Single exhausts will out HP and out Torque duals below 7,000 RPM. FYI: A single 3 inch exhaust with a 3 inch flowmaster and clean turns throughout, will support well over 350 HP.
Shorty Headers will cost you torque at streetable RPM ranges. Another area to kill things is by header tube diameter. Stick with a 1 1/2 inch header primary pipe diameter. The only exception would be a step header with the primary starting a 1 1/2 and stepping up to a 1 5/8. Keep it simple and run a 1 1/2 inch primary.
Since I design and make cams, I can help you there. I'm also a full competition race engine builder and carb blue printer.
Cams are the engine's air flow governor. Until you change the cam, you are not going to change the engine's air flow capabilities or demands. Be advised that 302's are VERY sensitive to cam timing. A little goes a LONG way. And they accelerate much harder with 108 Lobe Separation Angle cams than 112's or 114's. Especially as you add more duration in an engine with less compression. 108's will also give you far more of that cam sound, if you are looking for that characteristic.
Also keep in mind that stepping up to 1.7 ratio rocker arms will most certainly cost you low RPM torque and idle vacuum when you install them on the exhaust valves specifically. They also change the rocker arm to valve tip contact/footprint which is known as rocker geometry. To correct that, you have to change pushrod length. And if the contact at the valve tip is off center much, it will side load the valve in the guide and eat valve guides for breakfast.
I am telling you the rocker arm info because the Mustang crowd is comprised of many younger people who seem to like to throw a bunch of bolt on parts on their engines in search of cheap and easy performance gains. But most of the time, the don't see the whole picture. They'll care about peak numbers but don't realize what much of what they are about to do is going to do to their drivability or fuel mileage.
Your exhaust system needs to be set up for the HP out put of your engine. Stock cam, etc, I'd be messing with a 2 1/4 -2 1/2 single exhaust or you'll give up power with anything bigger. If you just HAVE to run duals, absolutely nothing bigger than 2 1/4 and an X pipe or you'll knock a TON of bottom end torque out of it that you'll NEVER make up for on the top end well enough to justify the loss on the bottom end.
Carb tuning is also where you can find some numbers and mileage to boot. That factory carb likely has some very high signal amplification rates on the boosters. And that's a very good thing. Fuel needs to vaporize in order to burn fast and make peak cylinder pressures early. Therefore, don't simply go out there and throw on a bigger CFM carb because CFM is only half the story. Like I said, if the carb does a poor job of shearing the fuel, it does not matter how well it flows because if the fuel is not vaporized well, the manners, mileage, power and response will all go in the toilet.
A 3.55 to 3.73 rear gear ratio is also a very nice set up for your car.
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