old guy dumb question time again.
i am building a 3" x-pipe out of 22 1/2 degree mandrel bend tubing chunks.
the question is, how big of an area does the intersecting hole have to be ?
can it be too big ?
can the x-pipe be too narrow width wise, at the intersecting point ?
a big TIA for those of you that are WAY smarter than i.

On mine I just cut and ground back on some 90 degree bends as in the 'photo. The "slot" produced was a 6" x 3" oval (maximum points), I figured that was enough area for a dual 4" system. I've also seen them where they are just a pure "X" with 90degree intersects. It's steel and now painted black, I plan to do it in stainless eventually. We didn't pick up anything over open headers but we didn't lose anything either. It also made the car quieter, levels went from 124.5 db to 120.1 over open headers. We have a 120db limit at the track. HTH's

How much does it weigh any idea? I know mine is heavy 30-40 lbs.

There may be an area calculation, I don't know.
We also got a Db rule at Sears Point in the '90s; so I got a Dr. Gas X, added a 3.5" system to my 3" collectors, Dynomax race Bullets, & turndowns before the rear axle that slightly faced each other.
Much quieter, though still kinda loud, didn't slow the car down, sounded great.
Funny thing is how it changed the pitch: In the burnout (6,000 max), it sounded like 10,000 RPM - people would back away from it
You can Google the Dr Gas X, and look, but none seem to have a huge common area.

I did weigh it when it was done (5 years ago) and I cant remember exactly but I reckon it was as above, around 35 lb. Makes little difference to the et running with or without IIRC but we have been running with some sort of silencers since 2008.

thanks for the picture Tig.
i used a 2 1/2" hole saw to cut a 2 1/2" x 3" long window in the pipe sections. i guess this will be enough.
these sections are also just plain steel and will be painted. when done, this exhaust system will outlast me, so i saw no need to do it in stainless.
i will post a picture or two when i get this done.
not sure of what mufflers i will be using. as of now, my car has a pair of the original flowmasters, when they first started making them many years ago.
i have a pair of single chamber flowmasters on the shelf, and i'm thinking of maybe getting a pair of race bullets to try instead.
also, not sure if i want to put the mufflers in the standard place under the floor pan, or putting them at the rear of the car.

I never measured the sound level, but it made a big difference using DynoMax Bullet mufflers on a 3" TTI X-pipe system on my small block.

As for performance difference, it did help my combination despite the added weight. Not more than a tenth, but positive nonetheless. Try jetting 1-2 numbers leaner with the X-pipe vs open headers. But it depends on how close you were to optimum with the open headers to begin with.

I seem to recall Engine Masters testing X and H pipes and the H pipe was better ?
I could be mis remembering , but this resonates with me.
Sorry for the lame pun.

On that particular setup the H pipe may have been better. Do the test on a different set up and the X may be better. It's all build specific as to what they want on exhaust.

an update.........
after cutting 2 1/2" holes, blending them together, and matching up the pipes, i ended up with a 1" x 4 1/2" window, top and bottom.
so i now get to fab two pieces to close those holes. the only good thing i can see about this goof is, i can get in there and smooth the internal mating surfaces some.
i hope i didn't screw this thing up.
an H pipe would have been MUCH easier to do !
all this work, and it will probably not make any difference.

So, that's 1/2" on each pipe ?
You should be able to see - I think - that if you make those cuts more oval (lengthwise), the top & bottom will butt up.
I think. Can't see through my screen worth a damn.

i can see that now. through my screen............
i know, no pictures. i am a dumazz for sure !
if i ovaled them lengthwise more, as you suggested and i should have done in the first place, i will end up with a very thin cross section pipe in the middle.
i'm just trying to salvage what i have done [screwed up !] because i don't have any other pipes to use.
this is certainly a learning experience for me ! i will know better the next time !


ps. pictures will come in a day or two. i tried to load pictures with this new operating system i have, and lost them. i have to practice THAT skill as well now........ it never ends.

No worries - call it the prototype, buy some more tubing, and conquer.
I'm a visual learner for the most part, so sometimes I save myself some grief by sketching out what I want before the cussin' starts.
Just kidding - there's cussin' even before I start

Technology ? Heck, I can only shoot a video on my phone by accident, then 10 minutes of fumbling trying to make it stop and go away.
And I have no more of an idea how to work a Bluetooth or any of that Play stuff than I do brain surgery...

How much of an "X" depends on how much you slice off the outside of the bend (as below, but I used two 90 degree bends). IIRC the engine master episode concluded that the X pipe gave more H/P higher up the rpm range but it wasn't much over the H pipe (my reasoning for making it). Both were better than twin / non connected pipes.

What Tig said. Depends on the bend. Here is mine 3.5 to 3 inch

i should have done a "straight line" cut like you show Tig, instead of using a 2 1/2" hole saw on each side of the bend center. i will certainly know better the next time !
i now have this fiasco "saved", so after a little prettying up, it won't be too awful.
i'll get some pictures so everyone here can laugh at a blind old dimwit's attempt at welding after an almost one year hiatus from touching a gun.

I made my own X pipe out of 45 degree mandrel bends also. Everyone alway comments about how good my Chargers exhaust sounds.

well, here is my salvaged fiasco.
after lots of "prettying up", it doesn't look too bad for a blind old man with little welding "skillz".
as to the inside shot, do i need to get in there and do any blending ? it looks like there may be flow issues at the intersecting points.
next time, i will remember the mistakes i made on this, and hopefully won't repeat them.

any opinions on the internal flow of this thing ?

The photo is kinda hard for me to make out, but the basic rule is:
Airflow doesn't like projections or interference, especially a sharp edge and cavity like what I think I'm seeing.

I suppose you could stick a garden hose in there, crank it up, and see if any water blasts back at you...
Not that it would be definitive, but would illustrate the principle.