Mustang MonthlyHow To Engine
How To Build A Better Boss 302 Engine
JGM Performance Engineering Shows How To Get More Horsepower From Ford's Trans-Am Powerplant
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.