Editor's Note
If your existing hot rod has a gremlin that just won't quit, you could be one of the lucky candidates chosen for a hands-on thrash. Drop us a line at pitstop@hotrod.com, and put "Rescue" in the Subject Line. We won't repair factory stockers or build you a whole car, but HOT ROD and industry experts will fix the immediate problem.
The Combo
More than 10 years ago, Milwaukee, Wisconsin's Gregg Gunta restored his '73 Z28 Camaro LT. The refresh included a brand-new GM 350 HO with Vortec heads (old GM PN 12486041) that he purchased in April 2002. The original M21 Muncie four-speed and 3.73:1, Positraction, 81?2-inch 10-bolt rearend were retained.
The Problem
As is the case with many of us, it took Gunta a while to complete the project. The car was not up and running until one-and-a half years after the engine was installed. From day one on initial fire-up, there was a serious vibration between 3,500 and 4,800 rpm in all gears under cruise and at wide-open throttle, as well as in Neutral when Gunta revved up the engine. It was impossible to make a full power run. He couldn't drive the car over 70 mph.
With the engine warranty expired, Gunta trekked to one independent auto repair shop after another. One place said,"It's just the engine breaking in." Another said it "has to be the driveshaft" and put a worm-drive hose clamp around the unit. "It didn't fix anything," Gunta says. "So then they sold me a whole new driveshaft." Still nada. Yet another shop insisted, "It's got to be the clutch or the flywheel," and replaced it with different parts. No joy.
By late 2011--four shops, eight years, and 10,000 miles later--the problem remained unresolved. In the interim, Gunta had learned to live with it, but now the clutch was acting up again. "It was starting to chatter like crazy," Gunta says. Thinking it was still a clutch or flywheel problem, we had Gunta limp the car into southern Wisconsin's shop of last resort, Norm Brandes' Westech Automotive. Brandes has been around for decades and performs full-service performance repairs and upgrades on virtually any early- or late-model car--old-school carbureted as well as the latest electronically managed vehicles.
The Diagnosis
Brandes observed vibration in all gears and in Neutral, even with the car off the ground with no weight on the wheels and tires. Therefore, the problem was not likely to be the wheels and tires, the rearend, the driveshaft, or the trans. That left as possible perps the harmonic damper, the clutch and pressure plate assembly, the flywheel, or (ultimately) the engine.
Because the clutch was slipping anyway, Brandes and assistant Jim Daley first replaced the tired parts with a new Centerforce clutch and pressure plate. The engine used an '86-and-later block with a one-piece rear main seal, which requires an externally balanced, reduced-bolt-circle flywheel. All factory one-piece-seal flywheels and flexplates have the same amount of unbalance, but some aftermarket one-piece-seal cranks are internally balanced, requiring a corresponding reduced-bolt-circle, neutral-balanced aftermarket unit. A previous shop had already replaced the flywheel once, so there was a slim chance that it had the wrong balance. Covering all bases, Centerforce sent Brandes a new unit with the correct amount of unbalance.
Both units turned out to have the same amount of unbalance, ruling out a mismatched flywheel. However, the new 40-pound Centerforce flywheel was 8 pounds heavier than the 32-pound flywheel installed on the car when it rolled into the shop. The new, heavier flywheel reduced but did not fully eliminate the vibration problem. For a final check, Brandes went the other way, installing a 4-pound GM auto-trans flexplate with the correct unbalance. Brandes reported that the vibration got noticeably worse: "It vibrated enough at 2,000 rpm that the ring gear hit the starter drive gears. The vibration even shook the hood."
Brandes next inspected the harmonic balancer. Although a one-piece-seal block has an unbalanced flywheel, it still requires a neutral-balanced damper. The one on the car was observed to be correct with no counterweight. Brandes saw "no visible delamination, and the timing remained constant"--so it wasn't a failing balancer, either.
Could it be engine misfire or a dead cylinder? The spark was normal, and a compression test was within parameters at 150 to 155 psi across the board. This left some sort of internal engine problem as the only remaining culprit. Brandes now knew "there was nothing that could be resolved on the outside. We had to look internally to find the problem--so we pulled the engine for disassembly."
While tearing down the engine, the Westech team visually inspected all parts, looking for obvious defects, damages, or assembly errors. No gross breakage was noted. However, the main bearing lower shells showed excessive wear. Nos. 4 and 7 pistons were not swinging freely on their pressed piston pin due to galled pin bores.
Based on consultations with industry experts and his years of experience, Brandes saw the damage as symptomatic of a more serious problem: a major internal engine balance snafu. So next the Westech team weighed the pistons and rods individually and as an assembly. There was no major deviation--all were within 5 grams.
Only the crank remained. Placing it, the flywheel, and the balancer in the spin-balancer along with the correct bob weights at last revealed the problem: The crank was excessively lightened, to the point that it was 93 grams underbalanced! Usually even mass-produced factory crank balance jobs are within 2 to 3 grams, and you can usually slide by on the street even if the crank is off by 5 to 6 grams. Neither Brandes nor other industry experts had ever seen such a gross balancing error, obviously a highly unusual aberration on the typically reliable crate engine.
The Fix
In consultation with Gunta, Brandes decided to replace the stock pistons with lighter pistons and rings that made it easier for Brandes to bring the crank into balance, which he did by welding metal back into the balancing holes. Gunta also gained a little power because the new pistons' smaller dish volume, combined with decking the block, the thin Fel-Pro steel-shim head gaskets, and the Vortec heads' 64cc chambers, raised the engine's static compression ratio from 8.8:1 to 10.28:1.
Although it was not strictly necessary to fix the immediate problem, Gunta sprang for a Comp Cams Thumpr hydraulic roller cam in place of the wimpy, flat-tappet hydraulic cam. He had Brandes modify the Vortec heads to support the higher-lift cam and upgraded valvesprings. The ignition and carb were also upgraded, and Westech dyno-tuned the refreshed combo.
Results
With the engine properly balanced at last, the Camaro's ride is now glass-smooth in all gears. As Jim Daley put it, "I'm falling asleep running it on the dyno. It's so smooth. There's no vibration whatsoever." It now puts out 300 hp and 342 lb-ft to the wheels, compared with 235 hp and 245 lb-ft recorded by the same model crate motor installed in Gunta's other car, a '75 Corvette. You can bet he's a happy camper; for the first time in years, Gunta says "I'm getting the real feel out of the M21 trans. I wasn't realizing its full potential. I really like the car's newfound torque and responsiveness. I'm no longer embarrassed to let my friends ride in the car, and my wife loves the new Dual Friction clutch's easy pedal effort and smooth modulation."
"Before, it shook like a Cracker Jack box. Now, the driveability and power are wonderful; everything's smooth." --Gregg Gunta
Lessons Learned
Projects often fall behind schedule. In the case of a major component such as a complete engine, it pays to at least get it up and running on a test stand sooner rather than later to make sure there are no defects before the warranty expires. If there's an in-vehicle problem, applying logical deductive thinking can usually eliminate some potential causes without requiring you to turn a wrench. Finally, don't let things slide; you'll only end up with a series of cascade failures in the long run (in Gunta's case, shortened clutch life, damaged pistons, and scoured bearings).
