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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.
One dyno test involved switching from a dual-plane Edelbrock Per-former RPM Air Gap intake manifold to a high-rise Parker single-plane manifold, available from Trans Am Racing. A dual-plane intake has long runners, which translates into good low- and mid-range torque, which is needed on the street. The Parker single-plane high-rise manifold has shorter runners, which changes torque and adds horsepower at high rpm.
With the Parker intake, horse-power went from 485.2 to 495.1, a gain of 9.9 hp in the same 6,500-rpm range. However, we lost torque-some 8.9 lb-ft at 5,200 rpm. Torque comes on strong at the same rpm, which means the single-plane Parker loses very little twist.
"After installing the Parker manifold, our fuel requirements changed," Mark comments. "Normally, a dual-plane like the Edelbrock Air Gap requires a slightly leaner mixture, which is fine for fuel economy. Our first pass with the Parker gave us an air/fuel ratio of 14.1 to 14.5:1. This put us right back where we started." This is when Mark went up three jet sizes for the primaries and up two for the secondaries. After the jet swap, the air/fuel ratio went back to 12.6:1-perfect for dyno testing. With Mark's objective being 500 hp, he was close at 495.1 hp at 6,500 rpm.
A day at the dyno is more than just flogging an engine under load at high rpm. It's an opportunity to find out what an engine is made of. If an engine is going to fail, it will fail during jet-check or the first solid pull. During a dyno pull, an engine works harder than it ever will in your Mustang.
When Mark began his Westech dyno session with our 347 stroker, peak horsepower was at 479.2 at 6,500 rpm, with torque coming in at 416.5 lb-ft at 5,200 rpm. With fine-tuning that included rocker arm swaps, going from a dual-plane to a single-plane high-rise manifold, and jet swaps to ensure proper fuel mixture, Mark was able to close the gap, achieving 502.4 hp at 6,500 rpm. Peak torque rose to 425.1 lb-ft at 5,200 rpm.
For a street stroker, peak torque should occur around 3,800-4,200 rpm. However, when that happens, peak horsepower isn't going to be 400-500. This is the trade-off between horsepower and torque. If you want high-rpm horsepower, you're not going to have good low- to mid-range torque. If you want strong low- to mid-range torque for the street, you're going to sacrifice horsepower. And fuel economy? Don't kid yourself. At 400-500 hp, you're not going to have fuel economy no matter how gently you drive.
What we learned from Mark's dyno experience is to know your engine's mission. This is not a mild-mannered small-block, but decidedly aggressive at over 500 hp with the Parker single-plane and roughly 475 with the Performer RPM Air Gap. Your 347ci stroker doesn't have to be this aggressive and you don't really need 500 hp in a weekend cruiser. With a milder hydraulic roller cam, Edelbrock Performer RPM cylinder heads for 351W, Performer RPM Air Gap, and a 750- to 830-cfm carburetor, you can still bank on more than 400 hp at 6,000 rpm and roughly the same amount of torque at 4,200 rpm.
|Budget 347 Parts List|
|Summit Racing Equipment|
|5.0L Roller Block (4.030-inch overbore)|
|ARP Oil Pump Drive||PN ARP-154-7904|
|ARP Head Bolts||PN ARP-154-3601|
|ARP Cam Bolt Kit||PN ARP-154-1001|
|ARP Crank Bolt||PN ARP-150-2501|
|ARP Stainless Engine Bolt Kit||PN ARP-554-9603|
|Fel-Pro Gasket Set||PN FPP-2804|
|Fel-Pro HP Intake Gasket Set||PN FPP-1262-R|
|Melling High Volume Oil Pump||PN MEL-M68HV|
|Ford Racing Steel Distributor Gear||PN FMS-M-12390-F|
|Fram Oil Filter||PN FRM-HP-1|
|Total Seal Gapless Piston Rings||PN TSR-M100135|
|Speed Pro Rod Bearings||PN FEM-8-7160|
|Speed Pro Main Bearings||PN FEM-129M|
|Trans Am Racing|
|347ci Stroker Kit (Nodular Iron Crank, H-Beam Rods, Forged Pistons)|
|Street Oil Pan: 7-Quart with Pickup||PN 9619-7P|
|Street Demon Valve Covers||PN 8505ST|
|Custom Bent Stainless Steel Fuel Line||N/A|
|Super Street Main Stud Girdle||PN 2965SSK|
|Stainless Steel Intake Valves||PN 1208+ .100-inch|
|Stainless Steel Exhaust Valves||PN 9373|
|Custom Billet Coil Bracket||PN 1165-B|
|Performer Air Gap Intake Manifold||PN 7521|
|Fuel Pump||PN 1725|
|Victor Jr. 70cc Heads||PN 1725|
|Aluminum Water Pump||PN 8841|
|Head Bolt Bushings||PN 9680|
|California Pony Cars|
|Timing Cover||PN ENG-000-409|
|Flexplate for AOD||PN TRA-650-710|
|HOLLEY PERFORMANCE PRODUCTS|
|830-cfm 4150 Carburetor||PN 0-9381|
|Billet Distributor w/Vacuum Advance||PN 8749|
|Blaster Coil||PN 8202|
|8mm Ignition Wire Set||PN 31183|
|Dash 6 B-Nuts||PN EAR-581806|
|Dash 6 Sleeves||PN EAR-581906|
|90-Degree Dash 6 to 1/4 inch||PN EAR-982266|
|3/8-inch Stainless Steel Hard Line||PN EAR-100034 by the foot|
|Dash 6 T-fitting||PN EAR-982406|
For our Budget 347, Mark port-matched the intake manifold and cylinder heads to achieve smooth airflow. This is one of the single greatest steps you can take when building a street driver small-block. Port matching and cleaning up the runners yields abundant airflow, as proven by our chart. Mark achieved a whopping 26.45 percent and 81.7 cfm on his best runner with port work. He also cleaned up the plenum, which improved flow.
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What Is Rocker-Arm Ratio?
There are two kinds of lift-lobe lift and valve lift. Lobe lift is maximum lift or rise at the cam lobe. For example, if lift is 0.450-inch lobe lift, the lobe itself gives us 0.450-inch (nearly 1/2-inch). Valve lift is what the cam lobe and rocker arm combined give us via the pushrod. If we're running a 0.450-inch lift cam with a 1.6:1 rocker arm, we're getting 0.720-inch or nearly 3/4-inch. Rocker ratio-1.6:1-takes 0.450-inch cam lobe lift and multiplies it 1.6 times. When stepping up to a 1.7:1 rocker ratio, we're taking cam lobe lift and multiplying it 1.7 times.
Dyno ResultsPull One
Edelbrock Performer RPM Air Gap intake manifold, 830-cfm Holley 4150, and 1.6:1 Trick Flow Specialties rocker arms
Switch to 1.7:1 Comp Cams Magnum rocker arms with Edelbrock Performer RPM Air Gap intake and 830-cfm Holley 4150
Parker single-plane, long-runner intake manifold, 830-cfm Holley 4150, and 1.6:1 TFS rocker arms
Parker intake manifold w/1.7:1 Comp Cams Magnum rocker arms