The S197 Mustangs are the...
The S197 Mustangs are the most technically advanced ponycars to hit the streets. One of the biggest features on the Three-Valve engine is the use of Variable Camshaft Timing (VCT). We tested Comp's latest camshafts designed to get the most from the newest member of the modular motor family.
In prison, when the general population gets out of control, guards institute a lockdown. When those bells go off, inmates are secured and activity is frozen. It helps the guards maintain order and keep control. The dictionary defines lockdown as "a state of containment or a restriction of progression."
Recently, Comp Cams instituted its own form of lockdown when engineers designed a special backplate for the Ford Three-Valve engine's camshafts. By limiting the advance/retard capabilities, Comp was able to introduce a new lineup of camshafts with aggressive specs. The Cam Phaser Limiter kit is not a total lockout of Ford's variable cam timing events, but it severely reduces the cam's ability to be retarded by the computer system at higher rpm levels.
Variable camshaft timing (VCT) in the Three-Valve engine is a big advancement for Ford V-8 engines. As you know, the cam (or cams) in any engine control the opening and closing of the valves. Therefore, cams are designed to optimize airflow into the engine, as per the given application. By allowing the cam timing to be altered while the engine is running, you essentially can make the engine more efficient over a wider range of rpm. This is what VCT does, and to accomplish it, Ford designed a series of oil channels through the heads and camshafts to feed pressure to camshaft phasers at the end of the camshafts. The engine-management system commands a pair of solenoids to advance or retard the camshaft timing (by as much as 60 degrees in relation to the crankshaft) based on driving conditions.
Jim D'Amore III's Saleen clone...
Jim D'Amore III's Saleen clone features a 298ci engine. It has JDM ported Three-Valve heads, a stock intake, a steel crankshaft, Manley rods, and forged pistons (11:1 compression). It made 350 rwhp and 366 rwtq, with the stock cams in place. The stock sticks were seriously hampering performance.
Adjusting camshaft timing while the engine is running offers several unique advantages over conventional fixed camshaft timing. On top of Ford's list is fuel efficiency. Nowadays, that's critical, as manufacturers are working towards the federal government's mandated 34-mpg fleet average fuel economy. This is to be accomplished by the year 2012. Your author's '07 automatic-equipped, near-stock Mustang GT regularly gets 25-26 miles per gallon in strictly highly cruising scenarios, showing its effectiveness.
Performance-wise, VCT is effective because valve timing (the opening and closing of the valves) can be optimized for a partic-ular situation. Have you ever wondered why Three-Valve engines have a broader torque curve than Two-Valve and Four-Valve modular engines? The bottom-end torque gains aren't the only advantages, as Three-Valve engines are rev-happy as well.
"The cam timing is advanced from the factory to help low-end torque," says Jim D'Amore Sr. of JDM Engineering. "The factory sets the cam timing base at 7 degrees."
Cutting right to the good...
Cutting right to the good stuff, our test vehicle picked up 51 peak horsepower. The real story is that we saw 81 more rear-wheel horsepower under the peak! Swapping to the Comp SPR Stage 3 camshafts helped our test vehicle go from an 11.51 at 114 down to a best of 11.15 at 119 mph.
As the engine climbs in rpm, the computer retards the camshaft to gain top-end power. It retards the camshaft 9 degrees in the upper rpm levels, bringing total camshaft timing to negative 2. The general rule of thumb is that advancing the camshaft timing creates better low-end power, while retarding the cam timing will help the engine make greater top-end power. Ultimately, this leads to more efficient cylinder filling over a wider range of operation. Pushrod, Two-Valve, and Four-Valve engines are stuck with fixed camshaft timing, forcing you to make a compromise when advancing and retarding the cam. VCT allows us to have our cake and eat it, too.
Now that we're hyped up on the variable cam timing, it's time to deflate its too-good-to-be-true abilities. VCT works great in the stock application, but is it the best thing to happen since sliced bread? One downside is that because the timing events are altered, installing big-lift or duration cams can cause piston-to-valve clearance problem. Obviously, the factory doesn't care much about aftermarket camshafts, so the clearance with larger camshafts wasn't factored in when Ford designed the Three-Valve mod motor. All is not lost, though, as Comp has now released a limiter kit, enabling your engine to continue utilizing VCT, while keeping the valves clear of the pistons when they near top dead center. It merely limits the cams' advance and retard movements to prevent interference with moving pistons.
The Comp Phaser Limiters don't allow the cams to vary more than 20 degrees (in relation to the crank) in either direction. Remember, the unrestricted stock combination allows as much as 60 degrees. The Comp Phaser Limiters allow for a more aggressive profile, allowing the camshaft to unlock more horsepower from your mod motor from greater lift and duration. According to Comp Cams' Brian Reese, "Limiting cam movement through just the computer is only good on paper. Mechanically limiting them to 20 degrees of movement prevents any problems. We've found that phasers are less controllable at low oil pressures or during startup. Another uncontrollable situation is during aggressive transient times--think shifting, dropping the clutch, panic stops, and so on. The cam phasers can move beyond their commanded position because of mechanical inertia they experience. During these times, they can do things they're not programmed to do."