Throttle Bodies

When selecting a throttle body, there are a number of considerations. You need it to flow enough to support your engine's horsepower.  Generally, you want to take the throttle body from an engine that made similar horsepower to your engine.


However, there are some drawbacks to a too large throttle body:


At low rpm, you go from low kPa to 100kPa with very little throttle movement, making driveability 'worse'. For example, with a very large throttle body you may get 100 kPa at 20% throttle at 2000 rpm. This means if you want to hold it at 40 kPa for cruise, you have to be very steady on the throttle, as small movements may produce large changes in engine output (so it's harder to be smooth), and
A small throttle movement (and a small V/sec TPS signal change) can result in a very large change in MAP (as mentioned above) at low rpms. The result is little (or no) accel enrich when the engine needs it most. However, you can usually tune around circumstances like this by richening the VE table at low rpms and higher kPa (say < 2500 rpm and> 70 kPa) by about 5-7%. This has a negligible affect on fuel economy, since you likely never see 70 kPa while cruising.



To calculate how much horsepower you can make from a given throttle body size, you can use the estimator in the megamanual.  http://www.megamanual.com/v22manual/minj.htm#throttle
BMW throttle bodies are made differently than any other throttle body I've seen.  Instead of having the butterfly actuated directly, BMW has a progressive opening butterfly using a 3 bar linkage.

3 common throttle bodies you find in US junkyards are from e30's


1 - 318i throttle body

2. 325e throttle body

3. 325i throttle body.

These all measured with a .001 accurate dial caliper.

First dimension is the air inlet Inside diameter

318i - 52mm
325e - 55mm
325is - 64mm

Butterfly diameter - I actually took each one out of the throttle body for this

318i - 45mm
325e - 51mm
325is - 58mm

Outlet - This is the part that goes to the engine
318i - hard to read because it turns into an oval - 50mm in the small length
325e - 53mm (yes 2mm less than inlet)
325is - 63mm (yes 1mm less than inlet.. I triple checked)

Since the butterfly is the important part I calculated the estimated horsepower for each throttle body in the megamanual estimator.

318i - 116Hp
325e - 150Hp
325is - 193Hp