turbo350 / th350 automatic trans HP limits ..

[Mike Engler]

I have a couple of questions about vette/chev trannys

is the Turbo350 and the TH350 the same automatic transmission?

if no, how does one tell the difference?

Also, I've heard, whispers and rumours that turbo350/th350 tranny's cannot reliably support more than 300 horsepower... I've heard that the Turbo400/TH400 is a more robust automatic, and can take much more HP and abuse.. Are any of these rumours true?

Thanks,
Mike Engler

[Clem Z.]

Re: turbo350 / th350 automatic trans HP limits ..

TH stands for turbo hydamatic so they are both the same. yes the TH 400 will take more HP because the internals are much stronger but also heavier which slows down acceleration. a lot of drag racer use built up TH 350 because of this. a TH 400 in stock form will take the output of a well modified BB without any problems.

[Duke W.]

Horsepower isn't the issue

Transmission capacities are rated by maximum torque and maximum input RPM, not horsepower. Since maximum engine torque occurs at a lower speed than peak power, it's tough to come up with a "maximum horsepower" spec, unless you use the torque capacity at the maximum input speed, but this isn't the way engines work.

I've seen some peak torque/max input speeds on some manuals, but never on an automatic, but, no doubt, GM had them and that's how they choose whether to install a TH400 or HT350 behind the various engines.

Duke

[Clem Z.]

Re: Horsepower isn't the issue

duke is correct as that why they call it "torque management system" on the C-5 corvettes that allows the basic same auto trans that is use in my wifes V-6 2000 200HP impala to be coupled to a 350HP,350 #ft of torque engine without it spilling its guts all over the highway. the computer kills certain clys for a millisecond during full throttle upshifts to lessen the torque load on the trans.

[Duke W.]

Interesting point, Clem

I know maximum engine torque on A4 C5s is limited to a bit less than the M6, but I always figured that the PCM closed the throttle a bit in the peak torque range to restrict airflow. Does it really kill some power pulses at the cylinder by not injecting fuel? If I recall correctly A4 peak torque is reduced about five percent, so one out of twenty power pulses would have to be disabled. I would think that would be noticeable by the driver as a slight missing or roughness.

Duke

[Duke W.]

More transmission rating info for gearheads

As stated in my earlier post transmissions are rated by maximum torque capacity and maximum input speed. Another interesting point is that the taller the first gear the greater the torque capacity because of less torque multiplication.

I don't have any data for Corvette transmissions, but I do have data for the Borg-Warner T-50 five-speeds that were used in seventies vintage H-bodies.

In 1975 there were three versions. The first, with a first gear ratio of 3.41:1, has a maximum torque rating of 175 lb-ft and a maximum recommended input speed of 8000 RPM. A second version, with a 3.10:1 first gear, has a maximum torque capacity of 205 lb-ft with the same maximum input speed. There was also a "heavy duty" version of the 3.41 first gear box with shot peened gears, which was also rated at 205 lb-ft, and only this version was produced after 1976 as a version/parts consolodation measure.

From this we can probably conclude that CR versions of B-W and Muncie four-speeds have higher torque capacity than WR versions. The M-22 had even higher torque capacity due to the lower helix angle on the gears and the roller bearing first gear. The latter feature helps with durability at sustained high input shaft speeds - like in road racing. I learned how nice a feature this is when the T-50 in my Cosworth Vega seized. First gear welded itself to the mainshaft while I was cruising at 70 MPH. The car only had 7K miles, and a lot of CV T-50s seized in a similar way, but since only about 800 CVs were built with the T-50 GM and NHTSA just ignored it. The high revving CV engine made for higher average and peak gearbox input speed. Seizing wasn't an issue on any other engine.

So I learned first hand why input shaft speed is important! The data I quoted for the T-50 is from a 1975 SAE paper on the T-50 that I unearthed about ten years ago and my GMPD parts catalog research.

Duke

[Clem Z.]

Re: Interesting point, Clem

duke that is how i understand it works. i read that somewhere when the 97 C-5s came out. the SAE tech lit would be good place to search this out.

[Clem Z.]

Re: More transmission rating info for gearheads

duke i was told that the first M-22s did not have the roller 1 st gear and there was a kit for a change over. the early citations had a strange setup in the trans,they used the gear teeth as a oil pump to push the lub thru passages in the case to oil the gear/shaft interface because they ran steel on steel,no bushings. i had on of these trans go bad while i was in fl. the thing squeeled like a pig except in 4 th gear. i got the car home to pa and since it was still under warranty on my way to the dealership i ran it as hard as it would go thru the gears to see what would happen,and guess what the squeel went away. they R and R the trans and you could see where the gears had gaulded to the shafts.the problem came from using the wrong sealer at the trans factory on the trans and it plugged up the oil passages in the case.

[John H.]

Re: Interesting point, Clem

"Torque-Management" has been a very common control strategy for improving automatic transmission reliability since engine computers, fuel injection, and electronically-controlled transmissions arrived on the scene, replacing the previous generation of mechanically/hydraulically-controlled transmissions; it also has avoided or deferred billion-dollar investments for new, stronger transmissions in many cases as engine performance has increased. Typically, at WOT acceleration, the spark is retarded by the PCM at the instant of gear change for a reduction in torque for a few milliseconds during the shift; this interruption is so brief that it's transparent to the driver, but it significantly reduces internal shock loadings to the clutch packs, planet carriers, and input shafts. Good thing Microsoft doesn't make PCM's

[Roberto L.]

Re: Interesting point, Clem

You know the joke, the auto trans would stop in the middle of nowhere and a check panel message would say "Unknown error..."

Roberto, NCRS #30019, RMC