6.9L And 7.3L Naturally Aspirated Diesel Y-Pipe Exhaust Modification

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By Roger Felling

This is the procedure that I used to rework the
Y-pipe on my 1986 F250 4×4 with a 6.9L diesel, non-turbo. The factory apparently
did not consider flow when they designed these pipes, because the flow angles
are terrible. Mine actually extended into the other pipe at least 1/4″ so that
definitely would disrupt flow from the pipe. Why do a single exhaust instead of duals?
Well, cost can be one factor, and with these 4×4’s space certainly is. Also, it
has been said that a single 3″ will perform better than dual 2.5″ due to pulse
of the exhaust helping to draw the exhaust out. The original setup used 2 -2.25 pipes emptying into a
single 2.5″ pipe. It doesn’t take a math genius to figure out that is a
restriction, and on these diesels restriction is BAD. The angle that the right
side pipe comes into the left pipe is the other problem,
it was too sharp a curve to be able to flow efficiently. Then coupled with the
fact it was sticking into the left pipe about 1/4″ didn’t help.

Ok, enough chatter, time to get to work. I removed the
Y-pipe, laid it on it’s right side on a big piece of
cardboard. Then trace the original pipe and flanges, for a reference for when
you tack the new assembly together. The reference between angles of the flanges
and the exiting pipes is important to getting it right for putting back into
the truck. The right side is fairly straight, so that is the one to reference.

  To correct the flow
angle, I used a piece of pre-bent 45 degree 2.5″ exhaust pipe from a local
parts chain shop. It isn’t as good of quality as the original using the
aluminized pipe, but the exhaust on the diesels aren’t as corrosive as the gas
motors, so doesn’t tend to rust from the inside . This came from an exhaust
shop manager when talking to him about a new exhaust for my sister’s truck. I
cut the right side pipe about the pint that the pipe continued below the outlet
pipe. Remember, the pre-bent slides over the original pipe. I put it backwards,
the end that would normally go inside the replacement pipe. While it was
laying there, I placed the 45 degree pipe over the original
to get a good feel for position. I then marked out the cuts and used a chop saw
as it was available, and would be a fairly clean cut. I cleaned up the left side entry point and
took a piece of the right side pipe to make a patch for the hole. This was when
I discovered the right side pipe was sticking into the left as far as it was. I
tacked the 45 degree pipe onto the right side flange, then flattened the 3.25
pipe I was using for an outlet pipe a little to be able to get the two pipes
into it, mostly.. This was the pipe I had available,
3.5 would be better, but this worked out pretty good for me. I had the right
side pipe go into the top, and then the outlet pipe partly cut out to be able
to fit the left side pipe to it, and have angle back for a more smooth flow.

 At this point, it was
only tacked together, so it is time to test fit to be sure it will fit
properly. I neglected to pay attention to the length of the pipe, and after
installing I had to take the cross member loose to be able to drop the Y-pipe
back out. I was only a few inches long from being able to remove properly. So
that is something to keep in mind when building yours.  It can go just past the cross member and be
ok, but that is about it, just enough to get clear for working the pipe for
hooking up the rear exhaust. The factory point would probably be a good

  Ok, back to the
point.  I put the Y-pipe back in to be
sure that it fit properly. Make real sure that the flanges are seated right,
and do as much welding while it is held in place to prevent warping. I didn’t
apparently check the right side good enough, as it was leaking badly when I
test fired it to check for leaks. I had to cut a notch into the pipe to get the
proper angle, but I was able to save it without a severe hack job.

  I pulled it back out
and finished the welding and then put it back in for the final fit. That was
when I had discovered my oops, so had to do it again – but the second time was the winner.

  Added to this
modification, I made a muffler for this that was a very high flow, not actually
a straight through, but had the capacity of a 5″ muffler. It gets a bit loud
when I stomp on it, but no more than any semi on the road. I don’t want to post
it as I am considering a patent on it.

  I used all used pipe
as my funds are kind of tight, and it isn’t the easiest or cheapest to use
these bigger pipes, they can be hard to find. The pipes were 3″ to the muffler,
and I had scavenged some 3.5″ pipe from the local heavy truck salvage yard that
I used from the muffler back. I missed out on the chrome stacks that they had
there, because I wasn’t sure they would work. When I went back, they were
gone – oh well. I found the bends I needed to get over the axle, and the
stainless steel band clamps were salvageable as well.

  When I was done, I
had a high flow exhaust, and a 4″ tip extending out at the stock location. It
sounds real nice, and the power and efficiency went up right away. Total cost
the way I did it besides time was under $20. I have been running it for about 6
months at this time, and am absolutely happy with how it came out.

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