TRANSMISSION REBUILD VIDEO THREAD, Input sought! Options

[Dr Evil]

The video is getting closer to happening as Eric (cupomeat) and I just work on firming up a date in the near future. The video will cover the full tear down, inspection and rebuild of both side and tail shift (both /4 and /6) transmissions, but the video can not cover everything that I wish to. As such, I have taken the last 4 days in the middle of nowhere MN to start writing the notes that will accompany the video. The notes may be made available for free as well at a later time, but for now I need to make sure that they are as complete as I can make them. I have much of them done already, but I am still in the editing phase. If you have some info (factual or anecdotal) or experience with any of the topics, feel free to post them so I can either shoot them down or include them (IMG:style_emoticons/default/wink.gif)

Here is the outline so far:

Transmission rebuild video supplement:

Forward – What the deal is

Chapter 1 – Do I need a rebuild?

What external things to inspect

*Engine mounts
*Trans mounts
*Bushings
*Cone screws
*Clutch cable adjustment and issues
-How to adjust cable
-Broken tube

How to adjust the shifter

Inspecting core tranny

*Input shaft and output flanges turn?
*Input shaft play?
*“it shifts fine on the floor”, Oh goody, big deal.

A word on reading used transmission oil

Chapter 2 – Do I need that?

Myths and musings

*Billet intermediate plate
*Through bolting the intermediate plate/case/tail cone
*Lubricants
-Dino vs Royal Purple vs Swepco
*Galvanic chart: Stainless hardware FACTS
-SS hardware will not prematurely corrode your case and why not
*Corrosion protection
-Original factory coating
-Options for aftermarket

Chapter 3 – Tweaking your gears and diff

Gear selection

*Considerations
*Engine dependent gearing
*Interpreting gearing
*Intended usage of the gear box
*Flipping gears

Limited slip differentials

*The main idea - what they do
*Torque biasing
*Friction biasing

Welded differentials
*Don't do it!


Chapter 4 – Troubleshooting, care, and feeding of your 901

Problem list

*Reverse gear grinds
*Pops out of gear
*Grinds
*Whining
*Clutch will not disengage – resurfaced flywheel usage?
*Clunk and shifts funny
*Pivot ball leak?

General maintenance
*Oil changing

Appendix

Torques and values

*Case nuts, 8x1.25
*Speedo retainer bolt
*Throwout bearing guide tubs nuts, 10x?
*Detente package retainer bolt, 14mm
*Pinion bolt
*Input shaft nut

Special tools list

[Dr Evil]

Chapter 3 – Tweaking your gears and diff

Gear selection:

Considerations - What gears will work best for you depends on several factors: engine torque curve, HP, what the car will be used for. Ideally, you want to choose your gearing so that your engine RPM in every gear will be set up to extract the most use of the torque. For example, say you make peak torque between 4k-5k RPM. If you choose gearing that utilizes this band then you will get the best use out of your gear box.

Engine dependent gearing -The engine that is being hooked up to the gear box also determines how the gear box would best be setup. For instance, engines that make high torque at low RPMs, like V8s and large displacement Porsche 6 cylinder engines, require a taller set of gears than stock 4 cylinder engines which make their torque at higher RPMs (tall and small are slang terms that refer to how many RPM are needed to move the car. Small is 1st, tall would be 5th). As such, stock 1st gear in the 901 is useless and even dangerous to use with engines like V8s as it can cause damage if used. Stock 5th for the high revving /4 engine will also be too high for any engine that produces torque at low RPM and as such most people option to install taller gears in the 5th gear position.

Interpreting gearing - Porsche utilizes the alphabet to designate where gears lie on a continuum with “A” being the shortest stock gear, and “ZD” being the stock tallest

Stock gearing for 914/4 all years:
A, F, N, V, ZD

Stock gears for 914/6 all:
A, GA, O, V, ZA

*Note – the gearing for the /6 engine is a good example of how tweaks to what gears you have make better use of the torque provided by the engine, and how the shorter 5th (ZA vs ZD) provides for better cooling through higher RPMs.

Intended usage of the gear box - How you intend to use your gear box is another important factor to keep in mind when you are choosing gears. A very popular and readily available gear selection for courses with lots of turns, limited straightaways, and with a 4cyl with a normal torque curve is A, F, M, S, X. The first two gears are stock for the /4 while the 3, 4, 5, gears are all closer together than stock and are shorter overall. The MSX setup would absolutely suck on a course with long and plentiful straightaways, or on a freeway to and from a course. The stock gearing is fairly decent overall for a mixed race course. Other than these two popular configurations, the sky is the limit (or your wallet). I wish for you reading this to keep in mind that me telling you what you will enjoy best in terms of gears is like me trying on a pair of pants and telling you that you will like them based on my experience. As such, take the advise here in the gearing section as just that, and see what works best for you.

Flipping gears – Flipping gears is done for two major reasons: 1) To gain a ratio that is not available in the normal A-ZD offering. 2) To make a gear ratio that one wants without having to shell out the money for the actual gears (for instance, a flipped “M” gear is the same as a ZA which is stock 5th on a /6 box). Flipping an “H” gear and sticking it into 5th to gain a taller gear than ZD is a very popular application used in V8 configurations because they produce torque at lower RPM and cruising at 3.5K-4K RPM at freeway speeds with a ZD 5th gear is not very enjoyable in a V8.

Limited slip differentials (LSD):

The main idea – LSDs are great when used correctly. They are not cheap so getting one just to get one is something I would recommend against. To understand what an LSD does one should first understand what the standard differential does. The differential's primary job is to allow the wheel on the inner side of a turn to turn slower while the wheel on the outside of the turn turns faster. For example, a left turning car will have the rear left wheel turning slower and the right rear wheel turning faster. The problem seen with a stock differential occurs when one of the rear wheels slips. It is at this point that the stock differential will transfer all power to the slipping wheel while the gripping wheel gets no power. This seems retarded, but in normal everyday driving this is a perfectly fine design and not problematic. On the track, when rounding a corner and your wheel starts to slip you lose power to the drive wheel. This cost you time which is not acceptable in racing. It is this scenario that the LSD is designed to address. LSDs keep power to the wheel with the grip thus keeping the power to the road around a slide or corner. LSDs come in two flavor that are addressed below.

Torque biasing (TB) – The quintessential version know to most people of a TB LSD is the one manufactured by Quaiffe. TB LSD use a gear package inside the differential to divert power to the sticking wheel. These are my preference as they are readily available and require little if any maintenance when compared to the FB type of LSD. The only complaint that I have heard from a few people regarding the TB LSD is that around corners is can feel kind of ratchety. Some do not notice this at all.

Friction biasing (FB) – The stock LSD that came in some HB designated (case code on bottom rib starts with HB) 901 boxes was a FB type. A FB type is constructed much like a positronic differential in that it uses a wet clutch comprised of several friction disks and pressure plates. The thickness of the friction disks and pressure plates sets the percent of torque bias, typically 40 or 80% (percent diverted to drive wheel in a slip). These are smoother than the TB LSDs, but they have a serious problem in that they require constant replacement of the internal parts in order to work. Depending on who hard one pushes their car and how often, the FB LSD may need rebuilding once a season or more. The LSD has 4 friction disks, 6 pressure plates, and 2 wave plates/belleville washers in it. Each part can be purchased from vendors like Guard Transmission. The wave plates/belleville washers tend to be in fragments by the time you rebuild the LSD. As the LSD parts wear, the % of bias it provides decreases until it behaves just like a regular differential.

Welded differentials:

Don't do it! - I am still not sure why anyone would weld a differential. It is closer to an LSD than the stock differential, but when welded the differential no torque biasing in that the wheels always turn at the same rate. ALWAYS. This is freaking dangerous! Why? Picture that you are trying to turn, but you have a welded differential. Your car decides that it wishes to continue going straight (under-steer), nope, wait, now it wants to turn really fast (over-steer), nope, wait, now it wants to go straight again. Sounds like a recipe for disaster. I will not work on welded differentials because I don't trust them and don't want anyone's blood on my hands. Proceed along the welded differential path at your own peril.