The bodies have a reasonably large frontal area to offset the lower Cd.
PU limits are fine - use negative quality to get more cylinders into the game. Not all supercars and race cars of the time have been engineering masterpieces. It is possible to use different engine layouts with the current setup.
There are no absolute wheelbase limits. Meme cars will be disqualified. Stay reasonable.
Your assumptions regarding fuel and cats are wrong: If there is no rule, there is no rule.
On another note:
How does cooling airflow affect the car in the BRC? does it overheat if the slider is lower than 50? or does it only have to do with the reliability stat
Iām not seeing any notes of the change in vehicle difficulty calculation anywhere other than that one comment on discord. 
It was already mentioned in the BRC Guide, but I also added a short changelog to the challenge description in the OP now.
I will add this section in the reliability section of the BRC Guide today.
If we could get, or somebody make the formulas in the guide onto an excel spreadsheet that would be great. Trying to understand what is going on is a little difficult, so entering values and testing would be a great help.
Talking about this kind of thing
partReliability is engineReliability
partModifier is engineCoolingMultiplier:
if engineCoolingFraction < 0.5 then
engineCoolingMultiplier = 1- (0.5-engineCoolingFraction)
else
engineCoolingMultiplier = 1+ 0.2 * (engineCoolingFraction-0.5)/0.5
engineCoolingFraction is the Cooling slider divided by 100
A few questions about drivers:
If a driver has a skill of 0/10 in something, will the race day status do nothing or is the scale extended inside the sim?
Is the overtaking skill used against lapped cars or does the blue flag negate it?
The excel file says +/-20% for tire wear and breakdowns for the Care skill, the Youtube video and Guide say 10% breakdowns/5% tires. Which is correct?
Will driver abilities be in effect for practice?
Rolling start?
EDIT:
Does TC reduce tyre wear in BRC?
Are automatic lockers still a (anomaly) thing in automation race cars?
A question;
Does Downforce get included in the axle loads? If so does it pull the downforce graph down or just the maximum value?
If the driver skill is already 0 and you have a negative race day condition (RDC), you will not lose skill points in this skill but one or more of the other skills.
Same goes for maxing out a skill: If you already have 10, you will only gain in other skills when you have a positive RDC.
No, lapping is not affected by the overtaking skill. Lapping is the same for everybody. The overtaking skill only applies to direct competition.
The excel is what counts and is more up to date - I added more information there where it is possible to specify concrete numbers. Will have to re-do the video in the futureā¦
Yes. Iāll deactivate the Race Day Condition for the pre-season test though.
It depends. Some have reported inproved lap times, some have not. Maybe Iāll ban them in upcoming BRCs, if I donāt forget again. 
For now automatic lockers are legal.
Downforce is currently not included in that calculation, just the static wheel loads at standstill. I didnāt understand the second question, but itās probably not important now anymore, right?
We do have rolling starts right?
On another note, I do worry one lap heroes might do damage to 107% cut, no?
Realistically, nobody is going to enter a one lap hero, Even with times that people have reported i havenā't seen anyone that is too slow that theyāre cut off. The pace will most likely be around 2-4 seconds slower anyway.
I do not worry - and if there is some really weird case, I will not take that one into account and just ignore that one for the 107%.
If there isnāt a rule about it, then there isnāt a rule. The only quality limitations were on Tyres (has to be 0) and aero (max +5)
There is PU and ET limits to do the rest.
OH
I had seen the PU/ET limits for the engine but I managed to miss them for the Chassis, alright