Thats a different question than the original?
You need to have about 8 times (800%) of cylinder displacement, not just 20% extra. (120%)
"a quiet one without hurting the engine" ? You will get rich if you can do that!

For least power loss, you need >1800% volume, placed directly behind a 120% volume first chamber. (dammed exhaust wave system) Forget the after muffler, and you have a "damned loud" system.