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Air flow theory gives a 5% hp advantage to 4v over 2v, but few production cars optimized 4v heads for HP, usually targeting low end torque with modest hp gains.
Hence the dilemma, stock each has some advantages, with the edge going to 4v, but do any mods and the cost is close to double for a 4v vs 2v for at best 5% at the top end. Performance per $ and 16v is a clear winner, cheaper to buy, cheaper to hot rod, cheaper to fix.
Lots of dyno numbers floating around, Euro S with cats is around 245 rwhp, which is very close to a 85/86 32v, and maybe 15 rwhp less than a S4. replace the Euro S stock iron manifolds with US 85/86 which are stepped headers like the S4 uses but with brackets that fit the 16v and pick up maybe 10 of those 15 RWHP, plus the early cars are about 200 lbs less.
More important I think is to look at what your target HP is, and what your budget is. I don't see a cheap path to 325 to 350 rwhp other than 16v. Goals of over 400 rwhp and it shifts back to 32v.
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