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

Induction System

  Subaru engines all have the throttle body at the flywheel engine, pointing towards the gearbox, and upwards at an angle. In the T25 / T3 / Vanagon model, this is not a problem, as there is enough room to fit the induction pipe. 

Standard Orientation Induction Manifolds:

P1160038  Subaru’s have plenty of room above the bellhousing, allowing part of the induction system to be positioned there. The induction pipe fitting on the throttle body is positioned directly above the bellhousing / engine join as standard, as shown taped up here.
  In T25’s, there is also a bulkhead in this location. However, they are further forward, and do (just) leave enough room for fitting a hose.


  Conveniently some Subaru’s have an induction hose which looks as if it is made for the job of a T25    conversion, although they are still a tight fit up against the bulkhead. These hoses are fitted to Legacy’s made between 1990 and about 1995. Later model engines have an air box located behind the engine, where the intercooler would be on a turbo. These are too bulky to fit in a T25, but the pipe from an earlier model can be fitted instead.



Subaru Induction Pipe - side

Subaru Induction Pipe - side

Subaru Induction Pipe - top

Subaru Induction Pipe - top

Installed in a T25 - Late

Installed in a T25 - Late

Installed in T25 - early

Installed in T25 - early





  The pipe which fits so well is shown above - note how they even fit around the electrical junction box as if they have been designed for the job.
  Also, on T25’s, the ‘bulkhead’ does not actually do anything - in the middle it is only a few inches deep, and there is nothing in front of it on 2WD models, so it can be cut away if required. Syncro’s have the petrol tank located immediately in front of this bulkhead, so the bulkhead is best left in place.


Relocating the Throttle Body:

  On Beetle and Bay window bus conversions here there is a bulkhead in the bodywork right in the way here - there is not enough room to get a hose on the throttle body without major body modifications. For these, a more radical fix is needed to get the induction system in. How this is achieved depends on the age of the Subaru engine:

Throttle Body Reverser - MY90 - MY99:

throttle body reverser03  From 1990 to 1999 Subaru engines had their idle speed control valve mounted on the induction manifold. The positioning of the valve means that if the entire manifold it turned around 180 degrees, there is not enough space for the alternator. Because of this, the manifold must be left where Subaru intended, however, the remote location of the ISC valve relative to the throttle body means that the throttle body alone can be reversed using one of our throttle body reversers, as shown to the left. This positions the throttle body above the coil pack.
   Our throttle body reversers use the tightest bend radius tubing available, which makes them compacte enought to fit in both bay window and split screen buses with no bodywork modifications. The photo below shows a 1966 split screen bus with a 1995 EJ20 engine installation. The throttle body reversers also have a built in a throttle cable bracket, positioned so the throttle cable can follow the most direct route to the VW chassis fitting.RIMG0012




  Note there is not enough height available to reverse the throttle body in a T25 / T3 / Vanagon, but these models have enough space to not have the throttle body space problem. The only T25 models which do have the bulkhead in the way of the induction pipe are those with early 4 speed gearboxes such as the DK box, which mount the whole engine further forward in the chassis. 




Reversed Induction Manifolds - MY00 -->:

  From 2000 - 2002 all Subaru’s had the idle speed control valve relocated to on top of the throttle body. The new stepper motor ISC valve is much smaller than the older manifold mounted pulse width modulated valve. This relocation of the valve means that when the entire induction manifold is reversed in the engine, there is enough room for the (relocated) alternator. This is the only option for moving the throttle body on a 2000 --> engine, as the complex extra passageways for the ISC valve mean that reversing just the throttle body (as with 1990 - 1999 models, above) is impractical: P1160030


  Subaru themselves (or rather Prodrive, the WRC car builders) reversed the manifold on the group A spec Impreza rally cars, as these had a large front mounted intercooler, and reversing the manifold tidied the plumbing.

  Reversing the induction manifold can be advantageous in a Beetle or a Bay window bus, as a means of avoiding major bodywork surgery. However, it is not as straight forward as it sounds though, for a number of reasons:

  1. The throttle body will be where the alternator would usually go
  2. The entire engine electrical system is mounted to the manifold, and needs to retain it’s original orientation
  3. The fuel pipes are attached to the manifold, and can not be reversed easily. They are likely to foul the coolant outlet manifold when the induction manifold is reversed

 P101018 We make a manifold reversal kit which solves the above problems by providing a bracket to relocate the alternator to where the air con compressor would have originally have been located, and manifold spacers which raise the induction manifold by 16 mm. This make the job a ‘bolt’ on conversion, apart from having to extend one or two branches of the wiring harness. It may be possible to reverse the manifold without using the manifold spacers, but fuel system modifications are likely to be needed, and the alternator may foul the throttle body - it depends on the exact engine configuration that you have. The photo shows the prototype alternator bracket fitted to a bare long block engine. P1010015




  The alternator brackets supplied in the manifold reversal kit are zinc plated and gold passivated, not bare steel, as shown. The come with mounting bolts, fit all SOHC (twin cam)and DOHC (quad cam) EJ series engines, and need no modifications to the heater return pipe or dipstick tube.



Drive By Wire some 2002 -->

  From 2002 some Subaru models have a ‘drive by wire’ throttle. This is an electronic system which does away with both the throttle cable and the idle speed control valve.

 Inventors of the Subaru - VW conversion bell housing using Subaru flywheel, clutch and starter

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