10-31-2012, 10:46 PM
Join Date: Oct 2006
Location: Springfield OH
My Ride: '00 323i + '75 2002
| I've been second guessing myself for a couple of months as to whether or not the S54 ITB's are really a good fit for the engine, or more directly the 885 head. While they do give an easy solution for certain aspects of an itb setup (common vacuum rail, fuel injectors and fuel rail, proper spacing) I still couldn't ignore their overall size, and how that would effect the power curve of the engine. So long story short, I bit the bullet and ordered some new parts. Then I started machining the old 'i' manifold from my donor engine. When I got done it looked something like this:
The throttle bodies I will be using are from a late model BMW (keeping it in family of course) 1200RS. They are 38mm in diameter, and while some may argue they are to small for the engine, they will have no trouble out flowing the intake ports of the 885 head. This will help to keep charge velocities up, something that would have been lost with the monsterous S54 bits. Another plus of these throttle bodies is that they are mounted with 2 bolts on the top and bottom, so re-spacing them to fit the 91mm center distance for the M20 was as easy as drilling some holes in a piece of aluminum. Finally, because I'm using the stock M20 intake to interface with the head, I'm able to keep the stock fuel injector/rail location as well as the oil vent tube that runs down into the block.
Since they come in sets of four, I had to purchase two sets. Seeing as how I had two left over, I gutted one so that I could see how it would line up with everything without the throttle plate in the way.
Here I'm holding the TB in place roughly where it will be. It looks fairly far away from the head in the picture, but it's actually only ~0.5" from the edge of the manifold. That will leave me enough room to weld the aluminum tubes in place that I will be mating to the throttle bodies. As you can see from the picture the port leading to the head will need to be opened up slightly and rounded out. However it should be a very direct and smooth path into the head.
Then I took some aluminum angle and drilled the new mounting holes to properly space the rest of the throttle bodies. Then I bolted them onto the angle and held it in place.
I still need to weld on the new tubes onto the manifold, as well as extend the throttle tabs that translate the rotation from the center of the assembly to the outer throttle bodies. Overall I'm very happy with how they will look. The assembly will take up much less space than the S54 castings and throttle bodies, mostly because it doesn't have to open up the port from stock to 50mm diameter. This will leave me a lot more room for a proper plenum, which will either be aluminum or composite. It also puts the throttle bodies themselves much closer to the head, which will lead to better throttle response.
Thanks for looking.
Tinker Engineering - 2014
- 2000 BMW 323i - The one that started it all
- 1975 BMW 2002 - The Definition of Project Creep
- 1988 BMW M5 - The piece of BMW history
- 2013 Subaru WRX - Stock, for now