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BMW Motorsport 48 mm Throttles vs. Ported 46 mm Throttles
Are they the same casting?


I have heard it said by some E30 M3 enthusiasts that the BMW Motorsport 48 mm throttle bodies are made from the same casting as the original 46 mm throttles. For a long time I thought that this might be true, which would imply that porting a set of 46's to accept the 48 mm throttle plates would be almost as good as the real thing. Well, this turns out not to be the case. There is "no free lunch" after all.

When I finally got some genuine 48 mm throttle bodies next to some 46 mm throttle bodies the differences were clear. The 48 mm casings are a lot bigger near the throat where they lead into the head. No amount of porting on a set of 46 mm throttles could ever make them the same. The following photo shows that the 46 mm casting has a throat height of 27 mm whereas the BMW Motorsport 48 mm throttles have a throat height of 33 mm. And these 46 mm throttles were not stock, they had been ported to the limits of the casting and were quite thin in the throat area. The 48 mm throttles were stock:


BMW Motorsport 48 mm Throttles vs. Ported 46 mm Throttles

The next photo shows the measurement technique for the throat height:

Measuring the throat height of the throttles

Note that I am not concluding that the 48 mm throttles are "better", only that they are "bigger". In the final analysis, which type of throttle body is better suited to you S14 depends on it's configuration. For my fairly modified 2.5L engine, swapping from the ported 46 mm throttle to the BMW Motorsport 48 mm throttles made a significant increase in performance above 4500 RPM. No comparison is complete without a dyno chart. The follow chart shows the results for my car with ported 46 mm throttles and BMW Motorsport 48 mm throttles - no other changes:

Click here for a dyno chart comparing ported 46 mm S14 throttles to BMW Motorsport 48 mm throttles

What a dyno chart can never show is how much better the engine "feels" with the 48 mm throttles. Plus my low RPM pinging issue disappeared 8^)

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More Myths Part 1