In the last post, I described the machining of the throttle body adapters. Once those were done, it was time to move along to make some brackets to hold the throttle bodies in position. Since it is a V-4 engine, the two rows of throttle bodies are at an angle to each other. They are nominally held in place by the adapters I discussed in the last post, but since those are circular, they don’t constrain the rotation of the throttle bodies around the intake axis. Those couplings are also slightly flexible, so it would be good to mount them more rigidly to each other.

The GpZ1100 throttle bodies have screw holes for mounting rails along the top and bottom edges. The bottom holes of the ones on the opposite sides of the “V” end up quite close to each other, so I designed a single mounting bracket that would hold all four at the correct angle. The top holes of each row can then be bolted to another bracket, providing further rigidity. Here’s a Fusion360 rendering of the entire assembly:

A rendering of the bottom side of the throttle body assembly, showing the three aluminum brackets that hold the throttle bodies in position.

The design of the angled bottom bracket was a perfect example of Fusion360’s “top-to-bottom” design style. I had already modeled the throttle bodies and how they attached to the airbox, so now I could design a bracket that mounted to the bottom screw holes. By mirroring this part across the centerline of the “V”, I then ended up with a part that had the correct angle and spacing to hold the throttle bodies in the right position.

Since this part isn’t flat, it couldn’t be milled in one piece (it could be done with a 4-axis machine… that would be nice…) but since it was modeled as a mirrored design, I could machine each half separately and they would then have the correct shape to be welded together.

Initially I had designed it as just a block of aluminum, but I figured I should leverage the power of Fusion360 and get rid of unnecessary material. The final bracket, as can be seen above, has much less material. Time to manufacture. I’ll let the pictures speak for themselves.

Since the bracket was to be machined on all edges, figuring out how to hold it took some thinking. The first step was to machine the areas where the throttle bodies would mount. This could be done with the part mounted in the vise.

This is a test fit of the bracket to make sure the mounting holes lines up correctly.

Once the flat areas where the throttle bodies had been machined, I could clamp the piece to the mill table using these areas and machine the rest. To avoid cutting into the table, I placed 0.75mm shims under the piece at the two points where it’s clamped down. I wasn’t sure this would be sturdy enough, but it worked fine.

After machining the two halves, they were test fit again before welding them together. They fit almost perfectly . There is a small gap between the parts, but this is well within the flexibility of the couplings that hold them to the airbox.

It’s been a while since I did aluminum welding, so I had to practice a bit before I dared to start melting the parts I’d just spent hours making. The welds didn’t exactly turn out spectacular, but I got the parts together with very little warping.

Here’s the final result, the welded-together bracket as well as the smaller ones that attach to the top sides of the throttle bodies. Everything fit beautifully. Compare this picture to the rendering at the beginning of the page. This CAD stuff actually works!

So this was a complete success, the throttle bodies are now held in the correct positions. The next steps are a bit more iffy: The holes where the fuel injectors mount in the throttle bodies aren’t the right depth for the Bosch EV6 injectors I’m going to use. This means I’ll have to actually machine into the throttle bodies themselves, which will be … exciting…

The intake ends also don’t fit into the intake rubbers on the engine, the OD of the throttle bodies, 40mm, is too large compared with the stock carburetors 36mm.  I was going to cut down the diameter of the throttle bodies, but there will be uncomfortably little material left if I go that route. It probably makes more sense to try to enlarge the holes in the rubber boots, although I’m not sure how to best do that. But that will be the topic of a later post.