I think that the lower you go in number, the louder the exhaust note is. However, I've heard of something called the Delta-Flow Flowmaster Exhaust, which reduces sound and somehow(according to the add) harness that sound to make more power, which I am somewhat suspicious about. To answer your question, the 70 series will not have as deep, throaty sound as the 50 series, I believe. If I were you, I would go with a 50 series(or even a 40 if they have one that works), but that is because I love the sound of exhaust. I have a 40 series and am very pleased with the nice throaty, deep sound that I am now getting from my V-6. Some people have actually asked me while riding in my truck if I have a V-8. So basically, decide how much sound you want out of your muffler and that will be your guide. The loudest one I know of is the 40 series(which I have, even though it's really not that loud). The so-called "SUV model"(ie the 70 series) is the least loud to my knowledge. Hope this helps...
Sean
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'95 XLT 4dr 4.0L V6 OHV Auto
Flowmaster 40 series Cat-back
K&N FIPK
Bosch 4+ Platinum plugs
Splitfire ignition wires
PIAA 9007 Super-White Headlights(adds 40 rearwheel hp )
Superchip (Coming Soon!!! Can't wait!!)
