th-350 Problems

I have a 78 Pontiac Trans am, with a 455 engine and a th-350 trans, just recently the trans has been acting up, it will shift as normal when the entire car has been sitting a while, but after 5 minutes or so, when the temp reaches 180, it wont leave first and will over rev, Is this a vacuum problem or what.

It could be the vacuum line to the modulator is plugged or loose. It's also possible the modulator has taken a powder. Remove the vacuum line, and if fluid comes out (even a drop or two), the modulator is the problem.
A 455 will make short work of that little tranny. You will hear people say the 350 can "take it". Not behind such a torque monster! If you find there's internal problems with it, save yourself a bunch of headaches and get a TH400.

I agree about the Modulator... definatley check that. But I do disagree on the strength of the TH350.... Granted they are not as strong as the 400, but can handle a stock 455 as long as you don't rag on it every day.

I don't see any reason for a swap to a 400 (hassle of wireing the kickdown, shortening driveshaft, moving crossmember, etc) unless you really race it or have it built over stock.

I know of a couple 455 cars on TH350's with no problems. I used one on my 11.80 Buick 455 in a '84 Cutlass 4 yrs with no problems (but that engine was basically stock... just a cam and headers).

A 400 will take constant abuse longer... and worth the switch if you're a weekend racer. If its just a daily driver that gets run every so often at the stoplight, then you don't "have" to. But the added insurance of not haveing to "take it easy" is nice. I did switch to a 400 when I started racing constantly.

Good Luck. 455 TA has to be a fun car:smokin:

Also try a trans fluid and filter change.

As t\for the 400.....yes its stronger, but its slightly slower.
Unless your 455 is heavily modified, the 350 is fine, and is actually slightly quicker in the quarter than a 400, all other things being equal.

A strong 455 will eat up a 350 in short order. A stocker may give it a fighting chance. Pontiac never put the 350 behind a 455, with the possible exception of the '76 T/A (455 had it's teeth removed for '76).
Another factor not often discussed is the power flow and shifting techniques used by the various transmissions. 400 is the most popular for more than just the strength. When an upshift occurs, in most automatics (including 350), two events must take place. A band/clutch must release while another applies. 400 simply applies another clutch pack to upshift, without the need to release. The result is a cleaner/crisper shift.
If building anything serious, the words "get away with" or "get by" should be deleted from your vocabulary...

I can say with almost complete cirtainty that the vac mod isn't your problem. A vacume leak in the line or a failed diaphram will cause a late and hard shift but it will shift long before it "overrevs". A properly build th350 is more that capable of handling a mild poncho 455 and is lighter, and has less paricidic losses as well.