swap a TH700R4 for a TH350 and Gear Vendors

[Ralph Curry]

I have a 1984 Chevy Van 20 with a 6.2 diesel and a bad TH700. I've always hated the TH700 since it has the huge ratio change between gears. When towing my boat, I can't make the 1st to 2nd shift on the hill to my house if I hit the light at the bottom red. But it'll easily pull it in 2nd (and in 3rd if I speed) at the speed limit if I hit it green.

Anyway, since the transmission went out, I've been thinking about trying to swap in a HT350 with a Gear Vendors overdrive. This would essentially give me the gear ratios of a 4L80 with overdrive (gear splitting) in 1st, 2nd and 3rd. Plus, if the numbers are to be believed, a 30 HP gain due to reduced parasitic drag. 30 HP when I only have 135 sounds too good to be true.

Has anyone every tried this, or have any thoughs on the idea?

[DmaxMaverick]

Welcome to the forums!

Most folks are trying to go the other way, getting rid of the 350/400's and install 700/4L80's.

Even with the GV, your economy will decrease, and your parasitic loss will increase. The 350/400's don't have a lockup converter, and generate more heat than a locked up 700.

Your best bet would be to have your 700 rebuilt properly. Search the forums. Several folks have done this.

In any case, your 6.2 will kill a TH350 in short order. TH400's hold up well, though.

[Ralph Curry]

Thanks for the reply. Maybe the best thing for me to do is get a TH700 and a GV.

but back to the original question...

I'll go with the assumption that a TH350 can be built at least as strong as a built TH700 (and much stronger than the 1984 TH700 that still managed to last 134K miles)

Since I'm focusing on towing, the proposed setup would give me a close ratio 6 speed instead of a wide ratio 3 speed (stoch TH700 can't tow in OD). A built TH700 might give me a wide ratio 4 speed.

My goal is to have a setup where all shifts keep the RPM between peak torque and peak HP, 1800 - 2800 RPM (35%). So a TH700 with GV barely qualifies with a 32% change between 1st-high and 2nd. The TH350 with GV keeps all gear changes under 23% and I don't tow in lockup anyway. Is there a TH700 setup where I could tow locked up?

[DmaxMaverick]

Towing in OD is not the problem. The trannies fall apart when they do a lot of "gear hunting". If you can get it into OD, and keep it there, it will be fine. The gear ratios of a TH350 are very misleading. The "open" torque converter skews the actual result. The key I've found is to get the TC locked up asap, and keep it that way as much as possible. Torque converters have a so called "torque multiplier", but they can never achieve anything above 1:1, and rarely get close to that.

Another factor to consider is your final drive ratio. What is your diff ratio and tire size? How much do you plan on towing, how often, and where?

How are you reaching your ratio spread numbers? The 700 is, IMO, ideal for towing. It is essentially a 3 speed, with an OD. 1st gear is lower than a 350, which requires a taller spread to get to direct 3rd. 3rd gear in both are the same, 1:1. The key I've found is to control the TC lockup. Mine is manually controlled with a switch on the shift lever. BTW, the 700 in my Blazer is the original tranny. I beefed it up in its young, undamaged life. It cost me about $400 back in '87 to do the mods. Keep the heat down and eliminate the gear hunting and it will last.

A built 700 is far superior to a 350, even if you mod the 350. A properly built 700 is comparible to the durability of a TH400. I don't think there is really any solution with a 350 and a 6.2. It will die young. If you insist on a non OD, the TH400 is the only way to go. Heat management will still be a problem.

[Ralph Curry]

Thanks again for your reply and strong endorsement of the TH700.

I was told that the problem with towing in OD on a TH700 is that it does not circulate enough fluid to cool properly. This is why so many TH700 vehicles cought on fire in the 80's. (This happened to 2 people I know, and I saw no shortage of tranny fires at the junk yard) The trans fluid reached its flash point and poof. If this is true, then you'd need a pump that flowed more than the original TH700 with 7 vane pump at low RPM.

My ratio calculations are just straight ratios and discount and torque multiplication the TC provides. But still, the closer I can get the ratios, the more torque I'll have at the crank after the shift, so the more the torque multiplication will benefit me.

Based on my experience towing my 4500 lb boat up a 18% grade, the TH700 is not optimized for towing, but can be a workable all around transmision. Nice 1st gear and rather tall OD. I just have to be patient when I don't have the right gear.

Given the cost equation and that fact that a new tranny will cost more than I could sell my old van for, I'm sure I'll just put in a built TH700 and wait fot the next thing to break.

I'd still like to see someone do next year's pull off with a GV equipped 4L80 and see if those half gears can beat an Allison 5 speed.

[Inspector]

With the GV you won't be able to use it in the firt two gears. The GV relies on driveshaft speed to operate the oil pump within itself to shift its gears. If you don't have the shaft speed it will not shift. The GV is oil pressure operated.
Denny

[Ralph Curry]

That's a good point, but doesn't sound entirely true. As I understand it, shifting is performed by an electronic valve that controls the pressure generated by the pump on the output shaft. They say it should work at speeds above 20 MPH.

So, on a 6.2, revving to 3200 RPM with a 3.06:1 first gear and 3.73 rear end, you might have trouble hitting 20 MPH, and thus it might not work in 1st. It should be fully usable in 2nd and above though. The math is fuzzy since they don't give an exact output shaft speed.

Sounds like another argument to use a TH350 (or TH400) since their taller 1st gear will easily get you over 20 MPH in 1st.

[Craig M]

I have a Gear Vendors in my dually. It is the old style unit and does not have the newer electronics. I can however manually use the overdrive in all three gears (TH400 main transmission). The "automatic" engagement of the overdrive only works in 3rd gear. 2nd over actually is not very different from 3rd direct. On a steep hill 1st over comes in handy when towing heavy.

[mhagie]

I haven't heard anyone say why not to use the 4l480.
In your first post you say a 350 with a GV will give you the ratios ov a 4l480.
then why not use a 4l480? the electronic issues can be overcome and you can build a strong 480 for the price of a 350 and a GV which is $2800 alone.
Just curious.
Merle

[ctmpwrdcamry]

Those with the GV think its worth the money? How did it improve your milage? I am looking at getting the 6.2lt and it looks like i could do the GV or the banks. With fuel being close to $3 a gallon, mpg is important.

[Craig M]

Hey ctmpwrdcamry; Question is the Banks or Gear Vendors good purchase for fuel economy? If all you are using your 6.2 equiped vehicle is straight driving (ie not load pulling). Then I would say a standard overdrive transmission (like 700R4) is your best bet. List price for the Banks and Gear Vendors are in the $2400.00 range. Used units can be had for about half that. You can buy a lot of fuel for that price. For straight driving you will not likely get enought benefit to justify the cost. If you do a lot of towing then you might consider one of the mentioned improvements.