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I just swapped my Spintech for that Ultra-Flo PN# WLK-17538 muffler on my 4" Mufflex last weekend. Man let me tell you it's like night and day. I had to add more fuel at WOT. I used to have consistant 113-114 mph trap speed in the 1/4. I went to the track last weekend and ran a 11.64 @ 120 mph with a 1.68 60ft on the first pass, I added more fuel at WOT and backed it up with a 11.66 @ 122 mph with a 1.80 60ft. So all you guys thinking about getting a 4" mufflex don't buy it with a spintech or flowmaster. I have proof in my hand what the Ultra-Flo can do. And it sounds pretty good, not to loud but just right IMO. Never got a chance to run the bottle because pressure was to low. So both passes were made on motor. I'm going back this weekend for some bottle passes and I'll post those times.
__________________ 91 Z28 w/383 eagle forged assembly, Accel Pro-Ram, ZEX nos plate w/Nano, 1200cfm TB, 847 cam, CNC Trickflow 195, Hooker LT coated w/Mufflex y-pipe and 4" Ultra-Flo, Bullet proof 700r4, ATI 3500 stall, Moser 12bolt 3.73's, Spohn suspension, 315 ZR1's rear and 275 ZR1's front, Weld Drag Lites w/ 27.5 Hoosier QTP's and M/T Sportsman up front. Stock 730 ECM w/NVSRAM and LM1 WB.
I run 2 of these on my race car. Absolutely no restriction of any kind. Open headers I needed to put in ear plugs just to fire the car up. With these mufflers, I don't even need ear plugs going down the track. You can see one of them under the passenger door in my sig picture.
i have the ultraflow too, its soooo unrestrictive it's incredible, i think it lowers the veloity of the exhaust so much its quiter? correct me if im wrong.... i just wanna kno
Hehehe. When I bought my Mufflex 3.5" catback system the only choices were the Spintech or the Slowmaster mufflers. I went with the Spintech because of the Slowmaster's reputation.
Anyways after looking at the Spintech I did not like how the air traveled through it and how much that thing weighed. Luckily Magnaflow came out with their very nice 3.5" straight through muffler so I was able to avoid the situation. The Magnaflow flows over 1100cfm which is good for 500 horsepower according to David Vizard's forumlas.
I have to say thats sounds pretty odd to me also, my car only loses
.015-.02 with my flowmaster on vs off - most of time at track I drop just the muffler and never lose more that .02 with it on.
I also run 4" mufflex. Car traps 125 ish with out and 123 with.
But I have been looking into swapping to another muffler though, maybe a lighter one with less restriction, but not much louder.
11.64 @ 120 mph with a 1.68 60ft on the first pass, I added more fuel at WOT and backed it up with a 11.66 @ 122 mph with a 1.80 60ft. So all you guys thinking about getting a 4" mufflex don't buy it with a spintech or flowmaster. I have proof in my hand what the Ultra-Flo can do. And it sounds pretty good, not to loud but just right IMO. Never got a chance to run the bottle because pressure was to low. So both passes were made on motor. I'm going back this weekend for some bottle passes and I'll post those times. 
The Magnaflow flows over 1100cfm which is good for 500 horsepower according to David Vizard's forumlas. 
