A piece of machinery is only as strong as its weakest link, and in a racecar, a machine comprised of hundreds of moving parts all destined for consistent abuse, it’s not hard to find problem areas.
For many drag racing and automotive enthusiasts that are pushing the limits of their vehicles into the higher echelons — north of 1,000 horsepower and sometimes well beyond — their transmission’s output shaft can quickly manifest itself as one of these weak links. To combat this, ATI Performance Products has developed a trio of one- and two-piece output shafts for the Turbo 400, Powerglide, and GM 4L80E and 4L85E transmissions, providing drag racers and those with everything from high-horse street cars to four-wheel-drive trucks with nearly bulletproof insurance behind their drivetrains.
For its strongest offerings, ATI has utilized 300M, a super-durable alloy steel often used in axles, in a heavy duty, two-piece output shaft. Gun-drilled to reduce weight, these Turbo 400 shafts are considered a must-have for heavy, high-horsepower cars or those with numerically-low rear gears.
“The OEM output shafts are of course old, and they really serve their purpose well until you get up near 4.50s in the 1/8-mile in a mid-weight racecar. We’ve since made this OEM replacement output shaft for the big-horsepower guys — Pro Modified, Pro Nitrous — as well as a 1,500 horsepower, heavy street car that might be a street car, but still has higher demands,” ATI’s J.C. Beattie, Jr. shares. “Let’s say a twin-turbo 427 that only turns 6,700 RPM … they put as much load on that output shaft as a 2,300-pound Pro Modified car, so they actually need an upgraded output shaft, as well.”
Also a two-piece shaft made of 300M, ATI has targeted the GM 4L transmission market with a shaft with OEM and Turbo 400 length variants for the aforementioned 4L80E and 4L85E.
The flange (containing the teeth that marry to the output carrier) that the 300M shafts are pressed into are manufactured from a heat-treated 4140, creating a very strong component that can handle the utmost in abuse. Beattie, Jr. confirms the durability of this part, noting “we’ve never failed one of these, ever.”
The two-piece shaft for the 4L is targeted at the growing number of 1,500-horsepower street cars out there. Beattie, Jr. shares that ATI shortened an OEM 4L80/85 shaft and paired it with a Turbo 400 tail housing, thus making it the same overall length as a Turbo 400. As such, those with 4L transmissions “have more space to play with,” as he puts it. Redline Performancem out of Pompano Beach, Florida is one of the shops that has used these shafts in their 7-second, street-driven Camaros that are heavy and have a numerically low gear ratio.
Beattie, Jr. explains that 300M is the next notch up in material quality from 4340 — noting it isn’t significantly stronger, but an appreciable improvement nevertheless — making it the staunchest of ATI’s offerings. These are the bad-boys used in Pro Mod, Radial versus The World, and the like.
From a manufacturing perspective, Beattie, Jr. says the issue with a one-piece shaft from 300M is that they must start with a piece of material that is over 4-inches in diameter and then machine 75-percent of it away.This is wasteful, but add to that that the cycle time is longer and they’re more challenging to make them perfectly straight. As such, from both a manufacturing and strength standpoint, two pieces makes for a better part.
A more recent addition to the catalog, ATI has also begun producing a 4340 forged, one-piece machined and ground shaft for the Turbo 400, which sport extra-long splines so they can be can cut down to Powerglide length or even the shortest 4×4 length (offroad “rock crawlers” and buggies often benefit from strong, aftermarket shafts such as these). Like the OEM shafts they replace, these accept the standard speedometer gears and come equipped with a bushing, but are void of provisions for the OEM governor.
A Turbo 400 is a longer transmission than a Powerglide (not counting “shorty” ‘glides), but the output shaft is actually shorter — a ‘glide shaft is nearly twice as long. That said, a Turbo 400 can be made to accept the same driveshaft as a Powerglide, you simply have to cut a 1/2-inch off of the output shaft and put a different extension housing on. To simplify this for customers, ATI has machined cut lines into the splines of the shaft.
On the 4×4 front, Turbo 400s have three different possible lengths — one of which is very short — and so the output shaft cut lines make it possible to stock one part and and a builder place them in a lathe to cut them to length for four-wheel-drive applications.
The 4340 shafts, while not as strong as 300M, are a direct OEM replacement, Beattie, Jr. adding “are certainly better than stock, you can get them, they’re new, and they’re nice.”
“It’s horsepower, gear, and weight. In a lightweight car with a lot of gear, you can go up to a couple thousand horsepower with an OEM-style output shaft. But if that same 2,000 horsepower has a 3.23 gear and it’s 3,500-pounds, then you’re in trouble. It’s just like buying a torque converter … you need to know those three variables to make an educated decision.”