can you downshift a rmvb th350
Thread Starter
Joined: Nov 2001
Posts: 3,406
Likes: 2
From: Tallahassee, Fl
can you downshift a rmvb th350 my m6 is on the way out and im strongly looking into a th350 swap with a rmvb and a brake. i had heard that with the rmvb your not supposed to downshift the tranny from third to second will you running. is this true. my car is not my daily but it is still very much a street car and i put about 5k miles a year and do alot of both dig racing and roll racing it in
thanks
thanks
I would see no reason why you couldnt . Your just replacing the govenor/modulator with you full manual valve body means just that you have o do all the shifting.
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you can, if you dont have a big stall and your shifting into a lower gear above the stall speed it may be a pretty harsh downshift. Its not really bad its just kind of annoying on the street. It helps if you try to rev up a little before you make the downshift, rev match a little.
Manually downshifting under deceleration is a sprag killer, especially when there is no intermediate band to help slow the drum.
Yep,
There is also the danger of the direct drum over-running due to the lack of band during decel and since it is made of cast iron, centrifugal force causing it to explode, as seen here.
I would think any performance transmission builder would be aware of this.
There is also the danger of the direct drum over-running due to the lack of band during decel and since it is made of cast iron, centrifugal force causing it to explode, as seen here.
I would think any performance transmission builder would be aware of this.
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FormerVendor
Joined: Oct 2006
Posts: 3,123
Likes: 3
From: D-F/W
Those are the same reasons to not ever slip a th400 or powerglide into neutral when going through the traps on a race track!!!! People have been hurt bad doing this... the only correct way to decel safely is to allow engine braking in high gear... and they break sometimes during that cycle as well.
"All ... 3 speeds using a simpson planetary system using production ratios can exceed 2X driveshaft rpm providing the input shaft stops but the drums are larger and the designs are more complex (than a powerglide) and full of stress risers which lower their burst speeds greatly over a glide, they also use sprags and sprags do not think much of high overrunning speeds without lube." recent quote from Hutch of Hutch's Transmission Service on another internet forum, discussion was about neutraling a tranny, turning off the engine in the shutdown after a fast pass. I thought it was applicable here.
There is a reason the racing authorities include transmission blankets, h/d cases, external shields etc. Just because you have never seen it yourself does not mean it does not happen... and happen often! As a matter of fact, that is the biggest reason the PCM does not allow the computer controlled unis to downshift when you try to downshift manually during decel.
Can we make this a sticky!!!!! I think this discussion and that pic are worth a thousand words and might save someones foot!
g
"All ... 3 speeds using a simpson planetary system using production ratios can exceed 2X driveshaft rpm providing the input shaft stops but the drums are larger and the designs are more complex (than a powerglide) and full of stress risers which lower their burst speeds greatly over a glide, they also use sprags and sprags do not think much of high overrunning speeds without lube." recent quote from Hutch of Hutch's Transmission Service on another internet forum, discussion was about neutraling a tranny, turning off the engine in the shutdown after a fast pass. I thought it was applicable here.
There is a reason the racing authorities include transmission blankets, h/d cases, external shields etc. Just because you have never seen it yourself does not mean it does not happen... and happen often! As a matter of fact, that is the biggest reason the PCM does not allow the computer controlled unis to downshift when you try to downshift manually during decel.
Can we make this a sticky!!!!! I think this discussion and that pic are worth a thousand words and might save someones foot!
g
Last edited by Gilbert@Ace Racing; Oct 15, 2007 at 07:27 PM.
Please excuse Frank here as I do not believe that he has ever built or raced these types of units. If he has he obviously never read the last page of the directions where it clearly states to never downshift one of these units until you come to a complete stop because of these dangers. I guess sometimes experience shines and other times it shadows. Vince
Sorry for saying this Gilbert
.....I agree with you that this should be a sticky!
Last edited by FLT; Oct 15, 2007 at 08:18 PM.
Thank you Vince and you are correct I have not spent alot of time with TH350s without the band in them. I have set up manual valve bodys in them however and never seen that happen. But looking at that pic and the inability to downshift saftley I certainly do not think I would want one set up like it without the band in my car and definatly not a street car. And since the OP says he wants to roll race he would definatley not want the band removed. Not being able to downshift while moving would put a real damper on roll racing I would think? But you are correct my primary performance expereince is with overdrive autos in the performance arena. I ussually try and talk people out of going back to a 3 speed auto even though we do carry them and they have done well. Me I like the OD trans that why I started playing with them in my own cars in the early 80s when they didnt work so well yet, You learn alot that way.
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Last edited by performabuilt; Oct 15, 2007 at 09:07 PM.
There is another solution. I had Rossler build my RMVB Th400 and keep the intermediate band, not sure if a th350 can do this though. I can downshift to 2nd from 3rd without worrying about overrunning the drum. Which is good in my case because I don't make the 2-3 shift until 110mph because I run 2.75 rear gears. I still make sure that I am on the throttle good before making the downshift as it lessens the harshness of the downshift. I try to make sure that I I don't let off in 2nd gear either, I always like to be in 3rd gear when I get off the throttle. It's good practice.
I also do 1-2-3 burnouts which is not reccomended, but as soon as the tires start to spin i click in to 3rd as fast as I can.
I also do 1-2-3 burnouts which is not reccomended, but as soon as the tires start to spin i click in to 3rd as fast as I can.
There is another solution. I had Rossler build my RMVB Th400 and keep the intermediate band, not sure if a th350 can do this though. I can downshift to 2nd from 3rd without worrying about overrunning the drum. Which is good in my case because I don't make the 2-3 shift until 110mph because I run 2.75 rear gears. I still make sure that I am on the throttle good before making the downshift as it lessens the harshness of the downshift. I try to make sure that I I don't let off in 2nd gear either, I always like to be in 3rd gear when I get off the throttle. It's good practice.
I also do 1-2-3 burnouts which is not reccomended, but as soon as the tires start to spin i click in to 3rd as fast as I can.
I also do 1-2-3 burnouts which is not reccomended, but as soon as the tires start to spin i click in to 3rd as fast as I can.
1-2-3 burnouts are fine as long as you don't go to heavy throttle in 1st, get the tires spinning, shift into 3rd ASAP, and then go WOT and let out right as the tires start to hook, don't let it completely hook and hop.
Downshifting from 3rd to 2nd under deceleration is very harsh on the sprag (even with engine braking/band) as the direct drum is being driven by the output shaft instead of the input and it causes shock loading to the sprag when the drum goes from engine speed to zero rpm almost instantaneously.
Downshifting from 3rd to 2nd under deceleration is very harsh on the sprag (even with engine braking/band) as the direct drum is being driven by the output shaft instead of the input and it causes shock loading to the sprag when the drum goes from engine speed to zero rpm almost instantaneously.
I have a question though. When I do my 3-2 downshift I get on the throttle, make the downshift and then let off after a few seconds to slow back down. Is that better or worse than just downshifting from 3-2 without getting on the gas? I usually do this when I race bikes on the highway.
Thread Starter
Joined: Nov 2001
Posts: 3,406
Likes: 2
From: Tallahassee, Fl
let me ask this question then, if i cruising at 60 mph in third and want to race a car i pull up on. can i drop the trans into second and race? what would have to be done to the a rmvb th350 to make this possible if it is possible
Honestly I have not played with one without the overun band however with it I have and have done that many times. However if you dont feel safe doing it buy a trans blanket. I mean if you cant do that in a street car whats the point.
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Can we make a list of DONT's? FOr example
-Dont downshift from 3-2 unless under throttle?
-Dont shift to neatral from 3rd?
ETC ETC, im still a lil confused as to what all you must not do.
Oh yeah, ive also heard if you shift into second then get off the throttle you have to be very gentle getting back into the throttle or you can break stuff, is that correct?
I have a RMVB TH400 (bought used how can i tell if the band is in it still or not???) and im wondering the same thing. Is it safe as long as i stay on the throttle during the downshift from 3-2? What if i want to shift 2-1 do i have to stop or can i do it from a slow roll?
Can we make a list of DONT's? FOr example
-Dont downshift from 3-2 unless under throttle?
-Dont shift to neatral from 3rd?
ETC ETC, im still a lil confused as to what all you must not do.
Oh yeah, ive also heard if you shift into second then get off the throttle you have to be very gentle getting back into the throttle or you can break stuff, is that correct?
Can we make a list of DONT's? FOr example
-Dont downshift from 3-2 unless under throttle?
-Dont shift to neatral from 3rd?
ETC ETC, im still a lil confused as to what all you must not do.
Oh yeah, ive also heard if you shift into second then get off the throttle you have to be very gentle getting back into the throttle or you can break stuff, is that correct?
You can downshift to 2nd from 3rd when on the throttle, only uinder heavy decel is it bad.
Same deal from 2nd to 1st. It is rough on the low sprag.
FormerVendor
Joined: Oct 2006
Posts: 3,123
Likes: 3
From: D-F/W
The goal is to not initiate a manual downshift while on deceleration at high rpm's. Severe damage can and will result.
If you are driving the car as a daily driver and are coming to a stop at low rpm and low vehicle speeds you will not destroy the unit.
If however you are trying to make a hit on the highway and you are at say 65 mph @ 3500 rpm and grab 2nd you are risking it.
If you are on a hard pass and grab 2nd and need to get out of the throttle for some reason upshift it to 3rd and then release the throttle if at all possible.
Hope that helps.
g
If you are driving the car as a daily driver and are coming to a stop at low rpm and low vehicle speeds you will not destroy the unit.
If however you are trying to make a hit on the highway and you are at say 65 mph @ 3500 rpm and grab 2nd you are risking it.
If you are on a hard pass and grab 2nd and need to get out of the throttle for some reason upshift it to 3rd and then release the throttle if at all possible.
Hope that helps.
g