The Shocking Truth behind Ford’s BANNED BOSS 429 Engine｜ Why？

The Boss 429 engine was built for NASCAR domination, featuring a massive 429 cubic inch displacement, hemispherical combustion chambers, and advanced engineering that promised unmatched high-RPM performance. But when it was adapted for street use, this powerhouse fell flat, struggling with low-end torque and sluggish acceleration. Its oversized ports and heavy rotating assembly, designed for track performance, made it underwhelming on the street, leaving enthusiasts disappointed. Despite its challenges, the Boss 429 found redemption in racing, particularly when modified by tuners like Bob Glidden. This video: MOST MISUNDERSTOOD Boss 429 Engine dives deep into why the Boss 429 became one of the most misunderstood engines in muscle car history.
00:04 Introduction
01:31 Birth of the Boss 429
06:22 Why It Fell Short
12:09 Redemption and Legacy
16:21 Conclusion

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Under Section 107 of the Copyright Act 1976, allowance is made for "fair use" for purposes such as criticism, comment, news reporting, teaching, scholarship, and research. Fair use is a use permitted by copyright statute that might otherwise be infringing."

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[CookeZachery]

As a die-hard Mopar guy, I still find this topic fascinating. Back in the '70s, we didn’t have a lot of cash, so we stuck to simple go-fast mods cam, lifters, valve springs, gears, shift kits, stall converters, and headers. Mopar 6 Pack guys needed a Performance Kit or progressive linkage, while Ford guys had some killer performance heads. The Boss 429 was mild for the street, but with the right tweaks, it was a beast. At the end of the day, it’s all about what you love.

[trucking]

The street version of the Boss 429 that came in the Mustang was very detuned. Came standard with smallish 735 CFM carb, a mild 10 1/2 to 1 compression, a tame camshaft and restrictive exhaust manifolds. All those restrictions didn’t let those Hemi heads with those huge valves and ports breathe and wasn’t able to reach its full potential. Just putting a larger carb and a set of headers helped a lot.

[kenm]

If Ford had only kept focus on the Boss 429 Torino.

[AilaniMadden]

I only got to attend one NASCAR race in person, Michigan International Speedway, 1976. The sound of that engine roaring down the front stretch was unforgettable! I'm 68 now, still watching on TV, but nothing beats being there.

[RoyNoakes-od]

Unless you're from across the pond. Copper is what the cylinder o-rings were made from. Not cooper. And it's pronounced hem-e not hem-eye. Also I could deal with less repetition.

[English-mj]

Appreciate the insight on the Boss 429, Legends like John Kasse and Jack Roush helped push this engine to its full potential.

[golgothapro]

Gotta dig that Cooper ring head sealing.

[mitchblack]

The first 249 production Boss 429s were built with the 'S' hydraulic cam engines that had the heavier rotating assembly using the shorter version of the Nascar 1/2" bolt connecting rod and without rifle drilling for the piston pin. The remainder of the 1969 production switched to the 'T' engine with the same camshaft, but with a lighter 3/8" bolt connecting rod similar to the 429 SCJ, but with about 50 grams more material in the beams. All used the same 735 CFM carb, heads and valvetrain. The Boss is referred to as a semi hemi because the combustion chambers were not half a sphere and had two opposing quench pads. It did have a hemi style though with an opposing valve lay out. The 1970 production Boss' were all 'T' engines, but were built using the 429 SCJ mechanical camshaft. The heads, carb and intake were the same as the 1969 engines.

The 'T' engines were rated at 375 HP and using quarter mile times, actually produced in the neighborhood of 410 actual crankshaft horsepower. In Nascar form as built by Holman-Moody, the last Boss revision engines with 'D' intake ports made 635-640 crankshaft horsepower at around 6800-7200 RPM.

Contrary to popular belief, the Boss shock towers were installed on the Ford production line, not at Kar Kraft. The very earliest Boss Mustangs were shipped with the 428 SCJ engine which was later removed and the shock towers modified at KK, but early on, this modification was later done at Ford and the cars were then shipped to KK sans the engine, but already with the modified shock towers and export brace.

All Boss 429 Mustangs used a high nickel content, thick walled 4-bolt block good for 0.120 overbore. The Nascar blocks were good for 0.150 overbore. All of the street Boss engines used a 1046 cross drilled forged steel crankshaft. The early 'S' engines 1/2" connecting rods were assembled like the Nascar engines where the bolt stretch was used for the final tightening sequence.

Aside from putting a lot of displacement under the Boss heads, most did not understand the tuning requirements necessary to make the Boss 429 perform. First and easiest - that sealed hood scoop was very restrictive and cost significant power. Aside from this, this engine needed a fast action camshaft with acceleration rates not available for many years. For street use, intake port wedges were available that raised up the floor of the intake port while reducing the intake port volume and cross sectional area - giving improved intake velocity. This modification was incorporated into the last revision of the Boss head casting and was used in the later Nascar racing engines.

The Boss 429's heads were modeled after the 427 SOHC and it was an outgrowth of it as a pushrod version of this engine.

The Boss 429 Mustang handled well and was the first Mustang to use a rear sway bar. These cars had a good built quality and left Kar Kraft with better assembly fit and finish than regular production Mustangs.

The Boss Mustang's value reflects the fact that this car was produced to homologate this engine for Nascar racing and that this car had many unique features. Further, the car carries with it the lore of Ford racing heritage and represents a time when Ford financed and supported its racing programs. Appropriately, moving forward, its value will be at or near the top of all muscle cars from this era.

[thomasblankinship]

It was detuned for the streets. It was made for wide open throttle use. Not tooling around town at 1500 RPM. And a 735 cfm carb wouldnt be close to enough to feed the beast in a better tune than street tune.

[FunnyDay-]

The Boss 429 and the SOHC Cammer are absolute legends, both look incredible. Can’t forget the 426 Hemi either

[raphaelcolombani]

As a Mopar guy, as a guy who cherish the story that made America create a Pony and Muscle Car era, I do believe that this engine might have had way too many people involved in the process, with to many goal to be obtained by the same engine. A race car is not a street car, and many engine intended to do both failed. All drag racer using their daily drivers knows that a 440 six pack is more fun and enjoyable than a more expensive 426 Hemi. I do believe that the same rules applied to the 429 Boss engine. It’s a race engine, that should have been way more modified for the street, and not been put in a Mustang that wasn’t even racing in NASCAR events. People want to have in their garage what they see in the street.

[gregarakelian]

I had a 429 boss in my mock one when I was 20 and We street raised all the time back in the 80s in my car was faster than anybody’s in my town I never lost a race that thing was so fast it was crazy I kept breaking cam shafts I don’t know why probably needed a Winebar line line bore 1:38

[edgesword]

Smokey had 3 different versions of the NASCAR 429 back in the mid 60 's but it wasn't released until 1969 . Ford dropped out of racing in 1971 so that ended research and devolipment on that engine along with availability of engines and replacment parts . As far as the 326 hemi was concerned it couldn't even beat the 427 Tunnel port from 1967 - 1969 Daytona 500's and was 70 hp . short of the Boss 429 so they had to buils a body to stay in the game .

[yankeewatchdog]

Because Ford DE TUNED the Boss 429 for the street application. 375hp in the Mustang, as compared to to about 560 hp in NASCAR tune...2 completely different tunes...

[gregarakelian]

You need at least an 850 CFM double pumper on it to fill those big ports with juice and a four-speed transmission so you could launch it at 4000 RPM with 411 gears gears you just had to know how to drive it you couldn’t just push down on the gas and expected to do a burn out and take off but if you had a four-speedAnd knew how to drive you’re not gonna be beaten by anybody I never was never been beaten by anyone I even beat a hemi

[peteranderson]

Thank you !! Great information and insight.

[nemesisxrox]

Back in the day ... friend of mine bought a new one ... had definite low end response and he always complained it ran hot sittin' in traffic.
Only kept it a year til he got rid of it.

[jamielombardo]

Ford's biggest problem is not making enough of them and charging way too much money

[professorxavier]

Sounds like the perfect engine to apply a super charger upon..I would have slapped a Paxton supercharger on it and been done....That solves your low end power problem and it would be king of the street. The thing was about 7k then which was

[johnclary]

The Boss 429 was ahead of it's time because electronic fuel injection hadn't been invented yet. The intake ports didn't generate enough charge velocity to get a strong airflow signal to a carburetor at low RPM, so the engine was sluggish below its torque peak. That isn't a problem with EFI which was 20 years away from the Boss' 1969 introduction.