Towing and rear diff ratios

[SirRobyn0]

Ok this is what I've got. 1984 C-20 350, 700R4 according to RPO code it has 3.73 gears. This is the semi floating axle. I use this truck to commute to my day job as well as for farm use. Not so much on the farm but for farm runs. Feed, supplies, equipment ect. I have recently purchased a 18' trailer, it is technically an open car hauler, it has custom built short side I will use it mainly for a large utility trailer maybe a few times of year my old Cadillac will get a ride on it.

On Saturday I will be headed over the pass with the truck and trailer to pick up a small tractor that weighs in at about 2,000, making the trailer right around 4K gross. My feed runs, I'd like to be able to get 2 tons at time so that would mean the trailer would gross 6K. I know with a 350 I won't be breaking any land speed records, but I'm pretty sure from having run a couple 2K loads on flat land I'm not going to be happy in the pass. So after this Saturday I'll know better how bad it does in the mountains, but I'm debating going either to 4.10 gears or 4.56. Your thoughts?

 

[Frankenchevy]

What size tire?
Do you have a temp gauge installed in that 700r4?

 

[HotRodPC]

Since you have OD, I'd go 4.56's. 3.73 to 4.10 just isn't worth the money, and not much change. With OD, and stock 31in tall tires, you're still in good shape with 4.56's and the 700r4 will love you for it.

4000lb isn't all that much. I'd just keep in D while towing, not OD. If you're trans doesn't lock the converter in D or 3rd, then occasionally when I had my speed built up pretty good safely so I'm not lugging the engine or straining the transmission, like maybe on a slight down hill or wind behind you, then I'd use the OD just a bit occasionally so the converter locks up and give the trans a chance to cool down a bit. This is a good scenario when it's nice to have lock up in D. Where you can tow in D and still have the converter locked up to prevent heat. When the converter is locked up you have alot less friction in the converter and therefore runs cooler.

 

[Blue Ox]

You definitely don't want to tow with the converter unlocked. If it locks in 3rd on the hills that would be a great thing, but running unlocked will put temperatures on the moon.

 

[SirRobyn0]

What size tire?
Do you have a temp gauge installed in that 700r4?

Yes I have temp gauge for the trans. stock 235/85-16, I know tire size does make a difference and I've considered dropping down perhaps 235/75-16, I also may not, so there is that

Since you have OD, I'd go 4.56's. 3.73 to 4.10 just isn't worth the money, and not much change. With OD, and stock 31in tall tires, you're still in good shape with 4.56's and the 700r4 will love you for it.

4000lb isn't all that much. I'd just keep in D while towing, not OD. If you're trans doesn't lock the converter in D or 3rd, then occasionally when I had my speed built up pretty good safely so I'm not lugging the engine or straining the transmission, like maybe on a slight down hill or wind behind you, then I'd use the OD just a bit occasionally so the converter locks up and give the trans a chance to cool down a bit. This is a good scenario when it's nice to have lock up in D. Where you can tow in D and still have the converter locked up to prevent heat. When the converter is locked up you have alot less friction in the converter and therefore runs cooler.

4.56 is really what I was already leaning towards. I understand 4,000 pounds isn't much, but when I do have the caddy on the trailer it'll be close to 7,000, but that won't be a frequent tow scenario, it'll be a couple times a year. Always in D when towing or with much of a load in the bed, I'm well aware of the consequences of towing in OD, and that applies to a lot of the newer trannys as well.

 

[shiftpro]

I wouldn't tow anything larger than a utility trailer with a 700. I don't care what the gear ratio is. And you live in the hills... never.

 

[SirRobyn0]

I'd just keep in D while towing, not OD. If you're trans doesn't lock the converter in D or 3rd, then occasionally when I had my speed built up pretty good safely so I'm not lugging the engine or straining the transmission, like maybe on a slight down hill or wind behind you, then I'd use the OD just a bit occasionally so the converter locks up and give the trans a chance to cool down a bit. This is a good scenario when it's nice to have lock up in D. Where you can tow in D and still have the converter locked up to prevent heat. When the converter is locked up you have alot less friction in the converter and therefore runs cooler.

You definitely don't want to tow with the converter unlocked. If it locks in 3rd on the hills that would be a great thing, but running unlocked will put temperatures on the moon.

Both of you probably know my day job is a mechanic, but the shop doesn't really getting into modifications, or tranny work. So I understand why towing in overdrive is a big no-no. I also understand why there is excessive heat generated in drive without lockup, I also understand why you shouldn't put to much pressure on a locked up converter. The way I see it's kind of damn if you do damn if you don't. My tranny will lockup in 3rd, actually it'll lock up in second as well. You guys may already know this already but it's completely non-computerized, the over drive circuit will lock up the second it hits OD, it's only release is down shift or brake apply. The 2nd 3rd gear apply is with the vacuum switch. This is factory, going of the wiring schematics for my truck. However currently the lockup for 2nd 3rd is unplugged at the switch. The reason being is that the transmission does not seem to like being locked up in second, and while it will shift 2nd 3rd locked up I'm not sure about the long term effects of that. I've done plenty of in bed hauling and light towing in drive without the lockup hooked up and I'm not as of yet had a temp spike in the transmission. Perhaps the short term solution at least would be a toggle switch so I can lock it up in 3rd if I want to, like for example when towing.

 

[SirRobyn0]

I wouldn't tow anything larger than a utility trailer with a 700. I don't care what the gear ratio is. And you live in the hills... never.

I'm aware the 700R4 is not the strongest tranny in the world, but I've got a temp gauge and it's in good health at this time. I'm I concerned sure a little, but if I were to launch it, I'd be more likely to have a stronger 700R4 built than to go to the 400 or 300, for the simple fact of the benefits of over drive.

 

[SirRobyn0]

Side bar on the tranny life thing. So I would welcome input as to the lock up scenario on my tranny, I'm thinking that it might be a good idea to leave the 2 - 3rd lockup vacuum sensor in place and simply wire a toggle switch inline so I can have 3rd gear lockup when I want it and none when I don't and be able to avoid 2nd lockup and the 2nd 3rd locked shift.

On another note. I just ran last weekend 200 miles with the trailer with about 1 ton of feed, so similar gross to the tractor about 4K, but this was not over the pass it was to the south, it's not really flat, there are hills, but nothing compared to the pass. I ran in drive (3rd) the entire trip without lockup and saw only the slightest increase in temps over running empty. I have no idea if or when the trans may have been rebuilt and if it did or didn't have any improvements done, keep in mind I've had this truck only 8 months.

 

[Blue Ox]

I can only relate my experience, and if you have a trans temp gauge you're in a better position than I was. But towing a 7000 lb race car and trying to maintain highway speeds on a grade, when it would downshift into 3rd the coolant temp would skyrocket. My 3rd didn't lock. Also had 3.73s at that point. Probably wouldn't have needed to downshift with that extra 10%.

 

[75gmck25]

If you get an aux cooler, look at the stacked plate coolers called the Long tru-Cool Low Pressure Drop (LPD) with the high GVW rating.

They were an OEM supplier to GM and Ford, and B&M used to repackage the Long design. Some models have a temp sensitive valve to allow cold fluid to bypass, and then go to full cooling flow when warm. The model I used on my Ford came with metal mounting brackets that made the installation very stable - not just plastic ties through the radiator.

Bruce

 

[SirRobyn0]

I can only relate my experience, and if you have a trans temp gauge you're in a better position than I was. But towing a 7000 lb race car and trying to maintain highway speeds on a grade, when it would downshift into 3rd the coolant temp would skyrocket. My 3rd didn't lock. Also had 3.73s at that point. Probably wouldn't have needed to downshift with that extra 10%.

Well I really appreciate it you relaying your experience. That maybe the difference between 4K and 7K. What did you do to solve the temp issue?

 

[Blue Ox]

Part of the problem was an incorrect type thermostat in the engine. However, even after I fixed that problem it would still develop significant temperature creep on downshifts. I had a big stacked plate cooler for the trans, and the trans never blew up, so I'm guessing I never cooked it that bad, or it was so intermittent that it wasn't going to kill it. But ultimately the cure was the team owner got a new tow vehicle and my Suburban didn't have to tow the race car trailer any more. I ended up changing to 4.10s and a full floater later, but I never towed the trailer again so I don't know how much of a difference that would have made.

 

[SirRobyn0]

Part of the problem was an incorrect type thermostat in the engine. However, even after I fixed that problem it would still develop significant temperature creep on downshifts. I had a big stacked plate cooler for the trans, and the trans never blew up, so I'm guessing I never cooked it that bad, or it was so intermittent that it wasn't going to kill it. But ultimately the cure was the team owner got a new tow vehicle and my Suburban didn't have to tow the race car trailer any more. I ended up changing to 4.10s and a full floater later, but I never towed the trailer again so I don't know how much of a difference that would have made.

Just out of curiosity I assume you must have had a temp gauge for the trans, I'd be curious what some of your higher readings were.

 

[HotRodPC]

Now I never heard of Lock up in 2nd. I wouldn't want that at all. I'd really only want lock in 3rd is when I manually selected D. Meaning I rather go to 1 through 4, then Lock up, or 1 through 3 then Lock Up. There's several ways to do it and accomplish whatever you want using the pressure switches in the VB, but it sounds like you have worthy plan in mind.