Do Dimpled Pistons Really Offer Huge Performance Gains In Diesel Engines?

When it comes to engines, everyone is always looking for more. More horsepower, more torque, more efficiency. One company has a rather interesting take on how to allegedly get a lot more performance all around, just by giving your diesel engine dimples. 

That company is called Speed of Air. Based out of Reno, Nevada, it specializes in designing pistons that might remind you of golf balls. That’s because they’re covered in lots of little divots—which the company refers to as “turbulators” for their effect on the air inside the engine. With their eye-catching design and purported performance benefits, they’ve been making some waves online of late. You might have heard of them on YouTube, where they’re routinely advertised by big diesel channel Dave’s Auto Center.

The physics of why dimples make golf balls fly better is well understood. But does applying the same technique to pistons have any real effect on performance or emissions? Could they really provide huge boosts to horsepower and slash fuel use by huge percentages? Let’s talk about it.

Dimples

The crux of the whole dimples phenomenon comes down to fluid mechanics. Weird structures and shapes can do all sorts of things to the flow of a fluid and create positive or negative effects for a given application. Sometimes, it’s intuitive—it’s pretty obvious why a smooth, streamlined bullet train can go faster than a slab-sided locomotive, for example. And at other times, it’s not intuitive. For example, take the common golf ball and the characteristic dimples that improve its performance. Why do they matter?

We can understand how the dimples affect the ball by examining what happens as it flies through the air. If the golf ball were a regular sphere, the small boundary layer of air flowing around the ball would remain smooth as it passed over the ball and experiences laminar flow. The laminar boundary layer separates from the golf ball quite easily as the ball flies through the air, leaving a large low-pressure area behind the ball—a wake—which creates a lot of drag, slowing it down. However, if the ball is dimpled, the immediate boundary layer around the ball becomes turbulent. This turbulent boundary layer sticks to the ball a lot better, helping guide the airflow around the ball and creating a much smaller low-pressure region behind it. The smaller wake comes with much less drag, and the ball can fly farther.

Golf brand Titleist has a great explanation of how dimples make its golf balls fly farther.

Of course, a golf ball is not a piston. Nevertheless, the idea behind Speed of Air’s invention is that dimples could be used in a similar way—controlling the boundary layer of air on the piston as it moves through the combustion process. Hence the dimples being referred to as “turbulators.” They’re there to generate turbulence and control how the air inside the cylinder interacts with the top of the piston itself. It’s not about cutting drag or anything like that, just about changing how the air moves inside the cylinder itself. The company says it combines the dimpling with special proprietary thermal coatings on the pistons to further improve performance.

The company primarily offers pistons for common diesel engines. The product range includes piston and ring kits for Cummins, Ford Power Stroke, and GM’s Duramax engines, as well as the Mercedes-Benz OM642. Speed of Air also offers a limited range of pistons for small-block Chevrolet gas engines.

Ultimately, these dimpled pistons are supposed to lead to performance gains in diesel applications, which are the company’s prime focus. “On average, our customers experience fuel savings greater than 20%,” reads an FAQ on the company’s website. The company also says improved combustion leads to maintenance benefits. “Speed of Air Hyperformance Pistons allow for a more complete combustion process, which in turn reduces soot deposits in your engine lubricating oil,” reads the website. “For a well-maintained engine, we recommend extending your oil change intervals by 50% after your break-in period.” The company also references testing by a company called DFC Diesel which allegedly found “5% increases in torque and horsepower” and nitrogen oxide emissions reduced by up to 80%. Other claimed benefits include reduced exhaust opacity from lower soot, and quieter engine running.

The question is—how does creating turbulence on top of a piston help in an engine? Speed of Air explains in somewhat vague terms on its website:

Speed of Air’s patented technology influences the thermo-physical boundary layers within the combustion chamber of the engine. As is the case for the golf ball, dimpling the surfaces inside the combustion chamber, coupled with our proprietary mix of coatings, creates a thinner and well attached turbulent boundary layer between the air-fuel mixture, its flame front and the combustion chamber walls. This thin, turbulent layer allows the combustion flame front to progress much closer to the cool metal walls of the combustion chamber thereby increasing overall combustion efficiency.

It’s a little hard to parse. The explanation appears to be that the dimples on the pistons are supposed to allow the flame front inside the combustion chamber to reach much closer to the piston and chamber walls, which apparently increases efficiency by some mechanism.

That explanation seemed vague, so I went looking for further resources. I found a video the company posted to YouTube in 2021 which explains its technology. “Speed of Air technology works by affecting the fluid boundary layer within the combustion chamber,” states the narrator. “Much like a golf ball, the thin boundary layer inside the combustion chamber remains attached to the surface longer, resulting in better homogenization of the air fuel charge and greater stability and duration of the flame front.”

The basic idea seems to be that a more stable attached boundary layer on the piston helps create a more homogenous mix of fuel and air in the cylinder, leading to more complete combustion—thus boosting efficiency, power, and torque, while cutting carbon buildup in the engine.

It’s a little strange to have two such different explanations for how the technology works, but nevertheless, these are the materials that Speed of Air has made available. The latter explanation perhaps feels like it has a stronger basis in some sort of physical reality. Engineers are always tweaking engine designs to finesse how air and fuel mix and burn inside the cylinder, with an eye to ensuring the fuel burns as cleanly and completely as possible. The video from Speed of Air doesn’t appear to explain how dimples and a turbulent boundary layer precisely achieve this better than traditional techniques, but there’s a basic logic behind what’s being said.

[Ed Note: Indeed, optimizing fuel-burn is an extremely complex process. Back when I was an engineering intern at Cummins, I learned about just how precisely the company designs the tops of its pistons to optimize combustion. Based on all the simulation and testing Cummins has done over the years, I strongly doubt they’d have missed a simple dimpling solution, and the fact that this tech isn’t in production should give everyone pause. – DT].

Without running a full computational fluid analysis on the Speed of Air piston design, it’s not easy to determine the exact fluid mechanics at play inside the engine. The explanations are also simplistic and vague enough that it’s hard to really get a good sense of whether the concept has value. However, there are other areas we can look at to better understand what’s going on with this technology.

There are some doubts around the technology from commentators online.

Supporting Documents

As you’d expect, Speed of Air provides a range of testing documents to back up its claims. Prime amongst them is a report on an unspecified 5.9-liter Cummins diesel engine, tested in both stock form and again with Speed of Air pistons. The report contains some graphs with what appear to be astounding results, indicating the engine gained somewhere in the realm of 30-40 horsepower by switching to Speed of Air pistons, quite a wild figure indeed. Other graphs indicate fuel use dropped by almost 8% under certain conditions, while hydrocarbon and nitrogen oxide emissions were both reduced as well. Speed of Air’s website indicates this was an independent test with the report prepared by Olson-Ecologic Engine Testing Laboratories.

Another test was apparently undertaken by the SEMA Emissions Compliance Center. The testing concerned a 2015 Chevrolet Silverado 2500HD fitted with a Duramax 6.6-liter LML turbodiesel engine. Graphs state the Speed of Air pistons achieved a 60.5% reduction in output of nitrogen oxides, a 93.5% reduction in visible black smoke, and passed a battery of emissions tests without incident. It’s also worth noting that Speed of Air has received a SEMA award for Best New Performance Street Product in 2023 for its piston designs.

Speed of Air also has a list of individual anecdotal data provided by various customers. These consist of dyno sheets from various trucks built from 2001 to 2014, which have been equipped with Speed of Air pistons. Some show minor gains to torque, others show gains of hundreds of horsepower—but it’s not clear if these comparisons are being made after solely changing the pistons in a given vehicle. They also contain customer anecdotes that report positive experiences with the product. For example, a report from Matt, the owner of a 2010 Ford F550:

This is my 2010 Ford F550 that I put a DFC Diesel engine with Speed of Air Hyperformance pistons. The previous engine had upgraded turbos, camshaft, ported heads, AFE intercooler and a Spartan tune on the 210HP Tow Performance setting. The truck has a 3 inch lift and is on 37 inch tires and with camper weighs nearly 19,000lbs and the trailer with my SXS is another 5,000lbs. Prior to the engine the truck was averaging about 40 lph (5.8 mpg) and after the engine has been averaging 34 lph (6.9 mpg) a 16% increase. The engine is noticeably quieter and I have seen a drop of peak EGT’s of around 150-200 degrees. There is less turbo lag and I get up to speed quicker at 1/3 throttle than I did at ½ throttle before and is easier to maintain speed on hills using less throttle input as well. I have put on nearly 10,000kms (6,000 miles) since installing the engine and based on my results am expecting to save over $12,000 within 100,000kms (60,000 miles). Just how well the truck works now alone was worth the investment the fuel savings are just an even greater added bonus I would definitely recommend to anyone needing an engine replacement.

Overall, there’s a collection of figures to peruse that seem to support Speed of Air’s claims. However, it’s always important to take things with a grain of salt when you’re reading them on the same website that’s trying to sell you a product. Unfortunately, my research didn’t turn up a whole lot of tests out there from third parties that aren’t directly involved in selling the product itself. Thus, it’s hard to get a balanced idea of just how good or bad these pistons might really be. It would be a different case entirely if we were seeing various automotive magazines or other outlets testing them in the absence of sponsorship and the like, but it doesn’t appear there’s a lot out there yet.

First impressions on the SOA pistons for the M57.
byu/Xallyeah inE90

One Reddit poster claims to have obtained prototype Speed of Air pistons for the M57 BMW engine, with positive results in terms of low-end torque output and reduced exhaust gas temperatures.

It’s perhaps no surprise that there aren’t more tests being done out in the wild, given the cost involved. A set of Speed of Air pistons for a V8 diesel like a 6.0-liter Ford Power Stroke can cost in excess of $4,000—more than double what you might pay for a traditional set from another manufacturer.

Market Realities: Why Isn’t Everyone Doing This?

It’s difficult to say whether dimpled pistons work or not solely based on the resources available online. Speed of Air presents some graphs with great numbers, but it’s always hard to trust an “independent” report published on the company’s own website. The company’s explanations of its own technology are also limited enough that it’s hard to glean what the real mechanism of improvement is, or what the benefits might be.

However, we can speculate based on what we see in the broader automotive world. The simple fact is that Speed of Air claims huge fuel economy gains—on the order of 20%—from its piston technology. Given that it largely involves drilling a few small divots into a piston, it’s perhaps shocking that no other OEM has explored this area before. Indeed, we’ve seen other manufacturers such as  Volvo boast about gains from their own advanced “wave” piston designs, which involved far more complicated geometry for fuel economy gains of just 2%. If they could achieve more gains just with a few vertical hits on the piston milling machine, you’d think they’d do it.

If it only took drilling a few dimples to improve fuel economy by 20%, you’d think everybody would be doing it. Automakers spend huge sums on computational modeling and real-world testing to optimize their engine and combustion chamber designs. It’s hard to imagine that in over a century of development, something as simple as a few dimples didn’t occur to anyone, given the apparent outsized gains on offer.

Dimples on pistons sound great—if they’re good on golf balls, they must be good for engines, too, right? However, you don’t have to go far to find skeptical comments online, on hypermiler forums, diesel forums, and beyond — many asking why dimples aren’t prevalent in the auto industry. It’s one of those cases where extraordinary claims require extraordinary evidence, and it doesn’t necessarily feel to some that Speed of Air has it. After all, one might expect every second engine builder out there to be raving about piston dimples if the 20% fuel economy benefits alone were there to be had. “Independent” studies published online by the manufacturer itself generally aren’t enough to convince the broader masses.

As for why OEMs haven’t jumped on the dimpled pistons, there are some theories. “It could come down to a cost versus benefit question that the aftermarket doesn’t typically need to answer,” stated Motor Trend in an article on Speed of Air in 2023. ” The extra time cost of machining a piston with these features potentially outweighs the benefits of how a piston performs without them.” However, these statements seem to ring a little hollow when stacked up against the stated 20% benefits to fuel economy and additional emissions benefits that Speed of Air claims. OEM pistons already undergo delicate machining in the production process; it’s hard to imagine that a few dimples would cost so much as to negate such huge savings in fuel economy.

An Expert Take

As part of our due diligence, we asked an independent engine expert for their comment on the Speed of Air technology. They hadn’t seen the product prior to our email, but instantly spotted some areas of concern. Speaking on condition of anonymity (because they prefer not to have their name associated with something negative), they stated responded to the Motor Trend article with the following:

I hadn’t heard of this one before, but it is contrary to design practices I have used my entire career.  Surface area-to-volume ratio in a combustion chamber has a profound effect on efficiency and emissions through heat transfer and wall wetting mechanisms.  And yes, lowering surface area is the preferred direction, contrary to the impact of adding dimples.  The radiator in your car is a perfect example of increasing surface area (via fins) to increase heat transfer.  In an internal combustion engine you do NOT want to transfer heat away from the combustion chamber.  Indeed, thermal efficiency is highest when combustion is adiabatic (no heat loss).

The primary source of unburned hydrocarbons is surface quenching in crevices.  Minimizing combustion chamber surface area where quenching can and will occur is the preferred direction.  I am quite surprised by their claim that carbon deposits will decrease.  Again, I would expect carbon to deposit readily in the cavities created by dimpling.  I also don’t like the sharp edges produced in the combustion chamber [in some Speed of Air designs].  These edges are going to get very hot and likely produce undesirable combustion effects during prolonged high-load operation.

Finally, it is difficult for combustion chamber surface details to have a significant impact on global performance parameters like power.  The claim of 15+% increases in torque and power are ludicrous.  I am instantly turned off when I see ridiculous claims like that.  The argument that OEMs don’t do it because of cost is equally absurd.  I did engine development in an OEM for 13 years, and anything that would produce the power and emissions benefits quoted would be adopted immediately.  The cost of dimpling surfaces pales in comparison to the cost of precious metals for emissions compliance.

Bottom line, I’m not at all interested in adding dimples to the pistons in my Chevy truck!

[Ed Note: I sent this expert’s thoughts to Speed of Air for comment. I haven’t yet heard back. -DT]. 

Ultimately, if Speed of Air’s technology really is as good as it claims, it will come to dominate the market in the next few years. Diesel engine manufacturers would be lining up around the block to license the company’s patents if 20% fuel economy gains are on the table, to say nothing of the claimed benefits to emissions and everything else. Double-digit gains cost tens or hundreds of millions of dollars in research and development. Speed of Air could make huge sums overnight just by selling access to the technology without ever having to manufacture another piston again. The fact that you can open up just about any OEM diesel engine and not find dimpled pistons suggests they’re maybe not quite as magic as the claims suggest.

I can’t say for certain whether Speed of Air’s pistons have huge benefits or not; I can only present the evidence available as it stands. In any case, when it comes to new automotive technology, it pays to cast a careful eye over what you see. Examine the claims, explore the science behind it, and rationally assess what everyone else is doing. That’s how you spot the technologies that work from those that don’t.

Image credits: Speed of Air, Volvo, Titleist, Cummins