Throttle body conversion- work in progress!

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

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

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

Pre-2000 750

• 44mm ID at inlet

• 40mm ID at exit

• Injector 'pockets' in the barrel of the body

• 190cc/m injectors (which are angled more in line with the bore of the body than later years)

• 1 piece fuel rail

• No secondary butterflies

• See http://forums.turbosport.co.uk/bbs/showthread.php?t=83594&page=1&pp=20 for a full conversion on a Mk2 Escort

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:

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:

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.

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.

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 indiction 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.

10th November: Won an auction for some GSXR750 TBs in Canada, the winning bid was for just over £70. They are of 2000 vintage so should have the 270cc/m injectors, and also the plastic & aluminium fuel rail that can be modified. Postage is about £15, so given that these are now regularly selling for £120+ in the UK, I should save some money.

17th November: Made a start on re-profiling the ports on the manifold. Once this had been chopped down, it was obvious that there was a small step in the internal profile about 15mm before the cut, ie the port gets narrower just before the point at which the silicon hose will attach. Given that the walls of the runner are about 5mm thick, it seemed sensible to reduce this restriction, at the same time reducing the step in internal profile from the hose to the manifold. I made a start on this using a combination of dremel and drill attachments. Progress was quite slow, but I want to take it easy on this first port!

19th November: Picked up the TBs today from ParcelForce. I got stung for VAT on them, plus a ridiculous handling fee, which has just about cancelled out the saving from buying abroad- all together they have cost about £110. You live and learn! At least the TBs have arrived safely, and appear to be in very good condition.

22nd November: Spent an hour or so working on the same manifold port. I've now practically removed the internal ridge, and as you can see from the pictures the wall thickness of the stub is now significantly thinner than before (but not too thin!). This has to be good, as the step into the manifold from the silicon hose will be smaller.

I then cut the silicon hose into two pieces and push fitted it onto the manifold stub- it fits perfectly, and the path looks fairly smooth. I think I will end up cutting down the other end of the hose to bring the TBs closer to the manifold, mainly to get some clearance to the engine cover, but I will do this at a later stage.

28th November: Had an email chat with Andy Sayle, fellow Mojo builder, who reckons he might get the opportunity to make me a custom manifold for me (he is planning to do likewise for his 4AGE motor). That would be great! I am persevering with the original plan for the time being, as the custom manifold may be some way off, but as a long term solution it will be ideal.

1st December: Tonight I separated the TBs from each other. Firstly, I unbolted the fuel rail and injectors, and then separated the fuel rail into it's component form. It really is a very simple solution, ideal for modifying to the new spacing.

Then I removed the 3 long through bolts from the TBs. I had expected the TBs to then pretty much fall apart, but they are actually quite a tight fit onto each other. With a little bit of gentle persuasion, I separated them first into 2 pairs. This enabled the removal of the secondary butterflys and shaft, which then made the final separation of the TBs easier. I now need to get hold of some M5 threaded rod to respace the TBs with an additional 11.8mm between them (the Zetec ports are at 91.8mm centres, compared with the 80mm spacing of the GSXR TBs).

4th December: Extended the throttle linkages.  I did this by making up 3 aluminium plates from some angle aluminium I had lying around (B&Q normally have decent supplies of this).  I trimmed off almost all of one leg of the angle section, just leaving a 2mm return to give the plate some extra bending stiffness.  I then rivetted these to the original linkage plates using the smallest rivets that I had in stock.  A quick re-assembly proved that the linkages worked perfectly.

5th December: Blocked off the holes from the secondary butterfly spindle with bolts and chemical metal, then left overnight to cure (in the warmth of the house...).

6th December: Assembled the TBs onto the 5mm threaded rod I bought from B&Q.  Conveniently, the nuts just sit into the recessed faces on one side of each TB, which locates each TB more securely.  I trial fitted the bodies to the manifold and then measured from the front face of the TBs to the cylinder head flange to compare to the existing carbs.  I then trimmed down the silicon hose so that the TBs protrude a similar amount from the cylinder head.  I think they are starting to look quite good!

18th December: Picked up a new Bosch fuel pump from a local ebay seller.  It is a Bosch part, PN 0 580 464 070, which has barbed inlet and outlet connections to take push on hose & clips (I am avoiding threaded connections to cut down on expense).

22nd December: Finished opening up the ports on the original Ford manifold.  This took a good couple of hours, but I'm pleased with the end result.  The wall thickness of the runners is now 2-3mm rather than 5mm, so the step from the silicon hose is much reduced.