I have an 87 IROC with the 5.7 350 and I was wondering what kind of transmission I have in it. It's an automatic but it is running at about 4000-4500 RPMs at about 75 mph which shouldnt be happening. I have read in about 3 different places that my car came with the 700R4 but I think it only has a three speed. If it is the 700R4 I have an idea of why it might not be shifting into fourth is because my car is running slower than it is supposed to like it is probably only running at 190 when it is supposed to do 225. Can anyone clear this up.

 

Running cool wouldn't prevent the 3-4 upshift. To determine what you have, look at the shape of the trans pan. If it is about square, with a bit of a ripple across the back end, then it's the 700R-4. The TH350 pan is a square with a corner shortcutted, making it a 5-sided pan. TH350s use a vacuum modulator at the rear of the passenger side, 700R-4s don't.
Now, your rpm is too high for how your car should be geared, unless you have 2.77:1 axle gearing and are running in second gear.
TPI engines don't like the kind of gearing you would have to be doing that much rpm in third. If you knew for sure your rear gears, that would help.
How many miles on the car? Has the axle lube been changed since you've owned the car? If not, crawl under and see how many bolts are holding the rear cover, Then go buy 2 quarts and a cover gasket. While you're changing the lube, you can look at the perimeter of the ring gear, and look for a number stamped in, like 36-11 (3.27:1) or 11-36 (still 3.27:1).

 

Im not sure on how it is geared but my engine right now has 20,000 miles on it since the rebuild and the odometer reads 153,500 miles. Will check the pan and the numbers later because I cant right now as there are dangerous thunderstorms going on but I will try and report by tomorrow. Only owned for about a month so I have not checked the lube. Where is the axle number located? Just checked the pan and from the angles I looked from it looks square so according to Atilla it is a 700R4. Since it is any other ideas on how to get it to shift to fourth?

 

So anyone have any ideas?