Welding a roll cage

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I am putting a roll cage in my car and I was doing some reading on different gases.Currently I am welding .135 mild steel with Argon/CO2 mix of 75/25.  I read that is I use just CO2 I can get more penetration.  I would think since I am using mig and on .135 steel that is what I would use.Any thoughts on this?Thanks!
Reply:Can't help with the answer but just wanted to point you to http://www.millermotorsports.com/mbo....php?forumid=3where some other gear heads hang out with their welding questions.I like that 4 link pro street in your other thread too.
Reply:Youll be fine with the 75/25 mix, if you want more penetration, crank up the power some.  What machine are you using?www.jmfabrications.comLincoln Precision TIG 375Lincoln Precision TIG 275Lincoln Power MIG 255Lincoln Power MIG 140CMiller Dynasty 350 (x2)TD Cutmaster 80
Reply:I agree with JMckeone - 75/25 should be great, and you are only working on .135 wall, so unless you have an undersized welder, just crank up the power and you will be fine.  Plus 100% CO2 will give you more spatter.
Reply:Thanks for the link.  I am using a Lincoln Pro Mig 175.  Voltage setting E and wire speed 7.  I have plenty of power.  I guess my main concern is that the mig wire speed is so fast you need to move quick and I am thinking I might not get good penetration.Maybe I am over thinking and just need to go with it.MikeBuild a Barstool Racer at BarFlyRacers.com!
Reply:You should do some test welds on the same material you intend to use and cut them up afterwards to ascertain the penetration.  A roll cage deserves this scrutiny, especially if you are not experienced with welding them.
Reply:I did a roll cage out 1-3/4" .095 DOM and I did some test pieces by welding a bunch of 6" pieces into a T shape. Then I slipped the top of the T on one of those big 6' steel 'farm' pry bars, layed the bar on the ground, stood on the bar, and proceded to beat the crap out of the T leg with a BIG sledge.I know its not scientific but it was the best I could do at the time. I did find out that when the sledge glanced off the tubing and hit my ankle it really hurt.  Oh yeah, the test welds didn't break. So much for materials testing 101.