Changing gear ratios: opinions/suggestions wanted!!

[79_GMC_C1500_jj]

Had similar situation. 79 GMC c1500, crate 350 (installed 2004) w/TH350, stock 3.73 rear ratio. Man, tac'ing 3,000-3,200 doing 55-60. That crate motor eventually wore out. So, last year, installed another (this time Jaspers, so far running great, about a year) 350 motor. I'm like.. i need to either swap rear end gears or get different transmission. thing is I like the "pep" in city driving. (I mean, I'm not uhm doing any burn-outs or drifting or anything ;-) as I have a shell, it's a lot (of weight) to haul around, and the low gears seemed to help. So, I decided to go with a non-electronic (manual switch, like I'm launching a missile), 700r4 transmission. Four gears, but I don't use the fourth gear, unless I'm popping on the FWY or long drives. Around town, (with the switch not engaged), it performs just like my old TH350. But man, the fourth gear is great. I get up to like 45, on FWY, take my foot off the gas for a second, flip the switch (I mean launch a missle) and Beee-BAAAAUWW, drops into a very nice Overdrive. 65-70 at like 1,900 RPM's. amazing. As it should be. The motor will not scream at me, and hopefully both transmission and engine will last a bit longer.

So, with going from TH350 to 700r4, couple things happened. They (shop) said OEM driveline was fine. (even tho the 700 is longer (inch or so) than the TH350 housing. I think they just flopped the OEM crossover, I believe holes were also there for this at the factory. I was like, that's not right. (same driveline) they assured me it was. Well did run fine. but.. after like 3 weeks, when i parked on a grade, I came out to a huge puddle of ATF under it. Turns out, the stock yoke for the TH350 in 1979, at least mine, had a little breather hole in it. not sure exactly what for.. - uhm breathing? ;-) . The OEM TH350 never leaked (at least from the yoke) so, I looked around and turns out the 700r4 yoke does not need a breather hole. So we ordered a new yoke, and while that was going on, did have local driveline shop take a measurement and cut it to the proper length. (Also we replaced all three u-joints while we were at it and balanced it. No more leaks, and I'm able to drive on the FWY now. (I've always wondered what it would be like to be in those far left lanes).

* UPDATE. Ooops... what tires.
I've always run them pretty much stock, on steel rims.
The tires on it are "General Grabber HTS 60"
255/70 R 15 108 S M&S

 

[MikeB]

Before swapping the gears,, look into a 4l80,, a lot of donors available. I use the MSD atomic controller and it has been flawless for 10 years now.

Hey Chris, that's good to hear about the MSD controller. I have a 69 C10 with a 350/TH350, but even with 3.08 gears and 29" tires, I find myself looking for a taller gear at 70mph. A 4L60E might be reliable, but an 80 would be bulletproof. I dealt with several issues with an 85 700R4 in a friend's hot rod. No issues with TV cable and converter lockup, but all kinds of other stuff. Turns out it was a cheap rebuild by a trans shop, and it must have had a lot of miles on it before the basic rebuild.

 

[Ricko1966]

Hey Chris, that's good to hear about the MSD controller. I have a 69 C10 with a 350/TH350, but even with 3.08 gears and 29" tires, I find myself looking for a taller gear at 70mph. A 4L60E might be reliable, but an 80 would be bulletproof. I dealt with several issues with an 85 700R4 in a friend's hot rod. No issues with TV cable and converter lockup, but all kinds of other stuff. Turns out it was a cheap rebuild by a trans shop, and it must have had a lot of miles on it before the basic rebuild.

You have a roller cam or flat tappet? Use a gear ratio calculator,use some common sense,like are you at 70 plus most of the time,or just occasionally,etc. I fully believe part of tge flat cam problem is too low of rpm on flat tappet cams,they are splash lubricated. Too low rpm I don't think the cams get enough oil.