will my TH 400 be ok inmy C4

I

ironman417

Member
i have a TH400 that i want to swap in my c4 big block swap insted of the tired 700r4. my question is with the factory rear gears and the 3 speed trans will my car be ok on the highway at 70 mph?
or will the rpm's be way up there???? and another question i have is there a tailshaft housing i can use to keep the torque arm?
i dont have the cash right now to get a 4L80E right now with the proper stuff to make it work right so i am stck with the TH400 for now.
any help on this would be great thanks guys....
 
Since I don't know what rear gear ratio you have or tire diameter, I setup a table in Excel to do several calculations. Basically you see 3 sections in the table, each one is a different tire diameter of 30", 32" and 34", hopefully this will cover your setup. The 700R4 has a final drive ratio of 0.7:1, while the TH350 is 1:1, you can the the difference it will make in RPM, just look in the "Difference" column. Will it be Ok with the 3-speed on the highway is a personal preference to me, but with the table you should have a better idea what you will think about the new setup.

I have no idea about the tail shaft questions, but I bet Grumpyvette will have some input on your questions.
 

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that's a really useful spread sheet showing the difference in rpm and gears

as to the TH400 most guys are forced to build a custom connector beam

http://forums.corvetteforum.com/drag-ra ... rsion.html

http://www.transmissioncenter.net/gforc ... mbers_.htm

viewtopic.php?f=32&t=151

http://vettextc.jiminees.com/jeff/C4_BBC/

http://www.the-transmission-center.com/6l80__6l90.htm

sometimes you need to fabricate adapters
careful measurement, a drill press and a welder in even limited skilled hands can fabricate a decent adapter
adapter.jpg

adapter6.jpg

some more welding grinding and some paint can do a lot for the looks
adapter1.jpg

adapter2.jpg
 
that's a really useful spread sheet showing the difference in rpm and gears
Click to expand...
If you need something in particular, it wouldn't take but second to make changes. Have you talked with your sons about having a download section for the forum???
 
grumpyvette said:
http://forum.grumpysperformance.com/viewforum.php?f=99

there still working out details, but for now , lets stick as much useful info thats related here in this section
Click to expand...
Ok, think I will expand the table above and post it in "spread sheets and engine related forms". Any suggestions for making it better???
 
Indycars said:
grumpyvette said:
http://forum.grumpysperformance.com/viewforum.php?f=99

there still working out details, but for now , lets stick as much useful info thats related here in this section
Click to expand...
Ok, think I will expand the table above and post it in "spread sheets and engine related forms". Any suggestions for making it better???
Click to expand...

other than suggesting "the MORE EASY TO UNDERSTAND , INFO THE BETTER" as a general guide, in all posts
I can,t think of anything ID change, I think your doing great
 
ironman417 said:
well what are the factory rear end gears for an 84 automatic???
Click to expand...
The C4 rear axle ratio RPO numbers decode like this:
RPO Rear End Gear Ratio
GH0 3.54
GM1 2.59
GU2 2.73
GW4 3.31
G44 3.07
G92 3.07 "Performance Rear Axle"

If your not sure what the RPO code is, then a quick way to find out approximately is jack the rear of the car up and turn the rear wheels. If they turn in the opposite direction then let one tire touch the ground. Now rotate the one rear wheel TWO turns. Count the number of times the drive shaft turns. For example if it turns 3 times, then you probably have a 3.07 rear gear ratio. If it turns a little over three, then it's a 3.31. It can be hard to tell the difference between two adjacent ratios.

If the tires turn in the same direction, then you have a limited slip differential. Turn the rear wheel ONE full turn and count the revolutions of the drive shaft.
 
ironman417 said:
where is the RPO sticker inside the glove box??
Click to expand...
I can't talk from experience, but after Googling "Locate Corvette RPO", I would say it's in the left rear storage compartment. Starting in 1999 the sticker is located in the glove box.

Let us know what gear ratio you come up with!
 
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