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Just wondering what the differences are between a 03 to 04 Hemi and a later model. Why do the later model Hemis have more horsepower and torque?
 

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Well the 03 is in a class by itself. 03 has a separate TCM (for trans) and ECM (for engine). This is why trans tuning is non-existent and engine tuning limited for 03's. Starting in 04 the 2 modules were combined into 1 PCM, which gives you more options, with 05 being even better. All 5.7's from 2003 to 2008 are rated the same 345hp, with the 06-08 (1500 only) having MDS. In 06 the wiring/communications system was changed. The 2009+ 5.7 is more powerful because it is a redesigned engine. The heads are similar in design to the 6.1. The intake manifold is active and can switch between short and long runners when necessary. This engine also has variable valve timing, MDS and I think a hair more compression and cam. All of that together adds up to 45 more hp.
 
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