mark kibort

I think the main components that need to be beafed up probably are in the first stages of the gear box. at the CV joints, there is already up to 4000ft-lbs of torque right, so only in first gear will you see those forces. at the drive shaft and 5th gear, you only see engine toruqe (say 400ft-lbs). anderson was breaking 3rd and i think he may have broken 5th as i did, but my failure was due to having a nasty vibration on the dyno due to having the e-brake partially engaged. ( we lost 100hp on that run before it broke 5th gear input)

as soon as you get to 2nd or 3rd gear, the parts out to the CV joints are probably plenty strong. i think the only way to break a CV joint is drag racing on drag slicks and getting a good "bite" .

mk

Ketchmi

The 10psi 89' auto I play with has 1.5 years under this level of boost, is around 500rwhp and has had no transmission modifications other than Redline synthetic D4 fluid and an additional cooler. It has yet to slip or act up in any way, it also had 70k miles on it when we started.

We did have a rebuilt axle give up and went back to a new one with no problems since. The rebuilt ones are not hardened when redone and they do not last even 1 year on that kind of horsepower.

pmotts

There are many of us running 5 speeds that will tell you this is not an issue if you have control of your right foot

mark kibort

exactly!

does this mean if you are doing 30-80mph, its like doing it 18,432 times?

i think the main thing we dont think about, and sometimes speed tv references this in their TV reporting of races, is that in our races, we shift 15 times per lap, so x 20 laps of a race at Infinion (a 2.3mile track) 200-300 times a session. thats 600 to 1000 shifts a weekend. i got about 75 race days on the Holbert car now, so thats near 75,000 shifts, all at WOT, all with 5000rpm engine blips for downshifts. no mecy shifting!!! in the wear of the holbert car, i wouldnt even consider my spririted street driving a factor. maybe a rounding error at best! Ive put 25,000miles on the holbert car now to date.

I think if you are using the 928 for street, there are no worries for using 500 to 600ft-lbs of engine torque, especially if you are not using 1st gear and drag slicks.

mk

Herr-Kuhn

Guys, you are talking in terms of torque here...it is torque that limits the capacity of the parts from a stress standpoint. F=ma and T=Fd. The HP is simply the product of torque and speed. You could have 500 HP at 6000 RPM and a different engine with 500 HP at 3000 RPM. The 3000 RPM engine would be making 2X the torque of the 6000 RPM engine...same HP. In this case the 3000 RPM engine would require more robust driveline. 400 ft-lbs running a race is a lot different than a weekend warrior 30 seconds of manifold pressure wet dream boost run. The way you use it does matter and plays a huge role in the life of the parts... The higher torque for extended periods of time will fatigue parts faster...add heat and you can be looking as scrap metal in short order.

With that said I have placed over 475 ft-lbs through a stock 1981 driveline in "weekend mode" (minus dream time thank you) without any issues.

Now, who wants to talk about high temperature strength of a cast piston?

H2

I don't care what kind of car you're using, the above kind of abuse is awesomely impressive. These cars are tanks to take what they sometimes are given. I can get 200K-250K relatively troublefree miles out of my wife's Camry, but it would fly apart in a single track day with this kind of treatment I believe. Not sure the Chevy's much better. Nor even my Audi TT.

Harvey

animal8526

but doesnt overcoming inertia actually cause more strain on DT parts than sustaining speed? I mean, I imagine those types of sustained speed are really tough on the engine itself, but not so much on the DT.

Mark Anderson

On the race track you are never trying to sustain speed. You are almost always trying to gain speed or lose speed as rapidly as possible.

animal8526

mark, my bad... I mistook "road america" for "open road racing"... I regress.

DFWX

I deliberately sidestepped the issue of piston/con-rod integrity as those are solveable with $$.
Certainly it is torque that breaks things and I only mention horsepower as that is what most tend to speak in terms of.
Obviously, the drivetrain could withstand 5,000 horsepower - as long as you never use it and the question of a street car, not race car - meaning more weight, lower gears action and, in a world,
drag racing - ie hammering it.
I'd rather have a 400 hp car that I can do nearly anything I want - and immediately - rather than a 500 horsepower car (or more torque car), for which I have to ginger the accelerator and slide the clutch so not to bust it.
What I am reading seems to confirm my inclination that the 928 motor has far more horsepower potential even in stock long block form than the drivetrain can withstand if that power is fully used. But this also raises issues, then, I never faced in other car I had. For those, the issue was making horsepower. For this 928 project, the question is deciding how not to make too much horsepower (meaning torque).
Long ago I learned that a street car that is short endurance or overbuilt to the point of breaking itself is not the route to go.
There are also concerning comments that support limiting horsepower. For example, "as long as you do not have drag slicks on it" and of being careful with the accelerator. The response is "damn, I was hoping to take it down to the run-what-you-brung 1/8th mile track just for the fun of it in the only non-USA V8."

Oh well...

Mark O

DFWX

On the otherhand, transmissions and torque tubes aren't THAT expensive on the used market, so I suppose there is only one real way to tell how much the drivetrain can take on launching... $300 to $1200 experiments. I better start rounding up more spares...
Mark O

heinrich

post-86 5spd gearboxes are unobtainable

Mark Anderson

I've been conducting these experiments over the past 20 years (in my own way). How many broken torque tubes and gear sets would you like to see?

adam928

I broke my TT... and Im running the original 320hp
On the other hand.. Im not sure yet why it broke.. It seemed like someone had been there before trying to weld it together.. but im not sure. It was kind of hard to tell just by looking at the broken ends of the TT.

Bill Ball

Does this look familiar?