Imperial Home Page -> Imperials by Year -> 1990 -> Modifications -> 58 mm Throttle Body (TB)
An other big gain modification for this engine is a 58mm TB. The stock TB has a 52mm diameter hole. Increasing this diameter increases the amount of air the engine takes in, and thus increases everything from HP to gas mileage. In fact, the only difference between the original 3.3 used in the 1990 Imperial and the late 90's 3.3/3.8L V6 in Mopar mini vans is the design of the manifold and the size of the TB. Yet this simple difference accounts for most of the HP gains made throughout the different model years.
The 58mm TB was used on all 1998 and newer 3.8L V6 Mopar engines, used in vans like the Town & Country. Some Caravans got them as well. These TB's go for just under $300 new from the dealer, but also under $100 used from yards (if you can find them). The 58mm TB uses the same design as the 52mm TB, and thus it simple can replace the stock TB.
But in order to gain any true benefit from this larger TB, the intake manifold must be ported to accept a 58mm TB. Basically the manifold's intake "hole" is still 52mm. Using a grinder to bring it up to 58mm allows for the TB to fit perfectly. This takes a couple hours, a high speed grinder, and grinding discs. It is recommended that the manifold be removed for this porting, as the metal fillings would be harmful for the engine.
If you plan on also replacing the manifold with one from a different year, it may be a good idea to do that at the same time (thus allowing you to only port one manifold, as opposed to doing the original, and then an other).
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