How To Build A Better Boss 302 Engine
JGM Performance Engineering Shows How To Get More Horsepower From Ford's Trans-Am Powerplant
Sometimes we stumble onto engine projects in the darnedest ways. In this case, we have a Boss 302 that has been sitting around for more than a decade in at least four different machine shops, as well as in my garage. It's a good example of how not to plan and execute an engine project. The engine has been consuming more than its fair share of space lately, so it became a priority.
It belongs to Ron Bramlett of Mustangs Plus in Stockton, California. He initially wanted to build this engine for one of his race cars. After one of my visits to Mustangs Plus, Ron drove me and his Boss 302 engine to Los Angeles. We dropped it off at a local machine shop, which sleeved the block and did much of the machine work. I eventually hauled it to other machine shops where there were several false starts but no progress.
Most of the delays were rooted in hard-to-find parts. A Boss 302 engine isn't the same as the average 289/302. It has a four-bolt main block, a steel crank with a 3-inch stroke, and C3AE-style 289 rod forgings with broad shoulders and larger 3/8-inch bolts. Although the Boss 302 uses 351C head castings, they have different cooling passages. It also uses a unique dual-plane, high-rise intake manifold, which was an issue for us because we didn't have one.
When I told Jim Grubbs of JGM Performance Engineering about the engine, he offered to build it. He thoroughly inspected the C8FE Boss block and D0ZE head castings. Everything checked out. Our block had been successfully sleeved and remained leak tight. Jim measured the bores and pistons, concluding that the block needed needlepoint machining and blueprinting. Our Crower Sportsman rods were ready for inspection and massaging, and Ron's Scat crank with 3-inch stroke was sent to the balancer along with pistons, rings, bearings, rods, a 28-ounce Centerforce flywheel, and a Fluidampr harmonic balancer.
When you build an engine, whether it's a stocker or a high-revving screamer, blueprinting should always be done. Professional engine builders teach us that it's important to look at your engine and see things you've never seen before. Never install parts right out of the box; always inspect and massage them as necessary.
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A mock-up can save a lot of grief when it's time for assembly and fire up. Mocking-up an engine early on establishes dimensions, enabling you to correct irregularities before final assembly.
A mock-up involves temporarily assembling the engine-crankshaft, bearings, pistons, and connecting rods-to measure tolerances. It should also include installing the cam and heads to check piston-to-valve clearances. To be absolutely certain of every aspect of an engine build, you need to check everything, including:
What is Blueprinting?
Blueprinting is engine building with strict discipline and attention to detail. True blueprinting is methodical and challenging. It gets in the way of getting the job done quickly, which is why most don't bother with it.
Think of your V-8 engine as eight individual engines on a common crankshaft. Each cylinder bore has its own personality, making a significant contribution along a crank it shares with seven others. Each is important to power and smooth operation. When blueprinting:
The Extra Mile
Modern shop speak teaches us to degree a camshaft during an engine build to ensure the camshaft is all the manufacturer says it is. That's not the only reason for degreeing a camshaft. It also ensures valve timing events as they relate to piston timing. Most engine builders stop at the No. 1 cylinder when degreeing a cam. If you want to be sure, degree on all eight cylinders.
To understand the Boss 302 cylinder head, you have to go back to 1968 and the 302 Tunnel Port small-block. The 302's tunnel port head had huge ports with a pushrod traveling through them. Although it flowed well at high rpm, torque didn't come on until well above 8,000. The problem was, the engines weren't designed to rev that high. During the 1968 Trans-Am competition, valvetrains failed, not to mention bottom ends, making 1968 one of the worst years for Ford in SCCA competition.
Desperate for solutions, Ford engineers looked to the upcoming canted-valve 351 Cleveland head being developed at the time. It had the same cylinder head bolt pattern and bore spacing as the 289/302, making it a bolt-in for the 302 with minor water jacket modifications and revised pistons. Ford engineers learned the poly-angle valve head delivered better torque at lower rpm ranges.
We haven't tried anything unique with our D0ZE Boss 302 heads. They haven't been ported or fitted with larger valves. Jim installed hardened exhaust valve seats and did a radius valve job to improve airflow without adversely affecting low- to mid-range torque. This approach, coupled with state-of-the-art valve seals, makes the JGM Boss head adequate for street and strip use.
In The Dyno Room
Nothing compares to the dyno to determine whether or not you've chosen the right parts. When we planned this Boss 302 engine a decade ago, we were going to experiment with cylinder heads, camshafts, and intake manifolds. When we decided to blow dust off this Boss 302, we were faced with several critical decisions. Because we wanted to build a streetable Boss, we decided to go with our dusty collection of parts. The cam chosen then was the Crane 27-C9ZZ-A mechanical flat-tappet version with 0.502-inch lift and 292-degree duration-essentially a stock Boss 302 camshaft with a pinch more lift.
We learned on the first few pulls that we had too much carburetor with our 750-cfm Holley 4160 and too little camshaft. Those of you who know Boss 302s know Ford fitted these engines with a 780-cfm Holley but not because that's what they needed. It was what the SCCA mandated for street homologation. The truth is, a box-stock Boss 302 is happiest with a 600 to 650-cfm Holley four-barrel. Unfortunately, we didn't have one.
Jim decided to reduce jet sizing from 0.72 (primaries) and 0.76 (secondaries) to 0.68/0.72. As the engine warmed and rings seated, performance improved considerably, but it was never optimum due to carburetor size. Large-cfm Holleys are too much for stock Boss 302 engines, which is why they struggle in street trim. They lack low-end torque due to a poor combination of large-port heads, excessive carburetor size, and insufficient camshaft.
Jet swaps and carb swaps, along with ignition-timing adjustments, can make significant differences in performance and driveability. We're convinced our Boss 302 would have 10-20 more lb-ft of torque and 10-15 more horsepower with a properly jetted 650-cfm Holley.