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Discussion Starter #1
I read the exhaust system questions with interest when they appear. I see one common reoccurring theme that starts with "I have a 2 1/2 inch exhaust system...".

As I will soon be buying an exhaust system for a 289 Mustang, I can't help but wonder why people are using a 2 1/2 inch system on an engine that doesn't need it and then spend more money trying quiet it down? Bigger pipes amplify noise and drone. This is especially true when the tail pipes are the same diameter as the rest of the system. I would guess that a 2 1/4 inch system with 2 inch tail pipes would be ideal for what a 289 flows.
For racing bigger is usually better, but for "spirited" driving on the street it can be a PIA if you don't like unnecessary noise.

So, is there any realistic reason to run 2 1/2 inch pipe on a 289 making less than 350 hp?
BTW, 2 1/2 inch pipe is what I use with a 425 hp Chrysler 440 that sees 6,000 rpm occasionally.
 

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No. 2½" is a fad, like MSD (or Accel before that). The vast majority of cars now wearing 2½" exhaust would run just as well or better with 2" exhaust.

Even the cars that do benefit from 2½" exhaust, the benefit is all at WOT at high rpm. How much time do you legally do that?

This present-day obsession that "you gotta have 2½" exhaust" is driven mainly by magazines, pushing their advertisers' products.
 

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No. 2½" is a fad, like MSD (or Accel before that). The vast majority of cars now wearing 2½" exhaust would run just as well or better with 2" exhaust.

Even the cars that do benefit from 2½" exhaust, the benefit is all at WOT at high rpm. How much time do you legally do that?
This is all making me feel better about my choice to stick with my 2 1/4" when I install the long tubes later this month. I'm not expecting much of an improvement in performance but maybe slightly better sound. It will all be seat of the pants feel but I will report back
how it sounds/feels. I do agree that in most street cars, 2.5" is likely overkill.
 

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I agree too. I went with 2.25" and Patriot/Pertronix mid-length headers with an H-pipe. Turbo mufflers and turn-down tips. Perfect, I think FWIW.
 

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At JBA we didn't really market the 2.5" systems for the 65/66. It was really oriented
toward the 67-up that took the generally larger engines. When we finally marketed
stuff like the 2.5" tailpipes for 65/66, it was 2-3 years later and mainly because folks
asked for them.
I think the numbers supported 351 cars and 2.5" systems on the street. Definitely
on the track. There were some limited needs for even 3" systems, but not on the
Mustangs. Most of that stuff was for (Jon) Kaase-sized engines in drag cars.
 

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currently I have 2" dual on my 289, I have 2.5 inch system sitting in a box in my garage but that is for a future 331 stroker.
 

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Any educated opinions about my 67 GT390? 1 5/8 primary Tri-y's, X piece, 2 1/2 to turbo mufflers, 2 1/4 over the axles and out the back. Kind of a sharp bend out of the mufflers. Engine is currently stock cam, Perf RPM, ignition, very healthy running. 600 CFM vac 2nd's, considering 750 Vac 2nd's. Mostly around town driving. Occasional race off the line. Some freeway use. 3.00's, 4 speed. 3100 lb car. No immediate plans to cam it up in the near future. Lunati 2nd up Voodoo if I do.
 

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Any system will not flow any more volume of air than the smallest diameter in the system regardless of the size of the rest of the system. This is the reason I had my OEM GT trumpets cut to match the rest of the 2 1/4" system on my '66 GT recently. HiPo manifolds, Magnaflow X-Pipe, Magnaflow XL-3 mufflers. Nice sound, not loud at all, no drone.
 

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AFAIK factory duals were 2" even on the 390 67 and 68. Consider the engine like an air pump. At 6000 RPM a 300 cube inch is pumping as much as a 450 cube at 4000 rpm.

IMO even the stock 2" factory or dealer add duals are overkill on a D, C, A or J code.
 

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So, is there any realistic reason to run 2 1/2 inch pipe on a 289 making less than 350 hp?
None whatsoever. 2.25" is more than enough, especially if you are running duals. Smaller exhaust diameter = higher exhaust velocity, which = more exhaust scavenging, which = better throttle response. People often call this backpressure. LOL no. It's not backpressure. There is no pressure moving backwards. The pressure is moving forward (away from the engine) and at a higher velocity than it would in a larger diameter opening.
 

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Discussion Starter #11
Great info!
I'm new to the Ford restoration world. I've seem where other manufacturers downsize the tail pipe to match the volume decrese of cooling exhaust gases to keep noise in check. I don't know if Ford did this from the factory or not, but it can help make for a quieter exhaust system.
My days of "louder is better" have changed to just wanting to hear the hiss of air in the carb when the 4 barrel cuts in. Yea, I'm getting old and realize very few people want a quiet performance car, but I can enjoy the rumble of other people's cars.
 

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AFAIK factory duals were 2" even on the 390 67 and 68. Consider the engine like an air pump. At 6000 RPM a 300 cube inch is pumping as much as a 450 cube at 4000 rpm.

IMO even the stock 2" factory or dealer add duals are overkill on a D, C, A or J code.
SO many people miss the whole "air pump" concept. "Bigger is always better".
 

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exhaust

74, you're misunderstanding the way exhaust works. Bigger is not louder OR more likely to drone. Many folks are warned about certain mufflers being more likely to drone, but they buy them anyway. Don't fall for the 'velocity' arguement, those folks don't understand how your exhaust works. They are confusing primary pipe size with what happens after the collectors. Don't lall for the 'larger is louder' arguement either. Those folks flunked, or maybe never took, physics class. As an example, look at the 3&1/2 or 4 inch ventilated tips some of the factories are putting on diesel trucks. Very large, with vents strategically placed to break up sound waves, and sometimes amazingly quiet. Large size pipes, in and of them selves, are not the cause of drone. If it were my engine, I'd get twin 2 &1/2s for a 289. LSG
 

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I'm an electrician by trade. I tend to look at things like this as an electrical circuit since the basic theory is the same. When you run something on 240 volts vs 120 volts, for the same amount of work, watts, it will cost the same to run. The advantage of 240 volts is the size of wires, conduit and cost. When you double the voltage you cut the amperage in half for the same wattage. Since you cut the amperage in half, the size of the wire CSA can be reduced in half to do the same. The other advantages are voltage drop. If I keep the same size wire with the amperage being cut in half, I now can run that wire 4 times the distance and keep losses, voltage drop with in 3%.

Applying that to exhaust systems since the gases at the manifold or headers are at their hottest. They can flow flow in a smaller CSA of pipe with acceptable pressure drop. As the gases travel they cool, become denser, lower pressure and slower moving. A good example is to take a look at the AC plumbing under the hood. The high pressure side is hot and the tubing is small. The low pressure side is cool and the tubing is much larger but the volume of gas flowing, CFM, is exact the same. And like wire, tubing has resistance to flow.

My thinking is the head pipes can be smaller then the tail pipes. In the end it comes down to what practical.
 

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2 stroke engines do perform better with the proper exhaust dynamics. Scavenging. The cam and RPM in a 4 stroke will dictate how the exhaust dynamics will affect it. This is probably all much more complex and dynamic than we are aware of. An RV cam will probably not care much because of the close to top dead center valve timing. Higher RPM and cam specs will dictate ideal exhaust specs. And as Huskinhano mentioned, gases cool further down the pipe. Probably more of a factor at lower RPM. I am sure there are experts out there who could lay some serious theory on us. Ideally, a valve with variable opening in the pipe would serve both low and high RPM. Just like a retrofit variable timing cam timing gear would serve all RPM's better. When is someone going to come out with one of these?
 
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