3 Inch Exhaust

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Joined
Apr 9, 2024
Messages
33
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28
Location
Lawrence KS
My Car
72 Mach One, 4 speed.
Hello, new 72 owner here. Like the title says, I'd like to put headers and a dual 3 inch exhaust system on my new to me 72 Mach One. It's a 4 speed car if that makes a difference. Does anyone sell a dual 3 inch exhaust for our cars? Also what headers would work with the 4 speed that fit nicely??? Thanks in advance and nice to meet everybody!
 
Hooker Comp 6915 fit perfectly.

3" exhaust will have to be custom bent. The staggered shocks prevent you from using anything off the shelf (Flowmaster, Pypes), which is only 2.5" anyway. Factory style 2.25" kits are available for staggered shock cars.
 
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I went with 3" on mine. It already had a set of long tube headers on it, no clue what brand. I had the local midas bend and weld up a custom exhaust last year. These are the dirt cheap summitracing "race" mufflers.

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Are you planning on running over 500 hp? 2.5" dual exhaust is good to 500, so if you go with 3" and you are not in that range you may lose some power due to back pressure losses.
 
FPA headers fit the best.

I ran 3”, but just because I had the parts in my stash if hoarded parts.

Supercharger on the 351c so I might need 3” over 2.5”

I had to flatten my pipes at the transmission crossmember for clearance as the FPA’s tuck up so well.

I have hooker maximum flow mufflers and turn downs

Phone app says 110db at idle sitting in the car with door open. Tone is DEEP and rattles stuff in the garage.
 

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FPA headers fit the best.

I ran 3”, but just because I had the parts in my stash if hoarded parts.

Supercharger on the 351c so I might need 3” over 2.5”

I had to flatten my pipes at the transmission crossmember for clearance as the FPA’s tuck up so well.

I have hooker maximum flow mufflers and turn downs

Phone app says 110db at idle sitting in the car with door open. Tone is DEEP and rattles stuff in the garage.
Did you have to flatten because it's an C6 auto or is it because that just the way the car is?
 
Are you planning on running over 500 hp? 2.5" dual exhaust is good to 500, so if you go with 3" and you are not in that range you may lose some power due to back pressure losses.
I thought the backpressure fuddlore myth had been debunked long ago. Smaller tube can help you keep better exhaust velocity. But you aren't getting more power from backpressure.
 
I agree, back pressure is not needed nor wanted in a performance application. I believe that where this myth started was people reducing back pressure (i.e. as with headers and/or removing exhaust pipes) without retuning the carburetor. The faster flow can result in a lean condition, which putting the exhaust (and back pressure) back on cures. So, I guess in that scenario back pressure is needed.
 
Depending of what you want. I have 2.5" exhaust and close to 500 hp. I also have exhaust cutouts right after the collectors. I was able to pick 10 hp between 5,000 and 6,000 rpms, but lost a tad of torque below 3,500 rpms, when I opened the cutouts so no "backpressure". However, the torque below 3,500 rpm was a bit wavy which may have biased the results. I think in my case I definitely gained HP and it was expected since the 2.5" exhaust may be slightly choking the system. I don't care much about it because when I need performance I just open the cutouts. Also, no way I can fit a 3" exhaust past the staggered shocks and my Panhard bar. In any case, if someone wants 3" exhaust by all means do it. I think the point is that with a stock engine your gains may be negligible.
 
It won't be stock for very long. I plan on pulling the heads and porting where they need it, bumping the compression and installing a roller cam. Motor has been freshened up recently so hopefully no full rebuild. I need it to somewhat keep up with my Buick.
 
I thought the backpressure fuddlore myth had been debunked long ago. Smaller tube can help you keep better exhaust velocity. But you aren't getting more power from backpressure.

Velocity is what gets you power. No velocity = no lower end torque.
 
If you really want to spend some money, the cool kids these days are running 3" collectors into oval exhaust. That lets you get a bit more ground clearance and fit through tighter spots.

The 3" round tube goes into 2.18 x 3.5" oval. Then tack together the pie cuts to snake through the tight spots.
https://vibrantperformance.com/304-stainless-steel-straight-oval-tubing-v1275/

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Very nice!! I've never seen those. I really am thinking about the Pypes SS for our cars. It's 3inches then goes down to 2.5 after the mufflers
 
Hello, new 72 owner here. Like the title says, I'd like to put headers and a dual 3 inch exhaust system on my new to me 72 Mach One. It's a 4 speed car if that makes a difference. Does anyone sell a dual 3 inch exhaust for our cars? Also what headers would work with the 4 speed that fit nicely??? Thanks in advance and nice to meet everybo
I can't see a necessity for 3" I ran 3" on my GTO it was a .060 over bore 455 12 1/2 :1 comp w/real Ram Air heads, just about 600HP. I couldn't tell the difference between 2 1/2 mandrel and 3" I was running Hooker racing headers on it with 2" primaries. That being said, you can run 3" all the way to the back. If you use any muffler shop bends instead of mandrel bends, you will be defeating the purpose. Muffler shop bends take 35-40% of your total radius leaving them with 2" flowability.
 
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I used Hooker Long tubes, then 2.5 inch pipes, and 40 series mufflers.... but the best thing I did was installing a crossover to get rid of the drone.. and getting the 2.5 in was a struggle on space and clearance.
 

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Muffler shop bends take 35-40% of your total radius leaving them with 2" flowability.
Blanket statements are not always accurate. Some of my bends measure 2.6". Some of them are 2.5". None of them are close to 35% smaller. Half in inch off 3 is 16% reduction. If anybody asks, I tell them those bends are there to help keep up my backpressure for maximum horsepowers. :D
 

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