In the never-ending search for horsepower, exhaust systems are one of the first places performance enthusiasts look. It's no secret that a free-flowing exhaust makes more power than a restrictive system. It's also no secret that factory exhaust systems were certainly not designed for horsepower. They were designed to be quiet.
Often on our classic Ford cars, the factory dual-exhaust system (if it even had duals) was composed of 2-inch-or-smaller pipe, and passenger-car mufflers. Even this was considered to be a high-performance system, since the standard Mustang exhaust system for V-8-powered cars was a single pipe leading to a transverse muffler.
Here's our '67 Mustang fastback on the Dynojet rollers at Superior Automotive during the baseline tests. The car has a replacement 302 and an AOD four-speed automatic transmission. The engine is equipped with the standard log-type exhaust manifolds found on both A- and C-code Mustangs. Two-inch pipe runs from the manifolds back to generic passenger-car mufflers that are very quiet.
One exhaust upgrade we've seen a lot is the installation of factory K-code-style exhaust manifolds developed for the '64 K-code 289 Fairlane. They're easy to install, work in tight situations where headers won't fit, they're quieter than headers, and leave the engine with a factory appearance. We have often wondered how much horsepower a pair of K-manifolds is worth. The other common exhaust upgrade we're curious about is full-length headers. How much power might they be worth above and beyond the K-manifolds? The only way to find out was to find a classic Ford car with a factory-sized dual-exhaust system-in this case, a nice '67 Mustang with 2-inch pipes-install the equipment, and then check the results.
Superior Automotive in Anaheim, California, is one of the best places we know of for professional testing and evaluation on any classic Ford project. We rely on the company's expertise with its Dynojet chassis dyno for consistent and accurate rear-wheel-horsepower measurements.
Join us as we baseline our '67 Mustang. We'll then test both the K-code exhaust manifolds and full-length Hedman headers for power production.
Stock
On the Dynojet at Superior, the numbers were as follows:
| RPM | HP | TQ |
| 3,000 | 126.1 | 221.3 |
| 3,200 | 135.9 | 223.1 |
| 3,400 | 146.1 | 225.7 |
| 3,600 | 155.2 | 226.4 |
| 3,800 | 162.7 | 225.0 |
| 4,000 | 169.1 | 222.1 |
| 4,200 | 173.9 | 217.4 |
| 4,400 | 178.1 | 212.7 |
| 4,600 | 179.8 | 205.2 |
| 4,800 | 180.8 | 197.8 |