I'm sure I'm going to get extremely "flamed" for this since I'm picking on a person who "claims", rather I was lead to believe he's claiming this, because I didn't read anywhere where he actually was, to be a great welder, but...
The "tin" welded muffler, it looks like all you did was do a great big "oval" back step maneuver... Not bad as it will allow for the dissipation of heat better, but it does make one horrible looking weld, especially if you don't control your spacing or oval shape consistency all that well... Chicken scratch, but serves the purpose... Most prefer a "stop / restart" setup with thin steel like this, but what you did is the best way, only it could look a slight bit better...
As for your big welded together pieces... What were you thinking... Just get it done as quick as possible? I noticed you not only used a weak weave maneuver, but you also didn't even bother to prep the steel, such as beveling... Look at your weld "how it's bubbled up or rounded", this means almost no penetration... You cut this and do a bend test and you fail right then and there... NO measuring tears for allowable acceptance, just fail... This will be because of a few reasons... First, no beveling = no to little penetration / Second, not enough heat = the big mound you've got, even with no beveling a weave shouldn't have this big of a mound / Third, you weaved = you never weave on something you rely on for strength "especially something that has this big of a weave" - you use nice small "packed" stringers
They will teach you all of this, if you take any sort of "high pressure" welding test such as a boiler welding test or any other "bend" type test, not the cheapo xray test... Bend test actually stresses your welds, by bending them completely around... Xray just looks for inclusions such as slag or air bubbles = much easier to pass...
Just my thought on the subject... OH yeh, I personally, like most tig or mig welders that rely on welding to make a profit, prefer Miller... Just my $.02... Lincoln always seems to tear up much worse, don't get me wrong, they all have their ups and downs and I at one time would only use Lincoln, until I got to using them to make money, instead of as a hobby welder, then realized just how easily they tear up and how much better the Millers layed out a bead... This, all of course, depends on the options you get on any welding machine...
The only pics I can currently provide of any of my welding abilities is of a test I took in a class I had, of round to round and round to flat, then a gusset... This was for learning the best way to weld roll bar (steel tubing)... The best thing I liked about this class was that my instructor admitted to me welding better than he, but I did learn a lot of nice things from him on the best welding procedures for certain situations... Oh these welds that I will be posting are nearly 1&1/2 years old - "when I took the class" and I won't have any pics until I take them and get them put on a CD to load to the computer and hopefully they will turn out clear and good, so you can comment with all of the bad comments on (I like hearing the bad, you learn from them)...
Other steel piping I'm great at welding is any type of boiler welding (extremely high pressure pipe), this is probably because I've got all 6 of my boiler welding certifications... I've always been able to weld and work with my hands, in fact gifted in the manor... I've worked as head welder for two different places, first a propane shop (don't recommend for anyone) and then I was head welder for Motor Sports Image, a little show car / nascar shop in Mooresville, NC (don't really recommend working there either - going through some management issues, or were when I left)... I did all of their Tig welding "both steel and aluminum" and any Mig welding where peoples lives might be Dependant on... The shop manager wouldn't let any other welder in the shop near it in this situation, other than me... I kinda liked that, but boy I don't like having to run around in circles trying to do 10 or 12 things at once... Welding should be something you take your time on, not rush...
Sorry for the long post and what seems to be a bunch of jibber jabber and self "brag"
I'll try to get the pics as soon as I can then you can let the bad comments roll
Greg