Have a 2013 3.7 f150. Plan on replacing the stock muffler with the magnaflow sido. But that stock muffler is a staigth tru 3''all the way, and the magnaflow has 1 3" inlet and 2 2/2" outlets ,but the exhaust has to go tru 3 chambers and than to the two side outlets , seem to me like there would be more of a restriction. what do you all think?

 

Magnaflows are straight thru mufflers, you can see right thru them. On my 13 with a 5.0 the stock ford muffler was 2 1/2" in and out not 3"

 

You don't have a magnaflow if it has chambers.

 

Replacing the stock muffler is a noise/sound decision. Performance gains are minimal > non-existent for stock/not-uber-mod'ed vehicles.
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Yeah, no Magnaflow muffler has chambers, should be able to see straight through it.

 

go to Magnaflow.com and click on xl mufflers and it 'll show you that the offset is not a straight true , it has 3 baffles or chambers and the sido cannot possibly be as straight tru either , that is what the muffler shop is telling me too. Unless its a straight tru muffler or single center inlet and single center outlet.

 

Straight thru is straight thru. If it y's but has no chambers its still considered straight thru.
I had a maggy that was 3"in dual 2.5" out and i could see right threw it.

 

Thats what i wanted to hear. Thanks i'm going for it now.

 

Alright glad we could get you situated. only way you would hinder your exhaust flow would be with a flowmaster style muffler.

 

I second all of this and would've said it myself. SIDOs just have an internal Y pipe, just like many DIDOs have an internal X pipe.