Ford 347 Stroker Small-Block Build Part 2 - How To Build A Budget 347
At The Dyno, We Learned How To Get More Than 500 Hp From Our 347 Stroker Small-Block
Last month, we shared a cool recipe for an affordable 347ci stroker small-block from Trans Am Racing and Summit Racing Equipment. We learned that you don't have to sell the farm to get good street power in a classic Mustang. All you need is a solid, well-machined 302 block with wide main caps and ARP studs, a stud girdle, and a stroker kit. Our logic is simple. You can fit 294 or 306 ci of displacement within eight 4.030-inch bores. However, for about the same amount of money (by eliminating the machining costs on the stock crank and rods), you can stuff 331 or 347 ci in there to make more torque and horsepower. And torque is what counts on the street.
When increasing an engine's stroke, you gain torque by mechanical advantage. It's like moving the fulcrum to gain leverage to lift a heavy load. By increasing displacement to introduce more volume above the piston, a larger mass of air and fuel is ignited, which is like packing more gunpowder into a bullet shell.
Mark Jeffrey at Trans Am Racing knows how to make power. His successful dyno experiences net tremendous amounts of power, not to mention reliability. For Mark, it's good, old-fashioned horse sense. The Budget 347 here isn't an extraordinary engine with special parts. It's a run-of-the-mill small-block that Mark is building for a customer using off-the-shelf parts from Trans Am Racing, Summit Racing Equipment, Edelbrock, and Comp Cams.
When Mark goes to the dyno at Westech Performance Group, he's not afraid to try different things-timing adjustments, jet changes, cam and rocker arm swaps, carburetor and manifold swaps, valve lash adjustments, and more. Mark uses his own custom-made dyno headers because he wants to make the most of breathing and scavenging. He understands that small primary tubes can choke off horsepower. He also knows that going too large can lose backpressure and torque. Mark's headers, designed specifically for dyno use, achieve a nice balance between torque and horsepower.
One cool thing about Trans Am Racing's 347 stroker is its 10.8:1 compression ratio for 91-octane pump gas. Mark achieves this by thermal coating the piston domes and valves to protect them from extreme heat. Although this seems like a solution for a high-compression engine on pump gas, it's a gamble in terms of durability in a street engine you intend to run hard. If 10.8:1 compression makes you a little nervous, consider lowering the ratio to 10.0:1, along with a richer mixture and more conservative ignition timing for improved durability.
Mark says there are many elements to making power. "Searching for the perfect air/fuel ratio can be a little tricky, especially when choosing a carburetor." He also says that jet size, air bleeds, metering blocks, power valve type, and primary and secondary circuits must all be addressed in order for the fuel system to perform properly. One of the biggest challenges, Mark tells us, is the transition from idle to power circuit. He goes on to say, "We started our pulls by making short passes to establish a baseline fuel curve. This gave us a good starting point to determine what the carburetor is doing off idle and with a light load. We found we were a little on the lean side at 13.9 to 14.2:1. Westech determined we needed to go three jet sizes larger in the primaries. After performing two jet-check runs to verify our jet change, we brought our air/fuel ratio to a safe 12.7:1. This enabled us to make the pulls without worrying about engine damage from a lean condition."
Mark tells us there was more to this dyno test session than wide-open throttle pulls. He also had Westech Performance "drive" the engine in a simulated experience of acceleration and deceleration. Mark wanted to know how this engine would perform on the street under actual driving conditions.