A little interjection before I get into the project details; I’m not sure I put much stock in this, but all the same, here’s a fun throttle body calculator to play with.
I had read before about some of the C20GET (turbo 2.0) guys modifying a 54mm Fiero throttle body to fit, which involved some bellcrank grafting among other things to get it to work. I think it is far from a plug and play deal. I had also heard rumor that a Grand National throttle body would bolt right up, but since I happen to have one on hand, a quick study proved that it was not even close. Granted, I am sure you could make an adapter or drill and hack your intake to make just about whatever throttle body fit on there you want – they mostly all work the same. Anyhow, I figured it doesn’t get much more bolt-on-ready than a factory one, so I threw it on the CNC and started cutting. Well, after I disassembled it of course, which was rather different than others I had done before – you have to grind off one end of the shaft to get it all apart and then there is an odd shaft locking ring that is under a peened-in washer. Anyway…
The stock bore varies from 53.2mm at the inlet, tapering down to 50mm around the blade, then stepping to 52.5mm at the outlet, with the smallest area effectively making it a 50mm bore. As the link I posted above illustrates, there are a lot of different formulas to calculate theoretical ideal bore sizes. Not having any dyno time with this in particular, I’m not really sure what may work and what won’t, so this is just a future experiment and exercise of “Because I Can.” I initially was going to resize it to 54mm, but then my Ford mentality kicked in and I decided to go from 50mm to 55mm (most Mustang throttle body upgrades tend to go in 5mm increments.) So first I milled out the housing to 55mm. Then I halved the throttle shaft and filed it wider to fit the new blade. Then I designed a simple fixture to make the blade since it is angled around the outside to sit properly in the bore. I had my friend make the fixture for me, then set up the blade and cut it out of .080 aluminum. It needed some minor filing and sanding work to fit nicely and operate smoothly. Finally, I ground down some flathead bolts (top and bottom) so they would sit flush and provide no obstruction. The originals are M3.5x.6, but I tapped the shaft to 6-40 because they were readily available and the threads are very close (the tap did very little.)
A couple things I ran into while milling the housing are:
1. A thin wall with two small holes above the vacuum fitting (black part on bottom of first picture) is removed, leaving two large holes. I don’t think this should be a problem since the fitting seals into the housing.
2. Another thin wall which creates a passage for the idle air stepper (that can be seen in the picture on the right) is made much thinner as it was milled. It ended up being about .030 thick, and I think I would call 55mm the max you could get out of the stock housing due to this wall; I wouldn’t want to make it any thinner than that.
That’s about it! Someday I may do an update if I get around to trying it out, but realistically it will probably be better suited to a higher-revving and better-flowing combination than a stock motor for which the 50mm is probably adequate. It was mostly for fun, partially because of my future notions for the car, but also just to try out.
The pictures below show it put back together enough to show the function, but it will probably be coming back apart for powdercoating eventually.
55mm 2.0L throttle body
55mm half shafted