It’s likely one of the most durable components on any car, from a stock Mustang to a 4,000-plus horsepower Pro Mod, yet it’s rarely given a second thought until it fails … often at the most inopportune time. However, the flexplate – that hefty round disc bolted between the crankshaft and torque converter – is far too important to be looked over when building a race car of any performance level. Flexplates not only transfer the engine’s torque rearward from the crankshaft to the converter, they also serve as the contact point for the starter to crank the engine during startup. Performing these tasks once is quite a feat. Surviving pass after pass requires incredible strength and durability, as well as high levels of precision.

Three companies leading the charge to end flexplate failures are Meziere Enterprises, ATI Performance Products and TCI Automotive. While each may have subtle differences in their approach to building what it feels is the best flexplate, all strive toward the goal of increasing racer and spectator safety, while reducing those dreaded missed rounds due to teeth being sheared off the ring gear, or having a flexplate spin itself apart at high engine speeds.

Join us as we explore the basic flexplate functionality, delve into some of the recent advancements, and discuss the three main types of flex plates available on the market. We will also help you, the racer, learn what flexplate may best fits your needs.

Flexplate Basics

At its core, the flexplate is a simple metal disk that bolts between the crankshaft and torque converter with a ring of teeth that is engaged by the starter to physically spin the engine during cranking. While serving those two purposes requires tremendous strength and precision, there’s more going on with the flexplate itself than is immediately obvious. For starters – pardon the pun – the gears on the outer edge of the flexplate have to not only be strong enough to withstand modern 16-volt starters, and the added stress of spinning over ultra-high compression engines used in many modern applications, but they have to be the exact pitch and profile to properly mesh with the starter pinion to ensure proper, reliable cranking. While every car may not feature 16-volts or high compression, the flexplate must be designed for the most extreme situations.

The other task the flexplate must perform is joining the crankshaft to the torque converter. If you really stop and think about a high-horsepower Pro Modified car planting the tires and launching to a sub-one second 60-foot times, realizing there is a small, relatively thin plate of metal transferring 4,000 horsepower rearward at one of the key junctions in the powertrain, you begin to see the kind of stresses today’s flexplates must endure. Now, tally up the number of runs a racecar might make over the course of a season or more, and it becomes clear that the flexplate really is a workhorse component that deserves far more credit than it receives.

• 
• 
• 

Flexplate-to-starter mesh is one area manufacturers focus much of their R&D. Here, you have two views of the starter pinion in relation to the flexplate's ring gear, as well as a starter adapter. This one, made by ATI, helps ensure proper alignment.

Technological Advances

Meziere Enterprises’ owner Don Meziere discussed with us how flexplates have advanced in recent years, especially with the widespread adoption of automatic transmissions in higher-horsepower applications that used to rely on a clutch and manually-shifted transmission. “Of primary concern is the plate’s ability to transmit horsepower. Stock-style stamped and welded plates have failed racers thousands of times. They simply will not stand up to the demands of high compression engines and an RPM range that is triple the intended design application,” he says.

Meziere goes on to say that in 2003, noting an alarmingly high number of flexplate failures from several manufacturers, his team dove into the possible causes and found that a great many plates available at the time simply lacked the strength and precision that racers demanded from the other components in their drivetrain. “That discovery sent us on a trajectory toward offering the best flexplates for these high horsepower applications.”

“Our plates are made from 4340 steel and are typically machined to .170-inch thick. The increased cross section, coupled with superior alloy, has proven to be a good solution for engines that make up to 1,800 horsepower,” says Meziere. This range would encapsulate the bulk of bracket racers, Super-class cars, and even some heads-up classes.

While much of the advancement is geared toward strength and safety, ATI’s J.C. Beattie, Jr. is quick to remind racers that the “flex” part of the flexplate must be addressed as well. If a plate is too rigid, it can lead to damage elsewhere that may not point to the flexplate as the cause, such as bearing damage in the engine or a host of issues inside the transmission.

Different Types Of Flexplates

All three of the manufacturers we spoke with sell three distinct styles of flexplate: stock-style stamped steel, two-piece billet (either welded or bolt-together), and one-piece billet.

Stamped steel flexplates are exactly what you’d expect; stock-style plates stamped out of steel, however, aftermarket stamped plates are certainly better than actual OEM equipment. While it’s the most basic of the three, you will find steel plates on a large number of cars, from showroom stock sports cars up through some of the lower-powered sportsman classes, including most street-and-strip capable weekend warriors.

Our trio of manufacturers all agree that these plates are acceptable for use in applications making up to 400 to 500 horsepower. Some of these plates even meet SFI 29.1 certification.

Two-piece billet plates are utilized in most track-only race cars, though as Beattie, Jr. points out, it’s becoming more common to find them in street-driven applications, given the ease of producing big horsepower from relatively inexpensive turbocharged and supercharged combinations, along with high-compression engines.

These plates benefit from thicker, more durable center plates with more robust ring gears attached, either by welding or by bolting them together. Meziere warns that the extreme temperature swings from the welding process can warp the plate, leading to starter misalignment and eventually broken teeth. This never seems to occur during testing or time trials, but instead, rears its head as you’re preparing for the final round.

One-piece billet plates are the strongest and most precisely-machined flexplates available. TCI Automotive’s Jonathan Lunati summed up these high-quality pieces. “The ultimate flexplate for drag racers is a one-piece billet flexplate. On these flexplates, the ring gear is integral to the center section (that’s engineer speak for “they’re one solid piece). These flexplates are often SFI 29.2 certified and are made of premium materials for the utmost in strength.”

One-piece billet flywheels bear SFI’s most stringent 29.2 safety rating and are approved for applications over 1,700 horsepower. Beattie, Jr. also alludes to safety mandates that require double-lined “cans” around the flexplate area in high-horsepower applications, resulting in a need to reduce the diameter of the plate from the traditional 14.2 inches to 13.8 inches, which also requires the adoption of a 10-pitch gear profile and relocation of the starter pinion to ensure proper gear mesh.

Lunati points out that for many drag racers, having a lower moment of inertia can be critical for a controlled hit off the line. Using a smaller diameter torque converter with a light-weight flexplate can provide that consistent, controlled hit desired. This attribute will be especially attractive to racers in classes where consistency is paramount, such as bracket racing, sportsman classes, and Top Dragster and Top Sportsman.

Which One Is Right For You?

There is a lot of information presented here, but choosing the right flexplate for your particular application really comes down to four questions you need to answer: How much compression does the engine have, how much horsepower do you need to transmit, what is the maximum engine RPM, and will the sanctioning body require you to run a plate that is safety-certified? The easiest criteria to determine will be your horsepower level, but that alone won’t always be enough to determine the correct flexplate.

For example, using Meziere’s reference of intended RPM range, if you have a 400 horsepower engine, the seemingly obvious choice would be a stamped steel plate, but if you plan to spin that powerplant up to 11,000 rpm, you’re going to want to look into a plate designed to withstand the rigors of high engine speeds.

While flexplate technology has advanced greatly in recent years, choosing the style of plate that best fits your combination is fairly straightforward. We strongly recommend you take a serious look at the offerings from Meziere, ATI and TCI when it’s time for you to replace your current plate or upgrade if you’re upping the horsepower of your car. There may be some performance gain in selecting the correct flexplate, but more importantly is the increased reliability and durability found in quality aftermarket plates.

Increased driver and spectator safety is also a major benefit offered by these companies, each of which strive to eliminate flexplate failure and give the racer peace of mind that when they reach for that “Start” button, they’re rewarded with a racecar roaring to life and not the dreaded screech of the starter pinion grinding against a bare spot on the ring gear where teeth used to reside.