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A Slug’s Life — Pistons take a lot of abuse in a high-performance engine. What causes them to fail?

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For builders and racers pushing engine combinations to the limit of their capability, piston failures can be an on-going concern. When a piston does kick the bucket, the type of damage that’s created generally falls into one of three categories: fractures, heavy abrasion, and melting. While that damage can provide some insight into what caused the issue to begin with, it’s not necessarily a straight-forward proposition.

“Some types of failures can lead to one another as well,” JE Pistons advanced product engineer, Clayton Stothers noted. “For instance, if you have a crack that’s forming on the crown of the piston, it can lead to a melt scenario because you now have a hot spot where that crack formed. Or in another example, sometimes when a piston seizes in a bore it starts to scuff, and then it starts to bind, and then it might break as a result of the binding. In our failure analysis research, the most challenging part of it is determining what the original failure was. The root of the issue doesn’t always make itself obvious.”

Here we’re going to take a closer look at the some common problems that can lead to piston failures, and find out what builders can do to address them in order to prevent a minor issue from becoming a much bigger one.

Not Exclusive To Big Power

It might be easy to assume that piston failure is really only a concern for builds that are putting up eye-watering horsepower numbers. But as Stothers points out, that’s just not the case.

“It can happen across the board. You can have a piston failure on a lawn mower. In theory it might be more likely to happen on high horsepower builds, but I would kind of counter that by pointing out that a lot of the guys making 1,500-plus horsepower already know that they are pushing the limits, so they tend to take a more cautious approach,” he explained. “The reality is that the street guys need to be just as careful—sometimes they’ll put together a combination, take it to their local tuner, and just kind of wing it from there. But you really need to take just as much care with a street build as you would with a race build, especially with today’s turbocharged systems.”

Lubrication is Key

Proper oiling is important for a variety of reasons, and it plays a direct role in the health of an engine’s pistons. Lack of adequate lubrication isn’t always just a matter of being a quart low, though.

“We see a lot of race setups with modified oil systems, modified pans, etc.,” Stothers says. “They might remove oil squirters, or modify those as well. But oil starvation can happen for a variety reasons. G-forces can be an issue, as can overheating of the oil. When the oil overheats, it breaks down and loses its effectiveness as a lubricant. At the end of the day, what we’re mainly concerned with is keeping an adequate amount of oil in the places where it should be.”

Tuning Makes A Difference

A bad tune can lead to overheating of the piston, either localized or as a whole, neither of which bode well for the part’s longevity.

“That can cause all sorts of different piston failures—cracking, melting, scuffing, all of it,” Stothers explained.

“That’s often because the tune is way too lean, but really any type of uncontrolled combustion event can lead to that. Detonation is a huge problem because it creates cylinder pressures that are far above what the components are designed to handle, so that affects everything in the system,” he added. “It can bend rods, it can chew up bearings, and it can also damage pistons. Many engines run at the edge of a detonation condition these days because a lot of ECUs have knock sensors, and we see a lot of pistons with pitting and erosion as a result of running at the ragged edge. A little bit of pitting isn’t the end of the world, but it’s easy to go from that into a more serious detonation issue that has the potential to break stuff.”

So what’s the advantage of running a lean tune?

“A lot of people think lean means more power,” he said. “You start rich and lean it out. As you do so, you start to find the air/fuel balance. When it’s running rich it will run cool, but you’re not maximizing the power, so there’s a natural tendency to go as lean as you can without causing detonation. But the reality is that it’s hard to tune for every operating condition—every load scenario, temperature, atmospheric pressure, etc. That’s why the OEs spend hundreds of thousands of hours on calibrations. It’s not too often that you see these types of failures in the early stages of tuning because people are usually more conservative at the beginning of the process. It happens on your fourth session of the day when it is 105 degrees at track, you’re running questionable pump gas, and you go into a 130 mile-per-hour turn. It’s in those types of worst-case scenarios where you usually see damage occur.”

Proper Installation

JE’s Nickolaus DiBlasi notes that improper installation is another common issue that can lead to piston failure.

“We see a lot of problems caused by people not really following the guidelines that we provide. People will put the wrong piston-to-wall clearance in, or they’ll use incorrect components so the locks won’t seat in properly, or they won’t set the pin-bore-to-rod-clearance correctly. When builders choose not to follow the installation procedures based on what they’re doing with it, that’s another external factor that can play a role. If you take a shelf product from our catalog and try to make 1,200 horsepower with it versus, say, 400 horsepower, you will have a different ring-end gap spec, a different clearance spec on your bearings, and a maybe a different spec on your rod-to-pin clearance. Those things change as you add heat.”

A lot of people also just throw away the ring installation instructions, he notes.

“That’s when you see situations where the rings have run right into each other, which causes them to start pressing against the cylinder wall, which kind of slows everything down and increases heat, which is then transferred back into the piston. That causes the piston to grow, then it starts scuffing on the top land, and the land starts pulling itself apart. That happens a lot.”

Engine Combination

While a piston doesn’t know whether it’s installed in a high-revving, naturally aspirated inline four cylinder mill or a turbocharged V-8, there are some combinations that tend to be more susceptible to piston failure than others.

“There’s a higher likelihood of failure in a forced-induction application versus a naturally aspirated one,” Stothers said. “Not only are you working with more power, there’s also more opportunity for there to be an issue because it’s a more complex system.”

Forced-induction systems introduce another set of calibrations that need to be done for parameters like boost control and pressure, in turn increasing the opportunity for error.

“Engines rarely blow up under their own power,” DiBlasi points out. “When you bring forced induction into the mix, people have a tendency to say, ‘Oh it’s tuned—what’s a couple more psi?’ It’s not linear like that.”

Nitrous, DiBlasi says, is in some cases even harder on the engine systems than forced-induction setups are.

“The power doesn’t ramp up as progressively—all of it is being delivered almost instantly. Going from 300 horsepower to 700 in literally one second doesn’t give you a chance to back off of if something seems wrong. It’s more like, ‘Oops, there’s a hole in the block!’”

Best Practices

We all know that using a high-quality piston that’s purpose-built for the application is a crucial element of reliability. But beyond that, having access to the right information and following it throughout the early stages of a build can have a profound effect on long-term success.

“JE does not make any recommendations on tuning. It’s implied that you know what you’re doing in that realm and you understand the basic principles of tuning,” DiBlasi says.

“However, what we do provide is physical installation guidelines. We tell people what the piston-to-wall clearance needs to be at a minimum, and information that says, ‘if you’re doing such-and-such scenario, we suggest doing this.’ We include a formula, depending on bore size, which explains that as you make more power, you need to increase it because of this,” DiBlasi added. “We also provide recommendations for pins based on power levels. We also provide information about the rate of thermal expansion, and in our ring installation instructions, we also provide a formula based on bore size and ring material that explains what you need to do as you make more power.”

While it can’t address some of the external factors that are inherently at play in a performance application, arming yourself with this knowledge early on will help set you on the path toward more reliable power, no matter what your combination and use case might be.

Source
JE Pistons
(714) 898-9763
Jepistons.com


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