system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do powerful, heavily tuned engines like those in big muscle cars, or drag racers idle so poorly, hunting around and spluttering and popping, only smoothing out when under load? Why can't they idle nicely too?
gpt
Because their camshafts are optimized to run well at high revs and full throttle. The camshaft is what controls the lift and duration of the valves opening during the intake and exhaust strokes of an engine. More lift and duration means more air/fuel mixture reaches the inside of the cylinder.

The hunting and spluttering you hear is the sound of an engine at idle whose valves are opening for too long. When that engine is under full load, the valves are opening for as long as is needed *at full load*, and that's why the engine works smoothly at higher revs.

It's possible to build an engine that works smoothly both at idle and at high revs - that's called Variable Valve Timing, and most regular passenger car engines have had it for a couple decades, in one form or another. But for race engines, it's added expense, complexity and unreliability. Drag racers and people who build competition engines don't really care that it works badly at idle - that's not important for them.

Of course, people think that race engines sound cool, and so you will sometimes see regular passenger cars that have been modified to sound like that at idle. That's just pure aesthetics. The best example is Harley-Davidson motorcycle engines - they sound that way at idle because it's traditional, and they're intentionally built to this day to run slightly badly at idle.