Richard Holdener
December 1, 2006
When it comes to finding extra power on your mod motor, take a long, hard look at the cam timing.

One Thing we've discovered about the OHC modular motors is that they are highly sensitive to cam timing. Unfortunately, the production tolerances are such that the cam timing can be off significantly from the factory-prescribed settings. This is especially the case with the Four-Valve motors, as they seem even more likely than their Two-Valve counterparts to be burdened with inaccurate timing. The most probable scenario is that the right and left bank cam timing differs, causing one side to be either advanced or retarded relative to the other. Both may be off from the factory specs, but the real key is that one side will produce significantly less power than the other. The key to balancing power production is dialing in the cam timing.

This procedure takes time and know-how, but the results can be significant. Dialing in the cam timing on one of our 4.6 test motors (a mild one at that) was worth as much as 22 hp. Balancing the cam timing side to side on a 4.6 Cobra motor was worth a solid 12-15 hp, from 3,500 rpm to 6,500 rpm.

Dialing in the cam timing on the 4.6 is obviously much easier with the motor out of the car, but it can be accomplished with the engine in place. This info will likely be more useful for engine builders planning on installing new or rebuilt motors, but rest assured that cam timing is more critical on the mod motors than the previous-generation 5.0 engines.

A word of advice: Determine whether your mod motor suffers from this cam-timing malady. Performing a compression test (something easily accomplished by even a backyard mechanic) will show if one bank of cylinders produces a higher compression reading than another. The adjust-ment procedure will be much easier with a set of adjustable sprockets, such as the ones shown from Fidenza, but we performed our testing using modified stock sprockets. The best method (short of using a degree wheel and dial indicator) would be to adjust the cams until both banks produce the same cranking compression. The Two-Valve 4.6 motor ran best after we balanced the compression to match the lowest reading (both at 155 psi). In the case of our 4.6 Four-Valve motor, the best power came after the compression of both banks produced the highest reading (200 psi). Once we have adjustable cam sprockets, we should be able to dial in the cam timing with the cranking compression gauge.

Unfortunately, we did not pay attention to the position when we removed it during the cam swap. This mistake allowed us to demonstrate the effect of changes in cam timing, as it was necessary to dial in the cam position to optimize the power output. When first installed, the right (passenger side) cam was equipped with a standard (nonadjustable) cam sprocket. The left (driver side) was equipped with the adjustable sprocket. We attempted to run the motor with the left cam advanced 3-4 degrees relative to the right cam. A compression check on each bank of cylinders resulted in readings of 175 psi for the left (advanced) cam and just 155 psi for the right (0 position) cam.

Though you might be inclined to advance the right cam to equalize the (higher) cranking compression offered by the left cam, the reverse actually turned out to be the correct action. Retarding the left cam back to the 0 position (so both sides produced 155 psi cranking compression) resulted in a significant power gain (as much as 23 hp). Obviously, dialing in the cam timing on the 4.6 Two-Valve is critical to maximum performance.

Effect Of Cam Timing (4.6L 2V)-0 Vs. 3 Degrees Advanced
This test was performed on a 4.6 2C PI motor supplied by Sean Hyland. The PI motor was equipped with stock compression, heads, and intake. The only modifications to the motor were a set of Kooks 151/48-inch headers and a set of Comp XE262H (non-PI) camshafts. While performing the cam swap, we noticed one of the cam sprockets had been altered to allow it to be adjusted slightly. By filing down the keyway, the sprocket could be adjusted over a small range.