Which is better? TH400 or a C6

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Re: Which is better? TH400 or a C6

Post by SupStk » Thu Nov 30, 2017 5:56 am

pdq67 wrote:
Thu Nov 30, 2017 4:54 am
I read somewhere years ago that the TH400 was good for like 800T(?) and 500,000(?) miles, but after GM's, "Bean-Counters", got done with it, it dropped some T and was good for like 200,000(?) miles!

I think it was after they started using plastic parts in it, but don't know for sure?? I bet an older tranny rebuilder guy could tell you for sure about this....

And I will say that MOPAR's 727 auto trans is a good one! Sucker shift's like, "the kick of a shotgun"!

pdq67
Don't know if they took a hit or not on torque capacity. The only plastic parts I recall are some thrust washers, spring locater, and the alignment sleeve for the one way clutch.

Colorado, didn't know the TH475 straight cut gear set was weaker. In the earlier class A motor homes with the helical gear sets, failures weren't uncommon. I'm guessing that is why GM made the change. My mention of the good stuff in the TH475 was slanted at the high element sprags over the normal roller clutches.
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Re: Which is better? TH400 or a C6

Post by Coloradoracer » Sat Dec 02, 2017 1:31 pm

SupStk wrote:
Thu Nov 30, 2017 5:56 am
Colorado, didn't know the TH475 straight cut gear set was weaker. In the earlier class A motor homes with the helical gear sets, failures weren't uncommon. I'm guessing that is why GM made the change. My mention of the good stuff in the TH475 was slanted at the high element sprags over the normal roller clutches.

No worries. Many thought the straight cut gears were the better gearsets but that has proven to be incorrect. The helical were and still are the strongest factory gears. Most of the failures of the gear sets in motor homes and heavy duty applications were from shock loading and not the gearset itself. But upgrades were done. I wasn't aware that sprags were used in any application after the late 60's, as GM went to the roller clutch in the earlier 70's. There were some sprag style clutches out there, but they only had something like 16 elements or so and were also not the best...The earlier 32 element sprags are the way to go if factory parts are used, but the outer race can fail. Newer aftermarket stuff has much heavier duty races and some also add additional sprag elements to improve holding capacity...

I'm at the point these days I either do a stock rebuild or race build, and leave it at that...Been a bit since I did my last 400 though, still my fav auto trans however.....
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Re: Which is better? TH400 or a C6

Post by SupStk » Sun Dec 03, 2017 12:12 am

Coloradoracer wrote:
Sat Dec 02, 2017 1:31 pm
SupStk wrote:
Thu Nov 30, 2017 5:56 am
Colorado, didn't know the TH475 straight cut gear set was weaker. In the earlier class A motor homes with the helical gear sets, failures weren't uncommon. I'm guessing that is why GM made the change. My mention of the good stuff in the TH475 was slanted at the high element sprags over the normal roller clutches.

No worries. Many thought the straight cut gears were the better gearsets but that has proven to be incorrect. The helical were and still are the strongest factory gears. Most of the failures of the gear sets in motor homes and heavy duty applications were from shock loading and not the gearset itself. But upgrades were done. I wasn't aware that sprags were used in any application after the late 60's, as GM went to the roller clutch in the earlier 70's. There were some sprag style clutches out there, but they only had something like 16 elements or so and were also not the best...The earlier 32 element sprags are the way to go if factory parts are used, but the outer race can fail. Newer aftermarket stuff has much heavier duty races and some also add additional sprag elements to improve holding capacity...

I'm at the point these days I either do a stock rebuild or race build, and leave it at that...Been a bit since I did my last 400 though, still my fav auto trans however.....
I worked as a dealer tech for 11 years starting in 1976. Did most of the trannys there and all the motorhomes and trucks. The failures I encountered weren't breakage but the planet carriers was toasted. Usually found pieces of thrust washers and pinion needles laying in the pan.
Think the sprag was 34 element but don't quote me on that.
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Re: Which is better? TH400 or a C6

Post by Coloradoracer » Wed Dec 06, 2017 8:07 pm

SupStk wrote:
Sun Dec 03, 2017 12:12 am
I worked as a dealer tech for 11 years starting in 1976. Did most of the trannys there and all the motorhomes and trucks. The failures I encountered weren't breakage but the planet carriers was toasted. Usually found pieces of thrust washers and pinion needles laying in the pan.
Think the sprag was 34 element but don't quote me on that.
That sounds more like a lube issue to the planets. What was done to correct that?
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Re: Which is better? TH400 or a C6

Post by SupStk » Thu Dec 07, 2017 12:15 am

Coloradoracer wrote:
Wed Dec 06, 2017 8:07 pm
SupStk wrote:
Sun Dec 03, 2017 12:12 am
I worked as a dealer tech for 11 years starting in 1976. Did most of the trannys there and all the motorhomes and trucks. The failures I encountered weren't breakage but the planet carriers was toasted. Usually found pieces of thrust washers and pinion needles laying in the pan.
Think the sprag was 34 element but don't quote me on that.
That sounds more like a lube issue to the planets. What was done to correct that?
I agree about probable lube issues. For correction nothing on my end, nothing other than rebuilding transmissions and replacing the needed parts under GM warranty. Now on GM's end, I don't know if mods were done to the lube circuits when they changed over to the straight cut gears. Had several other occasions to rebuild motorhome trannys but none had wiped gear sets after the switch.
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Re: Which is better? TH400 or a C6

Post by Pro Built Automatics » Fri Mar 02, 2018 4:09 am

The main reason the C6 eats more horsepower is that in the low/reverse area of the C6 they use clutches instead of a band as in TH400's, and that starts to eat horsepower as rpm climbs. Once the band is off, there is more drag even as rpm starts to climb, where clutches eat more horsepower because of drag. Turbulator steels help here and running less clutches helps eliminate drag also. All TH400's from 1965 to the early 1970's were 8 bolt pumps, and went to 6 bolt from there. If you are going to use a BOP case, the strongest are the Cadillac with the extra ribbing around the front pump. Starting in the early 1970's GM changed the intermediate sprag (smooth design) which can us the 34 element sprag on the direct drum to a roller clutch similar to the TH350 and not recommend for anything with any kind of horsepower. The strongest Chevy cases were the mid to late 1980's. The reason for "straight cut gears" were pinion thrust washer loading, which is high in heavy vehicles. Strength is not needed here as it is the load they are concerned with. Yes, the "helical cut gears" are stronger. In the 1970's the intermediate shaft has a restrictor plug in rear of them. Remove it for better lubrication to the rear area of the transmission. 175 psi. - 190 psi. maximum is all that is needed here. Use the GM Peek sealing rings from the 4L80E units for the pump and direct drum, much better than cast iron rings and easy to install. There is nothing wrong with plastic thrust washers, as they wear very well in the vast majority of cases. Double ring the pick up tube that goes into the case, to prevent picking up air. If all of the sudden the 2-3 WOT shift gets a little soft, chances are you are picking up air. I hope this helps a little.

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Re: Which is better? TH400 or a C6

Post by A_VAS » Fri Mar 02, 2018 1:23 pm

Pro Built Automatics wrote:
Fri Mar 02, 2018 4:09 am
The main reason the C6 eats more horsepower is that in the low/reverse area of the C6 they use clutches instead of a band as in TH400's, and that starts to eat horsepower as rpm climbs. Once the band is off, there is more drag even as rpm starts to climb, where clutches eat more horsepower because of drag. Turbulator steels help here and running less clutches helps eliminate drag also. All TH400's from 1965 to the early 1970's were 8 bolt pumps, and went to 6 bolt from there. If you are going to use a BOP case, the strongest are the Cadillac with the extra ribbing around the front pump. Starting in the early 1970's GM changed the intermediate sprag (smooth design) which can us the 34 element sprag on the direct drum to a roller clutch similar to the TH350 and not recommend for anything with any kind of horsepower. The strongest Chevy cases were the mid to late 1980's. The reason for "straight cut gears" were pinion thrust washer loading, which is high in heavy vehicles. Strength is not needed here as it is the load they are concerned with. Yes, the "helical cut gears" are stronger. In the 1970's the intermediate shaft has a restrictor plug in rear of them. Remove it for better lubrication to the rear area of the transmission. 175 psi. - 190 psi. maximum is all that is needed here. Use the GM Peek sealing rings from the 4L80E units for the pump and direct drum, much better than cast iron rings and easy to install. There is nothing wrong with plastic thrust washers, as they wear very well in the vast majority of cases. Double ring the pick up tube that goes into the case, to prevent picking up air. If all of the sudden the 2-3 WOT shift gets a little soft, chances are you are picking up air. I hope this helps a little.

good notes. I have 2 of the Cadillac '68 units

how does one identify the mid to late 80's chevy cases that are best ?
I have this one sitting around and not sure what year it is
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