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Transmission switch

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bvansic

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Howdy - I'm switching my 70 Custom20 Camper Special from a 4 speed manual to a 350 automatic transmission. First gear in the old manual was pretty much unusable on the road, 2-4 got her up and running but she was wound up pretty high at 55 on the freeway. Anyway, I'm wondering if I should be switching the rear-end gears at the same time as the switch to the 350. I'm looking for decent performance on the freeway and decent mileage, not to haul large loads. Any suggestions on a gear ratio to match the transmission and get the performance and mileage I desire?

Also, out of curiosity, does the 3/4 ton have heavy duty gears that are larger/thicker than your typical 1/2 ton?

Thanks
 
[updated:LAST EDITED ON Aug-20-04 AT 00:04 AM (CST)]HELLO,

GREAT QUESTIONS.
FIRST, YES THE REAREND GEARS ARE QUITE A BIT DIFFERINT. SAY A TYPICAL 1/2 TON HAS AN 8.5 DIAMETER RING GEAR AND THE TYPICAL 3/4 TON IS SAY, 8.875 OR EVEN 9". THE BEARINGS ARE MUCH HEAVIER DUTY THAN THE 1/2 TON.
AS FAR AS GEAR RATIOS GO, THE TRANSMISSION CHOICES YOU STATED HAVE THE SAME FINAL DRIVE RATIO AS YOUR 4 SPD, 1.00:1. IN FACT ALL TRANSMISSIONS WITH OUT AND OVERDRIVE HAVE THIS SAME FINAL DRIVE RATIO. MEANING IT WILL MAKE NO DIFFERENCE AS TO THE RPMS THE ENGINE WILL BE TURNING AT WHATEVER SPEED YOU ARE DRIVING WHEN YOU ARE IN EITHER DRIVE OR 4TH.
THE DIFFERENCE IS GOING TO BE IN THE FIRST GEAR RATIOS. JUST FOR EXAMPLE. TAKE THE TH350 IT HAS A FIRST GEAR OF 2.50:1 OR THERE ABOUTS.
YOUR 4 SPD HAS A 5 OR 6.00:1 AND THE SECOND GEAR IS CLOSER TO THE TH350'S FIRST. THATS WHY STARTING IN SECOND WORKS BEST.
YOUR REAREND GEARS IS WHERE YOU'LL NOTICE THE MOST IF YOU CHANGE THE RATIO. SAY YOUR TRUCK HAS 4.11:1'S. SHE'S REALLY WINDING IN 4TH AT 65MPH. CHANGE TO 3.73:1'S AND THAT WILL HELP, GO TO 3.08:.1 AND THE FREEWAY WILL LOOK GOOD, BUT YOU'LL LOSE YOUR BOTTOM END ESPECIALLY IF YOU ARE TOWING OR HAULING SOMETHING.
THE QUESTION IS WHAT TYPE OF DRIVING YOU DO WITH THIS TRUCK. THAT WILL DETERMINE WHAT GEARS TO RUN.

HOPE THAT HELPS

BRANT
 
I've got a Th400 transmission in mine with 4:11 gears in the rear end and the engine(350) really revs at 70. But my truck doesn't get driven on the highway a lot so it really doesn't matter. These gears are great for towing but not so great for highway use.
 
[updated:LAST EDITED ON Aug-22-04 AT 09:07 PM (CST)]HELLO ROBERT,

WITH 400 HORSES AND 4.11.1'S, I'LL BET THAT THING IS A KICK IN THE PANTS.
I'D LOVE TO RIDE IN IT!!!!!
IMPORTS BEWARE OF THE C20!!!

BRANT
 
well, I busted open the rear end and found a 4:10 (its a Dana 60), so your assessment was pretty much on the mark. I'll probably use this truck for cruising mostly, perhaps a little towing, but nothing heavy and not for long distances. My local tranny guy thinks I should look at two options:

1 - stick with the TH350 and switch to 3:54 in the rearend.

2 - switch to a 700R4 tranny and keep the 4:10 rearend.

Given the cost to switch out the rear-end, and the cost of a 700 tranny, its pretty much a wash.

Given these options I like the 700 with overdrive, my concern then is with the linkage and the steering column. I like to think I'm pretty clever, but I've never looked at the linkage that seriously and would hate to get in a situation where I can't get the two to marry-up. Ever tried this approach? Any thoughts on these option?

Thanks for the advice, you filled in alot of the gaps.
 
I have replaced my th350 with a 700r4. Originally I had 3.73s but swapped them for 4.56s. (I am running 33 inch tires)

The 700r4 has an advange of having a lower 1(and2?) gear ratio for quicker launches, and the overdrive is a must have on the freeway. But there are many drawbacks to them as well.

They REQUIRE an almost overkill cooling system and they need an electick lock up controll to be efficient (the simple brake switch lockup controll will work but you will be annoyed with the lugging caused by low speed lock-ups)

I strongly recomed the speed controled lock-up from B&M and set it to lock at about 40mph in BOTH 3rd and 4th.(you will spend a lot of time in 3rd gear when not on the freway and the lcok-up helps to keep the heat down.)
 
Sooo many things to think about. I'd really like to have the overdrive, but it sounds like the 700r4 will nickel & dime me into the poorhouse. 350 here I come! I guess I'll have to gear it down a bit to the 3.54, that ought to get me some decent highway use, and not kill me too much in the lower gears. Thanks for the advice, I'll post a note when I'm finished and let you know whether my decision was the right one.
 
HELLO,

THE 700R4 IS A GREAT TRANS. I'VE DONE THE SWAP A FEW TIMES AND WISH I WOULD HAVE DONE IT SOONER. THE LINKAGE IS THE TRICKY PART, ONE THAT IS EASILY REMEDIED IF YOU DON'T MIND THE IDEA OF A FLOOR SHIFTER.
IF YOU USE THE 700R4 YOU WILL NEED, AS WAS SAID EARLIER, TO USE SOME TYPE OF CONVERTER LOCK UP SWITCH. I'VE USED THE SPEED CONTROL TYPE AS WELL AS A TOGGLE SWITCH. INCLUDE A LIGHT OF SOME SORT TO ALERT YOU WHEN LOCK UP IS ENGAGED. ONLY USE IT AT SPEEDS ABOVE 40-45 OR SO.
ABOUT THE EXTRA HEAT, YOU WILL EXPERIENCE THE SAME IF NOT WORSE WITH THE TH350 AND HIGHER GEAR IN THE HEAVIER TRUCK, AND WITH A LOAD EVEN WORSE YET. IN ANY CASE, INSTALL A TRANS COOLER WITH AN EXTERNAL FAN. MAKE SURE YOU HAVE ENOUGH OF A GAP BETWEEN THE RADIATOR AND THE NEW TRANS COOLER, AN INCH + OR SO. ALSO, KEEP THE ORIGINAL FAN SHROUD BECAUSE THEY WERE DESIGNED WELL AND HELP THE FAN PULL AIR ACROSS THE RADIATOR BETTER THAN NOT WITHOUT IT.

IF I HAD THE CHOICE I WOULD DO THE 700R4.

GOOD THINGS.

BRANT
 
[updated:LAST EDITED ON Aug-27-04 AT 11:29 AM (CST)]I also had an issue with the shift linkage; Bow Tie Overdrives sells a nifty little kit that will hook right up to the stock column shift.

I personally never had a problem with heat in my TH350, and I did a lot of research on the 700r4 after my first one (custom built by Bow Tie Overdrives) blew.

Everything I read said the 700r4 produced substantially more heat then the TH350. I have since swapped my radiator for a "Be Cool" aluminum oversized one with internal trans cooler as well as an external cooler with fan. The tranny would still heat up after long days of low speed hill climbing; to remedy this I bought the biggest dual electric fan I could find, and no more issues.

I guess it all depends on what type of vehicle it is going in to, a street truck with smaller tires that spends most of its time just cruising, or a 4x with larger tires that crawls along at low speeds.

After spending well over 3k on the whole thing (First custom built tranny, rebuild, radiator, cooler, fan, lines, lock up switch) I would NOT do it again, I would go with a 4L80E (a beefier, newer version of the 700r4, kind of like the th400 is to the th350). Even though the 4L80E is computer controlled and the computer probably cost a few bucks, it would still have been less than a re-build.

Another thing about the 700r4 is the tools necessary for set-up. A pressure gauge is a must have to set the proper TV cable pressure. After my first tranny fried, I spoke with several trans shops and they said while it is possible to set up the tranny without the pressure gauge, the likelihood that is set slightly off (which is impossible to tell without the gauge) is high and will cause excess heat.

-there is my novel, but with all that I went through on this I thought I might share and hopefully eliminate someone else’s headaches. The 700r4 is a great upgrade but my whole point is that to do something right there are a lot of variables and extras to consider, so why not spend a few bucks extra for something like the 4L80E and have a really cool setup, or spend the extra to have it professionally set up.

Another quick note: there are 2 different versions of the 700R4, I don't know off the top of my head how to tell the difference, but I think the pre 1988? ones should be avoided (I believe they have a weak housing). A little research on the net should provide you with the specifics on this.
 
Good advice - I'd heard the same about the two versions of the 700R4, my tranny guy said look 88 - 92 and avoid anything else. I'll keep that Bow Tie Overdrives reference socked-away too, that may also come in handy some day.

Well, since my little "rebuilding" project has turned into a frame-off restore I don't think the truck will see too much off-road action, she'll pretty much be a cruiser or perhaps pull a small trailer on occasion. So I'm hoping cooling of the th350 will not be a problem (I'm not sure how effective Chevy's stock hd radiator is), but I suspect an external cooler is in my future, we'll see how that hand plays out.

This is my first venture into vehicle restoration so, when I started redoing this truck, my objective was to learn as much as I could and the chevy truck seemed like a great vehicle to do it. Trust me, I've learned much, but along the way realized that almost every answer opens up many new questions (perhaps thats the fun of it). Fortunately for me there are a quite a few out there who have been down this path that will share their knowledge. Thanks for the great lesson, I'll put it to good use.

BV
 
HELLO,

I AGREE WHOLEHEARTEDLY WITH SEAN ABOUT THE 4L80E. GREAT TRANS, BUT DEFINITLY EXPENSIVE WITH THE COMPUTER CONTROLS. ALSO, HE IS RIGHT ABOUT THE TYPE OF DRIVING AND RIG EACH ONE GOES IN.
HEAT IS THE KILLER AND ANY ATTEMPT SHOULD BE MADE IN ORDER TO KEEP IT TO A MINIMUM.
THE STOCK COOLER IN YOUR RADIATOR IS O.K. IF THE RADIATOR IS A 4 CORE. AN EXTERNAL COOLER IS BEST WITH ELECTRIC FAN.
TRUST ME, YOU CAN NEVER GO OVERKILL ON KEEPING THINGS COOL.
IN ANY CASE HAVE FUN AND REMEMBER THERE ARE NO STUPID QUESTIONS....WELL MAYBE JUST A FEW.

GOOD THINGS

BRANT
 
Yeah, it's fun to drive and it sounds cool with a glasspack in place of the stock muffler. The TH400 is from a '67 Caprice and the rear end is stock, it's got a lot of torque. It had a TH350 but it went out so my Granddad got the driveshaft cut and put in the 400. It used to be a cab chassis farm truck with a flatbed but it was converted to a long Fleetside in the mid 70's right before my Granddad bought it.
 
Just thought I'd camp in on this thread. Very interesting stuff on the 700R4, I visited my local tranny shop to get a quote to put one into my '68 Suburban. It was formerly a six cylinder and is geared accordingly. The guy before me swapped the engine and trans for a 350/ 400 combo but I am looking for an overdrive (tons of highway driving). Here's my question, can you overcool a transmission? My concern is not the summer, it's a Canadian winter. We saw temperatures at the -30C mark last year. I dont know that I will be driving the truck over the winter (the old Acclaim takes the brunt of that abuse) but I was just wondering.

Thanks,

Paul
 
According to the tech I spoke to at Bow-Tie Overdrives, it should maintain a temp of 130 or so (measured from the pan). I suppose it can run poorly if it is too cool, but i don't know the specifics.
 
i have nothing to back this up but in my opinion if the tranny got too cold the fluid would thicken up and then you may have a problem with it shifting on time, or shifting at all. but you might want to check with the pros try calling this tranny shop here in kansas city mo. it is called a & reds and their number is 1-816-483-7337
 
Hello guys,

tech1 you're correct on the thickening of the fluid. However the trans is that way on the initial start up and quickly warms the fluid from the friction and from flowing through the cooler in the radiator as the engine warms up. The best idea in winter is to allow the engine to warm up before moving. I would run the cooler lines through the radiator then out to an external cooler. Once the engine is fully warmed up, the trans will easily stay at around 130-150 degrees unless pulling a heavy load up a steep grade. That is where the external cooler helps out most.

By far the worst is heat, as in overheating.

GOOD THINGS,

BRANT
 
Good to know Brant thank you for the info on that subject!
 
OK this is a bit late, but I just ran across an old email from the tech line at the company who built my 700r4:

We have six years experience monitoring the pan temperature on every General Motors car or truck that came through our shop. From this monitoring, we have determined factory cars and trucks consistently run 140 to 160 degrees F or 30 to 50 degrees lower then the indicated engine temperature gauge. Our bulk transmission supplier indicates that fluid should be ran at 165 degree F or lower to achieve maximum fluid life which seems to verify our observations.

Word of caution, our information is based on a temperature probe located in the transmissions pan. This location only represents the fluid temperature at that location which is about the coolest location in the system. Do not deduce from my statements above that the fluid in these applications does not get substantially hotter inside the converter or the hydraulic pump. It does. We haven't figured a way to monitor the fluid temperature inside a spinning converter and really never found a reason to do so. We accept the fact that the fluid can get very hot inside the converter under heavy load situations.

Hope this information is useful.

Steve Holmes

Bow Tie Overdrives
 
Hello Steve,

Thanks for the heads up on the fluid temp and all.
I suspected the manufactures and reman places knew more than I did, but now I know as well.
Thanks again Steve.
 
i'd keep the 4 sp. unless u'd rather an automatic...
and change the rearend gears....
with granny low in the 4 sp you could go to 308's in the rear and make first gear useable + lower the rpm's on the highway 1500 rpm's
:P
 
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