Everyone wants to improve their car a little (or a lot). How much do power, grip, and aero improve lap times? There’s lots of anecdotal evidence out there, but not much rigorous study. One reason for that is that every day is different. Lap times in the morning can be more than 1 second faster than the afternoon in California. One way to test these parameters on equal footing is through simulation. So I decided to embark on such an activity with Assetto Corsa. Why AC? Because it’s easy to modify. I can simply edit the power.lut file to change horsepower. Aero is similarly easy. Tires are more difficult, but there are already 2 tire choices for most cars, so I did that.
The baseline car I started with is 2400 lbs, 120 hp, 0.40 CoD, 20 sq-ft. frontal area. This doesn’t represent any specific car. It’s not far off from a Spec Miata though. The track I used was Brands Hatch Indy. I almost always test stuff on this track because it has a very small layout, about half the size of most tracks, which lets me get consistent numbers in a short time. I also think the mixture of turns and straights represents the average race track pretty well.
Let’s imagine changing horsepower from 1 to 300. I didn’t do that exactly. I drove the car with 60, 80, 100, 120, 140, and 160 hp. Then I did a curve fit to smooth the data points and extrapolate from 0 to 300. The extremes may be inaccurate, but they are also somewhat unrealistic given the starting vehicle. In the graph below, the X-axis is horsepower and the Y-axis is lap time. You can see the diminishing returns with increased power. If your car has 50 hp, adding 10 more makes a big difference. However, above 200 hp, there’s very little to be gained. If your car is a momentum car, increasing the power will lower your lap times. If you’ve got a muscle car, you need to search elsewhere.
For this next study, I once again drove the car with 60-160 hp but with harder tires. The take-home lesson here is that the higher the power of the car, the more sticky tires are important. Let’s illustrate that with a couple data points. If you’re on the red tires and have 173 hp, your lap time is 57.000 seconds. Switching to the blue tires will lower you to 56.336 seconds. You could also increase horsepower to 217 to get the same lap time. It’s a lot easier switching to stickier rubber than finding another 44 hp. Let’s look at a low horsepower example. At 100 hp on red tires, the lap time is 60.454. On blue tires, it’s 59.940. You can also get to 59.940 on red tires with 106 hp. So, sticky tires could be worth 6-44 hp depending on your starting hp of 100-173.
Aerodynamics are modeled with frontal area and coefficient of drag. It’s kind of hard determining what these values are for your car. You can look these figures up online, but I’ve found really conflicting figures. You can also try to estimate these at http://hpwizard.com, which is a pretty awesome website. I urge you to check it out. Another great website is https://mycurvefit.com if you want to do some curve fitting. Below is the lap time as a function of CoD. This was actually done at 100 hp not 120 hp.
Fantasy Enduro Builds
Let’s imagine 2 builders, Mario and Ian, who decide to build endurance racers from Spec Miatas. Mario decides to perfect the aero while Ian decides to reduce weight.
Mario’s aero mods add weight, but since he’s no longer playing by SM rules, he is able to remove an equivalent weight. So his car stays at 120 hp and 2400 lbs, but his coefficient of drag is now 0.30 CoD. I kept the 20 sq-ft frontal area. The aero mods are worth ~6 hp.
Ian guts and chops his Miata, turning it into a freakish dune-buggy like thing. Same power and frontal area, but weight is just 2150 and CoD has increased to 0.45. The decrease in weight effectively increases hp by 14. But the CoD effectively decreases hp by 8. Overall gain ~6. (Note: I don’t believe simply summing up the hp losses and gains this way is very accurate, but for small values around the defaults, it’s probably okay).
So what’s the difference between these two builds on track? To test that, I loaded up Laguna Seca and input the new vehicle parameters. The two cars were nearly identical in lap times. The aero car had a very slight advantage at the end of the main straight. The lightweight car had a very small advantage in the infield. But after T6, the cars were neck-n-neck the whole way back to the finish line. It was a little surprising to me that the advantage of shedding 250 lbs, which is over 10% of the weight, could be completely mitigated by bad aero. I guess I had better add an air dam to my dune buggy.