Category Archives: Th350 Transmission

Parasitic Drag/Oil Shear Effect on Clutch Packs

so something i have been trying to find a happy medium on transmission builds is what is largely known as parasitic drag. this is the amount of power it takes to make the transmission function. on a stock th400 it is around 30-35hp it taxes off the engine, whereas a th350 is about 10hp less than a th400 on the average.

parasitic drag i always thought was due to rotating mass and power requirement of the hydraulic pump. simply put, a hydraulic pump needs power, and the resistance to flow creates a power loss as well. you also have slippage in the torque converter as it is a fluid coupling(essentially a pump). internal components also have some weight to it. basically the more it weigh’s- the more it takes to get it moving. i’ll spare you the law’s of physics.

there is also something i have been trying to deal with lately called oil shear. on your clutch packs, oil circulates from the center out for lubrication and cooling. when the clutch is not applied, the surface area of both the clutches and steels are still covered in circulating lubricant oil. now, if we are spinning a dissengaged clutch pack at any kind of an rpm, the oil will sling to the outside…..literally sucking the flat clutches and flat steels together and causing them to drag. i want to say it is fairly similar to a kind of capillary action caused by centrifugal force. obviously a dragging clutch pack will increase tranny temperatures, and if severe enough, clutch failure.

the solution is basically put a relief area in the clutch pack for the fluid to escape. the easiest way to battle this is to loosen up your clutch pack clearances. this though is a catch 22 as if your clearances are too loose you will have other consequences.

the use of radial grooved clutch discs and waffle clutch discs are the best solution to resolve the problems caused by oil shear on most of your older trannys. gm did it on different clutch packs in both the th400 and the th350.

later model 4L80E’s and 4L60E transmissions used what is termed as turbulator steels. basically they are steels that have an oblong hole in the middle to relieve the oil shear caused by centrifugal force. they are found in the 97 and newer 4L80E’s in the overdrive clutch packs.

on the 96 and newer 4L60E’s , they used turbulator steels in both the reverse input drum and the low 1/reverse drum. ironically it is erie similar to the same areas that fail on a th350 in a demolition derby….anyways the late model turbulator steels can be swapped into earlier 700R4’s as an upgrade.

now as an example of oil shear. on my 1st/reverse th400, i got a guy that gives it time to shift, but floors it from one side of the track to the other all night in forward. after 40 minutes of running the piss out of his suicide lincoln……he loses reverse or it starts to slip. upon teardown there is nothing wrong with the seals- the seal rings-the pistons- the forward clutch is fantastic- gearset is fine. the direct/forward clutch is burnt to a crisp- i mean no linings. on a 1st/rev the direct drum spins just as fast in the opposite direction but dissengaged. even when using stiffer trans brake springs to keep the clutches dissengaged- the discs still suck together!

this is chronic oil shear….and can happen on ANY transmission used for demo derby. even with the use of waffle clutches, chronic oil shear still can happen. we are experimenting even to this day with combinations of different linings-number of discs vs number of steels, different fluids/additives, and clutch pack clearances to prevent this. there is no answer at this time for derby transmissions.

TH350/converter service article

this is an article i ran across of a th350 tranny and converter service. now the refresh is nothing special but it is an excellent pictorial and brief  description of how a converter is serviced and how easily a lousy converter can screw up a transmission.

TH350/TH400:modulator block off plug mod’s

302049547Well, this is the first in a new series of posts in the name of performance upgrades you can do for your transmission. what you see in the picture here is an aftermarket aluminum block off plug you can get from most any performance autiomotive supply outlet for about 10 bucks. it not only makes for a much cleaner appearance on you tranny, but in demo derby application it keeps the modulator valve from ripping off on the floorpan and causing a major leak.  unfortunately as i have stated a few other times  throughout my blog, this creates a major pressure spike while in reverse causing severe internal damage to the tranny. usually the modulator valve will act as a blow off valve while in reverse, but with this rigid plug it doesn’t move and deadheads the psi. well, i am going to show you the simple mod’s to get around this.

396007828Basically, what we are going to do is render the boost valve of the pressure regulator useless.  on a th400 shown here, the pump is removed to expose all the oil passages. the pick is pointing to the oil supply port from the reverse circuit. this supplies oil to the boost valve of the psi regulator that is in the front pump of a th400. we need to block this oil. here i have tapped it and installed a 5/16″ set screw to block the oil. some builders simply jam a check ball in here with a punch. Aftermarket valve body kits will simply omit the supply hole in the valve body mid plate to accomplish the same thing.

400boost01Now, this is not a total seal to the boost valve when using a check ball or set screw. Even though it stops the signal pulse of the oil, some oil may seep through and slowly raise the boost valve off it’s seal and trap the oil in the circuit so even when it is suppose to dissengage the boost valve- you have a high psi situation causeing internal damage in all gears- not good.  To solve this issue we grind two small lands on the large face of the boost valve. this insures the boost will not come off it’s seat.

using this modification, your th400 system pressure will remain a constant 155psi or there abouts in all gears where a stock blue psi spring is used. your main regulator spring is the sole psi adjustment now and obviously can be inceased with a stiffer psi spring, which are available aftermarket. you can do these mod’s and not install a plug-leaving the modulator valve in place to go along for the ride- it will still maintain the constant 155psi. this is more than enough oil and psi to make a th400 do whatever you want it to in my opinion. if you have to run more psi to make it shift harder- you really ought to be looking at modify’n your valve body and/or mid plate rather than increasing system psi.

Now, these modulator plugs can be used on the th350 asth350 boost well, and yes they also need some internal mods as well, but ironically we are going a slightly different direction. pictured here is the psi regulator valve ocated in the valve body of a th350. Basically, the reverse/modulator boost valve needs to have a “slight” relief cut into it on the land the arrow is pointing to in the picture. what we are doing here is creating a small shallow groove on the large land of this valve to bleed some of the psi off the circuit while in reverse. now, this is somewhat of a trick as if the groove is too deep you will bleed way too much psi off in reverse and burn up the clutch packs: not enough and it won’t bleed enough and it will spike your psi and also cause damage. the goal is to create a slight bleed of psi in reverse to keep the psi realistic in reverse. don’t get the brilliant idea to just weld the valve in place through the sleeve cause then there is no movement and your pressures will do goofy shit in several different gear range.  i usually don’t mess with it on the th350’s as the short modulator valve sticks out the back of the tranny and really isn’t in the way anyways.

What transmission do i need?

what do i need to get for my transmission and how much will it cost. Well, i truly feel that everyone needs to have a transmission built for their particular needs and desire. this is why i truly believe no “off the shelf” transmission will make everyone happy. usually you end up paying a lot of money for something that gets the job done but really isn’t exactly what you want. i mean you have your engine built to your desire with a cam of your choice and compression/power/budget you want right…..well to me the same applies to your transmission AND rear axle. really they need to compliment what your trying to do otherwise you aren’t taking full advantage of your engine. you’re making due with a one size fits all tranmission. 

when it comes to the th400, it is a very sturdy platform to begin with so to beef it up to handle descent power is relatively affordable. although debatable, i feel in stock form it can handle up to around 400hp in stock form with good stock replacement clutches like a raybestos or borg warner. if you’re going to be using any kind of a shift kit, the next thing is to insure you have an aluminum direct drum piston as not all th400’s had aluminum pistons. when you push near the 500hp mark and are going to hammer on it at the drag strip or upgrade to a full manual valve body of some kind, the next thing is to upgrade to a 34 element performance direct drum sprag. this is the most common weak point of the 400 in 71-later transmissions…..and when it was retired and replaced with the 4L80, they went back to the older stronger sprag.

the next level is over 500hp+ and above 5000rpm, trans brakes, and nitrous oxide. this is a level that most derby engines never get to and if they do-they never apply the power fully. the first thing that needs to happen is to upgrade the input shaft and mainshaft must be upgraded to a hardened aftermarket shaft.  they loosen and snap. the direct drum and intermediate clutch should be upgraded to a 4 disc set up instead of the 3 disc stock….and you must modify or replace the direct drum and sprag to accomodate this. also thrust loading of the rear planetary can become an issue and planetary gears can snap when you get toward the 700hp mark and both straight cut and aftermarket 5 pinion carriers can be used as well. torrington bearings at the rear of the planetary and be machined and installed also.

now spinning a th400 over 6000rpm and up is somewhat of a rarety as a lot of bracket racers usually will opt of a lighter th350 or powerglide as they do not have to rotating mass and do not eat as much power as the 400 does. there are aluminum and lightened components out there to help the 400 in high rpm application hp, and you bend over and take it in the ass on price. so when you get to that point you should definitely consider all options on the table and look across the board at other transmission options for the price. you may want to go powerglide, th350, or even crower or a top loader manual at some point. i mean if the 400 was that great pro stock cars would use them….ya know what i mean? know when to say when.

the th350 pretty much was light duty from the get go and to put any real hp for anything over 250-300hp, you need to upgrade the center support to one out of a later model 4L60 or 700R4, and completely replace the direct drum and sprag assembly to an aftermarket sprag similar to the 34 element th400 sprag. also machining the aluminum piston down to accept 5 clutch discs on the direct is a good idea as well. there were numerous gearsets used on the th350 so you may or may not need to upgrade things like planetary’s and sun gear shell. BUT if you spend the money….the th350 WILL out perform the 400 as far as weight in the car and power put to the ground through the power range. here again- know when to say when.

the powerglide is the choice of transmission for bracket racing in my book. you pretty much have to gut it and use all aftermarket parts, but the consistancy is undeniable when you only have one shift point to worry about.

but as far as derby, the 400 can be built fairly affordable with stock parts and literally has the raw strength needed for the application over most other transmissions period. as i have stated in other parts of my blog, the th350 is not well suited for derby in my opinion, but some people have good luck with them. here again it doesn’t work for everyone.  a lot of guys like running a manual clutch, but for big shows most people go back the 400 and run their clutch set ups at the county fair to have fun. manual trannys for derby are inconsistant and to me it seems like it is a matter of when not if they will fail. the 727 chrysler is a descent tranny, and will work for derby. still not a th400.

1st/reverse only th400 works well for guys running less than a 4.56 gear in old iron and 4.10 in new iron. after that you wind the piss out of your engine. BUT i do know guys running 1st/reverse with 5.13 gears and scream across the track from one end to the other without a care. for the guys that like the 4.88 and up gears i suggest either a regular shift 400 that is modified for holding 1st gear(which is actually quite simple) OR a full manual valve body so you basically have a 3 speed manual transmission without a clutch. a lot of guys prefer the fulll manual over a regular shift as they can hammer 2nd gear to reverse and if they get to a point where they need to move a pile out of their way or their engine is overheating, they kick it down to 1st gear and bull doze around the track with the 5.13 gear. food for thought.

hope this helps guys.

Th350 Center Support Weakness

390707193Can you see the crack? this tranny placed 3rd after a 2 1/2 hr feature run in a demo derby, one of the most extreme things to ever put a Transmissions through. snow plow trucks are very similar as well as drag racing.  the TH350 was designed not to shift to low 1 at high rpm and/or road speed. This was done through the use of both a manual low control valve and an internal shift control valve that is part of the 1-2 shift valve in the valve body.

The shift control valve basically has line psi to the back side of it with the modulator valve unhooked. to engage reverse, oil flows through this valve to the rear holding clutch pack and does not need to move….giving you reverse. to get manual low 1(which engages part of the rear clutch pack to help hold low 1 in addition to the center support) it must shuttle this valve to expose the passage to flow oil to the rear holding clutch pack.

so what am i getting at here? in demo derby and drag racing, the rear low 1 clutchpack in a nutshell is not allowed to move because the tranny believes it is hauling ass down the road with the modulator valve unhooked. leaving the center support and rear sprag to bear the entire load of holding member for 1st gear. however, unlike the th400 line, the th350 and it’s later counterpart the 700R4 underwent many different variations of valve body calibration and control for different vehicles. therefore this is not a golden rule.

So, as a rule of thumb for the Th350, during a rebuild i always recommend retro-fitting in a center support assembly out of a later model 700R4/4L60 transmission, even on driver transmissions. they are a larger sprag and can take more abuse. also always run your modulator valve hooked up. if you are doing any kind of performance for street. strip an aftermarket valve body kit or shift calibration kit is recommended as well. for demo derby….just go get a th400 or run a 3 speed. yes th350’s can run and last for demo derby…..i have had guys swear by 700R4 trannys too for derby. don’t do it. the results are inconsistent.

Cutting a case for an aftermarket bell housing

390168847Well, you could say i have prepped a few 400 cases for the jw performance ultrabell. it is pretty straight forward, and the kit comes with instructions and hardware. this is pretty much how i have found to be the quickest way to get the job done.

The installation to me is done in two parts: 1- remove the bulk of the housing 2-grind the edges for an flush true fit. To remove the bulk portion of the housing, i like to use my 4 1/2″ milwaukee grinder with a cut off disc rather than a sawzall. 390168894 On a lot of cases like the one shown, there is usually some kind of impression left from years of oil pan gasket contact. use this as a guide and make your first two cuts above this mark….cutting through to the thin part of the bell. I then basically make a circle cut around the housing above or even with the top of the oil pump flange. keep in mind we want to remove a large portion of the bell to get it out of the way.

390168856390168859Here in the two pictures to the left, you can see the housing with the bulk of the bell removed. Also noticed i am pointing to two of the 8 bolt bosses needed to attach the case to the bell. Not all housings can accept the ultrabell. if your case does not have both of these bosses, don’t even bother cutting it. you need all 8 bolt bosses for the case to stay attached properly. the pump is no big deal however, just flip the pump over and drill out two holes to turn a 6 bolt pump to an 8 bolt pump. A lot of the ultrabell failures are due to two things happening, either not enough clearance when it was cut to fit the housing or not all 8 bolts were used because the case was cheap shitted. i have tore down ultrabell trannys from other builders where the housing ripped off……and find that it only had 6 bolts holding it on rather than 8, and they simply put a nut on the back side to make it look like it had 8 bolts in the case!!!! 390168849 At this point it is time to switch wheels and finish fitting the housing. i use a flap wheel style grinding wheel to knock the material off the housing. It doesn’t plug with aluminum like a metal griding wheel would, and removes material rapidly. wear your safety gear with this too cause it throws a lot of material all over. pull the wife’s car out of the garage first and save an arguement!!!

390168896390168868First thing you do is knock the 3 casting ears off so the bell sits flush. you then set your pump in the case….or what i do is use a tore up piece a shit as a dummy plug…..then set the bell housing down on the pump using two bolts as alignment dowels through the works. Now if you have a working tranny with a busted bell and just want to fit the housing, they say removal is necessary to install the bell housing. Although i have never done it, i think if you are careful you can do it without removal. i always cut for fitting during a rebuild. Basically at this point you take your flap wheel and keep knocking down the edges until the bell housing sits completely flush to the face of the pump. Basically trial and error fitting. You have the case clearanced correctly to the bell housing when you can hold it flush and still wiggle it against the bolts slightly. the only point you have to worry about taking down too far is getting into the oil pan gasket.390168903 You don’t want any part of the case touching the bell housing- just the face of the pump. Remember to refer to your instructions or call JW performance with questions. the picture to the right is a bottom view of a properly clearanced bell housing where it meets the oil pan.

after clean up and final installation, you then install the 8- grade 8 bolts provided. the o-rings go between the bell housing and the pump face to prevent leaks. i torque them to 18-20 ft/lbs. DO NOT OVER TORQUE these things. it is just aluminum. the bolts that come with the bell housing are actually a special length as well and thread into the case beyond a stock fastener. these bell housings may seam like they aren’t worth a shit, but they do work! i just wish you could run something bigger than an 11″ torque converter!!

I.D. your tranny according to oil pan shape

1 Aluminum Powerglide
2 TH200 (Metric), TH200C (Metric)
3 TH250, TH250C, TH350, TH350C, TH375B
4 TH375C, TH400, TH475, 3L80, 3L80HD
5 TH200-4R
6 TH700-R4, 4L60, 4L60E
7 4L80E

Th350 Lock-up /Non Lock-up

A lockup tranny basically is the short term used to describe a transmission with a locking clutch internally in the torque converter.

A non-lockup converter hydraulically locks at a certain rpm to create a drive source for the planetary gearset. together with gear reduction of different gears in the tranny, the converter provides an amount of torque multiplication to help get the car moving. it becomes unnecessary at highway speed for the converter to provide this added torque multiplication by slipping per say, but it still rotates a small amount under light load/cruising speeds. this eats away at your mileage

Iin the late 70’s is when we started seeing the first lock up clutch transmissions. it basically works just like a clutch in a manual transmission. At highway speed, usually around 45mph, the computer engaged the clutch in the torque converter to halt any and all excessive rotation of the converter to in effect create a solid drive source going to the tranny. obviously it can only be engaged at cruising speed cause if if didn’t ever release the engine would stall at the stoplight. also you need the added benefit of torque multiplication to get the car going. think of it this way…..a standard tranny has usually 5 speeds….an automatic for sake of arguement has 3. Since there is going to obviously be more of a gear/load jump the torque converter makes up the difference by creating torque multiplication. You can also start to realize how customizing how the converter is built and stall speed can really effect how your car performs can’t ya??

For derby use only, avoid running a transmission with a lock-up clutch. most of the time this clutch is already screwed up cause it came out of a car with years of abuse….especially something that was in around town a lot that cruised 45mph. the clutch dissintegrates and locks the torque converter up tighter than a crabs ass……and you’re dead in the water. even if you never engage the clutch it can still jam on ya as the tranny heats up.

When the Th350 first came out in the late 60’s, it is pretty obvious that they had planned on some kind of a lock-up clutch design in it’s future. In all there were 4 or 5 different versions of the th350. the the 250, 250c, 350, 350c, and 375B(somewhat rare). I believe the lock-up trannys started with gm in 1980 when the on-board computers came around, but i could be wrong. this line of transmissions underwent numerous changes every year of production. a dead giveaway you have a 250 or 250c is the output shaft is hollow….i avoid the 250 also because the clutch packs are pretty skimpy as it was a light duty tranny, along with a lousy gearset.

For derby use…..and for what i build the B-25 derby tranny’s out of, we want to use a non lock-up th350. Commonly they are longtail and have an input shaft that is fully splined to the end sticking out the front pump of the transmission. there are no electrical plugs in the case anywhere. Usually if it came out of a 70’s vintage tuna boat it’s a non-lockup.

700r4_establishing_bg A lock-up th350 transmission will look more like a 200R4 or 700R4. the input shaft sticking out of the front pump will resemble something like a manual transmission would have. instead of a fully splined input shaft there is a smooth arbor at the end of the shaft: narrower than the splines about 1/2″ long. this is the smooth arbor that engages the internal clutch of the lock-up converter. in addition, on the driver side toward the rear of the case there is a square electrical plug sticking out of the top of the case above the oil pan. if you find either of these things on a tranny, it’s a good bet you have a lock-up transmission. I have found that a lot of short shafts are prone to being lock-up trannys. now you can take the short shaft/tail housing out of a lock up tranny and put it in a non lock-up tranny, but other than that it is usually just easier to start with the right core to begin with. hope this helps.

J.W. Bell Housing

374721910Well, when the bell housing breaks or you want to go with a sturdier upgrade to your transmission the ultrabell from jw performance or coan engineering is an excellent option. Installation is pretty straight forward you saw off the bell housing and remove enough material to bolt the ultrabell flush to the face of the front pump of the transmission. i use a sawzall to rough cut most of the bell housing off, then a cut off wheel on the angle grinder and a flap-wheel style grinding disc to polish edges of the aluminum to final fit the ultrabell to the pump/case.

On the th350 there isn’t much more to it then that, but the th400 did not always have 8 pump bolts cast into the transmission housing. most of your older 70’s vintage case had all 8 bosses under the front pump even if only 6 bolts are showing. quite often you can drill the extra 2 bolt holes in the pump itself through the backside of the pump. however your 80 & newer chevy th400’s only were cast with 6 pump bosses. you must have all 8 bolt bosses to make it work. you MUST use the bolts provided with the bell housing kit. they are hardened grade 8 and a special length.374721907

The advantage to the ultrabell is that it will not snap from torsional stresses of forward/reverse shifting. This same action also breaks the bell housing in snow plow trucks for the same reason. In normal installation, it WILL NOT make the tranny stronger in reguards to vertical stress from a distributor protector or anything else as the car bends upward. The housing can simply rip off the front pump if it gets pinched in a lateral direction. I am currently working on some improvements to keep this from happening, but it’s top secret right now…
the bell housing is meant for drag racing, and is intended to run an 11″ torque converter. you can make a 12″ converter fit with some machining. for derby i prefer to have a custom built low stall 11″ built so the engine starts to pull around 1100-1300 rpm.