Please help me understand 727's a little better....
#1410057
03/28/13 12:43 AM
03/28/13 12:43 AM
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JoesMopar
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I have several questions about 727's, I feel like an idiot for asking because it's been addressed so many times, but I don't understand a lot of the answers I've read.
Here goes...
1. What exactly does the "front drum" do?
2. The sprag from what I understand is a one way roller clutch. How does it "break" or come loose just because the tires on the car hook right away? Why would it grab then but not when the tires are spinning? Wouldn't it be prone to failure when the car "bangs" into second gear with a hard 1-2 shift?
3. If the sprag, and LBA if available, slow the rotating mass inside the transmission so it doesn't get out of control, wouldn't that slow the output speed to the driveshaft? In other words, what's turning inside the transmission is turning the driveshaft, if that slows down because of the sprag or LBA, would it not also slow the speed the tires are turning? Or is it independent from the output shaft?
4. If 727's are so tough, it seems like they sound pretty weak to me during the part of taking off where it matters most....the launch. So where is a 727 strong where other transmission are weak and what does that do for the driver? What makes it stronger?
5. Many times I find myself spinning my tires when I leave a light enthusiastically. I know I'm not "doing it right" according to the proper way to do a burnout. I don't consider this a burnout, but does it still apply to the proper ways to not blow a 727? So, let's say I have it in drive (no LBA), take off and spin the tires and they hook before second gear? Did I potentially screw up my tranny?
I just feel like I have to baby this thing like it's an eggshell or something. It's a auto/manual valve body TCI Streetfighter with a bolt in sprag supposedly. But, I have other cars that have the factory press in sprag (if that's what you call it) so I'm also interested in how the questions relate to them as well.
Thanks for any help, I know it's a bunch of questions and they've probably been answered but I just don't understand all of the "science" behind a 727 so a lot of what I've read just leaves me clueless.
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Re: Please help me understand 727's a little better....
[Re: JoesMopar]
#1410058
03/28/13 03:28 PM
03/28/13 03:28 PM
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Rio Linda, CA
John_Kunkel
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1) The front drum (or clutch retainer) serves two purposes, it contains the clutch pack for 3rd/Reverse and the outer surface is where the kickdown band clamps the drum to stop it from turning in 2nd gear.
2) The sprag needs a load applied to it in order for the rollers to seat properly and serve as a clutch, when the tires are spinning in 1st gear there is a reduced load on the rollers so they don't properly seat; if the tires suddenly gain traction the shock can cause the rollers to "roll under" and fail....once the trans shifts to 2nd the sprag serves no function, it just rolls along like a roller bearing.
3) The sprag doesn't slow anything, it's supposed to stop the rotation of the rear planetary carrier which is necessary for 1st gear. The only time the rotating mass gets out of control is if the sprag/band fails to hold the carrier stationary....when this happens the cast steel front drum is driven at 180% of input shaft speed and can explode due to the centrifugal force generated by the overspeed.
4) Mopar fans love to talk about how "strong" the 727 is (it's called "brand loyalty") but it has weak points compared to some others...the sprag is only one of them.
5) Whether or not the sudden hook of the tires will cause sprag failure depends on a lot of factors including vehicle weight, horsepower and converter stall ratio. In many cases sprag failure is the result of cumulitive abuse, not a single incident. Repeated abuse can cause flat spots to wear on the individual rollers which will lead to failure down the road.
The bolt-in sprag only prevents the type of failure where the outer cam ring spins in the case, the most common failure is of the internal elements (springs/rollers). The pic below shows internal failure with the outer cam bolts clearly visible.
The best preventitive to sprag failure is a valve body with LBA and the use of manual low when spinning the tires or launching hard. A billet front drum is the best preventative to front drum explosion.
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Re: Please help me understand 727's a little better....
[Re: Bumbalawski]
#1410060
03/28/13 07:13 PM
03/28/13 07:13 PM
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Rio Linda, CA
John_Kunkel
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I have always questioned the wisdom of using lightened sun gear driving shells; prior to '67 the shell was stamped from a lighter gauge steel and failure occured enough to warrant the change to the heavier gauge steel.
The INTERNET, the MISinformation superhighway
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Re: Please help me understand 727's a little better....
[Re: JoesMopar]
#1410061
03/28/13 07:30 PM
03/28/13 07:30 PM
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JohnRR
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Quote:
It's a auto/manual valve body TCI Streetfighter with a bolt in sprag supposedly.
Unless a valve body is specifically built as a manual shift valve body all valve bodies are auto/manual.
As far as the bolt in sprag in your TCI transmission, it's more than likely just a standard sprag with a bolt in outer race. It will not stop the rollers from rolling under, only the A+A or Coan super /Ultimate sprags are designed to stop that from happening. It's a total waste of money and effort to remove the outer race from a trans that does not have a loose outer race and replace it with a bolt in outer race.
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Re: Please help me understand 727's a little better....
[Re: John_Kunkel]
#1410062
03/28/13 10:04 PM
03/28/13 10:04 PM
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Thanks for the good info. So what about the scenario where you take off, tires spinning all through first, you upshift to second and the tires stop spinning....is that considered the same as the tires abruptly "grabbing" and causing damage to the sprag even though it's in second gear at that point? I'm curious because I'm trying to picture what's going on and if the tires are spinning all through first, how can the sprag/whatever react fast enough from the sudden grab from the tires. In first gear I understand what you mean. I'm far from a serious racer, I just want to take the car out and have fun every now and then without worrying if I spin my tires a little enthusiastically. It makes me wonder how people drove these cars in snow and ice without blowing transmissions every winter. In this video, is what the guy removes @ 3:55 what I was calling the "front drum" and what causes the explosion in 727's? https://www.youtube.com/watch?v=fQrTqSdSCbM&list=HL1364522704So is the band that wraps around the drum what you guys are calling the LBA? Is that how the sprag is "bypassed" when the car goes into 2nd gear? So is basically what happens is when your spinning tires the sprag is spinning around crazy fast and when the car hooks it tries to stop from spinning abruptly and those rollers "roll under" the edge of the sprag?
Last edited by J5440; 03/28/13 11:13 PM.
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Re: Please help me understand 727's a little better....
[Re: JoesMopar]
#1410065
03/29/13 01:51 AM
03/29/13 01:51 AM
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JohnRR
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Quote:
Thanks for the good info.
So what about the scenario where you take off, tires spinning all through first, you upshift to second and the tires stop spinning....is that considered the same as the tires abruptly "grabbing" and causing damage to the sprag even though it's in second gear at that point? I'm curious because I'm trying to picture what's going on and if the tires are spinning all through first, how can the sprag/whatever react fast enough from the sudden grab from the tires. In first gear I understand what you mean.
I'm far from a serious racer, I just want to take the car out and have fun every now and then without worrying if I spin my tires a little enthusiastically. It makes me wonder how people drove these cars in snow and ice without blowing transmissions every winter.
The sprag is HOLDING the REAR drum from spinning in first(low) gear, with a low band apply valvebody the low band , which is the REAR BAND is holding the drum along with, or instead of, the rear sprag. EXTRA INSURANCE .....
You, and the people in snow, are not really making enough HP nor are you spinning the engine to an RPM that will cause the front drum to spin to an RPM ... over 7000, that will cause it to blow up. I wasted the rear sprag in the trans in my moms 72 fury by accidentally jamming the trans into reverse at about 50mph while manually shifting . she drove the car for at least a year after that incident . Years later after the car was mine and then sat in the backyard and then parted I disassembled the trans, when I flipped the trans over, bellhousing down, the outer race and part of the case fell out onto the floor. 
Quote:
In this video, is what the guy removes @ 3:55 what I was calling the "front drum" and what causes the explosion in 727's?
https://www.youtube.com/watch?v=fQrTqSdSCbM&list=HL1364522704
So is the band that wraps around the drum what you guys are calling the LBA? Is that how the sprag is "bypassed" when the car goes into 2nd gear?
NO , that video starts with the FRONT drum already out of the trans , the FRONT drum is in the FRONT of the trans closest to the engine.
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Re: Please help me understand 727's a little better....
[Re: JohnRR]
#1410066
03/29/13 01:18 PM
03/29/13 01:18 PM
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Columbia, CT
moper
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Guys- correct me if I'm wrong here...
There are two bands and two clutches in a 904/727. The combinations of them being applied give you the 3 forward gears and one reverse gear. The parts are named both for their various functions and for thier placement in the geartrain so it can get confusing:
- The Low/Reverse band is in the back of the case - It's only used in "Break-a-way first" (meaning standard valve body manually shifted and held in "1") in combination with the applied Forward clutch and the sprague; and in reverse when it's applied and acts against the sprague.
- The Forward clutch is actually the rearmost clutch in the geartrain - but it's applied in all forward gears, hence the name.
- The "Kickdown" band - or second gear band applies to the outside of the high gear clutch drum which is the most forward mounted drum and is applied with the forward clutch to get 2nd gear.
- The High gear clutch - inside the high gear clutch drum, which applies with the forward clutch to get 3rd gear.
There are some good books on 727s for under $20. Buy one and read it - it's easier to see the pictures and the flow through the various components.
Well, art is art, isn't it? Still, on the other hand, water is water! And east is east and west is west and if you take cranberries and stew them like applesauce they taste much more like prunes than rhubarb does. Now, uh... Now you tell me what you know.
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Re: Please help me understand 727's a little better....
[Re: moper]
#1410068
03/29/13 02:25 PM
03/29/13 02:25 PM
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JohnRR
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Quote:
Guys- correct me if I'm wrong here...
There are two bands and two clutches in a 904/727. The combinations of them being applied give you the 3 forward gears and one reverse gear. The parts are named both for their various functions and for thier placement in the geartrain so it can get confusing:
- The Low/Reverse band is in the back of the case - It's only used in "Break-a-way first" (meaning standard valve body manually shifted and held in "1") in combination with the applied Forward clutch and the sprague; and in reverse when it's applied and acts LIKE the sprague to hold the rear drum stationary .
- The Forward clutch is actually the rearmost clutch in the geartrain - but it's applied in all forward gears, hence the name.
- The "Kickdown" band - or second gear band applies to the outside of the high gear clutch drum which is the most forward mounted drum and is applied with the forward clutch to get 2nd gear.
- The High gear clutch - inside the high gear clutch drum, which applies with the forward clutch to get 3rd gear and reverse.
There are some good books on 727s for under $20. Buy one and read it - it's easier to see the pictures and the flow through the various components.
someone else may correct this ?
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Re: Please help me understand 727's a little better....
[Re: JoesMopar]
#1410071
03/30/13 01:32 PM
03/30/13 01:32 PM
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JohnRR
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Quote:
Thanks again guys.
In the 2 videos I posted, are those guys doing any serious damage to their 727?
No way of knowing because they don't say what kind of valve body they have. But I don't think they are doing anything real bad as they never really get good traction before letting off the gas.
I think you are over agonizing this ....
Part of a drum exploding is taking the engine over 7000 rpm , want to avoid that, don't rev the engine that high ... rev limiter ?, put in a 16 element sprag, put in a billet front drum, only do burn outs in manual low with the low band applied or drive the car like an old lady ...
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Re: Please help me understand 727's a little better....
[Re: JohnRR]
#1410072
03/30/13 06:37 PM
03/30/13 06:37 PM
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Quote:
Quote:
Thanks again guys.
In the 2 videos I posted, are those guys doing any serious damage to their 727?
I think you are over agonizing this ....
Part of a drum exploding is taking the engine over 7000 rpm , want to avoid that, don't rev the engine that high ... rev limiter ?, put in a 16 element sprag, put in a billet front drum, only do burn outs in manual low with the low band applied or drive the car like an old lady ...
I think you're right about me over agonizing 
I just read so much about these "problems" and I'm sort of a pessimist anyway so I was looking for a little closure and you guys helped out tremendously.
I didn't know you could spin tires as long as you didn't let up until after you gained traction.
Thanks again for all the info, I feel MUCH better now and have a better understanding on what exactly is going on.
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Re: Please help me understand 727's a little better....
[Re: John_Kunkel]
#1410076
04/02/13 05:13 PM
04/02/13 05:13 PM
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JohnRR
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Quote:
Quote:
4-speed................
R. 
Great idea, then all you gotta worry about is a flywheel or clutch exploding, bent valves from a missed shift and/or busted axles/gears.
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