Upswepts are as noted, very much in the way of any work on the engine, any time; AND you need a flapper cap to keep gravel from being slung into them as you are moving the car around with the engine off. Don't need to fill up a cylinder with pebbles or water ! There are some really good high perf 'racing' mufflers nowadays that actually enhance performance over an open system. A good engine builder/tuner has this(mufflers) in mind when working up the specs for an engine combo that will see mufflers.