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I'm not sure if boost changes things or not but I've seen guys making 750-900rwhp go from dual 2.5" to 3" and gain no horsepower.
It does and depends upon the level of boost...... directly related to how large the engine has effectively "grown" in CID at that max boost.
 

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1965 A code 4 speed coupe
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I have a question regarding this issue. I have a stock A code that I am converting to headers/dual exhaust from a single exhaust. I am planning on using the Arvinode system because it sounds so darn good! I think that is a 2.25" system, correct?

From previous experience (with BB Chevy's) I think 2" would be more than enough for a 289, even one that is modified and likely enough for a 351. I have installed a stainless Magnaflow 2.5 system on our 1970 Nova with a slightly warmed LS6 crate engine (1971 vintage with OEM but reworked heads and about 550HP) and 2.0 OD headers. and it seems to flow well. I've never put it on a dyno but it pulls really well up about 6000 RPM-I've never tried to go higher.

As a frame of reference, a stock LS6 Chevelle (450 HP and 6500 redline but max power and torque is usually in the 6200 range) had 2.5 inch head pipes but was restricted to 2.25 inch tailpipes going over the rear axle to the rear of the car.

Just my opinion but I believe that, as long as the bends are not restrictive, I think a 2.5 inch dual exhaust is sufficient for up to 500+ HP and 6000 RPM at least.

Oliver
 

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64 1/2 D Code Coupe,
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I bought an 2 1/4” kit from the H pipe all the way back from Mustangs Unlimited back is 2005. It’s still going strong. The kit came with turbo mufflers. It looks like the same kit from Sold by VA Vintage Mustangs. If I had to change anything I’d get mufflers that were quieter until I put my foot into it. I would also consider a single two in two out transverse mounted muffler similar to what I had on the Formula 400 Firebird back is the day.
 

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Years ago I was looking to redo my exhaust on my 1966 Convertible. I went with the JBA shorty headers and the Magnaflow 2.5" stainless system despite the fact it mentioned "not for convertibles". It was a real bear to install, every piece forward of the mufflers was modified. The convertible reinforcement plate is lowered using 1-1/4" square tube as spacers. I love the sound except when I am in the garage or at a drive through.
 

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… am looking at the factory style setup with…

Why not use Waldron exhaust. Full on factory style. From factory six, C-code, A-code, K-code and Early GT.
1965-69 Ford Mustang V8 Arvinode Base Model Dual Exhaust

My current ‘66 already had JBA shorties. I called Waldron and had them adapt to the H-pipe I got from JBA and it fit like a glove. The one I used was for the early ‘65 GT’s before Ford got cheap and went in-house. Talk about not sounding like every other FlowmasterMagnaflowwhatever muffler. No drone, wakes up when you get on it and sounds mellow when cruising. It seems the price may have gone up some in the seven or so years since I bought, but I would do it again today.
 

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One thing is if you go factory, you can get a hanger kit. Also, you should buy the floorpan reinforcement plates to really do it right. There are other good ways as well.

He is the LH for a '67 but NPD carries them all.

REINFORCEMENT PANEL, FLOOR PAN MUFFLER HANGER, RH - #0113A12-3B - National Parts Depot (npdlink.com)

There are articles on installing them. A lot of people use bonding adhesive.

After they are installed these drop through hanger bolts go through and are held down with a screw. Then you bolt the muffler hanger to that under the car.

Here's why I put them in my car:

67 Dual Exhaust Reinforcement Plates-Weekend Project | Vintage Mustang Forums (vintage-mustang.com)
 

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65 Mustang Caspian blue coup, disc brakes, equaloc rear, power steering, 289 auto >100k born Nov 64
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Hi Folks,

Going to order a factory style dual exhaust for my '66 A code 289. Converting it from the single exhaust configuration. And man, the OD I think on that single system is less than 2"...

I've done all the prep work (floor reinforcement, brake lines) and am looking at the factory style setup with Flowmasters. Vendor I'm looking at offers them with the hangar brackets welded on the muffler already.

Only ? remaining is diameter -- I'm thinking 2" will be adequate with stock manifolds and my 460CFM carb.

I'm really more into cruising this car than anything else. So, seem ok?
You should be fine. Keep in mind, your brake line is very close with a 2 in system. I have that exact system on my 65. Except I have the hipo manifolds. It sounds great. It comes apart easy it’s only clamped on, I found the stock hangers worked very well. I didn’t need to weld reinforcements before the axel. Good luck
 
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Unless your 289 is making big power above 6000, 2 or 2 1/4 will be fine. I wanted some mandrel-bent pipe and the pre-formed Flowmaster tails in 2 1/2 is all I found available for a reasonable price but 2 1/2 is hard to get to fit right on the driver side between the leaf spring and the fuel tank.
Yes it is, I have heard 2.25 works the best most of the time for modded 289s but Magnaflow's smallest kit at the time was 2.5 and a pretty good deal
 

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Discussion Starter · #37 ·
One thing is if you go factory, you can get a hanger kit. Also, you should buy the floorpan reinforcement plates to really do it right. There are other good ways as well.

He is the LH for a '67 but NPD carries them all.

REINFORCEMENT PANEL, FLOOR PAN MUFFLER HANGER, RH - #0113A12-3B - National Parts Depot (npdlink.com)

There are articles on installing them. A lot of people use bonding adhesive.

After they are installed these drop through hanger bolts go through and are held down with a screw. Then you bolt the muffler hanger to that under the car.

Here's why I put them in my car:

67 Dual Exhaust Reinforcement Plates-Weekend Project | Vintage Mustang Forums (vintage-mustang.com)
As part of my prep, I installed the floorpan reinforcement plates, and installed a new dual-exhaust accomodated OEM style rear brakeline kit from the master back wat all the to the drum cyinders.
 

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Discussion Starter · #38 ·
Well, just an update - The 2 inch Flowmaster system from Virginia Classic Mustang is now installed. I like the quality of the parts and it all installed easily. I haven't driven the car yet, but it sounds nice in the garage - a bit more subtle than expected, but still very nice. It replaced a rusty single-exhaust system (1 7/8"), so its a bit of an adjustment to my frame of reference... Hah!

As a recap; I converted a single exhaust A-code car by installing an OEM style dual exhaust (in a non-GT car). I installed rear seat floor reinforcement pans, hardware, and I installed new rear brake-lines (and new hardware) from the master back to the drums with a new flexible line & bracket welded to the body. Also installed Scott Drake GT Exhaust Conversion frame brackets (fit perfect!) as non-GT cars don't have the GT bracket holds in the rear sub-frame. Part number is #C5ZZ-5277-CR @ $43.32 a set.

All in, this project cost just over $1200. Price increases just before I ordered the exhaust proper, added about $200 here. Ouch!

Conversion prep (reinforcement pans & brakelines) were the hardest part of the project. Actual exhaust install was a breeze!

Could I have done it cheaper? Sure. But I would have had cut a lot of corners and made other compromises (quality, fit and safety) as well.

If you're going to do a similar project, research, plan well,and budget plenty of time.
 

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65 Mustang Caspian blue coup, disc brakes, equaloc rear, power steering, 289 auto >100k born Nov 64
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135 Posts
Well, just an update - The 2 inch Flowmaster system from Virginia Classic Mustang is now installed. I like the quality of the parts and it all installed easily. I haven't driven the car yet, but it sounds nice in the garage - a bit more subtle than expected, but still very nice. It replaced a rusty single-exhaust system (1 7/8"), so its a bit of an adjustment to my frame of reference... Hah!

As a recap; I converted a single exhaust A-code car by installing an OEM style dual exhaust (in a non-GT car). I installed rear seat floor reinforcement pans, hardware, and I installed new rear brake-lines (and new hardware) from the master back to the drums with a new flexible line & bracket welded to the body. Also installed Scott Drake GT Exhaust Conversion frame brackets (fit perfect!) as non-GT cars don't have the GT bracket holds in the rear sub-frame. Part number is #C5ZZ-5277-CR @ $43.32 a set.

All in, this project cost just over $1200. Price increases just before I ordered the exhaust proper, added about $200 here. Ouch!

Conversion prep (reinforcement pans & brakelines) were the hardest part of the project. Actual exhaust install was a breeze!

Could I have done it cheaper? Sure. But I would have had cut a lot of corners and made other compromises (quality, fit and safety) as well.

If you're going to do a similar project, research, plan well,and budget plenty of time.
Sounds like a good job! Pics?
 
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