Open mufflers are usually related to engines that live a life of high revs and high horsepower numbers, (as well as short piston stroke, high flow headers, and V engine configurations)
Where-as a muffler that produces some backpressure are more often related to engines that do lower revs and are more built for torque. (as well as long piston stroke, standard headers, and the inline 6 engine configuration)
fordman49450 is right, but we have the luxury of a 4 stroke engine pushing the exhaust out for us, fancy mufflers are way less crucial than, say, intake. If I had some money to throw at my tow rig I would invest it on the breathing IN side on the engine. Or, perhaps a auxillary tranny cooler? (if you have an automatic)
