[Chaplin]

Yes, all above.

There are really 2 different kind of redlines that are involved, and each is involved in determining the redline of any engine to some degree. One would be a redline imposed by design limitations- i.e., a small cam, dual plane manifold, small carb, small intake and exhaust valves, and the other redline would be what is imposed by the physical limiations of the parts you use, i.e., forged crank v. cast crank, 4 bolt main block v. 2 bolt main block, cross-bolted mains or non-cross bolted mains, etc. In the design limitation example with a small cam, your motor will only make so much power up to a specific rpm and then will fall flat on its face because the motor will run out of air, so for all practical purposes, that is its redline since there is no point of spinning it faster because you are not making more power. Using all forged internals and the highest quality components won't make a difference in that engine for redline purposes.

Then there is the redline imposed by the physical limitations of the actual parts you use. If you use a big solid roller cam with a single plane manifold, huge carb and free flowing heads, you might might be able to make power to 7500 rpm, but how strong is your bottom end? If you have a cast crank, 2-bolt main, non-cross bolted block, etc., you might not want to spin it much past 6000 or 6500 rpm for fear of it grenading, so that might be the redline you use. However, if you had all forged internals, strong block and quality valve train parts, you might be fine spinning that same motor to 7500 rpm.

So when you come down to it, redline is a function of both the design of the engine and the quality of the parts used. Finding the right mix for what you want to do with the motor is always the fun part.