2.79 to 3.55 swap

[md02geist]

Originally Posted by horsehead

As long as you got lots of cold beer to go along with that steak.

Hey i'll give you a steak and beer to rebuild mind. And I'm in VA too


As far as my driving it will see short jaunts on the highway, probably 20-30 minute drives from one end of town to the other. But for the most part it will be spending its time on back roads, city roads, etc.


I don't really like highways at ALL in a modern car, I certainly do not like them in my old car.

[wareagle]

Originally Posted by horsehead

As long as you got lots of cold beer to go along with that steak.

How about some ice cold PBR?

[wareagle]

Hey Horsehead, where are you in VA?

[ToneMT]

Im in the process of going from 2.7 to 3.25 (literally just got delivered the new gears + swap kit and all). Running on a rebuilt C4 w/ shift kit via mild cammed 289. I like running on the highway and cruising around town - so Im hoping that this isnt gonna shoot me on the foot - stealing from Peter to pay Paul and all...

[dzahm]

Actually I need someone to go thru the 3.50 TL pumpkin (9 in). I have war eagles rear (8in) here at my house- So if it is possible you might wanna run down for a Sat rebuild session- I would be interested- I have tools/shop, and can get a kit- and have both pumpkins ready for you. We are about 70 miles from the NC/VA line (right off 77) So let me know if this is a serious prop. OP may have the ability to get his rear to this area (still coming this way?) So maybe 3 to rebuild? Since you'd likely be full (of steak) I will pay cash

[wareagle]

Originally Posted by dzahm

Actually I need someone to go thru the 3.50 TL pumpkin (9 in). I have war eagles rear (8in) here at my house- So if it is possible you might wanna run down for a Sat rebuild session- I would be interested- I have tools/shop, and can get a kit- and have both pumpkins ready for you. We are about 70 miles from the NC/VA line (right off 77) So let me know if this is a serious prop. OP may have the ability to get his rear to this area (still coming this way?) So maybe 3 to rebuild? Since you'd likely be full (of steak) I will pay cash

+++++++++++1

[horsehead]

Originally Posted by wareagle

Hey Horsehead, where are you in VA?

I'm located about 30 Minutes west of Charlottesville, Virginia. Over across the mountain in the valley....

[wareagle]

Originally Posted by horsehead

I'm located about 30 Minutes west of Charlottesville, Virginia. Over across the mountain in the valley....

Looks like about a 4 hour trip to Statesville (where my and dzahm's rears are).

You up for it if we get the stuff needed?

[HoosierBuddy]

Originally Posted by md02geist

What would be fun around town without being insanely high on the interstate, coupled with a 4-speed toploader then?

Well...my take on it is that step 1 is to replace the transmission and then step 2 is to get a rear axle ratio that works well with the new tranny.

Here's was "logic" on it.

A C4 has a 1st gear ratio of 2.46:1
A 4R70W has a 1st gear ratio of 2.84:1

So...if you did not change your rear axle ratio, but just changed your transmission, you'd have an "effective" change of 1.154 X additional torque, in 1st gear, at the rear wheels. This, by itself, would be equal to you changing from your 2.79 to a 3.25 R&P (well...almost anyway).

And the overdriven gear is .7 on a 4R70W, which means a 30% reduction in highway RPM (it's actually more than that, because it has a lockup torque convertor unlike a C4 that doesn't).

So...you don't want that 30%, so you can back that down by changing your rear axle ratio.

Going to a 3.55 set at that point means 27% increase in RPM in all gears compared to the 2.79....giving back most of the 30%....but the lock up TC means 200 to 300 rpm less by itself.

MPG soars on the highway and the car now has 1.154 transmission increase X 1.27 rear axle increase = 1.468.

So....you now have 47% more effective gear off the line and BETTER (not worse) highway gear ratio.

That's how i looked at it anyway. I did my transmission conversion first and the 3.55's next. Then again, my c4 was pretty much toast...so that helped make the decision.

Phil

[wareagle]

Damn Phil, you come up with all those numbers off the top of your head

[md02geist]

Originally Posted by horsehead

I'm located about 30 Minutes west of Charlottesville, Virginia. Over across the mountain in the valley....

I'm in Richmond, you ain't far from me

[md02geist]

Originally Posted by HoosierBuddy

Well...my take on it is that step 1 is to replace the transmission and then step 2 is to get a rear axle ratio that works well with the new tranny.

Here's was "logic" on it.

A C4 has a 1st gear ratio of 2.46:1
A 4R70W has a 1st gear ratio of 2.84:1

So...if you did not change your rear axle ratio, but just changed your transmission, you'd have an "effective" change of 1.154 X additional torque, in 1st gear, at the rear wheels. This, by itself, would be equal to you changing from your 2.79 to a 3.25 R&P (well...almost anyway).

And the overdriven gear is .7 on a 4R70W, which means a 30% reduction in highway RPM (it's actually more than that, because it has a lockup torque convertor unlike a C4 that doesn't).

So...you don't want that 30%, so you can back that down by changing your rear axle ratio.

Going to a 3.55 set at that point means 27% increase in RPM in all gears compared to the 2.79....giving back most of the 30%....but the lock up TC means 200 to 300 rpm less by itself.

MPG soars on the highway and the car now has 1.154 transmission increase X 1.27 rear axle increase = 1.468.

So....you now have 47% more effective gear off the line and BETTER (not worse) highway gear ratio.

That's how i looked at it anyway. I did my transmission conversion first and the 3.55's next. Then again, my c4 was pretty much toast...so that helped make the decision.

Phil

That's pretty solid thinking, but I really have no desire to go to an automatic.

[lukep6470]

I've just done exactly this swap. I used Yukon gears with a TruTrac and it has turned out a little noisy but seems to be settling down with time. I would recommend getting any gears you buy (including new) lapped before fitting just to be extra sure they are quiet.

[Dave DeAre]

I have a 3:50 with mild modified 289 and C4. Strong off the line, no problem around town. Often go to car shows, longest 200 mi round trip. No problems but I seldom go over 70 mph; mpg is 16-17 same as new 2:79 (but not over 70 lol!)

[66DavesGT]

Try Joes High Performance. I've bought several center sections from him with no issues whatsoever. 1-724-459-5050 or 1-724-459-8562. I shopped around and he has pretty decent prices too.