Transmission question

[kingsolver72]

I am thinking about purchasing a 400 turbo trams for my 72 c-30. The truck has a manual 4 speed now. My question is, other than installing an automatic column would I need to shorten or Lengthen the the drive shaft? I'm pretty sure I won't be able to use the crossmember.

[tranz am]

You can use your crosmember just slide it wher you need it and drill new holes.
as far as driveshaft you will need a th400 yoke you will need to shorten the driveline or move the motor and trans forward.

[71meangreenc10]

You have to check the yoke coming out of the trans. I don't know on the 4spd, but the TH400 yoke is bigger than everything else I have worked with. You will have to measure the trans to see what to do with the shaft. Most swaps I have done I had to shorten the shafts. Don't forget about a trans cooler.

Smitty

[wbc409]

The TH400 also requires a kick down switch and associated wiring. It is NOT vacuum controlled like a glide or TH350

[SS Tim]

If it is a C30 4-speed truck, you will need to remove the crossmember at the bellhousing along with the rear engine mounts.
Because the factory 4 speed is a cantilever installation you will need to source a factory C20-30 automatic crossmember with braces, transmission mount and the frame spacers as well. This will convert it from four mounts to three.

[kingsolver72]

Thanks for all the help! Hey SS Tim I'm not sure about the term 'cantilever installation'? I need to replace the engine and thought a 400 trans would bring the rpms down. Sounds like there is a little more involved than I want to do for now. When I do the engine swap would it be easier to pull it with the trans or leave the trans in?

[frankslagoon]

ya know the high gear is the same in the 4 speed as the turbo 400 1:1

[SS Tim]

Originally Posted by kingsolver72

Thanks for all the help! Hey SS Tim I'm not sure about the term 'cantilever installation'? I need to replace the engine and thought a 400 trans would bring the rpms down. Sounds like there is a little more involved than I want to do for now. When I do the engine swap would it be easier to pull it with the trans or leave the trans in?

Sorry, Ok, the the engine sits on four mounts (four points), two front side mounts and two rear supporting the bellhousing. Kind of sits there as a power egg. The transmission is stabbed into the bellhousing and bolted to it alone. Meaning it just sticks out in space there without a transmission mount or rear crossmember. That is what makes it a cantilever installation.

On the automatic the rear engine mounts are not present and a singe transmission mount is used at the rear to support the trans and the engine as an assembly. That makes it a three point support.

Either the current four speed or a TH400 are a direct or 1:1 final drive. Meaning the input and output RPM are considered the same.

So changing the trans wil not reduce the engine RPM for a given road speed at all and given torque converter slippage will actually be a little higher.

To lower the engine speed the choices are limited to,
taller tires (limited change)
overdrive transmission, like a NV4500 0.73:1
add on overdrive, like a gear vendors 0.78:1 (expensive but allows splitting lower gears too.)
re-gear rear, with the choices of 4.11, 4.57 and 5.13 for a HO72 there isn't much to be gained there either.

http://www.summitracing.com/expertad...tio-calculator

[kingsolver72]

Lots of good info. I thought about taller tires, won't that change the speedometer reading? Is it hard on the engine if I drive 70 mph on the highway? Don't like driving 55 on the interstate,

[SS Tim]

Thanks!
Yes the speedo will be slow and the odometer will indicate short with taller tires because the taller tire rotates slower for a given road speed.

I'd suggest playing with the calculator I linked to above so you can see the effect on engine RPM by changing tire size or differential gearing.

As to what your engine is doing at 70 MPH... I don't know.

But as examples, with 30" tires a 4.57 differential and 1:1 trans ratio the engine would be turning about 3583 RPM @70 MPH.

Stepping up to 33" tires results in 3257 RPM

If we went 30" tires and 4.11 gears it drops to 3222 RPM

Or 33" tires and a 4.11 gears would give 2929 RPM

As I said above the actual change is limited with the gears and to an extent tires.

[tranz am]

if you want to drop rpms at cruise speed look into the 4l80e or 700r4/4l60.

[Steeveedee]

I have a '70 Chevy 3/4 ton truck with 400 engine/400 trans and 3.55 rear gears. I have yet to hook up the tach since I went to HEI, but that will be happening soon. Feel free to PM me if I don't give you an answer real soon. I'm working some home remodels/repairs for the foreseeable future, so don't have a lot of time to work on the truck.

[special-K]

What ratio do you have now? If not 4.10, grab a center section and plunk it in. Taller tires would be worth it. What do you have, 235/85-16s? I'm not sure you can run 255/85s w/o extra spacing/studs but if they fit that would be a nice improvement.

[kingsolver72]

Originally Posted by special-K

What ratio do you have now? If not 4.10, grab a center section and plunk it in. Taller tires would be worth it. What do you have, 235/85-16s? I'm not sure you can run 255/85s w/o extra spacing/studs but if they fit that would be a nice improvement.

Yes tires 235/85/16. Not sure about the gears. Been raining here in Ks. and my shop is too full to pull the truck in out of the rain.

On a different note, I was looking at the spid and noticed "Dr Wheel Conversion".
Anyone know what that means?

Thanks again for all the replies,they've been very helpful!

[SS Tim]

RPO "RO5" is the conversion of the base truck to DRW (dual rear wheels) by the factory during assembly. Basically a different rear axle/brake assembly and front drums or rotors with a spacer to allow the ~6" negative offset budd wheel (same as one of the rears) to run correctly.