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Coast High Performance 4.6 Modular Stroker Crankshaft - One More Reason
At 5.1 Liters, Coast High Performance's New 4.6 Modular Stroker Promises To Narrow The Displacement Gap
Back in the '60s, Ford built 427, 428, and 429 big-blocks all in the same year-and you couldn't blame a then-new enthusiast for wondering why. Now the newbie might wonder what the difference is between a 5.0 and a 5.1. The answer is "everything." There is no engine swap taking place, so things such as exhaust systems, sensors, wiring, hood clearance, and all that small-but-bothersome jive are not an issue
Modular engines have their defenders and detractors, but nearly everyone agrees it wouldn't hurt if the things displaced a few more inches. Size matters and all that, and with only 281 ci of working volume, the standard 4.6 Mustang engine has to rely on plenty of technique, efficiency, and plain-old charm to thunder with the rest of them.
True, there is the 5.4 V-8 with its willing torque and generally power-happy disposition compared to its little brother. But 5.4s are tall and wide and don't fit quite as naturally in Mustangs. Putting a 5.4 into a Mustang means an engine swap with its attendant hassles, a consequential weight increase, a shift upward in the center of gravity, a dearth of good intake manifolds, more cramped exhaust, and-well-you get the idea. The 330 ci are great, but they're a bit of a pain to gain.
Enter the stroker option. Made popular by 347 pushrod engines, strokers have come to dominate the Fox Mustang scene at rebuild time. It costs almost no more to re-engine with a 347 than a 302 these days, and modular engine builders have been studying the situation so they can do the same with the modular engines. One of those engine builders is Coast High Performance, which has just released its newest family of short-blocks, featuring a 5.1-liter derivative of the 4.6.
Advantages of the stroker 4.6 concept are many. There is no engine swap taking place, so things such as exhaust systems, sensors, wiring, hood clearance, and all that small-but-bothersome jive are not an issue. Intake manifolding options for the 4.6-based engines are, if not numerous, still distinctly better than for 5.4s. There is no weight gain or shift in center of gravity. Outwardly the engine remains visually a 4.6, so no one has to know what you've done. And now, the swap has been made easy with off-the-shelf parts.
For a big engine firm such as Coast, some of those special parts are easy to make, but a few are essential because of either cost or complexity. So, while Coast doesn't sweat pistons because the Probe Industries arm of the same company is one of the largest specialty piston makers in the United States, and strong connecting rods are avail-able in any configuration from China, the stroker crankshaft is the key to the modular swap.
While stock 4.6 cranks can be offset-ground to stroker dimensions, the process is time-intensive and thus expensive. Worse, the surface hardening of the stock crank is thin and removed during the offset-grinding procedure. That means expensive hardening is needed.
All said, a custom stroker crankshaft is quickest and easiest, provided the econo-mies of scale work out to keep the cost low-as they fortunately have in the 5.1's case. Also, the mechanical leverages and ratios that can make stroker engines short-lived are not an issue with the 5.1. It has a nice 1.586:1 bore/stroke ratio, and piston speed shouldn't be out of line.
While the engine shown going together in our photos is actually a Coast High Performance 5.1 prototype (that we'll get to use as a dyno mule!) and thus uses a reground stock 4.6 crankshaft, Coast could already tell us all about the goods on its new run of 4.6 stroker cranks. Sourced from Chinese foundries, the new cranks will be available in either cast or forged steel.
The cast-steel crank, which is due in July of this year, will easily be the most popular due to a low $450, or thereabouts, retail. It will offer a 3.750-inch stroke and stock Ford journal diameters (2.086 inches on the rods). Both six- and eight-bolt flywheel mounting will be available, and the crank should be handsomely durable. Until the cast-steel cranks arrive, Coast is selling the reground GT crank seen in our photos. These cranks sell for $894.32.
Eventually, an eight-bolt-only, forged steel crank should come about, likely in the $700 range. The current forged-steel option is to offset-grind a Cobra crank, which leads to a $1,700 crankshaft because the core Cobra crankshaft must be purchased from Ford before the machining begins.
Luckily, the stock 4.6 block, while lightly built, seems to take enthusiast power levels without complaining much because that's all we have at the affordable level. For 5.1 stroker duty, Coast simply buys new Ford replacement blocks; its only modification is ARP main bearing cap studs and a power hone to a 3.625-inch bore. All blocks are sonic-checked for cylinder-wall thickness. Coast allows no less than 0.185 inch at the top of the cylinder and 0.125 inch at the bottom on the minor side of the cylinder. This is enough meat to keep the block from glowing like a Japanese lantern when a droplight is lowered down a cylinder-sufficient for naturally aspirated applications, in other words. Big-boosted engines need to seek a stouter block from Ford Racing Performance Parts.
Other details are covered in the photos and captions, save for price. Coast's Street Fighter short-block retails for $1,999, the Pro Street version is $2,599, and the difference between a Street Fighter and a Pro Street is the crankshaft. The Street Fighter uses the cast crank, while the Pro Street uses the forging. And if you noodle around the Coast catalog or Web site, it doesn't take long to figure out the short-block prices reflect a substantial savings over buying these parts piecemeal.
All we need now is a 5.1 fender badge.