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TH350 shifting & fluid recommendations

Evolution1980

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OK...I'm doing top speed runs, manually doing the 1-2-3 shifts. Running high RPMs. Then I come off it for a few seconds, put the pedal back down to floor.

It downshifts to 2nd and just about buries itself at 7000 RPM. It'll stay there until I just barely lift off the throttle.

So then I start again from a stop. This time I just leve it in Drive (3) and let it shift itself. The 1-2 shift takes a bit longer, and the 2-3 shift doesn't happen again until I lift a bit off the throttle again.

I just changd the fluid about a month ago, which was just draining the pan. I know that there is still plenty of fluid in the system and that you need to pump it out or something to get it all out. The fluid level is correct. I had basic off the shelf Dexron3 (Mercron? whichever one is the correct one for ATX) put in. Is this my problem? The cheapo stuff doesn't like the excessive heat? Or does my problem lie somewhere else? I figure it has to a be a heat related problem because when it's cool or had a chance to cool, everything's back to normal.

What's the preferred hi-perf/HD tranny fluid that you like or have heard good things about?
 
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page62

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To be frank, you're beating the snot out of your drivetrain!

Now that's out of the way, my first question would be: Do you have an auxiliary tranny cooler? It'll do more for you than any fancy fluids!
 
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Big Fish

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I agree with the trans cooler idea, surprised they aren't standard equipment. The "cooler" in the radiator acts more like a heater than a cooler. How do you cool the trans with 160 degree water? As far as the shifting problem it might be your vacuum modulator. Adjustable ones are available from places like Summit, can't really go wrong if you are able to adjust for your engine and driving techniques.
 
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Evolution1980

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Page62:
It's a matter of opinion, but I wouldn't say I'm beating the snot out of the drivetrain. I don't consider driving a performance oriented car to it's abilities as 'beating on it'. I didn't put a ZZ4 engine and a HD torque conv in it so I could leisurely creep up to 65mph and stop there. The car was purpose built. And I sometimes like to drive it as such :D I wouldn't expect to be doing this kind of driving in a Saturn. On the same token, if I wanted to just drive around in a everyday fashion, I'd buy a Saturn for that purpose.
As for a tranny cooler, no I don't have one yet, but have been seriously considering it. I do have a higher capacity aluminum pan with cooling fins (heatsinks) on it, as well as magnetic plug to catch any debris. This helps some. I also change my tranny fluid after every season (about 5 months of driving).


Big Fish:
I'm no mechanic, but I believe I'm correct when I say... your tranny fluid gets MUCH hotter than 160°. It also get much hotter than your engine coolant. When it goes through the 'cooler' in the radiator, the 160° water is actually removing heat from the hotter tranny fluid. So, yes, it's still cooling the tranny fluid.
I don't think it's a vacuum problem as when the car is cool, the problem doesn't exist. But I don' tknow for sure. That's why I'm here asking :)
---------------

If it does turn out that I'm giving more to my rebuilt TH350 than it can handle, it's no big thing because next year I'm dropping in a built-up 700R4 tranny. At that time, I'll likely have the tranny cooler added as well. I'm just looking for other options in the meantime. I figured a better performing fluid could be a simple fix. I hear there is like a Dexron5 or something like that that handles higher temps better (becomes less viscous at higher temps as compared to the Dexron3...or something like that.)
 
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page62

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Sorry Evo, but your sig didn't provide any clues about what you've done under the hood. I based my comments on your running a stock/original configuration.

Tranny coolers are dirt cheap, and not too difficult to install. I'd strongly suggest getting one now to keep your TH350 alive until next year.
 
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Bullitt

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I agree with the additional external transmission cooler suggestion. If you're going to do it anyways, it might as well be right now. If you have the money, splurge and get one with a mounted fan. Those prices are around $150 or so. Even the cheaper ones are adequete. Consider that towing vehicles, which see higher strain on the tranny, come equipped with external coolers, not to mention today's high performance sportscars.

The transmission modulators affect the speed at which the tranny kicks into another gear. Therefore by adjusting it with aftermarket modulators, the gear change can happen at a higher or lower speed, whichever you perfer.

--Bullitt
 
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Big Fish

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You are correct in saying the trans fluid runs hotter than 160 degrees, of course the water temp in the radiator could be hotter than 160 as well. The external cooler would still be more effective (guess around 90 degree air, depending on outside temperature). If you are running any kind of stall speed converter you are heading for disaster without one for sure. I would still be leaning toward a vacuum problem however, the modulator is what tells the trans to up-shift. Could be the modulator is OK, but the engine is not making enough vacuum at high end to make it work. You could install a manual shift valve body that will allow you to shift into any gear no matter what your vacuum is. This is a relatively easy project that does not require trans removal.
 
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Evolution1980

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Big Fish said:
I would still be leaning toward a vacuum problem however, the modulator is what tells the trans to up-shift. Could be the modulator is OK, but the engine is not making enough vacuum at high end to make it work. You could install a manual shift valve body that will allow you to shift into any gear no matter what your vacuum is. This is a relatively easy project that does not require trans removal.

Big Fish,

But wouldn't vacuum be an "all the time" thing, meaning it either works properly all the time or it doesn't? When the system is cool, I have no problems with the shifting at the high RPM. It's only after a couple of top speed runs that it starts doing this. How would the heat buildup affect the vacuum? Hot or cold, I would think at xRPM, their's either vacuum or there's not. Unless...heat is causing the modulator to stick open/closed?
 
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Big Fish

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I guess I don't know all the answers, but you might try hooking up a vacuum gauge and rigging it up so you can observe what the gauge reads while you are having trouble, and when you are not having trouble. This could rule out a vacuum problem once everything warms up. If the vacuum readings stay the same maybe it's time for new seals, etc in the trans. The 350 really isn't a bad transmission, but they all get tired after a few years and a few horn runs. If the vacuum changes you may be getting a vacuum leak that only occurs when everything is hot.
 

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