolt rear and Pro 5.0 shifter are installed with the Centerforce clutch at Modern Musclecar. Clutch disengagement is much closer to the floor than before. Missed shifts are noted but are attributed to the fact that we've been driving a Buick with an automatic for too long. ("Bringing Up The Rear," September, 1999)
*Shift problems continue when the MTI 396 LT1 is installed. A modified stock flywheel (neutral balance) is installed and the stock driveshaft is found to be bent and is replaced temporarily with another stock driveshaft.
*We decide to have the T-56 trans completely rebuilt. It is found to have no fluid in it, the tailshaft bushing and rear seal are destroyed, and the driveshaft is found to be bent again. The shift effort is improved slightly with a trans rebuild, but the problem is still there. ("Transmission? Impossible!" Sept. 2000)
*A new hydraulic clutch actuation assembly is installed along with a DTS chromoly driveshaft. The shift effort improves enough to make a few dragstrip passes, but it's still bad. ("Here's The Beef!" January 2001)
*Installation of Star Performance Engineered Clutch. Shift effort goes down from 36 pounds of effort to 10 pounds of effort. No more calls please, we have a winner!
Why's It So Hard To Make A Good LT1 Clutch?
From the quantity and tone of many of your letters and phone calls, it appears that rotten clutches for LT1 (and LS1) F-bodies are the rule. Simply stated, clutch problems are epidemic among '93-'00 Camaro and Firebird owners. So why is it so hard for aftermarket firms to build a good clutch for these cars?
Exhibit A: Hydraulic clutch actuation. Many companies employ extra-stiff pressure plates which overpower the slave cylinder or even the master cylinder. When powershifted rapidly, the fluid simply bypasses around the piston and insufficient fork movement is the result. When using a pull-style hydraulic clutch, a stock pressure plate or one with a modest increase in pressure should be used. At high speed, add-on weights on the diaphragm fingers significantly increase the load on the hydraulic system.
Exhibit B: Air gap. In an effort to beef up the friction surface, manufacturers add extra material to the clutch disc. That's fine for a regular push-style clutch, but the GM pull-style has a very small air gap between the clutch disc, pressure plate and flywheel. This is a result of the increased mechanical advantage needed for the hydraulic actuation. When the clutch disc is fattened up by the aftermarket, it can become "perpetually engaged"--dragging slightly even when the clutch is fully depressed.
Exhibit C: It's not an efficient use of time or money to develop a clutch for 4th-gen F-bodies. Mustangs, musclecars and high-tech racing parts are the bread and butter of most clutch companies. Building a good clutch isn't rocket science, it just takes dedication.
How does Star overcome these problems? By using pressure plates with modest clamping force, and by using friction materials with a higher coefficient of static friction. The use of Kevlar is largely overlooked by other manufacturers because it's not "street friendly" (i.e., it shutters and stutters when you slip away from a stoplight.) Star gets around this by modifying the pressure plate to soften Kevlar's "hit" on the street, and by employing a sprung hub disc. The number of friction pucks is also adjusted to soften or harden the engagement per the customer's taste. All this sounds rather easy--almost in no-brainer territory--but incredibly, Star is the only manufacturer to have put substantial effort into 4th-gen F-body R&D. The result is a clutch that holds like a pit bull, disengages smoothly on the street, and powershifts like a hot knife through butter at 6000 rpm.
Star Performance Engineered Clutches
Stage I: performance pressure plate, steel-backed organic performance disc ($379)
Stage II: performance pressure plate, full-faced Kevlar performance disc ($429)
Stage III: performance pressure plate, 4-puck sprung disc with Kevlar, carbon or ceramic pads ($499)
Stage IV: (not recommended for street use) performance pressure plate, 3-, 4-, or 6-puck unsprung disc with Kevlar, carbon or ceramic pads ($499)
Stage V: 12.5-lb. race pressure plate with choice of disc ($649)