It's really hard to pick just one,so I won't. I like all kindsa engines. I like the Harley Davidson generator Shovelhead a lot.
I really liked my old Pontiac 389 tri-power engine,but not all the tickets it got me.
I currently havea 1968 Pontiac 400 in my '57 GMC that's also a great engine.
Much as I hate to say it in the circle of friends I run with though,my all time favorite is the Dodge 383 Interceptor I had in my old Power Wagon.That thing would pull like a BIG big block and rev like a smallblock,it was an amazing engine.
I'm also impressed by the 355 smallblock in my '62 tonner.Though relatively fresh,it's already passed the bulletproofedness test.
Speed
