Latest project - boring out a throttle body!

John V · 05-19-2013, 09:33 PM

We're ditching the stock BMW engine management for a Megasquirt (DIY fuel injection) system on the DSP car for a variety of reasons. One of the big ones is with Megasquirt we can get rid of the factory drive by wire (DBW) throttle for a mechanical throttle. The DBW is laggy and it's not easily modulated. With the car making really good power now, precise throttle modulation is important.

After researching all the available throttle bodies I found that an M50 throttle off of an E36 325/M3 would be the easiest to adapt to the manifold on the E46 330Ci. One problem though is it's a 64mm bore whereas the stock throttle body is a 68mm bore. Maybe not enough to make a difference now, but once this car is running E85 and pushing 250-260hp to the wheels it could be. Aftermarket big bore throttle bodies exist, but I wondered if we could make our own. The price for failure was low since these M50 throttles are a dime a dozen.

Here's the stock unit with the throttle plate removed. Not much wall thickness here in the cast aluminum. Also, the mechanism is very complicated and fairly difficult to reassemble properly.

We chucked the thing on a lathe (stripped down) and carefully bored out the center, making sure to go slowly and shim the unit in the lathe so it was centered. Here it is after a lot of work, now with an extra 4mm of diameter, matching the stock 330Ci throttle body.

Next challenge, making a throttle plate. I could have used the 68mm plate out of the DBW throttle, but it is 2.5mm thick versus the 2mm thick plate that this M50 throttle takes. I would have to mill it down to that thickness at which point I'd never be able to use it in the DBW throttle, making that an expensive paperweight rather than something I can sell to recoup cost. Making a throttle plate is tricky, though. As I said, it's a 2mm thick plate of aluminum. I didn't have any 2mm thick aluminum sheet, only some .125" thick (3.5mm) stuff. So I cut an oversize piece on the band saw, then milled it down to 2.10mm. I did a lot of wetsanding with 220 grit paper to surface grind it to 2.0mm.

Next, I cut it down to a slightly oversize disc on the band saw. The sharpie dots are roughly where the holes need to be to mount it to the throttle body. I transfer drilled it using the original throttle plate.

Next challenge. The factory throttle plate has a subtle angle cut into the outer edge. This is to give the plate a better seal against the bore of the throttle body. In other words, when the plate is fully closed it is not aligned with the radius of the throttle bore - it's slightly angled.

To replicate this angle, we angled a piece of hardwood in the mill and cut it flat. This gave us a piece of wood that was flat on one surface and angled about 7 degrees on the other surface. I cut it roughly circular on the band saw then screwed the throttle plate blank to it.

When mounted to the lathe with the flat surface in, the angled surface facing out allowed us to cut the plate down to a 68mm diameter measured at that 7 degree fully-closed angle. It is weird seeing something kind of wobbling back and forth on a lathe, but it cut cleanly and worked perfectly.

Test fit into the throttle - it slid right into the groove in the shaft and sealed the bore perfectly. Better than the factory one did!

We fitted some new screws, gave them a nice taper for smooth airflow and peened over the ends so they can't back out. Success!

05-19-2013, 09:33 PM	#1
John V No more BMWs Join Date: Apr 2005 Carmudgeonly Ride: Ram, MS3, CX-5, RX-8 Location: Glenwood, MD Posts: 14,753	Latest project - boring out a throttle body! We're ditching the stock BMW engine management for a Megasquirt (DIY fuel injection) system on the DSP car for a variety of reasons. One of the big ones is with Megasquirt we can get rid of the factory drive by wire (DBW) throttle for a mechanical throttle. The DBW is laggy and it's not easily modulated. With the car making really good power now, precise throttle modulation is important. After researching all the available throttle bodies I found that an M50 throttle off of an E36 325/M3 would be the easiest to adapt to the manifold on the E46 330Ci. One problem though is it's a 64mm bore whereas the stock throttle body is a 68mm bore. Maybe not enough to make a difference now, but once this car is running E85 and pushing 250-260hp to the wheels it could be. Aftermarket big bore throttle bodies exist, but I wondered if we could make our own. The price for failure was low since these M50 throttles are a dime a dozen. Here's the stock unit with the throttle plate removed. Not much wall thickness here in the cast aluminum. Also, the mechanism is very complicated and fairly difficult to reassemble properly. We chucked the thing on a lathe (stripped down) and carefully bored out the center, making sure to go slowly and shim the unit in the lathe so it was centered. Here it is after a lot of work, now with an extra 4mm of diameter, matching the stock 330Ci throttle body. Next challenge, making a throttle plate. I could have used the 68mm plate out of the DBW throttle, but it is 2.5mm thick versus the 2mm thick plate that this M50 throttle takes. I would have to mill it down to that thickness at which point I'd never be able to use it in the DBW throttle, making that an expensive paperweight rather than something I can sell to recoup cost. Making a throttle plate is tricky, though. As I said, it's a 2mm thick plate of aluminum. I didn't have any 2mm thick aluminum sheet, only some .125" thick (3.5mm) stuff. So I cut an oversize piece on the band saw, then milled it down to 2.10mm. I did a lot of wetsanding with 220 grit paper to surface grind it to 2.0mm. Next, I cut it down to a slightly oversize disc on the band saw. The sharpie dots are roughly where the holes need to be to mount it to the throttle body. I transfer drilled it using the original throttle plate. Next challenge. The factory throttle plate has a subtle angle cut into the outer edge. This is to give the plate a better seal against the bore of the throttle body. In other words, when the plate is fully closed it is not aligned with the radius of the throttle bore - it's slightly angled. To replicate this angle, we angled a piece of hardwood in the mill and cut it flat. This gave us a piece of wood that was flat on one surface and angled about 7 degrees on the other surface. I cut it roughly circular on the band saw then screwed the throttle plate blank to it. When mounted to the lathe with the flat surface in, the angled surface facing out allowed us to cut the plate down to a 68mm diameter measured at that 7 degree fully-closed angle. It is weird seeing something kind of wobbling back and forth on a lathe, but it cut cleanly and worked perfectly. Test fit into the throttle - it slid right into the groove in the shaft and sealed the bore perfectly. Better than the factory one did! We fitted some new screws, gave them a nice taper for smooth airflow and peened over the ends so they can't back out. Success!