Building a Better Boss: How to tap into the potential of F

Recreating the “shotgun” hemi

Perception and reality rarely align, and for the past 50 years, that has been the case with the street-production version of the Ford Boss 429 engine. Like Chrysler’s 426 Street Hemi, the Boss 429 was a civilian version of an engine intended for NASCAR and the sustained, high-rpm performance required for stock car racing — and it even offered hemispherical (or very nearly hemispherical) cylinder heads. But the competition version of Ford’s V-8 was much farther removed from the street version than Chrysler’s Hemi.

Nevertheless, it was easy and logical to assume that, with those mile-wide valve covers and football field-sized intake manifold, one of the era’s wildest production engines delivered thrust just slightly below a Saturn V rocket. Unfortunately, that wasn’t the case.

Although development had churned for several years on the racing version, the street version that was required to homologate the engine for NASCAR’s approval was largely a last-minute rush to production. And it was shoehorned into a comparative handful of 1969 and ’70 Mustangs, because the original plan of installing it in the Galaxie was deemed insufficiently sexy for such an exotic engine. Its lowball 375-horsepower rating was vexing, because the engine made more, but not that much more, thanks to a number of compromises to accommodate production.

In fact, all of the attributes that made the engine a winner on NASCAR ovals worked against the Boss 429 on the street, where its huge intake ports and restrictive production intake manifold effectively put a wet towel over the nose of a marathon runner. Performance was particularly disappointing at low rpm, as the poor airflow characteristics of that low-flow intake manifold design and inappropriately large intake ports failed to generate cylinder-filling air velocity. Also, a factory rev limiter kicked in just about the time the engine was starting to get up on the cam.

To address early complaints, Ford instituted some changes midstream, though these seemed to do little to significantly improve the situation. The early hydraulic engines are known by their 820-S engine code, while the later engines—still using a hydraulic cam—are known by the 820-T code (see sidebar). These later T engines have lighter rotating assemblies to help the engine rev up more quickly for livelier performance. Later still, for 1970, Ford switched to a solid-lifter camshaft setup for the Boss 429. At some point during the 1970 engine run, an 820-A designation was introduced, using the solid cam and lighter rotating assembly. While it’s true that when wound up, the Boss 429 engine would pull strongly until the factory rev limiter shut things down, in most daily driving situations, the Boss 429 was a rather anemic street-performance engine.

However, with the rarity of Boss 429 Mustangs (only 1,356 built), comparatively few enthusiasts have had the opportunity to quantify its true potential. That, however, was just what enthusiast Dave Freelander was looking to do, using contemporary technologies and parts to uncork the engine’s capability. He turned to Adney Brown and his shop, Performance Crankshaft, Inc. ( for assistance. In turn, Adney partnered with engine builder John Lohone to help develop and assemble the engine.

“We thought if we could take an engine beyond the 600-horsepower level, but with good low-speed and idle characteristics, we’d have a great, contemporary combination that would be competitive with modern crate engines, but one that’s going to draw attention when the hood is lifted.”

Using an original Boss 429 block and heads was out of the question. The specific, thin-wall casting of the Boss 429 block is all but impossible to find and, given the collector value of restored cars, the few out there are prohibitively expensive. The same goes for a set of original heads. Instead, a Ford Racing cylinder block and Jon Kaase Racing Engines “Boss Nine” cylinder heads were selected.

Fortunately, Ford Racing’s strong, 460-style block is readily available brand-new, and Kaase’s Boss Nine heads are designed to bolt right to the deck of a standard 429/460 engine. That’s significant, because the oiling circuits were different on wedge-head blocks from the Boss versions, resulting in different oil drain holes. Even if you were fortunate enough to find factory Boss 429 heads, you couldn’t just mount them to a regular 429 or 460—you had to have a Boss block. Kaase’s heads match the oil drain holes of production-style 429/460 blocks.

It would have been easy to hit the afterburner and use high-flow aftermarket heads with an engine displacement of more than 500 cubes, but the goal was to keep it as close as possible to the original dimensions, for a more accurate comparison with the original production engine.

“The bore is a hair larger, but the stroke, compression ratio, and basic setup of the engine is very similar to the original,” says Adney. “We hoped to make the most of the original Boss 429’s truly impressive specifications.”

The Kaase head design mostly mimics the production Boss 429 design, but with a few different features and optimized port configurations. The biggest difference is the combustion chamber. The Ford head got its “semi hemi” nickname from a chamber configuration that was based on a true hemispherical design, but with filled-in sides that provided better quench.

The Kaase head design eliminates the semi-hemi combustion chamber and replaces it with a more conventional fast-burn-style chamber that exhibits more efficient and faster burn characteristics. It also is designed to use “regular,” non-Boss 429/460-style head gaskets. The original heads used an O-ring style gasket insert around each cylinder.

Large, 2.300-inch intake and 1.900-inch exhaust valves are used in the Kaase heads – the intakes are only 0.02-inch larger in diameter than on the original engine, while the exhaust valve is the same size as the original. Adney and John used a flat tappet camshaft, like later Boss 429 engines, to actuate the valves. They specified a Comp Cams grind that delivers a whopping 0.650-inch lift on both sides, with 251 degrees of duration, also on both sides. That’s a huge difference from the 0.478/0.505- inch cam used on the later-style production Boss engines.

As the builders would quickly find out, bigger wasn’t necessarily better with their project engine.

Adney and John discovered the engine’s displacement wasn’t enough to satisfy the capability of the cylinder heads.

“The heads are modified, when compared with original Ford heads, but they’re very similar in design, especially in the intake ports, and we found they’re just too darn big for an engine of this displacement,” says John. “To build low-rpm power, we cut down the intake runners’ volume by about 35 percent, and they were still too big, flowing more than 400 cfm.”

The results reinforced why the factory engines left much to be desired on the street.

“You just can’t adequately fill the ports at low rpm with those big heads,” says Adney. “What you really need is about another 100 cubic inches of displacement to process what the heads are capable of flowing.”

Despite the challenging combination, the builders nonetheless achieved impressive results after experimenting with a couple of different camshafts and the aforementioned squeeze-down of the heads’ intake ports. They topped the engine with a Kaase single-plane, spider-type high-rise intake manifold (with welded-in “wings” to effectively lengthen the interior runners), a 1-inch double-tapered spacer and a 1,050-cfm Quick Fuel-built carburetor. The factory Boss 429, of course, used a lower-rise, dual-plane intake and a much smaller, 735-cfm carburetor.

“We even looked at the original-style NASCAR-style intake, and while it appears impressive, it’s totally wrong for a street engine,” says John. “It just doesn’t flow air at low speed, period.”

Nevertheless, in the mid- and upper ranges of the rpm band, the contemporary Boss engine performed admirably. Adney and John experimented with camshafts, header designs, and more on the dyno at Jim Kid Motorsports (, in a give-and-take learning session that saw peak horsepower and torque numbers vary widely. The best result they saw delivered 670 peak horsepower at 6,400 rpm and 556 lb-ft of torque at 5,600 rpm, with 32 degrees of total timing.

“Oh, it’s a good number, for sure,” says Adney. “But there’s a lot left in the engine if we could make the most of the heads.”

Despite their engine’s more-than-respectable performance, John and Adney are left wanting more from it.

“Ford had a great design with the Boss 429 heads, but they were never used anywhere near their potential,” says Adney. “As the dyno results show, our procedures helped bring out more of the power, but there’s still a lot left in there; and a roller cam would have easily pushed horsepower towards the 800 level.”

Untapped potential is a great thing to extract. In the case of the Boss 429 and its cavernous heads, there’s a goldmine in horsepower waiting to be mined. You just need a large cart to haul it all out— a really large cart.

There were multiple versions of the Boss 429 production engine, all rated at 375 horsepower. An updated engine was introduced during the 1969 model-year run with changes designed to boost its anemic feel. This included changing the rotating assembly from the extra heavy-duty components, like the connecting rods, to lighter pieces. The rod bolts also changed from 1/2-inch studs to 3/8-inch bolts. Later still, a higher-lift, solid-lifter cam replaced the smaller, hydraulic camshaft that was installed at the start of production.

The update delivered slightly better low-speed characteristics, but didn’t radically alter the car’s performance. Ford also specified a shorter, 3.90 axle ratio that improved low-speed acceleration. The early, hydraulic-cammed engines are recognized by an 820-S engine code and magnesium valve covers, while the later ’69 engines (the vast majority of production models) had an 820-T code and aluminum valve covers. The 820-A designation was used later still during the 1970 run.

Performance Crankshaft, Inc.
Detroit, MI


2,800 434 231
2,900 440 243
3000 445 254
3,100 451 266
3,200 455 277
3,300 459 288
3,400 459 297
3,500 456 304
3,600 455 312
3,700 459 323
3,800 465 336
3,900 471 350
4,000 472 359
4,100 468 365
4,200 466 372
4,300 467 382
4,400 468 392
4,500 472 404
4,600 483 423
4,700 493 441
4,800 506 763
4,900 520 486
5,000 534 508
5,100 543 527
5,200 549 542
5,300 552 558
5,400 553 569
5,500 554 581
5,600 556 593
5,700 556 603
5,800 556 614
5,900 554 622
6,000 553 632
6,100 552 642
6,200 551 651
6,300 551 661
6,400 550 670

Peak numbers in bold

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