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 Subaru Conversion Information Resource Navigation Bar - over 160 pages of info on Subaru powered VW’s! :

Bellhousing FAQ's
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Please click the questions below to see the answers:
 

Q1: Which Subaru engines does the conversion bellhousing work with?

Q2: Which VW transaxles does the conversion bellhousing work with?

Q3: What are the advantages of a conversion bellhousing over an adaptor plate type?

Q4: Are there any advantages of an adaptor plate / welded flywheel conversion over a conversion bellhousing?

Q5: What Subaru engines are recommended for the conversion?

Q6: Can the bellhousing be used to fit a Subaru engine to a VW automatic transmission?

Q7: Is it necessary to modify the bodywork to fit a Subaru engine?

Q8: Can the conversion be used to fit a Subaru engine into a Beetle?

Q9: What clutch does the conversion bellhousing use?

Q10: What flywheel does the conversion bellhousing use?

Q11: Which starter motor does the conversion use?

Q12: How is the VW transaxle input shaft supported in the Subaru Flywheel?

Q13: Can the conversion bellhousing be used with Phase I and Phase II Subaru engines?

Q14: What is the bellhousing made from?

Q15: How are the conversion parts made?

Q16: Does the conversion work with cable operated VW clutches (as used in the bay window 091 VW transaxles)?

Q17: Can Japanese spec (JDM) import engines be used?

Q18: Are all of the serviceable parts readily available?

Q19: What parts are needed outside those supplied in the kit to join the Subaru engine to the VW transaxle?

Q20: Where does the engine sit in relation to the original VW engine?

Q21: What does a Subaru engine weigh in comparison to the original VW engine?

Q22: Will fitting a Subaru engine affect the ground clearance of my VW?

Q23: Will the Subaru Clutch be up to the job of moving my heavy VW?

Q24: What is OBD I and OBD II?

Q25: What is the difference between Phase I and Phase II Subaru engines?

Q26: I’ve seen people running Subaru engines in VW’s with no cam belt covers, What is the advantage of this?

Q27: Why can’t the Subaru gearbox be modified for use in a VW?

 

 

 

 

 

 

 

 

 

 

Q1: Which Subaru engines does the conversion bellhousing work with?

A1: Any 4 or 6 cylinder SOHC or DOHC Subaru engine from a 1990 onwards manual transmission donor car, naturally aspirated or turbocharged. Engines from automatic Subaru’s can quite easily be converted to manual spec (by adding a flywheel and clutch, plus a couple of other bits and pieces). The conversion is slightly more involved for a Subaru turbo engine, as the clutches work very differently. It is also necessary to convert to a hydraulic clutch system (if your VW doesn’t have one already), if you are using a turbo engine. Note there will be a lot more complications when fitting a turbo Subaru engine due to the position of the turbo equipment LINK.

Engines can be found in quite a wide range of donor Subaru’s, although exactly what models depends on what country your donor is from. The options available different countries vary a lot.

The four cylinder engines are fitted to Legacy, Impreza, Forester, Loyale, and XT models, with the more powerful ones usually being from the high spec Impreza and Legacy models. The 3.3 six cylinder models can be found in SVX models, and the 3.0 six cylinder in Legacy’s.

Note that Impreza parts often cost more than the other car’s even if they are the same. This is because of the much larger market for Impreza parts because of their widespread modification and rallying use.

The bellhousings do not fit the pre 1990 engines with a removable flywheel housing. This includes the 1.8 and 2.7 models.

This table LINK shows a more detailed guide to the different Subaru engines

 

Q2: Which VW transaxles does the conversion bellhousing work with?

A2: The conversion bellhousing works with any VW bus manual transaxle as used from 1968 to 1992. That’s all the 002, 091, and 094 type transaxles, in all variations, including the five speed and 4WD Syncro models.

Type 25 (Vanagon) 1980 - 1992 All models, air and water cooled, manual 4 or 5 speed transaxle, rear wheel drive or 4 wheel drive Syncro models, and hydraulic* or cable operated clutch. That’s all 091(side mounted shift rod) and 094 transaxles.

Type 2 (Bay Window) 1968 – 1979 all models with a manual transaxle. May also fit some later model South African / Brazilian models, but not the ones with the ‘type 1’ Beetle IRS transaxle, unless you convert to a bus transaxle. That’s all 002 and 091 transaxles. A few minor modifications to the bodywork are needed (only visible from either underneath or in the engine bay).

Beetles (all Models) Beetle transaxles can’t have their bellhousings removed – they’re part of the main case casting. To fit a Subaru engine into a Beetle, you need to either use an adaptor plate / fabricated flywheel type kit, or fit a transaxle from a bay window bus and an adaptor bellhousing. However, putting any water cooled engine into a Beetle is never going to be straight forward as there is no easy place to put a radiator. Fitting a bus transaxle into a Beetle is far from a straight forward job, but relative to the rest of the work installing a Subaru engine, it is worth considering. The bus transaxles are a lot stronger than Beetle ones, mainly due to their much larger hypoid design ring and pinion. They are widely used in sand rails because of the extra strength, so there is a wide range of aftermarket ratios, diffs, and other uprating parts available. Bus ‘boxes into Beetles is a relatively new idea, and it is mainly complicated by the much bulkier design of the casing, but that’s because they’re stronger.

NOTE: Air cooled vehicles will obviously need extra work to add a radiator.

 

 

Q3: What are the advantages of a conversion bellhousing over an adaptor plate type?

A3: There are a number of advantages of a conversion bellhousing over an adaptor plate / flywheel type. The main ones are:

1. All service replaceable parts such as clutch pressure plates, flywheels, clutch discs and release bearings (for most combinations of engines and VW transaxles) are standard VW or Subaru parts, and are therefore widely available. This also means that aftermarket or upgrade parts intended for the VW or Subaru applications can also be used for the conversion.

2. All clutch parts needed for the conversion will either be on your original VW engine or new Subaru engine for the majority of the more common conversion combinations. This means that an expensive aftermarket heavy duty VW clutch is not necessary, as it is with an adaptor plate / flywheel type kit. Replacement of the pressure plate, disc, and release bearing with new parts is recommended, unless you know that the used parts you have are in good condition – it’s up to you. Note: some of the less common combinations of engine and transaxle do need some extra aftermarket or uprated parts.

3. From an engineering point of view, a bellhousing is a far superior design to an adaptor plate. This is especially true if a special flywheel and starter motor are also needed with the adaptor plate, and even more so when the special flywheel available for such conversions is sometimes fabricated from up to four pieces of steel. The quality of some of these conversion flywheels can at best be described as very poor. At least one is a very nicely machined two piece fabrication, but it is still a fabrication (not good).

There are a number of reasons why the adaptor plate design is flawed though. The details can’t be answered in a couple of lines, so please click the following links for more info:

Flywheels    Starter Motors    Engineering Details

4. A conversion bellhousing ‘looks right’. It does not look like an adaptor which crudely joins two parts which are clearly not designed to go together. Instead it makes the conversion look like the engine and transaxle are intended to be joined together. It has a much more ‘production car’ look. If the bellhousing were the same design but die cast from magnesium, like the VW bellhousing, it wouldn’t really stand out as a conversion at all. (Die cast magnesium is not used for production volume reasons).

5. An extra advantage for Syncro’s. A customer bought a bell housing to replace his adaptor plate after getting through six VW starter motors on his adaptor plate conversion in a Syncro. His direct comparison of the two exposed an advantage of our bell housings which we were previously unaware of.
   The combination of a heavy vehicle and very low speed off road means that flywheel inertia plays a bigger part in keeping a Syncro moving then usual. The customer reported that as well as starting without the motor sounding like it was struggling, his Syncro was far more driveable in the ‘crawler’ gear off road once he converted to an RJES Bell Housing in his EJ25 powered Syncro. The reason is the increased rotational inertia of the Subaru flywheel due to it’s increased radius. He reported that with his adaptor plate he was constantly balancing revs and clutch slip to keep moving, and with his RJES bell housing and VW flywheel he could just drive over the same terrain normally.
   For more info on Subaru flywheel types (there are many different types, with different weights, etc), please see Flywheel Types.
   Thanks to Yurik in Australia for his feedback on this.

 

Q4: Are there any advantages of an adaptor plate / welded flywheel conversion over a conversion bellhousing?

A4: Yes, one. The adaptor plate / welded flywheel type can be used on Beetle transaxles with their integral bellhousing. Conversion bellhousings can’t.

 

Q5: What Subaru engines are recommended for the conversion?

A5: The 2.2 litre flat four naturally aspirated engines (130 bhp) have always been the most common for use in VW conversions. Those from ’92 – ’96 are the most popular. They have the slightly simpler OBD I engine management diagnostic system. These engines can be very cheap now – from as little as 150 (240 ish euro’s) complete with all the wiring and ECU, etc. However, as the conversion is a big job, and you probably want better performance, fuel efficiency, and reliability from your new engine, without the risk of using a 10 year old engine with high mileage. Why not got for a lower mileage, more recent model? For around 2 – 3 times as much money (still less than you’d pay for an exchange reconditioned VW engine), you can get a more recent, lower mileage 2.2, or even the more efficient 2000 onwards 2.0 litre (126 bhp). These have the OBD II diagnostic system, and although the wiring is a bit more complex, the extra info you can get from the OBD II system may well be worth it when setting up your engine.

 

Q6: Can the bellhousing be used to fit a Subaru engine to a VW automatic transmission?

A6: No. The VW automatic transaxles do not have removable bellhousings, so it is not possible to make an adaptor bellhousing.

 

Q7: Is it necessary to modify the bodywork to fit a Subaru engine?

A7: That depends on what you’re putting the engine into. The four cylinder Subaru engine itself is a very similar size to a VW Type 4 engine that has had its rear mounted fan replaced by a Porsche fan. The bodywork modifications needed to fit the engine into your chosen VW will be similar to if fitting a Type 4 with a Porsche fan (a fairly common conversion), and will easily fit within the external bodywork of almost all VW’s, with minimal modification to the engine bay. The six cylinders are larger, particularly the 3.3, and would not fit within the external bodywork of a Beetle. They should fit into type 2’s, 3’s, Karmann Ghia’s etc. OK though.
   The DOHC Subaru engines can require body modifications when fitting into the small engine bay of a Beetle. The DOHC engines are a tighter fit due to the cylinder heads being much taller to accommodate the extra cams. The plastic cam covers fouls with the gutter around the engine lid, just above the rear valance. Running with no cam belt covers to gain some extra clearance in this area is NOT recommended. Accept the fact that a DOHC engine in a Beetle is a more extreme conversion, so more extreme modifications are going to be needed to get it to fit properly!

The bellhousing is larger then the VW one (due to the Subaru’s bigger flywheel diameter. This is not a problem on Type 25 / Vanagon’s, but some minor bodywork mods would be needed on a Bay window bus or Beetle in the area above the bellhousing. Note it is not necessary to fit a large metal ‘box’ in the luggage area behind the back seat in a Beetle for a nat asp Subaru engine. This would be necessary for a turbo Subaru engine though, unless you want to get into relocating the turbo and intercooler. Fitting a bus transaxle into a Beetle is a fairly serious modification though – not a small job.

 

Q8: Can the conversion be used to fit a Subaru engine into a Beetle?

A8: Yes, but not directly - the transaxle must be swapped for one from a bay window (1968 – 1979) bus. This is not a simple conversion, and can be done a number of ways, but the bus transaxles are much stronger than Beetle ones, so it may be worth the effort, especially for racing or turbo Subaru engines. We’re looking into developing a mount kit to put the Subaru engine / VW bus transaxle combination into Beetles.

 

Q9: What clutch does the conversion bellhousing use?

A9: The clutch is a combination of Subaru and VW parts.

 

Q10: What flywheel does the conversion bellhousing use?

A10: The original Subaru flywheel. Light weight aftermarket flywheels are available for Subaru engines too. These are a bolt on replacement for the standard ones, and would be worth using if you want to improve engine responsiveness with an engine used in a light weight car. Light flywheels should not be used with VW buses though, as the extra inertia of a standard flywheel would be of benefit in the heavier bus.

 

Q11: Which starter motor does the conversion use?

A11: Any starter, direct drive (nat asp) or geared (turbo) from a manual transmission Subaru. Starters from automatic Subaru’s are not suitable.

Q12: How is the VW transaxle input shaft supported in the Subaru Flywheel?

A12: It is supported by a sealed ball race, not an unsealed needle roller bearing as seen in at least one adaptor plate / welded flywheel type conversion. That’s not to say that all adaptor plate / welded flywheel kits miss out the VW felt seal, but at least one does.

 

Q13: Can the conversion bellhousing be used with Phase I and Phase II Subaru engines?

A13: Yes. It was designed around a Phase II. To fit the bellhousing to a Phase I Subaru engine, you just don’t use some of the bellhousing bolt holes.

 

Q14: What is the bellhousing made from?

A14: LM25 aluminium alloy, heat treated to condition TF. This is a UK equivalent grade to 356 T6, the US grade used for the majority of drag racing engine cases and cylinder heads (Scat, Autocraft, Pauter, etc). It is one of the strongest grades available.

 

Q15: How are the conversion parts made?

A15: The bellhousing is sand cast. The sand mould is formed around a cast aluminium pattern. The casting is then heat treated to strengthen it, before being CNC machined. The few steel plate parts are laser cut and TIG welded and zinc plated. The spring clips are CNC wireformed.

 

Q16: Does the conversion work with cable operated VW clutches (as used in the bay window 091 VW transaxles)?

A16: The kit for use with a naturally aspirated Subaru engine works with either cable or hydraulic VW clutches. If you are fitting a turbo engine, it will be necessary to convert to a hydraulic clutch. Note there will be a lot more complications when fitting a turbo Subaru engine due to the position of the turbo equipment LINK.

 

Q17: Can Japanese spec (JDM) import engines be used?

A17: Yes. There are plenty of different types to choose from. Many are more powerful than the models sold to the rest of the world, and often feature new technology such as variable valve timing before it is offered to the rest of the world. Bear in mind though, that most seem to require premium grade (97 RON or higher) petrol. Also parts for Jap spec engines are not available through the main dealers in Europe, and the availability of things like wiring diagrams may be a problem.

 

Q18: Are all of the serviceable parts readily available?

A18: Yes. The clutch parts are all either VW or Subaru, with the exception being the clutch disc for either a Subaru engine with a dual mass flywheel (as used on some naturally aspirated models from 2000 onwards), or some turbo models from 2001 onwards. These use aftermarket parts, as, to the best of our knowledge, there are no suitable production car parts available. In most cases, all the parts you need will be fitted to your VW or Subaru engine. Whether new ones are fitted at the time of conversion is up to you. It depends on the condition of the originals, and whether you know their history.

 

Q19: What parts are needed outside those supplied in the kit to join the Subaru engine to the VW transaxle?

A19: Non, unless you have to replace worn originals, or use all new service parts (recommended, but not essential). No expensive extras like heavy duty VW clutches are needed.

 

Q20: Where does the engine sit in relation to the original VW engine?

A20: The bellhousing is 121 mm thick, compared to 125 mm thick for the original VW one, so the engine fits 4 mm further forward compared to a VW engine. An adaptor plate / welded flywheel conversion would put the engine approximately 12mm further back in the VW compared to a VW engine.

Some off road users consider any move of the engine forward (even just a few millimetres, like this) to be an advantage, as it reduces the rear weight distribution balance in a rear engined vehicle. However, in reality it is very unlikely that such a small move would produce a noticeable or measurable performance increase.

 

Q21: What does a Subaru engine weigh in comparison to a VW engine?

A21: A VW air-cooled Type 4 2.0 litre engine weighs about 130 kg, depending on what emissions equipment, etc. is fitted.

A naturally aspirated four cylinder 2.2 Subaru engine weighs about 130 kg.

A turbo four cylinder 2.0 Subaru engine weighs about 150 kg.

A 3.0 six cylinder Subaru engine weighs about 169 kg (2000 – 2003), or 160 kg (2003 ). Note just 10 – 20 kg more then a four cylinder turbo.

A 3.3 six cylinder Subaru engine weighs about 180 kg (estimated).

 

Q22: Will fitting a Subaru engine affect the ground clearance of my VW?

A22: Yes – there are two things on Subaru engines which present a ground clearance problem when fitted to a VW – sumps and exhausts:

Sumps

Despite Subaru’s marketing info stating the advantages of a horizontally opposed cylinder arrangement in terms of a low centre of gravity height, Subaru engines actually sit high in the chassis compared to VW engines. This is not for ground clearance reasons, as below the engine block they have a sump which is very deep. Unfortunately all Subaru engines except the 3.0 Litre ‘ZE30’ flat six feature this deep sump. You lose about 70 mm of ground clearance if a Subaru engine is fitted to your VW with it’s standard sump. This is a problem for standard height VW’s, never mind lowered ones, or Type 25 (Vanagon) Syncro’s, with their under engine skid plates. The ground clearance problem can be reduced by fitting a shallow sump. Some of these are very crudely converted standard sumps with a couple of inches cut off the bottom. They run a reduced oil capacity, and higher level than a standard engine – both undesirable. A slight improvement is a fabricated sump which has the removed oil capacity added elsewhere. This initially seems a good idea, until you try to fit one. Some of the sump mounting screws are almost impossible to get at. Another problem with both of these types is that they have to be made from your original Subaru sump and oil suction pipe, requiring sending them away to be chopped up and modified, which is inconvenient.

What is needed as a proper solution to the ground clearance problem is a totally new sump made for the job which is easy to fit (both with the engine in or out), retains the standard oil capacity and oil level, and gives the same ground clearance as a standard VW engine.

Exhausts

Subaru exhausts also hang very low beneath the engine, but not quite as low as the sump. The low Subaru exhaust header on a naturally aspirated Subaru engine is not a problem, as its shape makes it no use on a VW conversion. A bespoke exhaust system is needed.

On turbo charged Subaru engines the very low exhaust header is a problem as it can not be swapped for a reshaped version with increased ground clearance without a major redesign. However, this is just one of quite a few things which will be a problem if you want to fit a Subaru turbo engine to your VW. These are mainly to do with the position of the turbo and intercooler interfering with the VW bodywork. Ideally, they both need to be moved, which would require a new exhaust any way.

 

 

Q23: Will the Subaru Clutch be up to the job of moving my heavy VW?

A23: The clamping force of a clutch pressure plate is determined by the engine power, not the vehicle weight. The amount of heat a clutch must absorb without being damaged to is determined by vehicle weight. If you have a heavy VW bus such as a fully loaded up Syncro, with a full camper interior, then yes, it will weigh a fair bit more than the heaviest Subaru Legacy. However, bear in mind that the Legacy is widely used as a towing car too. You should not have clutch problems unless you intend to tow with your fully loaded camper, or do ‘drag racing’ starts.

If you do want to increase the capacity of your clutch, then you could either convert to a standard one from a higher powered Subaru. These should be able to dissipate more heat without damage, so they are more abuse resistant in the really powerful Subaru’s. Alternatively, fit an aftermarket heavy duty pressure plate and / or a metallic clutch disc. Metallic discs do not slip much, so have a very on and off action. They will not be nice to drive on the street, but because they don’t slip much, they don’t generate much heat in the first place (which is why they are used for racing).

Note, it may be worth replacing a dual mass flywheel with a single mass one if you are putting your Subaru engine into a very heavy VW and intend to drive it hard. The single mass flywheels are a lot better at dissipating the heat.

 

Q24: What is OBD I and OBD II?

A24: OBD stands for on board diagnostics. Car manufacturers started to widely fit these systems in the mid eighties. Their principle function was to help diagnose faults with anything which affected the exhaust emissions. It is a very useful way of fault finding when splitting out the engine wiring from the rest of a Subaru harness. There are two systems, depending on the age of the engine:

OBD I:

Almost all Subarus from 1990 to 1996 have an OBD I system. The OBD I systems varied from one manufacturer to another, but all allowed a technician to plug diagnostic equipment in to obtain basic information about faults. Typically that information tells you what circuit has a fault. Some manufacturers included a built in system to allow you to read basic diagnostic info without any special equipment, Fortunately this includes Subaru. You read error codes as pulsed codes flashed on an LED. This is a very useful feature.

OBD II:

In 1996 it became mandatory for manufacturers to fit an on board diagnostic system which was more advanced then OBD I. The new system, ODB II, was also much more standardised across all vehicles. The plug in the vehicle is always the same, and it is always located in the dash, somewhere below the steering column. Some manufacturers (not Subaru) had been using the OBD II system from 1994 or 95. Although the plug is the same, there are three connectivity standards used, which are not compatible. Subaru, along with a lot of other manufacturers use the ISO standard.

The OBD II system can give a lot more information, such as what type of fault (open circuit, short circuit, out or range, etc.). It also expanded to cover a lot more than just the emissions control system. All electronic systems on most modern cars are diagnosed via the OBD II socket, and it can also be used to reprogram most of the ECU’s, given the right software. Some cars can have the engine management remapped through the OBD II socket, as an alternative to ‘chipping’. For a long time nobody had worked out how to re-map Subaru ECU’s, but it can now be done. Because OBD II is a lot more standardised, relatively inexpensive code readers have been developed. These are easily affordable by the keen amateur, and usually work via PDA or laptop. Some of these allow you to datalog what is happening with any of the engine parameters too, whilst driving. Fortunately, Subaru also retained a system for reading the error codes without any special equipment, again by reading codes off a flashing LED.

 

Q25: What is the difference between Phase I and Phase II Subaru engines?

A25: Subaru had a design update of the 4 cylinder engines around the 2000 model year introduction. Those before this change are known as Phase I, those after, Phase II. The Phase II Subaru engines feature a range of minor modifications throughout. The main ones which we are interested in here are the addition of three extra bellhousing flange bolts, and the introduction of dual mass flywheels LINK for some models.

Just to make it more confusing though, it seems that not all the changes cut in at once, and although the old 2.5 DOHC had it’s cylinder heads replaced with a new SOHC design, there are also some crossover models which feature a Phase 2 crank case with Phase 1 cylinder heads. PHASE 1 / 2 PHOTOS

 

Q26: I’ve seen people running Subaru engines in VW’s with no cam belt covers, What is the advantage of this?

A26: In a word, nothing! It is VERY dangerous, both for you and your engine. Do you want to risk losing a finger, etc when working on the engine?
   Running with no cam belt covers is NOT a good idea, unless you don’t mind breaking cam belts when dirt gets between the belt and the pulleys. Don’t be misled by reading that Subaru engines are of a ‘non-interferance’ design, so it doesn’t matter if the belt breaks. This is certainly true of some SOHC Subaru engines. The valves will not hit the pistons on a DOHC engine, however, they will hit each other unless both cams remain in phase at all times. Expect the damage to be at least eight bent valves if cam belt breaks on a DOHC engine.
   The DOHC Subaru engines can require body modifications when fitting into the small engine bay of a Beetle. This is a possible reason why people may remove the covers. The DOHC engines are a tighter fit due to the cylinder heads being much taller to accommodate the extra cams. The plastic cam covers fouls with the gutter around the engine lid, just above the rear valance.

 

Q27: Why can’t the Subaru gearbox be modified for use in a VW?

A27: We get asked this all the time, sometimes by people who don’t realise that in a rear engine application they will have 5 / 6 reverse gears and won forward, or by people dreaming up all sorts of exotic, but totally impractical methods.

  • There is not room inside a Subaru gearbox casing for the ring gear ans differential to be on the opposite side of the pinion gear.
  • Subaru’s along with most modern gearboxes use a hypoid ring an pinion, as they have a number of advantages. This includes all VW bus boxes, and all Subaru and modern Porsche boxes. To be able to reverse your existing diff, it must be of a straight cut bevel gear or spiral bevel gear design, like a Beetle. People dreaming up ways of reversing the diff either don’t realise this, or don’t understand the geometry of a hypoid R&P. See this link for more infoThere is a height difference between the pinion centre line and the ring gear centre line, do to reverse the diff would also result in a height change of double this difference. See The only way around this is to have aan opposite handed ring and pinion custom made.

 

 

Starter Motors:

The problems with burned out starter motors on adaptor plate type conversions are likely to be due to using the 132 tooth VW sized flywheel on a Subaru engine. The VW flywheel is considerably smaller than the Subaru one, resulting in less cranking torque. This is made worse on the naturally aspirated Subaru engines, as they have a relatively high compression ratio in comparison to a VW engine, and so require more cranking torque.

The usual work around for this problem is to either use an expensive aftermarket geared high torque starter, or more powerful ones modified from either Porsche or diesel engines. Why have all this hassle, when you could be using the starter and flywheel designed for the engine?

Which Subaru starter motors can be used?

 

Flywheels:

Welded flywheels will never be found on production cars, mainly because there is no need to fabricate them when mass produced, but also because they are subject to a lot of testing. Flywheels transmit and store a lot of energy. Engine manufacturer’s test each design’s safety factor by spinning one way beyond it’s expected rpm range – about 2.5 times faster. This is called a burst test, as flywheel’s fail by exploding due to centripetal forces when they either contain weaknesses, or are spinning too fast. Exploding releases all the energy stored in the flywheel, and is very dangerous.

Whilst there is nothing wrong with the concept of a fabricated flywheel, the welds would need to be of the very highest quality, with individual x-ray weld analysis and probably individual burst tests for every flywheel. Maybe some of the fabricated flywheels available are subjected to proper testing – if so the suppliers don’t seem to use it as a selling point though. If an engine manufacturer did have to fabricate flywheels for some reason, they would almost certainly choose laser, plasma, or friction welding to ensure 100% weld penetration. This would be performed robotically (not manually) to ensure the very highest quality. The outer surface of the weld would then be machined away to remove surface imperfections.

Most production cars use cast iron flywheels, with the higher quality ones using forgings for extra strength (including all German built VW’s, certainly up to the end of the air cooled era). Cast flywheels (on VW’s and others) can be identified by the fact that they have to have a steel ring gear shrink fitted, as cast iron gear teeth would not be strong enough. Cast VW flywheels are generally considered inferior, and are avoided by people building high performance engines. However, performance VW engines are often built to spin way above the speeds that VW intended them to. Subaru Flywheels are cast, but bear in mind that they’re designed and tested to run at up to the 7000 rpm that some Subaru engines red line at.

That’s not to say a welded flywheel can’t be made, or that all are made from four pieces welded together – there are clearly a lot of satisfied customers using then in adaptor plate type Subaru conversions. However, some certainly are made from four pieces. The extra welding involved greatly increases the chances of there being a tiny fault or crack in the finished flywheel. The four pieces of steel are totally unnecessary. The two piece designs use a new central ‘hub’ welded into a forged steel genuine VW flywheel – a very much simpler and neater design. Fabricating flywheels is a crude engineering solution to the problem of how to install a Subaru engine into a VW.

PHOTO OF 4 PIECE FLYWHEEL

 

Engineering details:

There are a number of details of the adaptor plate / welded flywheel type conversion which are really not examples of good engineering. Apart from the fabricated flywheel, the main one has to be the M10 x 1.5 bolts joining the machined adaptor plate to the transaxle. They are threaded into tapped holes in the plate, but the area that they are screwed into is only 12 mm thick. The generally accepted standard for tapped holes into aluminium for steel bolts is that they should be 2.5 times deeper than the diameter – i.e. 25 mm deep. You don’t see steel M10 bolts threaded into Subaru or VW aluminium or magnesium parts by just 12 mm.

You occasionally hear of people having threads pull out of the adaptor plate on the Subaru conversion web forums. Although not a common fault, it seems to happen more often with the heavier 3.3 litre flat six Subaru engines. It’s hardly surprising.

The RJES conversion bellhousing uses almost all the original VW and Subaru fasteners to attach it to both the engine and transaxle. The design features minimal threaded holes into the aluminium, with the critical holes feature solid steel threaded inserts. Note these are not Helicoils, which can unwind and then be very difficult to fix, but a solid steel ‘tube’ threaded on both inner and outer diameters, and screwed and Loctited into the casting. PHOTO

The alloy which the conversion bellhousing is made from is a very high spec – as good or higher than that which production manufacturers would use.

 

 

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