Wow, two wins in a row for our guys at SpeedSource in the new Mazda6! This is great momentum heading into a very busy 10-week stretch that includes five race weekends. Our Mazda SKYACTIV-D Clean Diesel is running great as development of the platform continues. The race against Porsche at Detroit was a close one and looking forward to another close battle come Mid-Ohio this weekend.
A lot of people have been asking what it's like to drive a diesel racecar. Other manufactures have tried it from Indianapolis (Cummins in the 1950s) to modern day prototypes at Le Mans (Audi and Peugeot). Those engines were quite big, being inline six-cylinder or V12 machines. In the case of our SKYACTIV-D Clean Diesel engine we are running a small 2.2-liter 4 cylinder engine which makes 400hp with 450 lb-ft. This is the spec for the GX class in Grand-Am competition.
The way I like to compare the styles of a gasoline normally aspirated engine vs. a compound turbo diesel is, where the gasoline engine is weak the diesel excels and where the gasoline engine is strong the diesel is not so far off. Now with that analogy a diesel engine has a few nasty tendencies that accompany the platform. Refer to my previous column where I talked about vibration.
Since the engine is not a normally aspirated engine, let's start there. Managing the turbo lag is an area that the drivers and engineers speak about often for gearing purposes. The SpeedSource-prepared Mazda6 uses a 6-speed sequential gearbox. In most cases a non-turbo gasoline engine using a similar gearbox will use gears second through sixth with first gear used to leave the pit box. In our situation with the SKYACTIV-D, we use all six gears on track. This helps keep the rpm values in a higher range, which produces less turbo lag.
Another way the drivers help to reduce turbo lag is to begin applying power earlier than what our butt in the seat tells us. I will admit this was odd the first time I did it at Daytona but it works quite well to get the turbos spoiled up. Say a given corner will produce two seconds of turbo lag. What we will do is apply power two seconds early in the corner and then power is there when we need it. Yes, we do “two-peddle” a little bit but we try not to as this upsets the handling characteristics of the car. As a driver we have to change our driving style when needed to fit certain applications, of which this is a perfect example.
I mentioned gearing, earlier which is another difference in driving the SKYACTIV-D platform. Remember when you take off from a stop light next to a big rig? They seem to be shifting gears forever (that is because they have 18 or more gears to pull but you get the point). The reason being is the low RPM range of diesels vs a range of 6000 rpm from gasoline cars. In our case we have roughly a 2500 rpm range to work in which makes the best power/torque.
What this means is constant shifting on the racetrack when exiting corners. Each gear, of course gets longer and longer but one thing I noticed is the driver's hand never leaves the gear selector until at least fourth gear. This is because those first few gears come so quickly, you might as well leave your hand on the gear selector instead of going back and forth to the steering wheel.
For our last race at Belle Isle in Detroit we utilized very short gears, so we were constantly shifting up two gears then down two gears. As I mentioned earlier gear selection is key to manage the turbo lag and is something the engineers monitor to produce the best performance.
Staying with the trend of gearing, let's move on to downshifting. One thing that diesel engines do not like at all is over-revving the engine. Very nasty things happen to the valves and internals when this happens. With that in mind downshifting becomes a time of patience for the driver. One of the first things team owner/engineer/teammate Sylvain Tremblay told me was to never ever over-rev the engine on downshifts.
So what does this mean in terms of driving? Going back to the gasoline engine, a driver would downshift early in the brake zone to help stabilize the rear of the car with engine braking. This was very common in Indy Lights, but it's a big no-no in the Mazda6 SKYACTIV-D engine, however. The Bosch system used in the Mazda6 GX car has an auto-blip system for the downshifts so the driver does not accidentally blip too much and inflict an over-rev. What we must do is brake for a given corner and then wait…wait…and wait until late in the brake zone before selecting the first downshift.
Sometimes the last downshift, in hairpin corners at the end of long straights, will occur just before the apex of the corner. Yes, the last downshift occurs almost before you start picking up the power again! Like I said earlier downshifting requires patients. The good news is the Mazda6 has a long wheelbase which helps braking performance and the car is not as nervous in the rear end under braking as one would think.
The best part about driving a diesel racecar is the torque. Pulling up the hills at Road Atlanta or under generic acceleration really puts you in the seat with 450 lb-ft of torque. This is where we outperform the gasoline engines. On top end the gasoline engine has some speed on us currently but that is something the engineers at SpeedSource are working toward reducing. With a diesel, you continue to make horsepower with more fuel but the engine reaches a critical heating point from this, which becomes the limiting factor. This is where SpeedSource working with Bosch tunes the engine for performance. The two turbos help with the tuning aspects of the engine as they act as the cooling device to offset the fuel, which heats the engine up. It is very interesting how simple yet complex a diesel is.
Without going into detail, another positive that comes with racing a diesel is it fuel mileage capabilities. The added fuel mileage that comes with an engine with performance such as the SKYACTIV-D makes it a great candidate for endurance racing. As I mentioned before. the majority of this engine is production-based and thus the street car version with have great fuel economy as well. I cannot wait to try it out when it becomes available.
This coming weekend the SpeedSource team will be back in action at Mid-Ohio Sports Car Course for a 2 hour and 45 minute race. The race will air live on SPEED or you can follow along via Twitter either by following @speedsource or @joelmilleracing. If you are at the track make sure to stop by our paddock to check out the new Mazda6 and our SKYACTIV-D engine.
Until next time!