A few exhaust system questions

[Steve McMahan]

I have purchased the Flowmaster Fx 2.5” off/center exhaust mufflers, the Flowmaster 15807 2.5” tailpipe rear exit pipes, and a 2.5” OD mandrel exhaust pipe kit (which has enough pipe to build the shorty header to muffler pipe). My question: is the fitment of the muffler, it’s position, set by having the front of the muffler clearing the floor pan and that clearance, sets the rear? I’m asking because, when I positioned the muffler, clearing that pan offset, the rear of the muffler was about 3” past the location where the rear hanger mounts. Now, I could build a bracket to extend the attachment point to be equal to the 3” issue. Or, is that what others have already experienced, and having the hanger extend back that 3” to the floor sheet metal is fine? Second question: how close to the floor pan should the exhaust system be, speaking about the straight pipe from engine downturn to the muffler? Finally, what tools did you find you needed to build your system that you didn’t plan initially. I have a good welder, average welding skills, no time demands on completion, and a desire to do this. Appreciate any advice.

 

[Don C]

I think I've needed a tail pipe expander on most put it together yourself exhausts
https://www.harborfreight.com/large-exhaust-pipe-expander-57184.html

 

[Hemikiller]

I've had the best luck working from the rear forward. Set the tailpipes and mufflers in their hangers, get the tailpipe angles right to clear the axle area and just snug the clamps so things don't move. Use stands to support the front of the muffler. This allows you to connect point A to point B between the header and muffler inlet.

FYI - the mufflers should be oriented offset inlet/center outlet for the tailpipes to work properly.

 

[Steve McMahan]

Right, but what about the issue with the rear of the muffler 3” past the hanger attachment? Have others done this and I’m worried about nothing? Also, I did dry fit the muffler and tail pipe and the tail pipe is excessively longer that needed. Do I determine the placement by matching the centerline if the axle with the high point of the tail pipe arch? Just trying to get as much knowledge from the forum as possible.

 

[Hemikiller]

If you're running the 14" case muffler, there's plenty of room to move it forward to clear the rear axle. Set the muffler location by the rear hanger. If you center the factory hanger on the muffler outlet, you should be good to go.

You'll need to trim the tailpipes as needed, since they also fit the 67-73 Cougars which have a longer trunk and underseat muffler area.

FWIW, I have a set of 21" case Dynomax mufflers on my 71 temporarily. They fit, barely. The 14" have room to dance.

 

[Steve McMahan]

Thanks Hemikiller, when I dry fit them, I didn’t have the hangers (I do now thanks to you). I’m back home in CA, so as soon as I get back to AZ, I’ll hang them. I appreciate the help.

 

[Steve McMahan]

You know, I had intended to get the 14” case, but I screwed up and got the 21” ones. That’s probably why the fit doesn’t look right to me. So, now that the truth is out, any new tips?

 

[Steve McMahan]

Well, I lied, I did have the 14” case mufflers. The 21” on the box was the total length including the pipes. So, I’m good there.

But I’ve been bugging Hemikiller too much with exhaust questions, and I am an equal opportunity pest, so here goes.

I’ve been rough fitting the muffler to rear exhaust pipes and I’m really having issues with the pipes coming out anywhere near the rear valance (I have the one with the exhaust cutouts). Now, the pipes I’m using are Flowmaster 15807 and muffler Flowmaster Flow Fx 2.5”.


I think the pictures illustrate the issue, the pipe is straight and level, it’s just 2” too low at the valance. I’m using the CJ pony EXHT3/4 pipe hangers. I’ve got I believe a good fit to the muffler with axle clearance (about 2”) and no interference in the axles arch.


Any idea or recent experiences in resolving this issue? Steve

 

[Steve McMahan]

So, I was able to solve some problems with the exhaust pipe. I ended up cutting the pipe hanger to move up the attaching point; goes from a two bolt to one bolt attachment, but the stock bracket I guess only works with the stock pipe, which I’m not using, 2 1/2” dual exhaust wasn’t an option, or if it was, the hanger I purchased wouldn’t have worked.

At least, I’ve got something close to start; I’ve got to get a longer bolt for the modified hanger to add a shim/washers behind it to better center the pipe in the valance opening.

 

[vintageman]

I feel your pain Steve, I have a Summit 2 1/2 inch header back system as well. My mufflers did not go all the way back to reach the hangers and I had to relocate them more forward. I also had to cut the hanger down as you did. My driver's side comes out lined up with the valence cutout but the passenger side is off center. I am going to have to add spacers or washers to the mounting point on the frame to pull it over toward the tank some. After I get them both lined up and tightened down better, I will add a heat blanket of some sort to the tank areas where the exhaust comes close. Good luck and your exhaust system is looking really good, keep at it.

Tom

 

[DAverkamp]

So, I was able to solve some problems with the exhaust pipe. I ended up cutting the pipe hanger to move up the attaching point; goes from a two bolt to one bolt attachment, but the stock bracket I guess only works with the stock pipe, which I’m not using, 2 1/2” dual exhaust wasn’t an option, or if it was, the hanger I purchased wouldn’t have worked.
View attachment 65885View attachment 65887View attachment 65886
At least, I’ve got something close to start; I’ve got to get a longer bolt for the modified hanger to add a shim/washers behind it to better center the pipe in the valance opening.

I have SS complete w/MagFlow offsets and although I have not installed my rear cut out valance (and maybe never will) the only way I see success in having them centered is to follow Hemikillers advice (and others) and lock down the tailpipes where they need to be and fab your way forward to hang high and clear the driveshaft and trans along with your choice of H or X crossover and then to your collectors. If you are “hung up” (pun intended) on OEM hangers, then you will more than likely have to redrill the floor board mounting holes or just refab the hangers from the existing floor holes to your mufflers. Again, just MO

 

[DAverkamp]

I have SS complete w/MagFlow offsets and although I have not installed my rear cut out valance (and maybe never will) the only way I see success in having them centered is to follow Hemikillers advice (and others) and lock down the tailpipes where they need to be and fab your way forward to hang high and clear the driveshaft and trans along with your choice of H or X crossover and then to your collectors. If you are “hung up” (pun intended) on OEM hangers, then you will more than likely have to redrill the floor board mounting holes or just refab the hangers from the existing floor holes to your mufflers. Again, just MO

I should also add that my set up is with Sanderson huggers and a C4 so again, getting the geometry “correct” had its moments!

 

[Steve McMahan]

Yea, that’s what I finally did, after a day of trying to mount the muffler first. I pined the pipe in the center of the valance outlet and worked forward. While maybe not the goal of others, I want to keep my muffler about 1” from the floor, so that the bottom of the muffler is above the bottom of the frame connectors. So far, I was able to do so, but every hanger part I’m using required drilling new mounting holes, cutting the opening of the top muffler hanger, and you can see what I did with the pipe hanger. I’ve got to do the same on the drivers side, so I’ll take pics with measurements in case it might help someone later.

I‘m also proud that I was finally able to use this fuel line insulator I found at a salvage yard, (you can see it in the final pic where the fuel line crosses the exhaust pipe). It’s a metallic cover, with insulation, followed by the traditional wire cover split tube. Cost me a whole $1 for a 2’ piece.

I should put this stuff in my build forum thing?