Given equal power and weight, what's harder on transmissions, drag or circuit?

So I've been slowly doing homework in preparation for the inevitability of my ZF320 grenading in my turbo E30, with about 780whp/670wtq. I've been looking at the T56 Magnum, and while outside of it's official factory rated torque, if it can really take 700 by the factory rating then I'm sure it could take mine too. But a trip down the rabbit hole of the internet put me onto stock car Tex and Jerico etc 4 speeds. A further spelunking put me in touch with a local pro Trans Am racer and race car builder who rebuilds these.

He told me that even a T101 would blow up at my output within one outing. Then this part really didn't make sense to me relative to what I've always thought. He said not to look at what the gearboxes are taking on the dragstrip because at the same power level, road racing is FAR harder on transmissions. His car makes 730/500 to the wheels and they use a current Cup transmission which are like 14k new... that's not happening.

At the same time, another racing team, Twins Turbo, has been insanely helpful in guiding me in my project and they race a 750whp Supra in NASA Super Unlimited. They use the Supra Getrag and as far as I can tell, haven't had issues with it for the 6 years the car has been beaten on - and beaten on in California and Nevada climate. The Getrags are only rated for like 370 ft lbs, which is why I figured something like the Magnum, factory rated at 700, is more than enough for me.

Where does the truth lie? I don't think that the Trans Am guy was trying to deceive me or anything like that. I do wonder if maybe it's because his experience is rooted in big power naturally aspirated V8s which don't come on softly like a turbo but hit like a dump truck from idle. Plus he races professionally so there is no taking it easy on the shifters.

Also I always figured that with the traction levels on the drag strip and the general disdain for transmission health in favor of lower shift times, would mean that that's where the true strength of a transmission is determined.

to answer the question you listed in OP title, I would say depends.

drag racing is a momentary application of force. The big determining factor is going to be instantaneous bending of the shafts on launch. If the bearings survive the initial torsional spike they should be fine.

in road racing I would say the big issue is the bearings. Bearings have a certain life with a certain torque and rpm for a certain amount of time. the transmissions don't see the instantaneous torque that a drag transmissions sees but the see more total torque over time.

Remember a dragster is only going 1/4 mile at a time. One road race covers allot of years of drag racing seat time.

Heat. Heat heat heat. Focus on removing that heat.

A lot depends on your weight and the traction afforded by your tires. An E30 is relatively light, so that's a plus. If you're not continously launching it on slicks, the T56 might last a while.

It's far cheaper for traction to be the fuse in the system than for the transmission to be the weak link. Some manufacturers (Nissan 350z comes to mind) use the clutch as the weak link because they're typically a wear item and not covered under warranty.

I'd love to see a build thread on your E30!

I'll say circuit just because of the number of gear changes. A lap of summit point in an M3 will see 7-8 gear changes per lap. A sprint race is 35 minutes and lets say 29 laps so... 232 gear changes in just one race. The heat put into the box is over a longer time. The force is sustained.

Compare that to a more violent 4 shifts per race... "Harder" might be the wrong word. It will see more wear assuming it's up to both tasks.

I can definitely appreciate that during a road race, the transmission will see many more shifts over time and much more heat. The failure modes I've observed were actual broken teeth and cracked housings, which to me suggests excessive force. And unless a road racer is being particularly belligerent to the transmission, a road racing transmission shouldn't see a lot of impact/jerk, which is far more violent than static force. I've never been one to rush my shifts.

Maybe that's the secret, why the more stock-like gearboxes last longer than the super duper 4 speeds - unless you are a flawless shifter, banging gears in a dogbox introduces intertial impact forces on the gears which hammers them apart. I've seen diffs grenade at a fraction of the torque they can handle statically with no problem, applied as hammering impacts (wheel hop). A friend of mine operates a trucking outfit and he said he's seen it too - the rig can pull monstrous masses but an unloaded truck can snap an axle spinning wheels through a snowy/icy parking lot and discovering traction.

dj06482: Here is my build thread, just starting on bodywork and aero this winter:

http://forums.bimmerforums.com/forum/showthread.php?1509062-E30-S52-turbo-track-toy-build

Broken teeth can be caused by heat, if the gear mesh goes all wonky as things expand/loosen up.

One way to make the 323GTX transmission last a little longer is to plumb in a transmission cooler. And one of the RX-7 circuit racers stopped eating transmissions when he installed an exhaust heat shield after he found trans oil temps to be >250f.

100% definite it circuit racing. As there’s rev load on the whole system going in to the corners that you just don't get drag racing.

Road on account of the heat buildup. Drag car is only concerned with massive shock loading through 1st when it launches and a couple hard shifts, per day. Less shock on the road car, but everything between the shift knob and synchro teeth is getting hammered on constantly, and the heat buildup leading to lube breakdown and the inevitable failure that goes with that. So run a cooler with whatever you end up using.

Drag is where you destroy a trans instantly. Road is where you wear it out. In drag racing you have more grip than you normally would. When you accelerate at peak power is the most load that trans will deal with. 1st is always the weak point (not necessarily the most common failure, but it's where it's easiest to over do it). If you overdo it on a race track you tend to spin tires, spinning tires is easier on the trans than your maximum acceleration. However, because of the prep work at a drag strip you have "extra" grip. So now when you over do it, the tires don't spin and you put more than the normal max load the trans ever sees, or was ever designed for (in a manner of speaking).

The other aspect is, that at a drag strip, power shifts, no clutch slapping, and half engaged shifts are VERY common, these are devastating to a trans. Aside from the launch, shifts are the only major factor in your times so you tend to push the limits. In road racing, you generally shift at your leisure before and after a corner. The .008 seconds you save on a brutal shift is nothing compared to the 1-2 seconds you can shave in ever single corner.

And yes, instant power is way harder than boost power because of torque application.