88 F-150 300 I6. Installed 4" cat-back exhaust last night. Here is what I used: Cherry Bomb 24" glass pack (2" i.d./o.d.), 2-1/8" ID to 4" OD adaptor custom built by THOMURE MFG., 6' of 4" ID stanless steel flex pipe, finished out with a 4" curved tip.
The result: ALLOT quieter than I thought it would be! I am going to remove the glass-pack muffler tonight and try it that way.
I'll put up a part 2 post to let everyone know how that works.
Just so you will know, I am aiming for a "big rig" truck sound.

 

Hey Brown, Where did you get that 4" exahust from ? I am considering the 3" cat back system from J.C Whitney, but I hear losts of arguments that the I-6 motor does not need those big pipes and you could even loose some tourque ?

LMK, Chet

 

Might be a little difficult getting "big rig" sound out of a gas engine. Could be out of line here but by using flex pipe, wouldn't that be like having a 6' muffler (flex being full of "baffles")
Mike

 

I purchased the 4" exhaust parts from THOMURE MFG. I is the stinless steel flex pipe. Expensive, but well woth the extra cost!
And, no, the flex does not act as a baffle inside. It is nealy smooth on the inside. The cheap 2" stuff that the well known auto stores sell may be rough inside, but this is not. I did NOT do this for preformance reasons, I did it for sound. I removed the glass-pack and ran the 4" pipe straight off of the cat. IT SOUNDS AWSOME!! Total cost including frt was right at $200.00

 

I sure hope your flex pipe doesn't rust out after 3000 miles like mine did. I've had numerous people, even girls, come up to me and say my truck sounds exactly like a tractor. One guy said a Farmall H on steroids, and it cost less than 50 bucks. lol

 

4" is definately way to big for a I6. I even think a 3" is too big. 2.5" seems to be the ceiling for the six. Like Boss50 said, I hope that flex pipe doesnt rust out. It is a very poor product for exhausts (even the stainless steel flex). I dont think you are going to get the big rig sound you are looking for, sorry to say.


Rich