The Return of the Big Block from Hell
Part 16: Driveline Digest
From clutch to rear end, what our hot rod needs to survive.
By Hib Halverson
It started six years ago as a story about building a motor. Then, we began looking to look for a car to put it in. Whoa - what a concept! Build the baddest Shark that was still streetable. Such was born the "Big-Block from Hell", a 1971 Coupe that's nothing more than a big, nasty hot rod.
Initially the project had a four-year run in Vette, consisting of a dozen technical articles and three touring pieces. In this business, change is a given and, at the end of 1994, we let the 'BBfH' lie low for a while.
Now, it's 1997, other distracting interests were not all that long-lived and you readers have asked to see the car in the magazine again, so the Big Block from Hell, the longest running project car in any Corvette magazine, is back.
From the project's start in 1990, we've fashioned this car as an all-round street machine. It had to go fast, do it in reasonable comfort and be somewhat reliable. Good, all-round, hot rods have bulletproof drivelines. Stick with us and you'll learn what you need to make yours that way.
One is Good; Two are Better
BBfH fanatics will remember power for our project car is a full-tilt-boogie, 460-inch, 532 horse Big-Block Chevrolet. Its 530-lbs./ft. torque, capable of pulling the largest stumps, is hard on clutches. The first part of the car's beefed-up driveline, a two-plate clutch, was added way back in 1992 for Part Five of the project. For that, we installed the McLeod 'Street Twin' clutch/flywheel assembly (P/N 63002-7). It has two discs, twice the area of a single-plate clutch, so its pedal effort is less compared to a single-plate unit of the same clutch pressure. We chose the optional, large-diameter, aluminum flywheel. It weighs a scant, 12.5 pounds, but a low-mass flywheel is not a problem with this engine's prodigious, low-end torque. Considering the abuse we've dished out to the Street Twin in five years of severe service; we think McLeod products are darn good stuff. We'd have nothing else in our project car.
The clutch installation included a roller pilot bearing (P/N 14061685) from the GM Performance Parts catalog. Additionally, a Street Twin in a production clutch housing may need a bit more clutch fork travel. Sometimes, all that's necessary to gain that is to optimize clutch fork geometry. The fork fulcrum should be positioned such that there is .030-.060 inch clearance between the pressure plate and the fork. View this by looking through the clutch fork hole. If necessary, alter fork position by changing the length of the mounting stud. The ideal length lay between Chevrolet's 1.375-inch (P/N 3887159) and 1.480-inch (P/N 3729000) units, so we picked the 1.48 and shimmed it with a flat washer. Though fork geometry was better, the washer spaced the head of the stud above the transmission-mounting surface, nothing a brief session with a belt sander wouldn't cure. We added a stock Corvette clutch fork and installed the production clutch housing with new, production bolts. McLeod also has a line of gear reduction starters appropriate for hot street and racing engines. Before we moved on to the transmission, we installed one of these units (P/N 810352).
Six to Go
For years a Muncie M21 (standard-duty, close-ratio) four-speed did yeoman duty in Big Block from Hell. Recently, 86,000 street miles, a ton of torque and countless testing passes on the drag strip had the M21 showing its age. During a trans removal-for-inspection earlier this year, we noted the countershaft was loose in the case, a sign of a Muncie's impending death. Since we had to replace the gearbox, we'd be better off with more gears. That would both improve performance and fuel mileage. It was no-brainer, we swapped the DOA Muncie for a Richmond six-speed.
The old Doug Nash '4+1', the first aftermarket five-speed for modified V8s, arrived in the late-'70s. Its 3.27 low gear, used in conjunction with a tall axle ratio, gave street high-performance enthusiasts the acceleration of a four-speed/short rear gears combination, as well as, more top speed and better fuel mileage.
The company that made the Doug Nash went bankrupt in the late-'80s and Richmond Gear acquired the design and tooling. Soon the Richmond five-speed, with engineering updates and improved quality control, came to market. In the early-'90s, Richmond began work on a six-speed derived from that five-speed design. The 'Richmond Over-Drive' (ROD) employed a trick first used when Borg-Warner developed the T10 for the 1957 Corvette. Richmond moved the reverse gear into an enlarged tail housing then added an additional forward speed to the main transmission case.
From a materials and manufacturing standpoint, the ROD is a stout piece. Gears (count 'em, there are 14!) are cut from billets of 4620 steel then are carborized and heat-treated. Main and countershafts start life as 8620 steel billets then are machined, carborized and heat-treated. The transmission case parts are machined aluminum castings and the speedometer gears are OE GM.
Motorsports drove Richmond to develop a special synchronizer ring design they call 'Road Race Brass.' Traditional synchros have three, circumferential slots that engage their respective dog-clutch assemblies. The slots are weak areas in the ring's structure. After a period of time in severe duty, if you were to accurately measure the synchro; you'd find it warped into a slightly trioval shape. Once that happens, synchronizer action is impeded and shift quality degrades. The synchronizer rings in a Richmond six-speed do not have slots. There are no weak spots, the synchros stay round and shift effort remains consistent.
At Richmond's factory in Liberty, South Carolina, most of these parts are manufactured by fully-automated, computer-numeric-controlled (CNC) machining centers and assembled. An ROD weighs 108 lbs. - heavier than a four-speed, but mass is the price you pay for the extra gears and strength Richmond brings to the party.
For GM applications, Richmond offers a wide choice of gear sets. All have an overdrive, sixth gear. To help us better select a ratio-spread; we used computer modeling software from Racing Systems Analysis called 'Quarter Jr.' which allows the user to project 1/4-mile acceleration times. You enter data about the car in question, then the software predicts performance. Quarter Jr. is available in versions that run under either the Macintosh operating system or Microsoft Windows.
Based on a session with Quarter Jr., we selected the 2.77/1.88/1.46/1.18/1.00/0.62 configuration (P/N 7021626ED) for the Big Block from Hell's ROD. That gear set is rated at 475 lbs./ft input torque for 1st-5th gears. Sixth gear is rated for less input torque, but in our application, sixth will be a cruising gear that will never see full engine torque.
We did not disassemble the Richmond six-speed before installation, but if we had; the job would have been a snap. Tear down is accomplished with basic tools along with large retaining ring and large snap-ring pliers. The transmission is a split-case design. Once the left half is removed, all parts are easily accessible there are no case gaskets. Sealing is accomplished with a high-quality RTV silicon sealer such as that marketed by Valco Cincinnati.
Fabulous Fab Work
The Richmond fits the tunnel of a 1971 and will probably fit most other Mid-Years or Sharks with little trouble, however, its length precludes installation with the engine in the car unless the center crossmember is removable. '68-'82 automatic frames have this feature, so if you are converting one of those to a manual; you're home free. Anything else has to be modified and that had us visiting Hansen Race Cars of Ontario, California, known for top-notch chassis fabrication work.
The mid-section of the center cross member was removed and the six-speed bolted in place. Gary Hansen designed the Big Block from Hell's new crossmember installation and Fabricator, Donny Matter, built it, starting with 2 by 3-in., .095-in. wall thickness, mild steel stock. That was cut into sections and TIG-arc-welded to a pair of short lengths of 4.5-in, 3/16 inch-wall, tube stock which form the exhaust pass-throughs, characteristic of the C2/3 frames. Their larger diameter facilitates use of a 3-in. exhaust system. Donny Matter's crossmember included a relocated transmission mount pad because the distance between the front face of the six-speed and the centerlines of the transmission mount bolt holes is longer than that of the Muncie.
Next, four end plates were fab'ed from 4130 chromoly steel. Two were welded to the ends of the crossmember and two to the 'stubs' on the frame. They were drilled and nuts were welded to the removable section. Grade eight bolts hold it in place and the angled end plates cause a wedging effect that strengthens the installation. The parking brake cable pulley bracket was welded to the removable piece as close to the stock location as possible.
This part of the Big-Block from Hell six-speed conversion required the special equipment along with engineering, metal fabrication and welding skills you'd find at a chassis fabrication facility. If you have to add the removable crossmember, see Hansen Race Cars or a similar operation in your area. Be prepared to spend $750-$1250 for the work.
We reinstalled the stock transmission mount, connected the speedometer cable and put the driveshaft back in. While the Richmond six-speed's overall length is half-an-inch less than a Muncie's, adequate yoke length is still available for the transmission output shaft, so driveshaft modification is unnecessary. The ROD uses the yoke that was OE-for Turbo Hydra-matic 400s or '71-'79 four-speeds. Installations on earlier cars will require that yoke. If you're changing output yokes, upgrade to the 2.5-in. diameter driveshaft at the same time.
Getting the shifter in place and adjusted was a most difficult task. Currently, the only gearshift mechanism available is manufactured by Long Engineering. Richmond includes one with every six-speed. Fitting the shifter into the tight space at the rear of the BBfH transmission tunnel is difficult. Small hands, the right tools and patience are required. Our car needed to have the rear edge of the shifter hole in the tunnel trimmed a bit to clear the shifter.
The shift lever that comes with the Long is a poor ergonomic choice. The shift knob is too high and too far forward, so we ordered Long's lever for '84-'87 applications which is an inch lower. We heated it to add a rearward bend, painted it with Eastwood's Chassis Black paint (P/N 1244Z) and bolted it in place.
Which lubricant you select is critical with this transmission in a Corvette due to heat radiated from the exhaust. Unless you have sidepipes; there is no way to avoid the exhaust running close to the transmission. Richmond Gear recommends a synthetic oil and we used MT90 from Red Line Synthetic Oil Corporation.
Our initial, short-duration test of the six-speed, Big-Block from Hell was the drive back to our shop. We found second and sixth gears not working. Figuring a linkage problem, we put the car up on stands for an inspection. While Hansen's had installed the shifter per Long's directions, shift rods interfered with each other and the rods' heim-jointed ends interfered with shift arms on the transmission. The solution was rebending shift rods, installing spacers and readjusting the linkage. This work was time consuming because of the confined space in the transmission tunnel. Of particular difficulty was setting the shifter stop bolts.
Bringing up the Rear
The ROD drove changes to the BBfH's rear axle and suspension. We dropped the axle ratio from 3.36:1 to 3.08:1. Run the math and you'll see that final drive in first gear with that axle and the ROD is 8.53, somewhat greater than the 7.40 we had previously. Obviously, this combo will blow the tires away even worse than before, so during acceleration tests, we'll use BF Goodrich Comp T/A Drag Radials. With a little shorter final drive and more aggressive rear tires, we think BBfH will perform well.
We pulled the differential assembly and shipped it to Tom's Differentials in Paramount, California, a favorite amongst hot rodders and racers in the Western U.S. Of interest to us is its extensive experience with rear ends from all Corvette model years. Previously, Tom's had set our axle up with performance enhancements to the limited slip assembly and other modifications to improve durability. This time, it only needed an inspection and the gear change. We replaced the 3.36:1 ring and pinion with a set of new, 3.08s (P/N 3961418) from GM. Additionally, Tom installed new Timkin bearings from General Bearings Company, checked the limited slip clutches and painstakingly set up the gears.
Back at our shop, we fitted the diff. with a HD cover from Vette Brakes and Products (P/N 51245) because of its extra bracing and thicker metal around the rear spring mount. We rented a Porta-Power and popped off the differential crossmember. We installed two new GM isolators (P/N 9750078), reinstalled the crossmember, then bolted in the rear axle using an Energy Suspension urethane pinion mount kit (P/N 3-1101). We filled the axle with Red Line Heavy Shockproof lubricant and added an ounce of Red Line's new Friction Modifier which eliminates the limited slip clutch chatter that can sometimes be problematic of diffs modified for increased performance.
A bad feature of the production '63-'82 rear suspension is a lot of camber change as the wheels move up or down and this becomes a serious problem on a car with wide tires. On acceleration, the rear end squats, the wheels gain negative camber and tire contact patch is reduced. This makes for less traction-exactly what we don't need with 530 lbs./ft of torque.
Vette Brakes and Products' 'Smart Struts' kit uses a strut rod mounting bracket having revised positions for the rods' inner pick-up points that are lower than those of either of the three, stock brackets. The new pick-up points are adjustable through a limited range and alter suspension geometry such that camber change is either reduced or eliminated. The stock, '63-'67 rear suspension has 3.5° of camber change in 2.5 inches of compression. The '68-'82 rear is a little better with 2.5°. Smart Struts can be adjusted for between 0 and 1.25° camber change in 2.5-in. of travel, a significant improvement. Our Smart Strut kit (P/N 52002) includes struts with spherical bearing rod ends.
Another traditional way to reduce camber change is a stiffer rear spring. The higher the spring rate, the less the rear end will squat and the less the camber will change. The camber change reduction that comes with Smart Struts lessens the need for a stiff spring. A softer rear spring will make for a nicer ride so we swapped our 360 lbs./in. spring for a Vette Brakes and Products composite spring (P/N 42333) of 330 lbs./in. rate. A composite spring also offers a 22 pound decrease in weight and better durability over steel counterparts.
A drawback of composite springs is they are not as heat tolerant as steel units. A minimum of 1.5 inches is necessary between exhaust parts and the spring. We were right on the edge of that, so we attached Thermo Tec's Aluminized Heat Barrier to the bottom of the spring above the pipes. This product is effective in addressing exhaust heat problems.
Previously, this project car was fitted with Bilstein, high-pressure gas shocks with 'Sport' valving. We swapped those for a set of Bilsteins (P/N's B36-0946 fr. and B46-0956 rr) with less aggressive, 'HD' valving. Currently BBfH sees more street driving than anything, so the HD Bilsteins will be a better choice. If we go racing, we can just switch back to the Sport-valved units. We like Bilsteins for their high-pressure-gas-filled design. It provides a great combination of high-performance damping, good ride quality and outstanding durability.
The final driveline link is tires. With the torque of a modified big-block and the increase in first-gear, final-drive ratio we got from the six-speed; we needed a serious tire for performance testing. For BBfH 15, the car had been converted to '94-'95, ZR-1 wheels using Vette Brakes and Products adapters. The best choice for 17-in. street radials on the drag strip is the BF Goodrich Comp T/A Drag Radial. They are available in a number of 16-inch and 17-inch Corvette sizes. If you are a regular drag racer using wheels that size, investment in a set of Drag Radials will significantly improve your car's performance.
While the Drag Radial is street legal, it is not practical for long duration street use because of quick tread wear and limited water channels. For regular street driving, we want a tire that has good traction in the wet and the dry but also rides nice and is quiet. We are going to try the brand new Goodyear GS-F1, introduced on the C5. Last summer, Goodyear released a replacement version of the tire in many 16- and 17-in. sizes. Goodyear spokesman, Scott Baughman, told us that the replacement GS-F1 incorporates all of the improvements in performance and ride engineered for the C5 except the EMT 'run flat' feature. Unfortunately, the tires arrived after our publication deadline, so we were unable to do any objective testing, but our opinion is that, on this car, the F1 is a small measure better than the old GS-C.
Back before it was a Vette project car, Big-Block from Hell got a Flowmaster exhaust system, however, Flowmaster's products have improved much in 15 years. Also, the old system had inadequate clearance between it and the new transmission and with the now larger exhaust pass-throughs, we eliminated the short sections of 2.75” pipe we had to use through the stock frame. All this meant a new exhaust so we headed for Mr. G's Tire and Muffler Center in Stanton, Flowmaster's R&D operation in southern California. Jerry Holmes, Mr. G's master of the tube bending machine, built-up a set of custom, three-inch pipes out of Flowmaster aluminized stock. To that he welded a set of Flowmaster's 30-Series mufflers (P/N 53030), a crossover pipe and muffler hangers.
Flowmaster mufflers are manufactured from aluminized steel using wire-fed arc welders. Aluminized steel does not rust, but the weld beads do. This presents a ascetic annoyance to some and the solution is to clean the mufflers with a good degreaser, such as Simple Green, then paint them with Eastwood Exhaust Paint (P/N 1211Z). We want this Big-Block car to be Hell in name only, so to reduce exhaust heat radiation into the interior; we used Thermo Tec Exhaust Wrap from the headers, back.
And Finally - Interior
The gearbox work had us stripping the trans tunnel of all its trim. What we took off was 26 years old and pretty scuzzy looking. A phone call to Eckler's had a plethora of new pieces headed our way.
The first item we installed was a lower shifter boot (P/N A2144). Because the rear side of the shifter opening in the tunnel had been opened-up, the metal retainer on the Eckler's boot also had to be modified. Peel back the boot then trim the retainer with a die grinder. We used Valco Cincinnati RTV silicone to restick the boot to the retainer and to seal the retainer to the tunnel.
We added an Eckler's reproduction shifter console trim (P/N 28241) assembly. We looked at trim from a few different manufacturers and found the Eckler's piece a standout for its predrilled holes, instructions and materials that were functional replacements of OE. Expect to spend time trimming the opening in the top of any reproduction console to fit the console plate and HVAC control bracketry. We suggest a test-fit of the console shift plate to the console trim before the final assembly in the car.
A difficult task is reinstallation of the front fiber optic indicators. In a job that took a couple of hours, we installed an Eckler's front fiber optic indicator trim plate (A9113) and reinstalled the fiber optic indicator assembly that is beneath the console.
Once the console soft trim was assembled and installed on the tunnel, we added an Eckler's reproduction console plate (P/N 31640). The reinforcing rib that surrounds its shifter hole must be trimmed on the driver side where it runs next to the shift lever because it interferes with the wider, Long shifter. We cut slots in the shift plate rib with an Eastwood cutting wheel (P/N 3440) fitted to a die grinder, broke away the offending pieces, then smoothed the remaining edges with Standard Abrasives sanding discs. Once the console plate was modified, it was fitted with an Eckler's upper shift boot (P/N A6420B) and installed. We added an Eckler's engine data plate (P/N 31640) along with a number of other small items.
We installed one non-repro piece, a handy item we found in the Eckler's catalog called a 'Console Compartment' (P/N A8787). This is a parking brake cover that incorporates a storage compartment between the seats. Its padded top as an arm rest. As a final, really cool touch, Richmond Gear provided a six-speed, shift pattern plate to replace the OE four-speed pattern.
Proper reassembly of the above items requires patience and persistence. Expect to spend as much as 8-10 hours on a console 'job'.
So - does it all work?
Before any aggressive testing, we put the Goodyear GS-F1s on and took a highway trip to break-in all the new driveline pieces. The 'VetteNet Southwest Tour' is organized each spring by Nina and Charlie Spitzer, two members of VetteNet, the Internet e-mail club for Corvette owners. The 1997 VNSW Tour took us over the best roads in central and northern Arizona. It started in Phoenix, went northwest to Wickenberg, then north into the mountains around Prescott. After lunch in Prescott, we headed for the picturesque, mountainside mining town of Jerome, and from there, to the final stop on the tour, Sedona and its breathtaking scenery. For us, VNSW included a couple of 450 mile stretches between our winter/spring base in Southern California and Arizona.
The trip was mostly highway cruising at 55-85 mph. The roads in the Jerome area are twisty, so we got in a lot of shifting. The Richmond's shift quality started out notchy and high effort but improved during the trip. This shifter is a close-gated, racing piece, so we missed a lot of gears. Clearly, we need to become more familiar with its use. We also felt we needed another shop session on adjustment.
A fuel mileage check proved the Richmond's 0.61:1 sixth gear was helping. Previously, highway rpm was 3000-3500. With the ROD, it was 1800-2300, which is better for economy. At speeds of 65-80 mph, we saw 16.04 miles to the gallon, quite an improvement over the, previous, mid-13-low-14s. On a later trip up to central California after some fine tuning of the carb's highway cruise calibration, we saw 17 mpg., very good for a carburetor-equipped, hot-rod, big-block.
Upon return from VNSW, we checked the valve lash, installed NGK R5674-7 V-Power racing spark plugs and we bolted the BF Goodrich Drag Radials on the back. On our very first hard pass, we ran into more trouble with the shifter. We found that the 1-2 and 3-4 shift rods were still interfering in a way that prevented rapid shifts at WOT. We ended up with about 6-8 hours of shop time, on two occasions, in reworking this shifter such that it would function properly in a racing situation. Unfortunately, the Long Engineering unit seems not as well developed as it could be for Corvette applications as it requires a significant amount of work to modify and adjust it such that it works correctly.
Back at the test venue, the car felt weak. We suspected the removal of the 2.75-in. exhaust restriction and the addition of newer Flowmasters had reduced exhaust restriction enough that the we needed to recalibrate the carburetor. We installed our Valentine Research 'g-Analyst' data recorder. It reads acceleration directly and provides one of the easier ways to properly jet a carburetor for performance. We also added 101-octane, High-Performance Unleaded gasoline from Trick Enterprises. Mixing it 25-30% with pump gas gives us a hedge against the detonation that could occur if, during the tuning process, the air/fuel ratio got too lean.
BBfH uses a Holley model 4781, 850 cfm four-barrel with mechanical secondaries, double accelerator pumps and secondary idle circuits. It was modified by Fuel Curve Engineering of Tryon, North Carolina with the 'Saturday Night Special' treatment. For jet testing, we use WOT pulls in second gear from 1500 rpm to 6000 rpm. We start with jetting with what we think are jets that are too rich then we jet progressively leaner until acceleration peaks then falls off. At that point, we go back one jet number rich. Using jets supplied by Holley we followed this procedure until we arrived at best acceleration according to g-Analyst data. After we got the WOT jetting correct, we fine tuned the idle channel restrictions, accelerator pump cams and shooters to get best drivability.
Finally, it was time to add some more Trick Performance gasoline, install our trusty Vericom VC200 Performance Computer and run some quarter mile tests. The bottom line? The Big Block from Hell ran an 11.91/126.1…more than half a second quicker than it's best previous pass of 12.43/126.3. Looking at the mile-an-hour, it's clear all the improvement is just off the starting line. In fact, the engine is a bit tired compared to the last full quarter mile tests we did three years ago, but looking at the e.t.; it's obvious that better gearing from the Richmond six-speed, improved control of the rear suspension from the Smart Struts Kit and the Drag Radials make up for that and then some.
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The Idaho Corvette Page would like thank Hib for this great article.Return to Articles