What is the cheapest way to make the car faster??
Now that I have my local course and my car on Optimum Lap, I can use a very cool feature that it has called batch run. Basically, you can, keeping all other things equal vary one parameter of the car model through a range of values. This will help us understand where the biggest gains are hidden.
You can see the tires hard at work on this photo from the August Daytona SCCA Solo CFR event.
The first parameter I decided to vary was the weight of the car. I did a sweep from 3600 to about 4500 lbs. What I found is that reducing the car weight by 400lbs will bring about .2s worth of time. Also, I could answer something that always makes me curious, what is the penalty for carrying a passenger? Let’s say the person is 200lbs, In my car today, this will be a 0.2s penalty. So once I have my technique together and don't need an instructor I will run alone. We can see that below a certain point the improvement from making the car lighter tapers off as we struggle to put the power to the ground.
The second parameter I varied was the tire rolling radius. This is a low-cost way of changing the final drive ratio of your car though it is limited in its impact. The challenger comes with 20” or 18” wheels so a reduction in diameter is possible. In this case, we can see that a rolling radius reduction of about .03m (1” give or take) or 2” in diameter will offer a 0.2s time reduction. But we can tell the advantage tapers off and we don’t get any more time below that. That’s just as well because the car only came with 18” and 20” wheels from the factory. To be street we could go down to 17” but there are no gains associated with that.
The third thing I changed was the longitudinal friction coefficient. This can be achieved by improving the grip under braking/acceleration. There are basically three ways to do this: 1.) Improve your tires or the pressures, 2.) Improve your suspension, and 3.) Improve your actual brakes. All of these should be considered, but the tires will bring the most impact, then the suspension, and then the brakes unless you’re severely under braked and your car can’t lock the wheels. This is not the case in the Challenger, which can lock the brakes with the TC off in sport mode. Longitudinal friction has a big impact, a 0.1G improvement in the longitudinal acceleration will equate to a .2s improvement in time.
The third parameter I varied was the lateral friction coefficient. This is by far the one that impacts run time the most: a .1G improvement in the lateral friction coefficient is equal to a 1.5s improvement in time. Again, you can achieve this basically by Getting better tires, setting up your car to use them better, improving the suspension, and generating downforce.
The next parameter I varied was the final drive ratio, this will affect how the power is used. Remember the first three gears in the car have a big gap. Using a taller final drive will improve how the tires put power on the ground. Here we find that swapping from a 3.08 final drive to a 3.3 final drive will provide around 0.2s of laptime. This is a complicated and expensive swap though we would have to swap into a rear end from one of the V8 Challengers and possibly change the driveshafts. This one might also interact negatively with the smaller rear tires so we have to be careful.
One of the biggest things everybody wants to change is power. We all want a more powerful car. Varying the power showed that there is some room for improvement on the Challenger V6 but beyond a 10% increase (30hp) putting this additional power to the ground becomes a challenge. A 30hp increase in power will provide a 0.3s improvement in time. So basically a V8 Challenger R/T is a better car for Autocross than an SXT, keep that in mind if you want to autocross in yours.
The final aspect that was interesting was aerodynamics. At the low speeds of autocross, it might seem like aero doesn’t help much. But I modified it with the minimum necessary for the car to stop producing lift and generate some downforce:
A splitter, a rear spoiler, and a diffuser. This is easier said than done and really getting that coefficient needs aerodynamic development on the car but I wanted to see how it looked.
What I found was that these mods would likely produce a 0.4s improvement in time.
It’s very important to understand that some of these gains obtained in one way might be negated by gains obtained in another area since the car is a complicated system. Therefore testing the combinations is very important. In the future, it would be nice to test some statistical methods like Response Surface Methodology to find the optimal car configuration but I will leave that for later.
Looking at these, it is clear that the car's parameter with the most impact on time is the lateral friction. So, most of my efforts in the short term will be dedicated to optimizing this parameter. I will work on understanding my tires better and learning how to align and drive the car to maximize this grip. Once I feel like I have a hold on this and have the budget available I will swap to 200TW tires, and once I do this I might also consider swapping to 18” wheels. But for now, I feel like there is a ton to learn by tuning the tire pressures, alignment, and the part that sits between the seat and the steering wheel.
Interesting links:
Knowing what your car will do without turning a wheel.
Creating the Digital Twin of the Daytona Kart Track
Creating the digital SS Conegeek
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