5.0 Mustang & Super FordsHow To Engine
347c.i. Small Block Engine - An Easy 500
Coast High Performance's budget 347 makes a quarter ton of power on the Westech water brake
In our July issue ("Half-Price Hardware," p. 50) we detailed the building of a 347 small-block by Coast High Performance. It was labeled a budget engine because it didn't go all-out for an aftermarket block or killer reciprocating parts and was fairly bursting with offshore-sourced parts. The goal was to replicate what many a hobby racer seeks-affordable thrust that works.
While we did a fair job of laying out the costs and hardware, this month we'll see if the combination works. And it does-so you don't have to turn pages as if this were a suspense story. That means it met the goal of 500 hp. Along the way, we did some interesting intake tests, so of course you'll want to check out the whole story.
The Parts List
In case you missed July's article, a quick review of the budget 347's hardware is in order. It begins in 1972 when the stock 302 block was cast with more meat than the later Fox Mustang 5.0 H.O. engines offered. Two-bolt castings, these late-'60s, early '70s blocks, especially the so-called Mexican blocks, are as solid a foundation as you'll get before stepping up to a Dart or Man-O-War block. This one was prepped with a 4.040-inch bore and a SCAT 3.400-stroke cast-steel crankshaft.
Because Coast High Performance and Probe Precision Products are basically one and the same company, many Probe parts were used. This includes the forged, non-finished connecting rods. They carry 31/48-inch ARP bolts and can be thought of as good sportsman-type pieces. Probe calls them machine-beam rods.
Likewise, Probe pistons were a given. Because high compression is a sure path to power-and doesn't cost that much to build into an engine-this time Coast reached for 12.5:1 compression slugs. They are SRS-series forged pistons with a 9cc universal dome that works with most inline heads-Brodix, Edelbrock, all the Chinese copycat heads, Roush, Darts, AFR, in other words, all the usual suspects-but not the all-out canted-valve racing heads from Neal, Blue Thunder, or Yates, nor the slightly different Twisted Wedge or Edelbrock Victor castings. They will, however, work with a Victor Jr. head.
That's a good thing, as this engine wears Procomp heads that are strikingly similar to the American Edelbrock Victor Jr. head. These Procomps sport 215cc intake runners and a 2.055x1.600-inch stainless steel valve package. Coast had Troy Bowen at Ford Performance Solutions set up the heads, meaning Troy final-adjusted the CNC porting job, did a valve job, set the spring heights, and all that.
Camming is another low-cost path to power, and Coast went plenty big using an Isky custom grind measuring 263/272 degrees of duration at 0.050-inch of lift, and 0.592/0.608-inch of valve lift with the expected 1.6:1 Probe roller rocker arms. The lobe centers are a close 110 degrees, so this cam has a fair bit of overlap and promotes high rpm.
If there is a downside, it's that with the older block, a flat-tappet mechanical cam is almost required. This can be touchy to break in with modern oils, and a flat-tappet cam requires stout valvesprings. In fact, it seems that given a ton of spring pressure to control the valves at high rpm, the chances of flattening a cam lobe are high, yet if the valve spring pressures are reduced to promote cam longevity, the valves float. The answer is a mechanical-roller cam, but that would be more money, so the flat-tappet got the nod.
Part of an effort to promote lobe life by Isky is its EDM lifters. These feature a small hole in their faces to increase oiling at the cam/lifter interface. The valvesprings measured 130 to 135 pounds on the seat, and in general Coast figured this engine is safe to as much as 7,800 rpm.
Oiling is wet-sump via a modest Elgin pump and a beefy Probe heavy-duty oil-pump drive. The pan is a standard deep sump from Canton.
Coast also crowned the budget 347 with something new to the U.S. scene, a Parker Funnelweb intake supplied by Procomp. This is a tall, single-plane intake with a seemingly built-in carb spacer. It looked rather racy and promised to make good power, but obviously a tall hoodscoop or bulge would be required to get it into a Mustang.
Because the Funnelweb was new to us, too, we decided to try a few manifolds as a comparison. An Edelbrock Victor Jr. intake seemed the logical choice because it too is an rpm-oriented single-plane and one that fits under many hoods. We also tried an Edelbrock RPM Air-Gap to illustrate how a dual-plane intake would "rock the curve" in favor of lower-rpm torque. Frankly, we didn't expect the RPM Air-Gap to do anything but choke at high rpm, but luckily we actually do these tests, otherwise we'd never learn just how ignorant we can be.
Running this engine on the dyno was a refreshing, low-key effort. Carburetion is typically a snap on the dyno-nothing broke, the engine had already been run in by Coast High Performance on its hot test stand that isn't a dyno, but it's able to run the engine and break in the cam/lifters. Generally, it was a simple case of bolting up the engine and making power.
As delivered by Coast, the engine lacked only exhaust and carburetion. The exhaust was handled by our usual Hooker long-tube headers, short collectors, and, at least to start, a pair of mufflers. The carburetion was via a 750-cfm Speed Demon. This is the most affordable Demon carburetor at $435 list, and it lacks the more fully contoured carburetor top of the slightly more expensive Mighty Demon. No matter, it ran fine and fits in with this engine's budget philosophy.
With the Hookers and Speed Demon in place, our first steps were to set the timing and verify the air/fuel mixture before leaning hard on the loud lever. The fuel was easy, as the Speed Demon proved spot-on with a 12.7:1 A/F ratio. The ignition timing was almost as easy, hunting between 30 and 38 degrees-none of which made much difference-and we ended up where we always do, 32 degrees of total timing.
At this point, simply making a full pass netted 503 hp, so our initial goal was met without really trying. Nice. Backing it up we saw 510 hp as the oil warmed up, so labeling this a 500hp engine is an easy call.