Orginal Exhaust Question.. 66 Coupe 289 V4

My understanding is that if it is a '66 A-code or K-code car, it came with duals. If it is a C-code car, it came with a single exhaust. I cannot imagine that the exhaust would be original after all these years, anyway. All dual exhaust cars came with reinforcing plates for the exhaust hanger studs on each side of the tunnel under the rear seat floor pan. If it is a GT, there are also reinforcing plates for the hangers in the frame rails at the rear of the car - these were for the trumpets (through the rear valence exhaust).

Only true GTs had dual exhausts from the factory in 65-66, although in 40 years, many cars have dual exhaust added. Not all the non-GTs have replaced the single exhaust style rear brake lines with the safer dual exhaust brake lines, however. KAR (karmustang.com) and others (like NPD) sell the correct conversion lines if you need them. Either the exhaust will have to be modified (the over the axle turn on the driver's side moved away from the rubber line) or (better) replace the single exhaust rear brake setup with one specific for dual exhaust (~$100).

If your car came from the factory with dual exhaust, it would also have come with the H-pipe. I'd add one to improve performance and sound and to equalize the side-to-side the backpressure.
Daniel

Not all 1966 A-code cars came with dual exhaust. They should have, but did not????

My '66 A-code had single exhaust (and "verified" by the second owner who purchased the car in '78). There is no sign of the extra plate in the rear frame for the exhaust hangers, reinforcing plates on the floorpan near the back seat, and repositioned rear brake line, but it did have the original Autolite carb with tag, correct cast iron 4-barrel manifold.

My '65 A-code came with single exhaust as well.

I also have a 66 A code and came from the factory with a single exhaust, but was changed out to dual sometime in the late 60s/early 70s. I'll have to check my receipts to see when.

Again, "nearly" all 65-66 A codes came from the factory with only a single exhaust unless they were a GT. As with other options, I've learned not to say "all" when talking about Mustangs, because there are so many factory errors. Officially the duals were only available with the GT option package (HD springs, fog lamps, front disc brakes, emblem and quarter trim delete/stripe add, and rear trumpets). However, you could often get many parts over the counter or dealer installed or even factory installed that weren't "officially" available. I've even seen the Boss 302 shock tower reinforcements factory installed on a "plain" 69 351 sportsroof.
Daniel