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Old 08-25-2006, 04:33 PM
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TH 400 low gear conversion

I saw this in Jeg's, it looks pretty interesting. Talked to a mechanic friend and he says this gear set was used in big RVs from the factory. In my (so far) limited research, all I can find is straight cut vs. helical cut gears as the difference between heavy duty and regular planetary gear sets, no mention of gear ratios.

http://www.tciauto.com/components/lo..._planetary.htm

I would just as soon use OEM heavy duty RV rebuild parts in my trans, rather than spend twice as much to have brand name "hot rod" parts, so I'm trying to find info along this line......It's going slow doing the detective work by phone and foot, so I thought I'd throw out a line here as well. I would have thought this would be popular among hot rodders if it is for real, but hardly anyone I've talked to is familiar with it.

My car is a '73 Vette with a 350+/- hp SBC and a 3.08 rear end. The idea is to get a little better hole shot while keeping a good cruise RPM. My trans needs a rebuild anyway so I thought this might be a little more economical way to go than my dream of a BTO 200-4R conversion as it's the same ratios but without the overdrive, plus the savings of not changing rear end gears to go with an overdrive.

Any thoughts, ideas, whatever?

John

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Old 08-25-2006, 05:42 PM
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John,
Your mechanic friend is incorrect, they never installed a 2.75 ratio from GM.
They did install straight cut planets.

Why would you have to change ratios to go to an OD? The 200-4R has a 2.74 low ratio, and a .67 OD, it would work well with most rear ratios.
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Old 08-25-2006, 08:08 PM
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Jake,
The reason I would change the rear end ratios for a 200-4R is to keep a realistic top end cruise RPM. Much as I would like to, a 150 mph top end is not realistic, and 1700 RPM at 75 mph {or whatever it figures out to be--I did the calc once, but can't remember at the moment} with a 3.08 is not realistic for my set up either.
My diff needs a rebuild as well, so if I were to do a 200-4R I would spend the money for a 3.55 or 3.73 gear set (and really improved hole shots!)....but then I'm lookin' at more than twice the $$$ than for a good TH400 & diff rebuild w/out gear set.
Just trying to find and weigh different options.....
Appreciate your reply, it's one more piece of info. I now have one "Yes, there is an OEM 2.75" and one "No, there isn't" and about six "Huh? I dunno."

John
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Old 08-25-2006, 08:38 PM
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John,
I assure you there was never a 2.75 TH400 gearset. You can veryify my info from an ATSG manual, GM Service manual, HP Books by Ron Sessions, etc. it just did not happen.
I'm sure some of the other builders can verify this also.

I just sold a built up 200-4R. You're right, they aren't cheap. I started figuring the total parts it takes to build one, it adds up.
Billet forward drum- $420
Rebuild parts- ~$200
Valve body kit- $75
Servo- $100
oversize boost valves $30
etc..
If you use the desirable valve bodies or cores, add $250-600 to the cost.

I ran the 200-4R with 3.73's and it worked well, I would probably use a 4.10 if using it again.
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Old 08-25-2006, 09:01 PM
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Ah, well, it was a nice little dream while it lasted! That TCI unit is neato, but for that much money, I could do other stuff that would probably make more sense. Or apply it to my 200-4R.

Thanks,
John
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Old 08-25-2006, 10:21 PM
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John,
I assure you there was never a 2.75 TH400 gearset. You can veryify my info from an ATSG manual, GM Service manual, HP Books by Ron Sessions, etc. it just did not happen.
I'm sure some of the other builders can verify this also.

I just sold a built up 200-4R. You're right, they aren't cheap. I started figuring the total parts it takes to build one, it adds up.
Billet forward drum- $420
Rebuild parts- ~$200
Valve body kit- $75
Servo- $100
oversize boost valves $30
etc..
If you use the desirable valve bodies or cores, add $250-600 to the cost.

I ran the 200-4R with 3.73's and it worked well, I would probably use a 4.10 if using it again.
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Old 09-04-2006, 12:09 PM
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i've read one of the GM performance Transmission Gear ratio books, and didnt the TH400 have a 3.08:1 first gear?
i checked with GM about the TH350 we built for my 4x4 and i learned it has a 3.07:1 first, and we did a Lockup Omition or something like that, so we could use a standard Non-Lockup Torque Converter
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Old 09-04-2006, 01:42 PM
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Gentlemen,
The TH350 came with a 2.52 low gear ratio from GM stock, period.
The Th400 came with a 2.48 low gear ratio from GM stock, period.

There were never any other gear ratios offerred from GM. I rebuild these units and have multiple references to this including GM service manuals, ATSG tech manuals, as well as other reliable information, not including the hundreds of each unit I have personally rebuilt. There are several other very knowledgable transmission guys who frequent here who can confirm this.

There are aftermarket gearsets available for both the Th350 and TH400 to achieve a 2.75 ratio low gear. This requires a different pinion gear tooth count as well as a different sun gear tooth count.

The 700-R4 came with a 3.06 ratio first gear, some call it a 3.07 ratio. Again not the Th350 or 400.
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Old 09-04-2006, 02:03 PM
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the benefit of the low gear 2.75 is to get same 1st gear ratio from a 3.42 differential that you would from 3.73 with a stock 1st gear. and no, no 400 turbo came factory 2.75 first gear.
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Old 09-04-2006, 03:13 PM
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John Kilgore is the man who usually is the first to design the different ratio planetaries. He makes a 2.75 and a 3.00 (1st gear) last time I spoke with him. You can get them for 600-650 straight from him or a reasonable custom tranny shop.
Crosley can probably get them for you.
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Old 09-04-2006, 03:17 PM
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Crosley works at Hughes so I'm sure they make or market some different gearsets.
Kilgore is def an innovator in the TH400 world.

I like Chris at CKPerofrmance for his custom parts, although I haven't used any of the low gearsets for a 400 yet.

My understanding is that you use parts from the lower geared 700 or 4L60 planets to make the low (2.75) gearset for a TH350. You could also reverse this and make a shallower gearset for the 700, Chris makes a 2.66 first gear ratio for the 700 IIRC.
That would be a pretty good gearset in the 700.
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Old 09-04-2006, 03:29 PM
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Quote:
Originally Posted by jakeshoe
Gentlemen,
The TH350 came with a 2.52 low gear ratio from GM stock, period.
The Th400 came with a 2.48 low gear ratio from GM stock, period.

There were never any other gear ratios offerred from GM. I rebuild these units and have multiple references to this including GM service manuals, ATSG tech manuals, as well as other reliable information, not including the hundreds of each unit I have personally rebuilt. There are several other very knowledgable transmission guys who frequent here who can confirm this.

There are aftermarket gearsets available for both the Th350 and TH400 to achieve a 2.75 ratio low gear. This requires a different pinion gear tooth count as well as a different sun gear tooth count.

The 700-R4 came with a 3.06 ratio first gear, some call it a 3.07 ratio. Again not the Th350 or 400.
everything jakeshoe said is it!
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Old 09-04-2006, 03:33 PM
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the planetary set for the 400 turbo lowers 2nd from 1.42 to 1.75 where the 350 turbo doesnt lower 2nd gear any.thats what i see at tci,
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Old 09-04-2006, 03:36 PM
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Quote:
Originally Posted by jakeshoe
Crosley works at Hughes so I'm sure they make or market some different gearsets.
Kilgore is def an innovator in the TH400 world.

I like Chris at CKPerofrmance for his custom parts, although I haven't used any of the low gearsets for a 400 yet.

My understanding is that you use parts from the lower geared 700 or 4L60 planets to make the low (2.75) gearset for a TH350. You could also reverse this and make a shallower gearset for the 700, Chris makes a 2.66 first gear ratio for the 700 IIRC.
That would be a pretty good gearset in the 700.
does it matter what spline the 700r4 is 27 or 30? and wouldnt it be a 3.06 ratio? or is it some of the sun gears you change?
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Old 09-04-2006, 03:39 PM
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id like to find out more on how to do that, ive rebuilt 350 turbo's before
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