Welcome to the RACER Mailbag. Questions for any of RACER’s writers can be sent to mailbag@racer.com. Due to the high volume of questions received, we can’t guarantee that every letter will be published, but we’ll answer as many as we can. Published questions may be edited for length and clarity. Questions received after 3pm ET each Monday will appear the following week.

Q: So what exactly is a plenum fire, and how did it occur within Pato O’Ward’s plenum at such an unfortunate time?

Jesse, Franklin, IN

MARSHALL PRUETT: Couple of quick things to know up front, with the first being that plenum fires are nothing new and nothing specific to Chevy. And they aren’t a common occurrence.

On rare occasions with turbocharged engines like the twin-turbo V6s used in IndyCar, something wonky can happen in the top end of the motor to cause a plenum fire. For those who aren’t familiar with the parts and pieces, the plenum is the sealed carbon fiber housing that bolts atop the engines which enclose the air intakes.

It can happen in a state of transition, like when we saw Pato accelerating hard out of the final corner and a moment of wheelspin appeared to arrive when it wasn’t expected. His engine sounded like it went from being under normal hard load while accelerating to a strange spike in power and wheelspin in an instant, and then the plenum fire took place. Weird deals also happen where for a millisecond, the timing of things happening inside the engine are thrown off, and that can also cause bad things to happen in the plenum. There are a number of scenarios that can lead to a plenum fire.

From a process standpoint, the turbos feed their highly compressed air into the plenum and into the V6’s six air intakes. And with two fuel injectors per cylinder, the compressed air gets drawn into the intakes and down into the port for each cylinder where the first injector sprays fuel and mixes with the air before entering the combustion chamber. The second injector, which sprays inside the combustion chamber, does more air/fuel mixing before that mix is exploded when an electrical spark is introduced by each spark plug.

So, with all of that stuff known, a plenum fire is something that is briefly triggered when the air and fuel catch fire and burn out prior to the combustion chamber. And when the air/fuel that was meant to go into the combustion chamber doesn’t make it there and burns up in the plenum, the motor stumbles and lags because nothing reached the combustion chamber to go boom. For the driver, it feels like someone quickly turned the engine on and off because there was nothing in the cylinders to compress and blow up to keep the motor revving hard.

Another important note to make is the plenum is always filled with air and fuel, so that’s not the issue.

It’s when something happens that shouldn’t that drivers get grumpy. Let’s say there’s a misfire and all of the fuel wasn’t burned in one of the cylinders, and that raw fuel got dumped into the exhaust. Depending on the timing of things happening in the motor, if there’s unburnt fuel that hits the exhaust, it will be set alight due to the high temperatures there, and then you have a momentary fire. If the exhaust valves open and some of that flame creeps back into the engine, it could also make its way into the plenum when the intake valves open to let the air/fuel into the cylinder.

Normally, all of the sparks and flames are set to happen when the intake valves have closed and everything’s working wonderfully and on time so that air/fuel in the plenum is safe and walled off.  When we heard about a plenum fire, we know that for a nanosecond, those safeguards failed and something sparky or flamey got up into a place where they don’t belong.

If you think of each mix of air and fuel being received in the combustion chamber to spark and explode like the engine inhaling and taking a breath, a super-fast plenum fire is the equivalent of the engine being unable to breathe, and as we saw with O’Ward exiting the final corner, his engine had to wait for the fire to burn out before it could reset and take another breath. It only lasted a second or so, but that was enough for Marcus Ericsson to motor by and win the race.

Drivers are trained to handle this exact scenario, and as I understand it, Pato was quick to rectify the matter, but even so, when the second-place driver is sitting a half-second off your gearbox, a one-second plenum fire can turn a win into a loss.

Pato’s problems began inside a carbon fiber housing that looked a lot like this one. Marshall Pruett Image

Q: I loved Pagenaud’s self-deprecating quote: “Avoiding wrecks is not my strong suit.” As an average online driver, I can sadly identify with his comment. Good to see all the IndyCar drivers take their knocks in stride versus endlessly playing the victim card like some other series drivers do.

David D Franzen, Franklin, TN

MP: Simon said avoiding wrecks is normally his strong suit, but that didn’t happen this time, so it was a nod to unexpected misfortune and not a self-deprecating note.

Q: I noticed during Practice 2 that Newgarden’s rear wing main plane had a cover on it when he was in to get yet another toe link replaced. I know back in the day teams completely covered the wings to hide angle settings, but that practice was banned. Can you tell us the purpose of the cover? A place to set tools?

Glenn, Renton, WA

MP: I didn’t catch that, but the series does allow teams to place a thin cover over the rear wings to keep tools and other car/crew related items from scratching the paint/wrap/carbon when work is being done at the back of the car, or when the car is being presented for technical inspection and various tools and items are placed atop the rear wing before it rolls onto the tech pad.

It’s a different thing from back in the day where teams were allowed to cover their front and rear wings — and anything else they wanted to — in order to prevent other teams from seeing their suspension and wing settings.

With the new cars and engines that arrived in 2012, IndyCar wrote that into the rule book, disallowing teams to cover the wings and engines and dampers and so on, and I’m glad they did. It’s hard enough to get the average fan to care about the technology contained throughout the car, and if everything’s covered up when the bodywork comes off, or the wings are hidden when the car comes to a stop, we may as well make everything spec and low-tech and give people zero reasons to care.

Q: Bring back the Thrill from West Hill. The Mayor of Hinchtown gives me information overload with the technical minutiae of driving an IndyCar during the broadcast. Looks like Andretti might have its mojo back.

Dino, New Hanover, PA

MP: I’d love to have PT back in the booth, but not at the expense of Hinch.
Prior to his arrival, the biggest absence in the broadcasts was a recent driver who could speak on racing today’s aeroscreen-era cars and, more importantly, fix the glaring omission of high-level race strategy observations into the shows. Thanks to Hinch, fans have less filler and more insights on what to look for throughout the race.

Happy for Andretti as they found their speed, but not the luck.