Throttle body conversion- background information

Introduction: I have been contemplating once again the switch to fuel injection. Much as the twin 40s give the car 'character', throttle bodies still give a nice induction noise, and you lose all of the annoyances of carbs: getting the idle mixtures set up the same on all 4 cylinders, grumpiness in cold weather, occasional popping and banging, not so great fuel consumption (although the 30mpg that I get on a regular basis is not tooooo bad...), the list goes on...

The latest trend for a budget EFI conversion is to use throttle bodies (from now on referred to as 'TBs' as I can't be faffed typing it all the time!) from a motorbike, the most popular being those from GSXR600, 750 and 1000 bikes. These are great in that the 4 TBs are separate castings, and can therefore be respaced with some minor jiggery pokery with the fuel rail and linkages. Here is a summary of the bike TB knowledge I've accumulated so far:

GSXR TB specs:

750 & 1000 from 2000-2003

  • 750 & 1000 are essentially the same

  • 80mm spacing between bore centres

  • 270cc/m injectors

  • 48mm ID, 54mm OD at inlet

  • 42mm ID, 48mm OD at exit

  • Some have a fuel rail that consists of 4 plastic injector connectors joined by separate bits of tube: the tubes can be replaced if respacing the bodies.

  • Alternatively, use a Jenvey fuel rail

  • Contain secondary butterflies that were ECU controlled on the bike, to improve drivability. They can be removed and the holes tapped and blocked with bolts

2000 GSXR750 throttle bodies

600 from 2000-2003

As the 750 & 1000 except for:

  • 230cc/m injectors

  • 44mm ID, 50mm OD at inlet

  • 38mm ID, 44mm OD at exit

  • Normally seem to have a 1 piece fuel rail that will need replacing if respaced

2000 GSXR600 throttle bodies

2000 GSXR600 throttle bodies

Post-2003 600, 750 & 1000

  • Around about this time the design was changed to 2 double throttle bodies (like a set of Jenveys) which makes them unsuitable for respacing

2004 GSXR750 throttle bodies

Pre-2000 750

1998 GSXR750 throttle bodies

Mounting options:
There are a few ways to mount the TBs to the engine:
1 - Custom manifold to bolt directly to the head. If you can't make one yourself (and I can't!), the people to talk to are Bogg Brothers, Altiss or Concept Racing- cost is generally in the region of 120-180. Then use hose to mount the TBs to the manifold:

Altiss manifold

2 - Re-use a DCOE manifold, with adapter stubs to suit. The GSXR TBs do originally come with stubs that will work, but the bolts that hold them to the head on the bike are prone to seizing, and hence they are difficult to get hold of second hand. New from a Suzuki dealer they are about 60 a set. Alternatively, you can get equivalents made:

These stubs were made by Nev Powell- contact via the Sylva Chat list

3 - Adapt the manifold from the original EFI installation. This will vary for different engines, but normally involves chopping down the original manifold, and then either connecting the TBs direct, or welding on some additional tubes, in either case using hose to mount the TBs. In many cases you may be able to re-use the original injector location, which is normally positioned to inject onto the back of the inlet valve(s), which saves the need to use the bike injectors and associated costs with changing the fuel rail.

Example of modified manifold- here for an Audi engine

4 - Use the stub manifold from a Ford Focus ST170. This is obviously only appropriate to Zetec engines, and is similar to option 3 in that you can retain the Ford injectors and fuel rail, with the difference being that you don't need to cut the manifold down- it is in 2 parts already. Discard the plenum section and use the Ford rubber hoses to connect to the TBs. The TBs will still need re-spacing, and the Ford fuel rail needs modifying to add a return line. The injector locations on the TBs can be blocked. However, in my case , the downwards slope of the engine cover would clash with the TBs sitting at an upwards angle, so this method was not an option for me.

ST170 manifold

ST170 manifold with 1998 GSXR750 throttle bodies

ST170 manifold with 1998 GSXR750 throttle bodies

ST170 manifold with 1998 GSXR750 throttle bodies

I have decided to give option 3 a try, with the theory being that I can switch to option 1 at a later date if I'm not happy with the end result. The original Ford Zetec manifold is a little awkward in that there is a 'sandwich plate' mounted to the head which contains the injector ports, and then a separate cast manifold/plenum. I considered whether I could mount the TBs direct to the sandwich plate, but this looked to be difficult. So, I got the angle grinder out and chopped the manifold roughly in 2, separating the plenum from the inlet runners. With the plenum now out of the way, I chopped back the runners further so that I had a 20mm stub protruding from the manifold face. The external webbing had to be cut away to allow a hose to be slid on to the stubs. 

I then took some measurements of the cross section of the runners at the cut-off face. The external circumference of the oval port was the equivalent of a 48mm diameter circle. I noticed that this was exactly the same as the OD of a GSXR750/1000 TB- a piece of 48mm ID hose should allow the transition from circular to oval.

I debated for a while whether the use of silicon hose was a good idea- I wondered if there was a chance it would collapse under the low pressure generated by a closed throttle at high revs. I decided that for a short run of maybe 50mm, the support at either end of the hose would stop it collapsing. 

I also read that silicon hose was not suitable for immersion in oil/fuel, so I tested a small offcut with petrol for a couple of days, and it seemed OK. Given that it won't be immersed in fuel in the induction system, but merely breathed upon by a mixture of air and fuel, I decided it was worth a try.

As for injectors, in theory I could use the original Ford injectors fitted to the sandwich plate. The main problem is, once again, a lack of clearance with the engine cover, principally the upward projecting fuel line return from the rail-mounted pressure regulator. Given that I'll get suitably sized injectors with the TBs, and that this is a better place to mount them for good air-fuel mixing (and therefore POWER!), I have decided to just block the original injector ports, although exactly how I have not decided.

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