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Clutch FAQ for VW and Audi TDI engines
- Clutch FAQ for VW and Audi TDI engines
- Choosing a clutch kit- quick suggestions
- Mk3 VW TDI, 1996-1997 passat, 1997-1999 jetta
- Mk4 VW TDI, 1998-2005 new beetle, 1999.5-2003 jetta, 1999-2006 golf
- Mk5 VW TDI 2005.5-2006 Jetta and 2009-2010 Jetta
- Symptoms of a worn clutch
- The purpose of a clutch
- Resurfacing the flywheel and new clutch break-in
- Clutch break in
- Flywheel and clutch types and materials
- Solid flywheel
- The dual mass flywheel (DMF)
- Clutch types and materials
- Clutch disk materials
- Bearing wear vs increased pressure plate clamping force
Clutch FAQ for VW and Audi TDI engines(top)This article explains how a manual transmission TDI clutch works, the basics of the VW TDI clutch, and tips for choosing clutches. The first section lists your stock clutch part numbers/info and suggested replacement clutches. The later sections describe flywheel resurfacing and break in, and different clutch styles and materials. This article discusses the clutch/flywheel - for more on how a manual transmission works, see 1000q: how an engine and transmission works. For more on how a DSG transmission works, see 1000q: DSG FAQ.
1000q: removing the mk3 TDI transmission DIY
1000q: DIY removing the mk4 transmission
1000q: how to remove the mk5 TDI transmission
Choosing a clutch kit- quick suggestions(top)
First, you need to figure out what the clutch will be used for and the estimated torque it needs to hold. The overwhelming majority of TDIs are used as daily drivers. The best clutch for this is a full faced street disk. In a gas car, torque capacity for the disk should be at least 10-20% greater than your target torque. Unlike gas cars, it's normal for a TDI to get large torque spikes so plan for excess capacity. About 25% should contain any spikes and allow for tweaking your setup in the future. Some aftermarket kits also rate their clutch kits by the highest possible torque they've tested, some rate their clutch kits by a more average sampling. Some rate flywheel and some wheel torque. Make sure to ask your vendor specifics about the exact clutch you are considering.
Also avoid mixing valeo and sachs g60 clutches with the other's brand pressure plate (PP) since the height may be slightly different and is designed to work with the same brand clutch/PP. Most clutch kits include both the clutch and PP anyways. If putting a new clutch kit on your old DMF, sachs clutches won't fit luk DMF flywheels, etc. If using a valeo SMF clutch kit, their throwout bearing is also slightly thicker.
For a quick answer to what clutch upgrades are available for your car, see below. If your car has more than basic power mods, consult your parts vendor to see what other clutch kits there are. Below are more clutch/flywheel FAQ.
If changing a clutch and you have higher mileage, I also suggest a new clutch fork/release arm, along with the normal clutch kit components. You can also replace the pivot ball or fork spring but these seem to hold up well in the TDI. The fork is inexpensive and sometimes fails under normal use - replacement requires transmission removal. Pictured below left is a VW TDI fork (it's supposed to be straight) and another car's fork broken through normal use, below right. If you are putting in a much stronger clutch/stiffer pressure plate or have high mileage, you should consider replacing the fork with a new one.
Suggested torque specs: If your clutch kit did not come with torque specs, here are some suggested specs for the G60/vr6 kit. These specs are superceded by any kit specific instructions. Medium strength threadlocker is suggested on the bolts and your new flywheel bolts should already have threadlocker on them. *According to the Permatex blue medium strength Loctite technical sheet, no adjustment in torque is needed. This is only for this exact product, always check your specific threadlocker's technical sheet for any adjustment torque when threadlocker is used.
G60 flywheel: stage 1: 22 ft-lbs. stage 2: 44 ft-lbs. final stage: additional 1/4 turn (90o turn)
VR6 pressure plate: hand tight, then tighten diagonally in stages to a final torque of 15 ft-lbs. (this keeps the pressure plate flat while tightening)
Mk3 VW TDI, 1996-1997 passat, 1997-1999 jetta(top)
Stock clutch: These cars came from the factory with a 228mm sachs clutch with a solid flywheel. The clutch and pressure plate (and part number) is specific to the TDI.
Misc parts list
stock pressure plate VW# 044 141 025 x or #074 141 025 b
stock clutch VW# 028 141 035c or 028 141 035bx
stock flywheel VW# 028 105 269 b (rebuilt so it has an "x" suffix)
clutch pivot ball pin VW # 02a 141 777b
clutch fork VW# 02j 141 719 c
clutch fork spring VW# 012 131 741
release bearing is VW# 02a 141 165 g
6x 12 point bolts (one use only) VW# n 902 061 03
6x flywheel bolts (one use only) VW # n 101 010 01
Stronger clutch kits A basic upgrade would be the same sized 228mm TDI clutch with a pressure plate from a VW vr6 (6 cylinder gasoline engine). It looks similar to your existing parts but they are not the same. It will bolt directly to your existing flywheel and I suggest reusing the flywheel if it can be reused.
If you are using the sachs clutch/pressure plate kit, the older suggestion was to use the vr6 pressure plate with a vr6 clutch. When used with a vr6 clutch/vr6 pressure plate, it can hold about 330 ft-lbs torque. Some report more driveline chatter in neutral. The sound is most noticeable when at a drive through. If you are not heavily modified, the current suggestion is to use the sachs TDI kit because it uses the vr6 pressure plate with a different, TDI specific clutch to reduce driveline chatter. This is the TDI/g60 kit from sachs. The stronger pressure plate gives more clamping force and the clutch disk is the same size. The coefficient of friction of the clutch should be about the same.
Clutch pedal weight will be slightly less than stock versus the stock pressure plate, read the full FAQ article for more details.
The 1990-1992 VW Corrado G60 solid flywheel will fit all model years and will also bolt up to the vr6 clutch kit. Note: you can use the vr6 clutch but cannot use the vr6 flywheel because 6 cylinder engines use a different bolt pattern. There are also clutch kits that use longer clutch hub springs that help smooth NVH.
Mk4 VW TDI, 1998-2005 new beetle, 1999.5-2003 jetta, 1999-2006 golf(top)
Stock clutch: ALH engine TDI came with 2 different clutches, both are solid hub and used with a DMF
1998-2000 cars used a 220mm Luk clutch, stock PP is VW# 028 141 025 p, clutch is # 028 141 036 L
2000-2003 cars used a 228mm Sachs clutch, stock PP is # 038 141 025 d, clutch is # 038 141 031 h or 038 131 032 dx
Model year 2000 cars may have either clutch but they probably have the sachs clutch.
The Luk clutch/pressure plate is not the same clutch used in the VW 1.8T gas engine but they look similar. The 1.8T engine 220mm PP is #06a 141 025 e, the 1.8T clutch is #06a 141 031 d. The 1.8T engine also used a 225mm clutch kit. The Luk TDI clutch can hold more torque (about 250 ft-lbs) than the Sachs TDI clutch (about 190 ft lbs) before slipping. These suggested torque limits are conservative and on the low end, some can go higher without slipping. A power chip alone may be enough to cause the later clutches to slip, depending on the specific driver, car mods, power curve, etc. Your car may be slightly different.
Clutch fork is VW# 02j 131 719 c, clutch fork spring is #012 131 741, clutch pivot ball pin is #02a 141 777, release bearing is #02a 131 165 a, pressure plate bolts (quantity:12) are #n 907 253 01, flywheel bolts (m10x1x22,3) #n 906 650 0
Stronger clutch kits The 228mm vr6 clutch is a good choice but will not bolt up to your existing DMF. The 228mm 1990-1992 VW Corrado G60 solid flywheel will fit all model years and will bolt up to the vr6 clutch and is the basic choice. Note: you can use the vr6 clutch with a G60 flywheel but cannot use the vr6 flywheel because 6 cylinder engines use a different crankshaft bolt pattern. Also double check the flywheel diameter. The clutch kit can hold about 330 ft-lbs torque. Clutch pedal weight will be slightly less than stock, read the full article for more details. There are also clutch kits that use longer clutch hub springs that smooth NVH but it will not be as great as a DMF. Also remember that you can't mix/match the valeo and sachs clutch kit with a different brand PP. Most clutch kits include both clutch and PP anyways.
Mk5 VW TDI 2005.5-2006 Jetta and 2009-2010 Jetta(top)
Stock clutch: The car came from the factory with a DMF with solid hub clutch. Note: you can use the vr6 clutch kit with the 4 cylinder flywheel but cannot use the vr6 flywheel because 6 cylinder engines use a different bolt pattern. The 1990-1992 VW Corrado G60 solid flywheel will fit all model years and will bolt up to the vr6 clutch and is the basic choice. There are also clutch kits that use longer clutch hub springs that help smooth NVH but it will not be as great as a DMF.
NOTE: there is a TSB on the sachs clutch wearing out early. See this Sachs clutch TSB .pdf file on coverage of the sachs clutch until 4 years or 50,000 miles, whichever occurs first. This does not apply to the Luk clutch. Cars made after production date: CW 08/07 should use the Luk clutch. The best way to tell what clutch you have is to look through the clutch inspection window. Above the right axle flange on the transmission, there is an 8mm bolt holding the inspection window. The sachs and luk clutch have different shaped clutches, one is rounded and the other is more square.
stock flywheel kit VW# 03g 105 264 [g or d] *(caution: #03g 105 266 be is the flywheel for the DSG transmission), 12 point flywheel bolts (quantity:6) n 903 207 01, clutch and pressure plate kit # 03g 141 015 n (luk) or 03g 141 015 k (sachs), clutch pivot ball pin is #02a 141 777 or 02a 141 777 B, clutch fork spring is #012 141 741, clutch fork is # 02j 141 719 c, release bearing is #02a 141 165 [a or m], 12 point pressure plate bolts for sachs clutch #n 903 207 01, pressure plate bolts for luk clutch #n 907 255 01
Stronger clutch kits Mk4-mk5 clutch kits are interchangeable.
Symptoms of a worn clutch(top)
The TDI clutch is pretty sturdy. The most common reason for replacement is because it slips or it's broken. Clutch slip is when you notice the rpm move up without translating into forward motion (obviously not when the transmission is in neutral or if you have an automatic transmission). If you notice the rpm flutter and then go up with speed, you are slipping the clutch before it catches. This is more likely when you suddenly mash the accelerator pedal down because it's probably due to a torque spike. Once the torque spike subsides, the clutch grips and the rpm stabilize. This is most likely to occur in high gear. The best way to avoid large torque spikes is to apply power smoothly instead of mashing the go-pedal down. Due to the torque curve of a TDI, shifting at too low an RPM and giving too much load can aggravate slipping. There could also be glazing or oil leaking onto the clutch from the rear main seal which lowers the coefficient of friction. Regardless of the reason, you have too much torque for the clutch. If it's slipping occasionally, try to not apply accelerator pedal too aggressively and some of the clutch glazing will wear off over time, restoring some of the lost grip.
If the clutch is simply not working, it could be a few things. If the clutch pedal goes to the floor or the shifter won't go into gear, it could be a problem with the clutch hydraulic system or shifter and not the actual clutch. The clutch springs (if using a solid flywheel) or spring holders can break off and damage or jam the clutch system as well. The clutch fork/bearing can also fail and cause poor clutch release/shifting. If quickly pumping the clutch pedal or bleeding the hydraulic system makes a difference, the problem is most likely with the master, slave cylinder, or hydraulic lines. The slave normally goes first because water and contaminants settle at the low spot which is the slave cylinder. Noise could also be the motor mounts, flywheel, or a loose slave cylinder. If the noise is "growling" only when the clutch pedal is pressed enough to make the release bearing touch the pressure plate, the sound is probably coming from the release bearing. The VW TDI clutch pedal does not have any adjustment screws or levers.
How long can a clutch last? In theory, the clutch disk could last forever if you perfectly rev match every shift. However, the pressure plate springs that clamp the clutch disk can get worn, oil can leak onto the clutch, your "friend" goes drag racing when you ask him to pick you up at the airport, etc... I wouldn't replace a clutch on the TDI unless it's slipping. On some high performance cars, clutches might not last past 150,000 miles, some exotics only get 15,000-30,000 miles out of a clutch. The TDI has a pretty reliable clutch when power is left at stock power levels. With mk4+ cars, the DMF sometimes breaks and causes problems and the mk5 cars with a sachs clutch may be defective, see the TSB mentioned above. When I removed my TDI clutch due to excess power modifications at 160,000 miles, it was thick and was in excellent condition.
Most pressure plate springs actually increase the amount of holding force as the clutch disk wears. However, if it passes the optimum point, holding force decreases. Glazing or oil on the clutch surfaces can further reduce holding force.
The purpose of a clutch(top)
A clutch is a brake like disk sandwiched between the flywheel and pressure plate. It couples power in a manual transmissions. The flywheel is bolted to the crankshaft, and the pressure plate is bolted to the flywheel. The clutch is the only part that is connected to the transmission, through a splined hub at the center of the clutch. If you separate the transaxle from the TDI engine, you'd see this: (flywheel-pressure plate on left)
In a TDI, when the clutch pedal is down, the pressure plate springs release the clutch and the clutch spins independently of the flywheel and pressure plate. When the clutch pedal is up, the pressure plate springs clamp the clutch, and the clutch, flywheel, and pressure plate are spinning as one. All TDI and most imports use a diaphragm type pressure plate. All TDI clutches are dry clutches, this article does not discuss the wet clutches used in the DSG "manual - automatic" transmission, please see 1000q: DSG FAQ for more details on the DSG transmission.
On left is a short video explaining more on how a clutch works. To the right is a long video on how a clutch works.
Resurfacing the flywheel and new clutch break-in(top)
If the flywheel is warped (extremely unlikely for a TDI), has excessive hot spots, or cracks, it must be replaced. If it's in very good condition you can use scotch brite to clean the surface and remove any built up glazing. If you have a DMF (see below for more details), I recommend replacement in most cases instead of reuse. When I removed my non-DMF TDI clutch (shown above), it had a few marks but no unusual damage or wear. After scrubbing it with medium green scotch brite and washing with brake cleaner, it was reusable as is. Wash until the paper towel comes back clean to remove grease.
You can use 3M "roloc" pads or scotch brite to clean the surface. I would recommended resurfacing the flywheel if you have any doubt if it's reusable as-is. If you have very high mileage, it's also a good time to replace the rear main oil seal. If the seal leaks oil onto the clutch, it can cause your brand new clutch to slip. If you have lower mileage I would leave the oil seal alone unless it is leaking.
A professional flywheel resurfacing involves removing the alignment pins and using a blanchard grinder on the friction surface that the clutch touches and the outer step/shelf that the pressure plate bolts onto. Below right is a picture of a resurfaced flywheel (non-TDI). You can see the pattern the blanchard grinder left on the surface. A lathe is not a good way to resurface a flywheel because the cutting tool can skip over hardened spots and leave an uneven surface.
Below left is a picture of an unusable flywheel with hot spots and cracks. A TDI doesn't rev that high and you should get wheelspin in drag style launches instead of excessive clutch slip, so most TDI flywheels can be reused. Note: wheel hop quickly kills differentials and shocks the driveline in a really bad way so if you feel the wheels quickly catching and bouncing while doing drag style acceleration, immediately let off the accelerator pedal. Never use the clutch to hold the car on a hill! It won't burn up the clutch to the degree shown below but it causes totally uncessary heat and wear.
When resurfacing a stepped flywheel, they must remove equal amounts of material from both the friction surface and the outer step/shelf that the pressure plate bolts to. This keeps the lever arm relationship between the pressure plate and friction surface equal. Failure to keep this relationship equal changes the force that the lever arm of the pressure plate springs. If you go to a machine shop, tell them that you want to measure the step and runout after they're done. Double check their work by measuring it yourself before and after. If the step or runout is off, you'll have a poorly operating clutch or you'll have to pull the clutch again to have them fix the step height. This can be prevented by quickly verifying the step height before and after.
If the flywheel step is too shallow or lower than the correct measurement, the pressure plate diaphragm compresses more and the springs are going to be more flat. This may lead to release problems if the travel of the clutch release fork and throw out bearing is maxed out when the clutch pedal is fully depressed. The clutch might also slip since the diaphragm is compressed beyond the point of its maximum torque capacity.
If the flywheel step is too deep or high, the pressure plate diaphragm compresses less and its fingers are going to be less flat. The clutch will disengage better (since the clutch fork will have enough travel left), but the pedal will be heavier as the maximum torque capacity point has to be passed when disengaging the clutch. The installed height of the pressure plate will decrease with the clutch disc wear and the maximum torque capacity will consequently decrease.
Shimming the pivot bolt of the clutch fork or excess wear on the clutch fork or pivot due to a heavy clutch pedal can also change the distance that the throw out bearing moves when you press on the clutch pedal.
Some general tips: when putting on the clutch kit, wipe down the flywheel and pressure plate with rubbing alcohol or brake cleaner to remove any grease or oil from machining or your hands. Use a clutch alignment tool when centering the clutch. Use a THIN layer of high temp grease on the input spline. When tightening the pressure plate bolts, tighten in a diagonal pattern and in stages. If you have an even number of bolts you can do 2 turns on each opposite bolt, moving around the plate, until almost at final torque stage. Once they're all loosely tight, tighten them to the final torque. "Skip 1 bolt, skip 1, skip 2" also works depending on how many bolts there are and in what pattern. This ensures that the pressure plate stays flat while you tighten it to the flywheel.
Clutch break in(top)
To break in the new clutch, use lower load/rpm operation with smooth engagements and avoid excess slipping. 300 miles of city driving should be enough. Harsher materials such as feramic or puck clutches will wear into the flywheel and pressure plate more than softer materials such as organic disks, so plan for longer break in before full clutch clamping force is available. Harsher clutches need to wear into flywheel and pressure plate before they will hold their full rated force. Some minor chatter upon clutch engagement is normal, especially for harder clutches, it should go away as the clutch breaks in.
Flywheel and clutch types and materials(top)
This is the type of flywheel used on most older cars. It is the factory flywheel on the mk3 TDI. It is a solid piece of cast iron with a starter ring gear that is heat shrunk onto the outer diameter. When you turn the ignition key to start, the starter engages the ring gear which turns the flywheel. It rarely fails but does not dampen any engine vibrations. Because of this, street clutches used with a solid flywheel have damping springs on the hub. The flywheel is typically lighter than a DMF but because the clutch has springs, the clutch is heavier. Even though it's very rare for the flywheel to break, the clutch used with a solid flywheel has springs that can break out of their spring windows and cause problems.
Here is a picture of a lightened racing aluminum flywheel. The starter ring gear is on the outermost diameter. This specific flywheel is made of aluminum with steel inserts for the friction surface. Since it's aluminum, the pressure plate holes use helicoils. The reason it has segmented plates instead of a solid ring is so the segments can withstand abuse and expand independently without warping. Lightened aluminum flywheels can warp the friction surface when overheated due to the differences between the inner and outer diameter. Since the outer diameter travels a greater distance in one rotation than the inner diameter, a slipping clutch will heat and warp the outer diameter more and decrease the amount of surface area that is gripping, leading to more slipping. This is not a factor for a daily driver, but if you plan on drag racing the car, a non lightened solid flywheel will act as a better heat sink and hold more kinetic energy, giving better 60' times in drag racing. A lightened flywheel is more suited for road racing or autocross. For daily driving, I recommend a stock weight flywheel since most will find it gives better drivability, comfort, less noise/vibration, and because it's more economical to reuse the stock flywheel. A diesel relies on compression to ignite the fuel and a stock weight flywheel will also prevent loss of rotational inertia.
If your car already uses a solid flywheel, my suggestion is to reuse it. Stock TDI flywheels weigh about 20 lbs and help act as a damper to the jerky power delivery of a diesel. This is also why you should avoid lightening the injection pump sprocket or other TDI sprockets. Heavier sprockets act like flywheels and help dampen the jerky vibration of the injection pump and crankshaft on the timing belt and other components.
Don't forget the pressure plate
Your clutch kit normally includes a pressure plate. The pressure plate clamps the clutch on the opposite side of the flywheel. The pressure ring, the contact surface of the pressure plate, is normally made of a single piece of cast iron (pictured below).
The dual mass flywheel (DMF)(top)
A dual mass flywheel (DMF) is a flywheel which is 2 pieces connected with a damping system. Because the flywheel handles damping, the clutch can be made without a spring hub which makes the clutch lighter. A lighter clutch reduces synchronizer wear. The tradeoff is increased complexity and a heavier flywheel because of the addition of internal dampers. If you replace the DMF with a solid flywheel, it is suggested to use a clutch with a spring hub. A solid replacement will chatter more than your old flywheel because even though the spring hub is helping absorb energy, the overall effect is not as great compared to a DMF. These low RPM vibrations are best dampened by a DMF. There is a harmonic balancer pulley on the other side of the crankshaft but these are most needed at higher RPM and won't dampen idle chatter.
Almost all new German cars (manual transmission) come with a DMF. Because they're 2 big pieces and many small pieces instead of 1 solid piece, it's possible for the dampers to fail. Typical failure modes in the TDI DMF are a separation of the DMF causing failure of the damping system and sometimes even "liberation" of fragments, flying pieces of the flywheel which can damage the transmission. Black leaking oil (actually DMF grease) could from a leaking flywheel instead of a oil leaking out of the rear main crankshaft seal. The real problem with the DMF is that they are tuned to a diesel's specific resonant characteristics, power levels, and can overpower the springs inside the flywheel if you raise the energy absorbed by the flywheel too much. A heavily modified car might exceed the original design limits so it's suggested to use a solid flywheel on a heavily modified car. If the flywheel is damaged and the layers slip too much, it can even break and cover the flywheel bolt holes, making removal a real problem. If this happens, you can sometimes use a prybar to move the bolt holes into the correct position. Other times it's completely jammed and you have to cut holes to access the bolt heads. An example of this from twotone is pictured below. If it's really jammed like this you have to cut away some metal to access the bolt heads.
If your car uses a DMF, my suggestion is to replace it as necessary with a direct replacement or a regular flywheel. Select a solid regular flywheel or DMF depending on your power modification level, personal tolerance for increased NVH, and car history/mileage and driving style. I do not suggest buying a used DMF with an unknown history because the internals are considered a wear part. Some DMF are rebuildable and DMF can be resurfaced, but the TDI DMF cannot be rebuilt with new internals. Most people report that the VW TDI DMF has very hard surface and successfully reused it after a good cleaning and scrubbing with scotch brite. Unless you are using the car for drag racing, it's likely that the flywheel surface is reusable. Almost all people who switch to a solid flywheel are happy with it but notice slightly more NVH in idle and a clutch pedal engagement slightly closer to the floor.
Below is a cutaway illustration of a manual transmission DMF and some pictures (click to enlarge thumbnails).
Below is a DMF from a 2006 DSG transmission (automatic) taken apart. Please note: DSG flywheels and not interchangeable with manual flywheels! (click to enlarge thumbnails)
Here's a video of a bad DMF in a DSG. If you need to replace a bad DSG DMF, the BRM 2005.5-2006 and CJAA/CBEA 2009+ flywheels are all interchangeable even though the CJAA/CBEA flywheel is 1mm thicker.
Other dampening factors
Something else to consider is the front crankshaft damper, the harmonic balancer pulley. This also has a small effect on reducing NVH, especially on the serpentine belt, alternator, AC, power steering, etc. Some engines can safely use a lightened pulley or underdrive (smaller diameter) pulley to reduce parasitic power losses on the engine. I do not recommend using a lightened or underdrive pulley on the TDI engine. The power pulses in the 4 cylinder TDI engine are abrupt and it's partly why a TDI engine has more vibration than a comparable 4 cylinder gas VW engine. I believe that the stock weight pulley is needed to dampen the greater stresses that a diesel engine sees. In addition, the moment of inertia is close to the pulley axis, so there is probably only a very small power increase by using a lighter pulley.
The mk3 pulley will slip over time and will make a chirping noise, especially on cold starts. If your mk3 TDI makes a chirping noise that increases with lower temperatures and cold starts and higher electrical load, the pulley and tensioner should be replaced with the revised pulley. See 1000q: mk3 cold chirping noise fix for the detailed procedure. The mk4 pulley used 2 different harmonic balancers, 1998-2002 pulleys are slightly heavier than the 2003-2005 pulleys and are interchangeable as long as you use the correct length allen bolts for the pulley. Early mk4 harmonic pulley allen bolts are slightly longer than the later mk4 allen bolts.
Clutch types and materials(top)
There are a few types of clutch designs. Keep in mind that clutch feel, chatter, drivability, and smoothness is very subjective. Don't buy more clutch than you realistically need since it's normally at the loss of drivability. If you are using a solid flywheel I suggest using a full face sprung hub clutch. If you are using a dual mass flywheel you should use a full face solid hub clutch. Most TDI are not being used for racing, so a full faced clutch is a better choice than a puck clutch for street use. A full face means that the clutch is shaped like a ring with the splined hub in the middle. All TDI clutches are dry clutches, this article does not discuss the wet clutches used in the DSG "manual - automatic" transmission, see the DSG FAQ here for details on the DSG transmission.
A full face sprung hub clutch uses springs in the center splined hub and a marcel spring (looks like a thin wavy ring) between the plates to help absorb energy transmitted from the engine and make clutch engagement smoother. It is held together with rivets that go though one face, through the marcel springs sandwiched between the faces, and to the other face. With a solid flywheel, a sprung hub with full face will give the smoothest clutch engagement. Pictured below left is a full faced sprung hub. The springs soften driveline shock. The only disadvantage is that the springs could break out of the spring window, pictured below right. A clutch like this one was used on all mk3 TDI.
Clutches used with DMF normally use a solid hub because the DMF does an even better job of absorbing energy than a sprung hub clutch. Because a solid hub clutch has less mass, it puts less wear on the transmission synchronizers. Since there are no hub springs, it's also less likely to break and damage the clutch system. All mk4+ TDI used a solid hub full faced disk. In theory, switching to a solid flywheel and using a sprung hub clutch increases transmission synchro wear due to heavier clutch disk weight, but the difference is minimal. The same basic transmission is used in the mk3 cars, and they all came with sprung hub clutches, so in reality, driving/shifting style is a far greater factor in synchro and transmission wear. A solid hub looks like the hub in the puck clutch pictured below.
A puck clutch (pictured below) is a clutch that has a star shaped friction surface instead of a disk. Some are solid hubs, others are sprung. It's better than a full face disk for racing use because it clamps faster and harder due to the materials used and the design. They could be used for street use but have lower lifespan and an on-off engagement because the design and materials used make it hard to slip. People watching you parallel park with a puck clutch may think you don't know how to drive stick because trying to slip one will cause the car to buck. I don't think many TDI drivers are drag racing enough to warrant a puck clutch so I do not suggest a puck clutch.
A twin disk system is a clutch kit that uses two clutch disks, more on this below. In my opinion, this is overkill for any 4 cylinder TDI. If you had a 6 cylinder AWD drag racing TDI, a twin disk would be a great choice to avoid constantly frying clutches.
Clutch disk materials(top)
Keep in mind that some disks have one side made of one material and the other side made of another. Keep in mind that clutch feel, chatter, drivability and smoothness is very subjective. Old cars used clutches that contained asbestos, I don't think anyone sells these in the US anymore but the general precaution when working on an old car, especially pre-1990, is to treat it as if dust inside the transmission bellhousing could contain asbestos. I don't use air tools on the flywheel to keep dust down.
Pictured on the below is a kevlar disk in yellow, copper disk at bottom left, and black organic face disk at bottom right. All use sprung hubs. Some clutch material terms are misnamed because of marketing.
Most street disks are made with organic faced material. This is similar to brake pad material. It has the smoothest engagement but can slip if it overheats. If you are shifting under high rpm and heavy load, this can overheat an organic disk, resulting in a burning smell and a lower coefficient of friction. Typical stop-go driving is not enough to overheat this disk but excessive slipping can. Full throttle drag races with heavy clutch slipping can glaze this type of disk. The nice thing about organic disks is that if it starts to smoke, let it cool and drive normally. It may regain most of it's clamping force. It is probably the best choice for a smooth, long life, lightweight clutch on a daily driver.
Feramic is a good choice for mild drag racing and street use. It is a mix of ceramic and sintered iron. It's normally found in a full faced disk and can become hotter than an organic disk without weakening, about 900o. Because of this, it's a good choice if you want a smooth street disk and plan on doing an occasional drag race. However, since it is also heavier than organic disks, it will wear out the transmission synchros faster than a lighter disk. It will also not rev as fast but the effect will not be very noticeable on a TDI. Some clutches also use feramic on the flywheel side to withstand slipping and organic on the pressure plate side for smoother engagement and weight loss. It is also a good choice for AWD vehicles used in drag racing because an AWD launch involves more slipping than a 2WD vehicle.
Sintered iron is harder than feramic. An iron clutch will have more grip than a feramic clutch but will wear out the flywheel and pressure plate faster and are heavier. An iron clutch will be slightly softer than a ceramic clutch but both are considered aggressive clutches. Expect a short clutch/flywheel life, I do not recommend for the TDI.
Kevlar clutches can be more expensive than an organic clutch. Engagement depends on temperature and like feramic, it can withstand a lot more slipping and higher temps than an organic clutch. This clutch is resistant to glazing but once it does, will have a hard time recovering. For a more modified car, this may be a good choice and it should have a longer clutch life compare to other aggressive clutches.
Ceramic clutches are harder than sintered iron and is part of what feramic clutches are a blend of. A ceramic clutch is considered a hard clutch and is very grabby. It is more of a race clutch than for street due to it's hard engagement.
Bronze alloy is often used for puck clutches. Because you normally find this on a puck clutch, it will have harsh engagement. It will also wear into the flywheel more and have a shorter lifespan. It will also have a shorter lifespan, not recommended for a TDI. This is more of a race clutch than a street clutch material.
Carbon clutches are normally used for racing applications where it is expected to slip a lot. They can withstand up to 2000o temps without destruction. Engagement changes a lot depending on temperature and they normally slip a bit under normal street use. This is a clutch that needs to slip and heat up before reaching maximum holding force. It's actually a clutch which you can slip without damaging the clutch material. Again, since "drivability" is a very subjective term, these suggestions are conservative. I would not use this on a street car.
Bearing wear vs increased pressure plate clamping force(top)
You may be asking yourself, to keep smooth engagement and increase clamping force, why not just use less aggressive clutch disks with a stronger pressure plate? There is room for the designers to adjust clutch material and pressure plate strength, but in general, you don't want to increase pressure plate clamping force too much for a number of reasons. The VW TDI engine is only 4 cylinders and doesn't make a huge amount of torque compared to much larger performance engines, but it's part of why you don't want a clutch kit that is overkill. Again, modify your car with a goal.
First is increased clutch pedal effort. Everything else being equal, your clutch pedal will be harder to actuate with a stronger pressure plate. However, all clutches and pressure plates are not equal. When moving from a stock TDI clutch kit to a stock vr6 clutch kit, you use the vr6 pressure plate. The diaphragm spring lever arms are not at the same fulcrum as the TDI spring arms, so it actually reduces pedal effort. Below is a cutaway picture from luk.com of a pressure plate. The side view shows the diaphragm spring pivoting on a fulcrum. By changing the fulcrum, you change pedal effort and clamping force.
A very heavy pedal also takes a toll on other clutch system components. If you are building a setup with a very heavy pedal, depending on mileage, you should rebuild the clutch/slave cylinders as preventative maintenance and replace the fork pivot ball with a hardened piece with the same dimensions. I've broken clutch forks and noticed wear on non TDI fork pivot balls/pins as well. Due to the relatively low power levels of the 4 cylinder TDI, there are not a large number of broken forks but it's a good idea to replace it at the same time as clutch replacement if you have high mileage. Clutch pedal bushing wear and flexing of the firewall where the clutch pedal is mounted can also be a big problem with heavy clutch pedals by affecting clutch pedal action. This is not a significant factor with the 4 cylinder TDI due to the relatively light clutches used.
Here are some pictures of the front and back of a pressure plate. The release bearing was placed on top of the PP springs. Your TDI clutch may look different.
Another big reason why it's better to use a more aggressive clutch material and a lighter spring is because it reduces axial loading on the crankshaft bearings. The disadvantage is that a more aggressive material tends to have a hard engagement and/or shorter life. A heavier pedal, especially on cold starts where the crankshaft thrust bearing is not getting lubrication, does wear parts more than they were designed for. The increased load on the crankshaft bearings and hydraulic system is only when the clutch pedal is down. This is because when the clutch pedal is up and the clutch is engaged, the force of the pressure plate is working against the pressure plate bolts which are attached to the flywheel. When you press the pedal down, the release bearing is pushing the pressure plate which is pushing the crankshaft. This is another reason why it's better for engine wear to leave the shifter in neutral and clutch pedal up when you are sitting at a stoplight. This also prevents the car from suddenly moving forward if the clutch pedal suddenly fails but it slows your reaction if you need to quickly move forward, like if you are about to get rear ended. If your car has a low oil pressure problem, this makes the problem even worse. Again, the VW TDI uses a pretty light clutch with a pretty stout engine, so this issue does not appear to be a major factor for the TDI.
To reduce pressure plate load and increase clutch strength, a twin disk is a good option. This doubles the friction surface area which doubles the torque rating with the same clamp load. It lets you use a less aggressive friction material which also increases drivability. In my opinion, this is way overkill for a 4 cylinder TDI. Here are two examples of twin disk clutch kits, from www.gmhightechperformance.com/tech/0708gm_2006_chevy_corvette_clutch_kit/photo_02.html and www.vetteweb.com/tech/vemp_0710_corvette_twin_disc_clutch/photo_06.html.
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