Setup Rationalization for High Downforce Cars (SFL)

Setup Rationalization for High Downforce Cars (SFL)

Optimizing setups for best lap times or "best/safe" racing is both an art and a science. There are many variables and the potential combinations are almost infinite.  So having a "rationale" and an "informed starting point" is important.

The Super Formula Light (Dallara 324) is a near perfect example of a high downforce car. This relatively high downforce makes an understanding of mechanical grip as well as aero downforce important.

While it is possible to use the iRacing setups as a starting point, I discovered that a much better way was to subscribe to RRSetups (RidgwayRacing). Robert provides a highly developed setup for each track. He accomplishes his optimization thru a series of intense testing iterations and telemetry analysis. For most, his setup is optimum, but for some, some modifications are helpful--sort of like tailoring a suit for a perfect or just more comfortable fit. To help in this area of learning, I engaged Scott Haddock (CoachHaddock) to run a bunch of laps together in a two hour coaching session at Imola.

Through a trial and error process of testing at Road Atlanta, Interlagos, VIR and Watkins Glen, along with a close examination of Ridgeway setups and testing with Haddock at Imola, I developed a setup that worked quite well for me. I am 75 years old with a physical handicap due to having two broken legs in an airplane crash.  I seem unable to take full advantage of the maximum cornering speed the SFL is capable of, so my "tailoring" was to reduce aero drag and downforce in exchange for higher straight line speed. I tested this in a race at Watkins Glen and called this starting point my "Base" setup. (Finished P7 starting from P20)

This "Base " setup worked satisfactorily at Imola, but I was sure I needed some setups with higher downforce-for Hungaroring as an example. 

My approach was straightforward. The "Base" setup provided what I found to be the optimum level of "mechanical" grip so my settings for springs and dampers remained almost unchanged. My changes focused on aero drag and downforce with attention to any needed ride height adjustments due to higher downforce. 

The aero options of the SFL are sophisticated and advanced: The "Aero Package" of High or Medium Downforce configuration; Front wing/flap angle; Front flap gurney; Rear wing/flap angle; and Rear beam wing of the undercar diffuser. iRacing provides an "Aero Calculator" which is useful to a point, but it requires telemetry data to determine the front and rear ride height at speed. 

The two most obvious data points provided by the calculator is "Downforce to Drag" and "Aero Balance".   Thru experimentation I determined that my best performance on most tracks called for an Aero balance around 43.5% - 44% for dry tracks and 40.1% for a wet track. This % is the percentage of total downforce on the front tires. A 43.5% Aero balance provides 56.5% of the downforce to the rear tires. 

To verify the accuracy of my "feel" during testing, an examination of tire wear and temps confirms the Aero balance and general setup is close to ideal. The figure above indicates tire temps confirming a slight understeery balance.

To find the correct numbers to enter into the Aero Calculator, the only way I know of is to use telemetry analysis like Motec using Mu.

I generally focus arbitrarily on figures at 130 mph as this seems to be the cornering speed where downforce is most critical. Figures change considerably with speed. Generally, since the Aero Balance is lower in the front, the rear will drop more as speed increases. In other words, the "rake" will be reduced at max speed closer to 150 mph. This requires you to test to insure the car does not bottom out. Generally the car is faster at top speed with negative rake (rear lower than front) and positive rake (rear higher than front at lower speeds 130 mph and lower). 

The Aero Balance normally shifts slightly more rearward at higher speeds--logical as there is more downforce proportionately on the rear and downforce increases with speed at the "square" of speed increase. 20% increase in speed provides 44% increase in downforce. 

Concern with excessive suspension drop at speed calls for the use of the "third" or "heave" spring--both front and rear. Many drivers will use setups having no third spring in the rear, but this requires stiffer suspension spring rates and higher initial rake. 

All of my setups use the third spring on the rear. My research indicates the undercar diffuser or rear beam wing is the most efficient and effective source of downforce on the SFL 324. My goal is to optimize the aero configuration of the diffuser as the primary goal with the front and rear wings secondary, using the wings to balance the front rear Aero Balance.

So, from the "Base" setup, with relatively low downforce, I created three (3) additional setups: Medium DF, High DF and Extra High DF. The goal as to have incremental changes in downforce to suit different tracks--at least a starting point with some additional fine tuning expected. I used Downforce to Drag calculations to optimize that while incrementally increasing the Rear Beam Wing Diffuser.

To test these, I took the SFL to Talladega Speedway and ran top speed tests. The total top speed range was 150 mph for the highest DF and 160 for my Base setup. 

I then took the SFL to the Centripetal Track and determined the smallest radius circle I could drive at full throttle in 5th gear.  I verified that tire wear and temps were in the desired optimum range.

I labeled each set as Base, MDF, and HDF with the top speed (mph) listed first and the Centripetal Track radius (ft).  The higher the downforce/drag, the lower the speed. The smaller the radius, the more grip-generally from downforce at 130-135 mph.

I ran a test with Coach Scott Haddock at Barcelona comparing the HDF sets against his suggested high downforce set and the 150-120 set was comparable and comfortable.  I also ran comparisons with several of the RRSetups. 

If you are a high iRating (above 4000) and/or just highly talented and don't want to spend any time fiddling with setups--just subscribe to RRSetups and run what Ridgway provides. If like me, you are 2000 range iRating driver with less than optimal ability/skill/reflexes, then having a range of setups tailored to you, that can be modified slightly to fit any track, the above process may be helpful.

Pedal Force Settings

 Pedal Force Settings

When developing "muscle memory" for precision braking, it is useful to know the forces.

The best way to measure is with a "Force Dyno or Gauge" like this one:

Most high end pedals have software that allows "shaping" of the force curve. Linear is almost always the best.

In iRacing, the calibration can be "fine tuned" by adjusting the settings in the "joyCalib" file. 

- DeviceName: 'GTpro Pedals'

   InstanceGUID: 

   AxisList:

   - Axis: 2

     AxisName: 'Y Axis'

     CalibMin: 10

     CalibCenter: 180

     CalibMax: 325

These are my settings and are unusual in that my damaged and handicapped legs cannot push the maximum much higher than 30 pounds. Normal settings would be well above 75 pounds. (CalibMax in the 500-600 range)

The CalibMin is set to reduce unintended brake application when inadvertently resting left foot on pedal. It creates a "dead zone". 

Use the input graph to train your ankle and leg to recognize and repeat the Min, Medium, and Max levels.  Essentially, my trail braking requires precision force input in the 20-30 pounds force range. 

The chart above shows the forces using an orange and black combination of elastic bushings on the pedal slave cylinder. The chart below (black line) shows figures with two black bushings. Note the lower force (18 vs 20) necessary for the 10%.  In other words, a slightly smaller dead zone. Lower inputs for higher forces as everything is stiffer with slightly less pedal travel. I noticed that moving from the orange black combo to the double black caused me to over brake and over slow too early and I did not trail off properly.  joyCalib could have corrected this but I did not like the firmer pedal. 

Pedal and Foot Geometry

 Pedal and Foot Geometry

Optimization of the angle between your feet and the pedals is critical for the fine inputs necessary to achieve your best lap times. The timing and precision of braking and throttle in corners is the difference between the fast and REALLY fast drivers.

My rig uses Ricmotech GTPro pedals with hydraulic brake master cylinder which then compresses a composite bushing. The pedal travel and effort are infinitely adjustable. Then with the desired configuration/calibration of iRacing software, the pedal effort to reach 100% braking is finalized. My pedal travel for 100% braking is about 1.5 inches with about 35-40 pounds of leg force. (I am handicapped from an aircraft crash where I broke both femurs and my left tibia so the force I can apply to the brakes is limited.  Before the accident, I had the brakes calibrated to reach 100% with 100 pounds of force and about 1" of travel. ) 

My leg is bent at the knee at about 30 degrees to insure I am using my quadricep as well as my calf and ankle muscles at max braking. The other critical angle is at the ankle. I am pressing on the pedal with the ball and big toe of the foot with the ankle 90 degrees to my leg. This allows for fine control of brake force even down to 10% braking or 20 pounds of force.

In order to get the correct geometry, I fabricated a semicircular block to support and position my heel. To insure my foot was always correctly positioned on the pedal, I fabricated a "cuff" for my ankle.  (see Figure above) The cuff was sculpted by carving a yoga block and is attached to the foot plate with HD Industrial Velcro.

The throttle is set to have 2 inches of travel.  To allow for heel/toe shifting, the throttle at "zero" is even with the brake pedal when brakes are applied. So at full throttle the pedal is 2 inches further "away". My heel support for the throttle is forward of the heel support for the brake. My leg is less bent as little force is needed for the throttle and movement is controled with the ankle. At full throttle, the right ankle is at 100 degrees and about 90 degrees at the most critical 50% throttle position. 

Once the geometry is fixed, it will take several hours of practice to sufficiently develop the muscle memory to apply fine pedal inputs accurately and consistently.

Belt Tensioner

IRacing Formula Vee

IRacing has introduced the Formula Vee to its offerings. While it is introduced as a “Rookie” car, it has and will continue to attract a community of experienced drivers as occurred with the Skippy. These experienced drivers will compete using an “open setup” where drivers use setup optimizations to achieve advantage.

This series of WS Speed Analytics will explore and test the optimizations of setup adjustments. 

First, here is the iRacing Manual on the iRacing Formula Vee. Click on link.

IRacing Formula-Vee-Manual.pdf

The Formula Vee has a very long history. Starting in 1962-63 it became and continues to be a popular car in SCCA and other worldwide venues.

Here is one of the best info manual to introduce you to the car. There is a lot of info that does not apply to the iRacing FV but it will provide a valuable foundation. 

Getting_Started_In_Formula_Vee 

Here is a website with more interesting info about the FV.

The US FV Enthusiast Website

The FV in real life runs a Type 1 air cooled 1200 cc engine. The Type 1 engine without cooling fan was offered in the Beetle in Mexico in 2003 with1600 cc. Displacement can be changed by replacing cylinders and crankshaft. According to the iRacing manual, the iRacing FV has a 1400 cc engine that produces 76 ft-lbs of torque and 69 Hp with a max RPM of 7400. Not sure where iRacing got this spec but no matter, "it is what it is" and that is all that matters.

Here is an excerpt from the "Getting Started" book. Note "a very good engine" on the dyno in a real life FV produced 58 Hp. Pretty sure my engine built by Neumeister was 60 Hp as it had set the track record at the runoffs the year before. Note peak Hp occurs around 5400 RPM and peak torque around 3700 RPM. In real life competition, drivers take the engine to above 6500 RPM but the risk of losing a connecting rod or bearing is high. My educated estimate is the iRacing FV engine has a peak Hp at around 6000 RPM and like the real life engine, optimum shift RPM is 6500-6600 RPM in 3rd which drops to 5000-5200 RPM in 4th depending on how rapid the shift. 

This screenshot is a cockpit cam of me in 2011 at Summit Point in my Vortech FV-note the GPS top speed of 110mph. The iRacing FV is slower despite having more power. I think the iRacing FV does not have as good aero design as the Vortech.

Neumeister's Vortech at Runoffs

My Vortech at VIR (note the half NACA duct)

Here is the car airborne

The biggest difference between the Formula Vee and most other race cars is the unusual rear “swing axle” design. Significant camber change occurs when the rear drops and rises, but the odd characteristic is that when the chassis rolls in a corner, the outside camber become more negative while the inside tire becomes less negative and often goes into positive camber range. The net result is the car has a natural tendency to oversteer on corner entry when throttle is released, the rear rises, and the chassis rolls.

The first step for building a good setup is to build a "first try" setup. Start with the iRacing baseline and then search in every available free forum/online resource including Garage61.net where setups are shared. (See what others are using.) Then test at a track of your choice. I chose VIR North and after examining four different setups and several iterations, I settled on my "StroutX3"

Since the FV is a very low HP car, the first order of business when building a "optimum" setup is to research the settings that produce the highest top speed. We can then test settings that might improve cornering to see their effect on top speed. For this we go to the Talladega oval.

Note the top speed of 99 mph and lap time of 1:36.932.  This was with zero toe in on rears and 1/32 toe out on front. I was quite surprised to see the top speed was considerably lower than real life FV's. 

Increasing tire pressure produced very modest improvement of about 0.05 seconds in lap time and given the probability that more tire pressure might decrease cornering power, I went back to the 17/17.5 psi settings and changed the front toe to zero. This changed lap time by a significant 0.164 seconds to 1:36.768 (still topped at 99 mph so less than 1 mph improvement). However, I returned to VIR North and saw significant improvement (0.4 seconds) on the long front straight. I could not see any significant loss of cornering power or turn in precision at VIR North with the zero front toe setting. (Top speed at Nordschleife testing was 104 mph)

So we have finished the "rolling resistance" testing. Now we test aero drag variables. The iRacing FV has no wings and a flat bottom. So the only adjustment for Aero is "rake" or the angle of the bottom of the car relative to the pavement. This is determined by measuring "reference" points on the chassis: front and rear.   

The StroutX3 set has a rake of 3.123. I dropped the rear to the max (limited to -2,5 camber) and dropped the front to 1.85 increasing rake to 3.156 and the car went slower. (This was good because front setting below 1.95 results in the nose dropping and hitting the ground during hard braking.) I then tried reducing the rake to 2.966 by raising the front, keeping the low rear and the car went slower. So the 5.082/1.959=3.123 is close to optimal. To check that I raised the nose to 2.012 which caused the rear to change to 5.067. This rake of 3.055  produced a very small increase in speed dropping lap time by 0.012 seconds. 

So let's conclude that the front spring preload should be 0.5 or 0 (1.959-2.012") and the rear spring perch offset should be 3.606" (5.082-5.067") to achieve least aero drag. I suspect the 0.5 front spring preload will produce a slight incline of the flat bottom and may have some minor downforce.

A word about Pushrod Offset setting of 1.187.  On a Zero Roll Resistance Suspension with only one spring and damper, ride height is adjusted with a combination of Spring Perch Offset and Pushrod Offset. There is droop stop rod on the iRacing FV as shown in the pictures which is assumed to be fixed, so then the total droop is affected by Pushrod Length. In maximum droop, the suspension will change the rear camber from negative to positive. Not enough droop and the rear wheels will lift off the ground too easily. Too much droop and the car can be very unstable under heavy braking.  My suggestion is to set the Pushrod Offset in the range of 1.0-1.2 and adjust the Spring Perch Offset to get the rear ride height of 5.082-5.067 with qualifying fuel load.

Now let's take the car to Nordschleife to check droop where we know the car goes airborne in Pflanzgarten. As the car is in the air, note how the rear tires go into positive camber. Even so, the car went thru this section very well.

Now we go to the Centripetal Track circle and drive long enough to burn 0.5 gallons. Third gear with left tire on the 100 meter line. Modulate throttle for steering--keep steering input constant. (This will illustrate to you the Zero Roll Suspension tendency of the off throttle oversteer, on throttle understeer-good practice too) See Short Video below)

Below is the tire measurements with -1.7 front camber. (Remember rear camber changes only with the rear spring perch offset which we will not change.)

Below is the test with front camber set to -1.8. 

It should be easy to see that the right front tire is working harder with the -1.8 setting as the tire temps are higher relative to the rear and there is 1% more wear. So -1.8 is more understeery than -1.7 front camber. The speed (around 77 mph) and lap times of around 18.55 were very close.  Given that -1.8 would offer slightly less rolling resistance and a bit more cornering force--the final decision should be on track testing and driver preference. 

Next we up the rear tire pressure to 18. Hmmm. Tire temps and wear almost the same. Given that in our Talladega test we say a slight increase in top speed, I would go with 17 psi front and 18 rear. The relative 2.2/2.0 pressure rise is slightly better than the 2.2/1.9 so there is better balance in the work the tires are doing. 

Now let's look at the rear spring. All tests to far have used a 200 pound spring in the rear. The FV uses a zero roll resistance rear suspension, the roll resistance is all in the front so the rear spring really only affects the rear compression under load. Changing the rear spring to 175 and keeping the rear ride height and rear camber to the same requires a rear spring offset of 3.370.

Not much change. Perhaps a very small increase in understeer but speed was almost the same.  Given the odd design of the rear axle where rear suspension movement creates a lot of camber change, I would go with the 200 pound spring. On a bumpy track, I would go back to the 17.5 psi rear tire pressure.

Zero roll FV rear suspension 

Now let's increase the front ARB from .625 to .750. This WILL change the roll (not rolling but chassis roll) resistance. Because all the chassis roll resistance is in the front, changing the front ARB changes handling differently than on a car with springs on all four wheels. A stiffer front ARB will reduce body roll and reduce the nasty adverse change to the inside rear tire camber a bit. It also appears to improve front tire grip (less sliding and less temp increase) by reducing the camber change in the front during cornering. Tire wear is reduced, hot tire pressure is lower.  Clearly there is more total "grip". 

Here is a video of the centripetal test

An unknown regarding the iRacing FV is how much bump steer and toe out in turns is in their design. These are important characteristics, and close attention is paid to them in real life. But we get what we get here. 

Now about the dampers or shocks. These are important adjustments that affect the "transient" handling. Off the throttle and on the throttle transfers weight rear to front and front to rear. Higher damper settings make the transfer occur faster. 

On throttle:  I have a tendency (due in part to injuries) to apply throttle very quickly so my preference is minimal front setting=0. Others who have better throttle modulating skills may like setting at 1 to be better. 

On braking: A high setting of 5 on the rear makes transfer from rear to front occur quickly. The dampers appear to be single acting so they only damp when extending.  The stiffer setting for the rear slows the rate that the rear axle drops so weight is removed from the rear and added to the front faster.  I like it to occur a bit slower so I prefer 4. (A setting of 5 would increase trailing throttle oversteer and oversteer on corner exit-maybe faster.)

Finally, the static front weight distribution is 46%/54%. During braking a lot of the rear weight is transferred to the front, so you want considerably more than 46% brake bias. You will see front brake bias set in the range of 61-67%. 

Here is video. 99 mph to full stop-100% brake pressure.  The first is Brake Bias at 63.2%. Good braking-handled well in real race with moderate trail braking. If trail braking deeper, raise to a slightly higher number.

Next is a test at 61.4%. Stopped in 15 feet less distance but a bit unstable--required steering control input. Only a Top Gun with fighter pilot reflexes or someone who never reaches 100% braking could use this low setting. 

 

Below is the iRacing Baseline setting of 67%. Front brakes lock and requires almost 25 feet more stopping distance than the 63.2%. 

Take the "StroutX3" settings that I raced at VIR North that are shown above-modify according to these tests (Change to 18 psi rear tire pressure, increase front ARB to 0.750 and test increasing front camber to -1.8) ...The rest is testing on track and adjusting the car to your personal taste—then call your setup FAST.

I tested the setup at several tracks. Ovals: Bristol, IRP, Milwaukee Mile, and Tsukuba Outer. (Tsukuba is excellent for testing and practicing trail braking, trailing throttle oversteer and corner exit neutral handling)  Road: VIR North and Nordschleife. I ended up being slightly faster with the stiffer rear damper setting. 

Here is my final result. (BTW this is very close to the setup shared by Brian Styczynski so much appreciation to him for that)

or more rear damper:

The above is for practice and a 15 minute qualifying session.  The one below is for a 30 minute race using 4 gallons.

or more rear damper:

A note about shifting. Because of severe leg injuries I use paddles for clutch. When shifting up I use right shift and lower left clutch; shifting down I use left shift and lower right clutch. (This requires a bit of programming on my VRS Pro Wheel) When driving the FF1600 with dog type transmission, as long as the clutch and shift overlap the shift occurs without lifting. The FV uses an old fashioned system that is more sensitive to the clutch needing to be fully engaged if power shifting at full throttle. Similarly when downshifting even when (especially when) manually blipping.  So it takes practice to begin pulling the clutch paddle a mini second before pulling the shifter. I got the timing wrong for the first 10 hours of testing and racing and missed many shifts. Using auto blip is slow. So being fast in the FV is mastering precision power shifting. You should also master doing this with a perfectly timed small throttle lift and fast enough action so as not to exceed 7000 RPM when the clutch is disengaged. A missed shift not only costs time but if someone is following close it can cause a collision.

Different opinions about shift points. 3rd to 4th at "4 bars" or around 6600 RPM (85 mph) will drop the RPM when into 4th to just above 5000 RPM. Any higher than 6600 RPM probably is counterproductive. Shifting at 84 mph to be in 4th early using the downshift into 3rd to help rotation into a corner sometimes is beneficial. 

Gear Choice

Gear Choice

There is often a debate as to which gear is the best for a particular corner.

A general rule of thumb is that the best gear at the time of throttle application is the one that provides maximum acceleration from apex to corner exit.  

Each transmission is different, but most transmissions have a 20% difference between 3rd and 4th gears.  Given the same speed, the engine in third gear will be running at a 20% higher RPM than in fourth.  

Example:  5800 RPM in the Corvette in 4th is about 7200 RPM in 3rd. 

This is a part of the actual HP and Torque curve for the Corvette GT3.  It has a very "flat" torque curve (blue line).  Most modern racing engines have a reasonably "flat" curve.  Lower performance engines have torque curves that fall off at higher RPM-racing engines less so.

The iRacing version of the LT6 engine has a max torque of 460 ft pounds at 6300 RPM. Let's estimate at 5800 RPM torque is 440 ft pounds. And, at 7200 RPM torque is 420 ft pounds.

Force=Mass times Acceleration.   Acceleration = Force divided by Mass.   

Transmissions essentially MULTIPLY torque so 3rd gear has 1.2 times the torque of 4th gear.

OK. The Corvette has 5% less torque at 7200 RPM than at 5800 RPM but 3rd gear increases the torque by 20%. Greatly simplified:  3rd gear at 7200 RPM has 14% more torque and acceleration than 4th gear at 5800 RPM.  So initially, the Corvette will accelerate significantly faster in 3rd gear.   (Torque actually increases from 5800 to 6300 RPM, so the average increase is probably closer to 10%.)

Now, redline is about 7800 RPM, so using 3rd gear at the apex with 7200 RPM will require upshifting much sooner than if 4th gear was used.  Shifting down to 3rd on corner entry and up to 4th before corner track out takes time.  The downshift tends to slow the car in corner entry and acceleration is reduced when upshifting.  These reductions in speed must be compared to the increase in acceleration by using the lower gear. 

Another factor to consider is that the 10-14% more torque in the lower gear, in addition to increasing acceleration, also increases the risk of wheel spin and traction control intervention. 

So, the only real answer as to what gear is best is only determined by lap time. Often, there is very little difference. 

The "keep it in 4th" driver will probably see a bit more understeer both in corner entry and exit. But, no time or mental concentration will be lost in shifting. The "downshift to 3rd" driver will see less understeer on corner entry and exit. If using 3rd results in an RPM below 6800 at the apex the time duration with higher torque at the wheels and higher acceleration will most probably more than offset any time lost in upshifting. 

The Corvette will not allow downshifting if it would overspeed the engine. In most cases, the advantage, if any of using 4th rather than 3rd is that often in the process of downshifting, speed and momentum is lost. This speed and momentum loss can be minimized by downshifting as close to the apex as possible--the increased load on the rear tires will rotate the car without having to scrub off speed with steering input at the apex. 

 Author's Note:  I generally use the lower gear only for a very brief period-just before the apex and just after for a "burst" of extra acceleration and assistance in getting the car to rotate without as much steering input. 

IMPORTANT: The discussion above takes on a whole different perspective when dealing with a low horsepower winged formula car like the F4.  In the F4, maintaining speed and momentum is of paramount importance. Corning grip increases substantially with higher speed-likewise corner grip decreases with lower speed. If you feel able to use a lower gear, you may be slowing down too much on corner entry. 

Keep in mind that aerodynamic forces from wings and underbody channels increase at the square of speed. For example a 5% higher speed increases downforce by 10%.

Corvette Z06 GT3 Part 1

This is a series focused on setup development for the Corvette.

Before you start, be sure to read the iRacing User Manual for the Corvette

Chevrolet-Corvette-Z06-GT3_manual_V2.pdf

Rule #1. NEVER EVALUATE A SETUP ONLY ON A FEW LAPS OF TESTING. IT TAKES A MINIMUM OF 25 INTENSE PRACTICE LAPS TO DRAW ANY CONCLUSIONS. EVEN THEN THERE IS A HUGE POTENTIAL FOR AN INCORRECT CONCLUSION. 

The first task in developing a setup is to select the gearing most suitable for the track/s.  You need to know that you are allowing the car to reach the best top speed on a long straight in a draft. You also need to know which gearing best suits the corner exits of the track you are running. In many race cars you can change each gear set--1st thru top gear.  iRacing offers two basic gear combinations: FIA and IMSA.

Below is a chart showing the max speed in mph for each gear in the set:

At this point, the question is “Where do I start?” 

Start with a setup that you trust is reasonably close to your goal and modify it. Use iRacing “canned” setups, setups from VRS, Apex, Coach Dave or your League Engineer

Note that the FIA set allows a higher top speed in 5th and 6th AND more torque (lower speed=more torque multiplication) in 1st-3rd gears.

To choose what is best, test the top speed possible at the longest straight of the track/s you will be running at. Let's consider Road Atlanta, SPA, Mosport, and Suzuka. Testing indicates max top speed not in a draft is:  Road Atlanta 162 mph, SPA 158 mph, Mosport 155 mph, Suzuka 159 mph. 

Add 3% to these top speeds to adjust for maximum 2 car draft: Road Atlanta 167 mph, SPA  163 mph, Mosport 160 mph, Suzuka 164.  Although the IMSA gearing would be suitable for the top speed requirments, you also have to consider how the gearing works in corners.

Note the FIA gearing torque is higher than the IMSA:  20% more in 1st, 8% more in 2nd, 5% in 3rd, equal in 4th, 4% less in 5th; and 4% less in 6th. Bottom line: The FIA gearing will allow significantly higher acceleration (and potential wheel spin) in 1st thru 3rd resulting in higher speeds at the beginning of long straights--this speed "carrying thru" the entire long straight. 

On the other hand, the IMSA gearing will allow more acceleration in 5th and 6th gears. 

The "best choice" depends on the track. In a theoretical "drag race" for example at SPA: after exiting the hairpin in 1st gear, the car with the FIA set would reach Eau Rouge sooner with the car having the IMSA set catching up by the end of the Kemmel Straight. A driver with the FIA set defending the entry at Les Combs would have an advantage. The driver with the FIA set would almost always have an advantage in a race. 

In hot lapping the gear sets most probably do not produce different lap times. 

As can be seen by the chart, the rear wing setting results in only a small change in top speed. The difference between 9.5 and 8.5 are not significant. The difference between 9.5 and 7.5 and especially 6.5 is significant and would make a difference in lap times if the driver is able to cope with the reduced grip in the corners. 

Testing grip at the Centripetal Track

The vast majority of downforce on the Corvette comes NOT from the rear wing but rather the sophisticated air tunnels under the car. Rake or the difference in ride height from front to rear and total ride height WOULD have a significant effect on downforce. (The car needs to be as low as possible--just so high as not to drag on curbs.) The rear wing tends to be more of a fine tuning of the understeer/oversteer balance. Running a test at the Centripetal Track with 190 foot, there was less than 0.5 mph difference between each incremental change from 9.5-8.5-7.5-6.5. The most noticeable difference was the car had an understeering tendency at 9.5 and an almost oversteering tendency at 6.5.  A driver unable to cope with the oversteer tendency would do better staying with the 9.5 on all tracks where top speed is less than 160 mph. 

So, once you choose the gearing and the wing, move on to the adjustments that determine: 1) Brake bias for max stopping power and optimum trail braking that fits with driver's preference; 2) Adjustments that affect the understeer/oversteer balance in all corners--giving priority to the corners leading to the longest straights; and 3) Adjustments that affect the "transitions" and weight transfer front to rear, and side to side, during corner entry and exit. See Parts 2, 3 and 4 of this "series" on the Corvette.