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The Four-Valve Swap Power Struggle Upgrade
When You Need 9-Second Muscle And Your Two-Valve Engine Can't Hack It, Swap It Out For A Four-Valve.
As much as Ford's modular engine family was maligned when it debuted, we have seen the performance envelope for modular-powered Ford street cars ripped open in recent times. Big horsepower and streetability now go hand in hand, but at some point, there is a limit. We found that limit when we met up with Bob Watson of Jacksonville, Florida.
MM&FF has known Bob since 2002, when he hauled his Mustang to our Bullitt Mustang shootout. He took First place running 11.95. Not a bad lap given that it was his daily driver at the time. The Bullitt would in fact remain his daily driver for four more years, logging some 92,000 miles on the clock. The dependability of the supercharged Stang was simply awesome.
Over the years, Bob sought to increase the Bullitt's performance at the track by adding a speed part here and there, and before long, everything from the intake to the oil pan had been modified. However, the more he pushed it, the more unreliable the engine became. Three different short-blocks all produced a number of issues, not the least of which were leaky head gaskets. The big-bore, wet-sleeve, stroker engine that Bob was running is suspected to be the cause. Head gasket swaps are not exactly the easiest job to do on a modular engine, step one being "remove the engine."
When it was running well, the 5.0L big-bore Two-Valve pounded out some 600 rwhp, but Bob, like most of us, was looking for a bit more power--enough, in fact, to get him into the 9-second zone, which he wasn't all that far away from having run with a best e.t. of 10.01 on a 40-degree, mineshaft-like day in Gainesville. The car normally ran 10-teens and 10.20s in the Florida heat and humidity. Bob conferred with a number of people regarding his options to get reliability and make the additional power needed to run 9s consistently. He chose to swap in a Four-Valve modular 4.6L engine.
Over the next few issues, we're going to show you what's involved in swapping a Two-Valve engine for a Four-Valve 4.6L powerplant. With our subject vehicle already highly modified, there are quite a few changes that need to be made that are specific to this Bullitt Mustang, and we'll be sure to cover those, as well. At the conclusion, we'll offer track results along with a reliability assessment.
For this month's installment, we expand on the theory of the project a bit, detail the engine specifications, and show you some of the parts that are needed for the build.
The Pros and Cons of the Four-Valve Swap
The Four-Valve upgrade decision was solidified after consulting some highly respected modular motor experts. Al Papitto of Boss 330 Racing (Vero Beach, Florida) and Kris Starnes of Starnes Racing Heads (Hastings, Florida) assert that the Four-Valve chamber configuration is far superior to the Two-Valve chamber. The four valves are distributed evenly around the cylinder, with the spark plug centrally located allows for much better combustion (flame spread) and less possibility of detonation in a forced-induction motor. The extra two valves per cylinder allow much better flow to the cylinder, and the smaller valves have less mass, allowing lower spring rates that provide power at a much higher rpm range.