Of the many changes facing IndyCar’s engine manufacturer this season, the move to larger turbos has been met with universal praise.
The series changed its rules for 2014, requiring the use of twin turbocharging, and also defined a new, larger turbo for Chevy and Honda to install with their 2.2-liter V6s. That update came at the request of the manufacturers after finding slight performance limitations with the smaller units that were used from 2012-’13, and as Ilmor Engineering Chevy IndyCar program director Wayne Bennett told RACER, the spec Borg Warner turbos offer increased performance where it’s needed.
“Being slightly bigger, it is more efficient, and it’s mostly at the top end,” he said. “What that will do is help the turbo to not work quite as hard at higher RPMs to make boost, and that will keep the air temperature down and increase performance that way. The only other change we saw was slightly worse [throttle] response.”
IndyCar and its engine manufacturers elected to forego intercooling when the Dallara DW12 chassis and its engine formula were being created in 2011, making any reduction in the air that reaches the intake plenum a valuable gain. The move to the bigger turbo, as one might expect, also led to a slight sacrifice in responsiveness while gaining more peak power.
Honda Performance Development technical director Roger Griffiths continued the discussion on why IndyCar went down the path of new turbos:
“The general reason for it was a while ago, when the topic of twin-turbo versus single-turbo came up, we debated merits of each, and asked Borg Warner what they had that was a better match for our engines,” he said. “What we were using were off-the-shelf items, and whilst they worked quite well, we’d hoped they had come up with something newer since we brought these engines out in 2012 that worked throughout the entire power band.
“They presented a variety of choices for us, and the best one was a bigger turbo that offered some improvement in the efficiency, but it’s subtle and comes from aerodynamic changes to the turbine wheel. So we said that if we were going to be forced to go from our single to the twin, we said we wanted to do it with turbos that pulled hard all the way to the redline. It meant some sacrifice down low, but it’s small. I was also looking at where we’re going in the future, and if we’re asked to make more boost, I wanted a solution that gave us some headroom.”
IndyCar engine manufacturers are hesitant to provide horsepower and torque figures, but Griffiths did concede the bigger turbos have brought the numbers up.
“It’s a couple percent in turbocharger efficiency, which is a step in the right direction, and it does give us more power,” he explained. “On the dyno, we see it. Will the driver feel the extra boost? I’m not entirely sure – it’s not 20 horsepower, let’s put it that way. It’s a piece of the power gains we’ve all made this year, but it’s not the biggest component responsible for it.”
Comparing model numbers and technical data, the Borg Warner EFR-6758 used by Chevy through 2013 has been replaced by the EFR-7163, as Borg Warner engineer John Norton explains.
“The move to the 7163 turbo for 2014 will be a bump up in wheel size for both the compressor (cold side) and turbine (hot side),” he said. “In a nutshell, the 2014 twin turbos will consist of a slightly larger rotor group than last year’s twin turbo. Through last year Honda used the single turbo, the EFR-9180, a 91mm compressor wheel with an 80mm turbine wheel, and now both manufacturers will use two of the same units with 71mm compressor wheels with 63mm turbine wheels.”