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What's your truck's current final drive ratio?
I know it has 3.73 gears, turning a little over 3000 rpm at 70 mph
It’s not so much a matter of wanting to, just something that was recommended to enhance the driveability of the truck by a local guy with a nice squarebody. Just exploring what’s involved and feasibility at this point.
Ok, you're good...One commonly overlooked aspect of doing these swaps is the gear ratio. Many have a very tall a gear ratio and dont upgrade to a deeper one before swapping in the OD trans. They then wonder why the OD trans keeps slipping when trying to accelerate in 4th gear.
www.gmsquarebody.com
I know you forgot that the 700R4 has a numerically HIGHER ratio in 1st/Low gear - compared to the 350 - right?good point i didnt think about that. a th350 vs 700r4 will have a very different first gear too, so the rear ratio could probably come down to 3.20 and be pleasant to drive still. but if you like big tires you could keep your likely 3.73 or whatever stock
I could of swore the 700r4 has a better first gear than a th350? Maybe iim thinking of a 200r4 being nicer than a th350?
First gear is irrelevant to the mechanical leverage problem at higher throttle angles that can occur in overdrive (4th gear) when paring an overdrive transmission with something like a 2.73 final drive ratio or less. The 3.08 to 1 FD is the tallest ratio that I have seen that still works with the .70-to-1 4th gear ratio when in OD without slipping under load; the factory equipped most vehicles with a 3.08 or 3.23-1 final drive or deeper when paired with the 700R4 or 4L60E. That said, towing in OD on grades is generally not advisable (not saying you do that, just putting it out there).
700R4 gears are as follows:
The PWM is a graduated apply but once the TCC is fully applied, RPM slip should be 5 or less. I believe greater than 10RPM slip for two consecutive drive cycles when the TCC should be fully applied will trigger P1870 or P0894 in ODB2 year vehicles and ODB1 equivalent (not sure off-hand what the code number is for pre-1996).
He probably means the converter is trying to multiply torque, so the engine is revving higher than it would if he was on level ground.
make sure to use a structural bellhousing dust cover with your k caseI'm in the process of installing a 700R4 between a 400sbc/NP203 in my 76' K20. I went with Gearstar for the transmission and they built it to handle the torque (Level 3). I'm extremely happy with the kit and quality of the work so far. I haven't driven it yet, so that review will come later. I don't intend to drive it hard and no extreme off-road climbing, so I expect it will survive just fine. They made sure to install the output shaft needed to work with the NP203. The stock TH350 adapter works with the 700R4, so mounting it up was no problem. Gearstar also used the "K" case for the transmission, which is said to have beefier wall thickness.
I still wanted to add some support, so I found there was a support arm on some trucks/blazers that went from the side of the 203 T-Case (2 large 5/8" bolt holes) up to the bellhousing. This helps with the bending load on the transmission case. So, I sourced one (on eBay), and cut it in order to lengthen it. I used a piece of tube to sleeve it for the 2.5" extension. I fixtured it in place and then welded the sleeve to the support. (see attached image)
Overall list of issues to resolve: (my status)
* Resolve the output shaft compatibility with the NP203. In my case, Gearstar took care of this by installing the Advanced Adapters output shaft. (DONE)
* Trans to T-Case adapter: If you currently have a TH350, then you're good. If you have a TH400, then you'll need to source the correct adapter (ebay) (DONE)
* Cooling: There are a lot of opinions on how to cool a transmission. Gearstar provided a cooler in the kit and said DO NOT run the fluid through the radiator. Cooler only. (DONE)
* Gear selector/Shifter: I am not using the stock column, so I don't know if there are any issues with the number of detents for each gear in a stock column. I'm installing a retrofit Ididit column and it has the correct # of detents for the 4 gears. I also picked up the gear selector that will show correct for the 4 gears. (DONE)
* The front driveshaft will need to be "stretched" ~2.5", which means they'll need to install a new tube. (NOT DONE)
* The rear driveshaft will need to be shortened ~2.5". (NOT DONE)
* The transmission-T-Case crossmember sits back (you guessed it, ~2.5"), so new holes need to be drilled. Easy. (DONE)
* The stock shifter will sit ~2.5" back in the tunnel. To avoid this, here is my hack: (DONE)
After doing some research, I found that a TH400 was offered with the NP203 in some trucks. The location of the shift shaft on the TH400 is about 2-2.5" farther back from the bellhousing than on a TH350, which makes it just about right for a 700R4. I used the shifter and the two rods from a TH400/NP203 combo (found on eBay). I had to modify one of the rods slightly to clear the transmission pan, but no big deal. Now, the shifter sits correctly in the tunnel, just as it did with the TH350.
I'll update progress on this in my build thread, here:
New Member and new owner of a 1976 K20
Greetings all, Good to be here. Earlier this year, I purchased a 1976 K20 after a long search for one with a VIN combination I was looking for, and one that was a good candidate for restoration. Man, it was sure an ugly turd when it arrived, but I've spent the last 6 months getting rid of all...
I dont think GM had PWM transmissions which used ALDL codes. The earliest I know them to have used them was 1994 and those models were the obd 1.5 which used a Pxxx code. However, since they were not officially obd2 complaiant, the definiiton of P codes in those 2 years actually can vary from car to car lol. apparently 1993 corvettes had it??
PWM was introduced in 1995 for the 4L60Es so any ODB-1 TCC/component slipping codes would have applied for that year. Those codes were all two-digit codes without alpha-prefixes. 1993 vettes had 700r4s; they transitioned to 4L60Es in 94.make sure to use a structural bellhousing dust cover with your k case
on the column thing, you can retrofit a gm column with 4 speeds OR the parts that on the column itself make it show 4 speed, so iif you want to keep yours maybe one day stab it back in
I dont think GM had PWM transmissions which used ALDL codes. The earliest I know them to have used them was 1994 and those models were the obd 1.5 which used a Pxxx code. However, since they were not officially obd2 complaiant, the definiiton of P codes in those 2 years actually can vary from car to car lol. apparently 1993 corvettes had it??
weird. i distinctly remember my uncles 95 vandura using the modern obd port, and having P- codes whii READ EHO
