The best replacement for v8 is v6.

The best alternatives to v8 are rated as following:
★★★★★  v6
★★★★  v12
★★★★  v10
★★  v9
★★  v6 4runner
★★  turbo
★  4 cylinder
★  ecoboost
★  juicing
★  inline 6
★  tomato juice
★  v6 engines
★  i6
★  v6 gas mileage
★  spidermonkey
★  v6 camaro
★  v6 engine
★  v6 mustang
★  straight 6
★  v10 sol republic
  jvm
  i4
  v6 challenger
  v6 truck
  6.8 liter v10
  v6 jeep grand cherokee
  v12 vantage
  ford v10
  i6 diesel
  straight 8
  rhino
  v10 r8
  v6 ecoboost
  turbo 6
  v8 fusion
  hemi
  i6 jeep
  triton v10
  fenn xt
  rotary
  v4
  i6 trailblazer
  h50
  v7
  5.3l v8
  v6 eco boost
  garden cocktail
  v6 turbo

Your alternative for v8 is not listed? Suggest!
Speaking about v8, one probably shouldn't forget about good gas mileage.

Other search terms related to v8 and their ratings are:
★★★★★ good gas mileage
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Google Trends

Scale is based on the average worldwide traffic of v8 in all years. Click on graph for more.
v8
v6
v12