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good point trk, you're a smart guy. to research further, i went to the ipd plenum page where a variety of dyno charts have been posted. if you go to the cayman page, they post dynos for an unmodified car, a car with an ipd plenum, and a car with an ipd plenum and larger throttle body. it's interesting to note the following:
- the larger throttle body does not make a difference over the plenum-only install until over 4900 rpm; presumably when the resonance tube and cams have switched over.
- both the plenum and the plenum with larger throttle body underperform stock up to 3500 rpm, by up to 10 hp less. i attribute this to the plenum. after that the ipd products do look to increase hp by 10 to 15 hp, and the overall area under the curve looks bigger with ipd.
conclusion? the larger throttle body doesn't have any ill effects, but the plenum causes a low rpm reduction in hp. in a cayman.
if you look at the ipd boxster products, they only offer a plenum with larger thottle body (74 mm thottle body, equal to the boxster maf diameter) and their dynos show hp improvement throughout the rpm range.
so, if there are no ill effects with a larger throttle body, why did porsche use throttle bodies smaller than their maf diamters?
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