Wayne Cook
August 2, 2007

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

K-Manifolds
When we compare these numbers with the baseline figures, there's no gain in horsepower and just a single lb-ft of torque. Since we know from experience the K-manifolds do produce some power, we can only conclude that our 2-inch tubing and quiet passenger-car mufflers are holding things back.

RPM HP TQ
3,000 126.7 221.8
3,200 136.9 224.7
3,400 147.4 227.7
3,600 156.1 227.7
3,800 163.2 225.6
4,000 169.2 222.5
4,200 174.1 217.7
4,400 178.2 212.7
4,600 180.9 202.2
4,800 179.7 196.6

Full-Length Headers
With the full-length headers in place, the numbers were as follows:

RPM HP TQ
3,000 133.8 234.3
3,200 145.3 238.5
3,400 155.5 240.3
3,600 165.4 241.3
3,800 173.4 239.7
4,000 179.0 235.0
4,200 183.8 229.9
4,400 187.5 223.8
4,600 190.6 217.6
4,800 190.8 208.8

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