Mustang MonthlyHow To Engine
1966 Shelby GT350 - How To Rebuild A 289 Hi-Po
In Part Two Of Our Rebuild, JGM Motorsports Wraps Up And Dyno-Tests A GT350's All-Original 289 High Performance V-8
The Dyno Room
Whenever you build an engine like this, it's easy to get drawn into thinking that this engine will make more power than it did from the factory. However, factory horsepower and torque figures aren't always indicative of real-world power. Ford sales literature from 1966 tells us the 289 High Performance V-8 makes 271 hp at 6,000 rpm. Of course, that's an average of what you can expect from a factory original 289 High Performance engine-with some higher and some lower.
When Shelby weaved his magic, horsepower went to 306 at 6,000 rpm-again an average-with a modest increase in torque. Shelby got 306 hp with a dual-plane high-rise aluminum Cobra intake manifold and a 715-cfm Holley carburetor. Contrary to popular myth, Shelby American did not swap in a hotter cam. For one thing, it wouldn't have made economic sense. Shelby did easy bolt-on mods that also included a Cobra T-pan and Tri-Y headers, the latter of which also contributed to power.
Our Hi-Po engine operates at a disadvantage. From an automatic car, it doesn't have the advantage of a big carburetor. Instead of the 715-cfm Holley, it has a 480-cfm Autolite 4100. That's our 289's handicap-not enough carburetor for a 300-horse engine.
When Jeff Latimer set up our 289 High Performance Cobra V-8, he fine-tuned everything he could to achieve optimum power and efficiency. He fitted Rolo's Autolite dual-point with a PerTronix Ignitor II electronic ignition to eliminate ignition points. The result is smooth idle and good high-rpm performance. Jeff also took one more pass at the valves, adjusting them to 0.018-inch cold before fire up.
Because we were running a flat-tappet mechanical camshaft, we needed to break in the engine and also work-harden the cam lobes. Jeff fired the engine and held it at 2,500 rpm for 30 minutes to break in the camshaft.
So what did we learn from these corrected numbers? Our Comp Cams flat-tappet mechanical camshaft yields a broad torque curve where peak torque arrives at 4,500 rpm and passes the torch to horsepower at 5,200 rpm. In fact, peak torque is hard to define because it's there at 3,400 rpm and doesn't begin to waver until 5,000. This engine makes torque across a vast rpm window, and that's its greatest asset.
How could we have improved our numbers? First, with a 715-cfm Holley for higher horsepower and torque numbers. Secondly, a more aggressive roller mechanical camshaft with greater duration, more lift, and tighter lobe separation is available from Comp Cams. It wouldn't have hurt us to opt for the 351W firing order to improve crank load distribution and power. Aside from the Comp Cams stock grind Hi-Po camshaft and Pro Magnum roller rockers, our 289 High Performance engine is a stock rebuild sporting common sense improvements such as dynamic balancing, better gasket technology, hardened exhaust valve seats, Viton valve seals, and PerTronix ignition. The result is 291 hp and 300 lb-ft of torque.