Smokey Yunick, a legendary figure in motorsports, was renowned for his innovative and often unconventional approaches to engine design and performance enhancement. His techniques, while groundbreaking, frequently sparked debate regarding their practicality and adherence to regulations.​

Hill Country Performance & Machine in Spring Branch, Texas, got ahold of a Smokey Yunick Hot Vapor engine and a boatload of other parts, and are releasing content on all of it. That begs the question: are Smokey’s tricks still relevant today.

What the heck is the Hot Vapor engine?

According to HOT ROD, who first reported on this”hot air” engine in the June ’84 issue, “Smokey developed a 2.5-liter (151ci) Iron Duke four-cylinder Fiero engine that met all ’80s emissions standards (with a carburetor and no computer), made 250 horsepower and 250 lb-ft of torque (compared with about 90 horsepower and 125 lb-ft stock), went 0 to 60 mph in 6.5 seconds (stock was 12 to 13 seconds), and managed to get as high as 51 mpg on the highway running 93-octane pump unleaded premium gas (the stocker got about 35 mpg on 87-octane).”

Other hot vapor engines were developed by Smokey Yunick as well — a three-cylinder Buick plus a Ford and Mopar (the latter being what Hill Country got a hold of). According to HOT ROD, the general idea was to recapture lost efficiency in the Otto-cycle by superheating the incoming air/fuel mixture to over 450 degrees Fahrenheit for a homogenous, fully vaporized mixture to “prevent detonation” and ensure “complete combustion.”

Hot Vapor Demonstration

Greg Banish at Calibrated Success

Offering his take on the Hot Vapor engine, Greg Banish of Calibrated Success says, “Smokey makes two big claims: more power and better fuel economy. More power should be obvious, it has a turbo. That turbo also helps fuel economy when pumping losses are reduced. Warmer air is less dense, so the turbo does more work to help fill the cylinders. This work comes from otherwise wasted heat in the exhaust, so it’s a net win on efficiency. Sharp eyes will also note his use of different tires, which can have a big impact on fuel economy. Making more power with the turbo is no secret. The only catch is working with high inlet temps. They cleverly omitted any discussion of what he did for wide-open throttle AFR and timing.

“The warmer inlet temps also help evaporate the fuel. Today, that wouldn’t be necessary because modern fuel injectors give us much smaller droplet sizes that evaporate easier than the liquid stream that was likely entering the venturi of that old carburetor. By doing a better job of evaporating and mixing the charge, less fuel makes it to the cylinder without being able to fully participate in combustion. (Although, we occasionally screw this up with DI.) The warmer temps also drive lower spark advance requirements, which again help compression work-losses. Today, we would see that with in-cylinder pressure measurements, like I did on a recent project. The name of the game is getting complete combustion in the shortest time possible. I often work with clients to help them get there by using these and other design factors.”

Joel Espinoza at Tuner School

Similarly, Joel Espinoza at Tuner School believes Smokey’s theories no longer hold up under a modern lens, but led to that lens being created. “The hot vapor engine may have been a good theory. However, given what we know now, blowing hot air into an engine is undesirable. In the tuning industry, cold, dry air is best,” says Espinoza.

“Smokey’s idea to achieve ideal fuel vaporization was sound. We have learned that air density and other conditions greatly affect the way an intternal combustion engine performs, as well as any given fuel’s brake-specific fuel consumption.  Some proven methods implemented in the high-performance world are as simple as installing a cold air intake, and then get more complex from there, such as intercooled supercharger systems and water-meth injection kits; the latter being an excellent way to make more horsepower by keeping the engine much cooler because of its latent heat properties.

“Smokey’s ideas were great, and I believe they have led to some great innovations in making horsepower. We have learned how to add more fuel to both cool the engine and make more horsepower, in lieu of a perfectly vaporized fuel mixture.”

Conclusion On The Hot Vapor Engine

Smokey Yunick was clever in his use of heat to more completely vaporize fuel, and in understanding the importance of vaporization. The use of heat, however, has been far surpassed by high-pressure, direct injection as we see in today’s engines. While not a favorite among many racing circles, if not for size and space constraints, it would have the highest potential for power and efficiency.

Piston Balancing an Indy Engine

Another trick found in the box of Smokey Yunick parts is balancing a set of pistons. Some have argued that grinding on a piston can cause cracking from weakening the piston, uneven heat distribution (hot spots), and premature piston failure under load. What do the experts say?

Nick Bacalis at Bullet Racing Engines

“Those old cast pistons, or TRW forged pistons, could easily be 15 to 30 grams different from one to another. Same with factory rods. Today, it’s hard to find differences from one to another more than two grams [pistons] and five grams [rods] in any top brand of pistons and rods. So, I never balance pistons or rods. It’s not worth the time. I don’t use stock rods or cast pistons in anything I build, but if I had to I would balance them that way or mill the towers to make them narrower. Occasionally, you have to clearance the bottom side of the piston to fit the rod in and give it enough space between the towers, but that’s all.”

Judson Massingill at School of Automotive Machinists & Technology

“Balancing the inside of the piston is still done today. With common sense and a 45-degree angle you can take some off – not on the deck where the pressure is at. Old pistons like TRW could be 2-3 grams off…even 5 grams. Now if you are turning 6,500 rpm, it may make no difference, but if you are turning 7,500 to 8,000 rpm, I like plus or minus a gram. We need all the pistons the same for equal forces on each side of the block. High-dollar pistons these days are within tenths of a gram. I think that difference is negligible when you consider most scales aren’t accurate within a half of a tenth. OEM pistons are now very close, too.”

Jack McInnis at PBM Performance Products

“Smokey Yunick was an innovator! I think that at the time, one way to make pistons more durable was to simply add material, and much of it would have been guesswork as to how much more was really needed. So there could have been a significant amount more than necessary, and grinding some away would be pretty safe. Today, pistons are designed with far more sophisticated development tools (software) and are really optimized for an ideal balance of weight vs strength. Modern alloys allow for thinner sections as well. With a modern race engine, I probably wouldn’t recommend this method.”

With todays manufacturing they typically match pistons closely with one another, so you don’t have to balance them – but it’s always good to check. — Andrew Hachmeister, SAM Tech

Andrew Hachmeister at School of Automotive Machinists & Technology

“While it’s ok to remove some material from that area of the piston, if it is a shelf piston that you are not pushing the limits on or if there is enough meat there. However, some piston manufacturers shy away from using the drain back holes from the oil ring to oil the wrist pin. I would much rather radius the corners on the bottom side of the piston.”

Bob Wise at RaceKrafters

“Back in the day of Smokey Yunick, when that was the only type of forging available, yes, we used to end mill that area for balancing and lightening purposes. The bottom pads below the wrist pin are your actual balance points. However, taking the material from under the pin bore centralizes things on the piston. On today’s pistons that area is almost always hollowed out now from the manufacturer or not even there. With today’s longer rods and strokes the pin bore area is so far up under the piston that there is almost no meat there. Pin piers are pulled in, wrist pins are shorter, etc.”

Eric Grilliot at MAHLE

I would qualify [Smokey’s method] as a good potential method, but not the right method for every application.

On one hand, we’ve designed and built ultra-lightweight custom parts that incorporate removal of similar material from the get-go when we’re scratching to shed every gram possible. But, also keep in mind, the pin tower is the pathway that transfers the load (cylinder pressure) from the crown to the connecting rod.  In some of today’s high-output engines, you almost can’t get enough support in that area. The shorter the compression height, the worse it gets (compare a modern stroker piston to the tall clunky part shown in the video — that real estate just doesn’t exist).

The general question of where to pull mass out of the piston for balancing is one that comes up frequently, but unfortunately, there’s not a clear black and white answer. Modern piston designs have come a long way toward reducing a lot of the unnecessary mass, but there will almost always still be some compromises that exist for a variety of reasons.  Some of that mass is obvious, some is not.  Attacking it properly is a combination of understanding the design of the piston, the tools and methods available for removal, and [most importantly] consideration of the intended usage or application.

Balancing Pistons Conclusion

While this technique is not terribly relevant today (unless you were rebuilding an engine and reusing original pistons, maybe), the concern Smokey Yunick had with precision in the balancing of engine is very much relevant. In fact, this mentality of taking copious notes and blueprinting an engine to the nth degree is widely used and accepted throughout the industry as the gold standard for engine building (sorry, Sloppy Mechanics).