A8 Z06 vs A8 Hellcat

[ilirg]

https://youtu.be/amd3M4_oupA
Hellcat didn't stand a chance even when jumping the gun.

[GretchenGotGrowl]

So, maybe the Z07 aero package is what is impacting acceleration after all. I guess it will remain an open question until we see some more matchups.

[newmoon]

Is this the same car that ran the 10:40s? Funny how every other matchup the two are very close.

[GretchenGotGrowl]

Originally Posted by newmoon

Is this the same car that ran the 10:40s? Funny how every other matchup the two are very close.

Different color calipers, so I am guessing they are different cars. GM did say the A8 would be faster than the M7 and that the Z07 produced more downforce than any car they had ever tested. If the Z07 package has 200 lbs of downforce (who knows what it really is) at 100 MPH, that's like having another full-size male occupant...not to mention the drag that must go along with it. That can significantly impact acceleration. So, maybe what we are seeing is that A8s with less aero accelerate faster. They won't handle the twisties as well, but they may be really good in a straight line.

[ilirg]

Originally Posted by newmoon

Is this the same car that ran the 10:40s? Funny how every other matchup the two are very close.

Nope not the same car, this is the first time I've seen a stock a8 z06 vs an a8 hellcat. All the haters are getting awfully quiet now.

[DGthe3]

Originally Posted by GretchenGotGrowl

Different color calipers, so I am guessing they are different cars. GM did say the A8 would be faster than the M7 and that the Z07 produced more downforce than any car they had ever tested. If the Z07 package has 200 lbs of downforce (who knows what it really is) at 100 MPH, that's like having another full-size male occupant...not to mention the drag that must go along with it. That can significantly impact acceleration. So, maybe what we are seeing is that A8s with less aero accelerate faster. They won't handle the twisties as well, but they may be really good in a straight line.

As far as grip is concerned? Yes. The downforce will increase the friction between the tires & the road (thats why its there in the first place). That increased rolling resistance might help (if the tires were in danger of spinning) or it might hurt (if the car already has enough grip to prevent the tires from spinning). But hurting high-speed acceleration because of apparent weight? No.

Lets go back to Newton. Net (or sum or total) force = mass x acceleration

In the case of a car, our mass is the car + everything in it. For forces, lets break this down into horizontal & vertical. In the vertical direction we have the cars weight + any aerodynamic downforce pointing down. This is met by the normal force of the earth pushing back up resulting in 0 vertical forces (otherwise the car would either lift off the ground or bury itself).

Along the horizontal, you've got the force of the tires (driven by the engine) pushing the car forward. You have the friction from the air (drag) & the road (rolling resistance) holding it back. You should notice that downforce does not factor into the horizontal acceleration, not directly. It increases both forms of drag as a side effect, but it does nothing by itself with regard to forward acceleration. Subtract the frictional forces from the propulsive force & you get the net force thats trying to accelerate the car (or truck or train or boat or plane or anything really).

The major impact will be felt in increased drag. I'm not sure on the ratios for lip spoilers, but on wing-types its usually a few pounds of downforce for every pound of drag.

[GretchenGotGrowl]

Originally Posted by DGthe3

As far as grip is concerned? Yes. The downforce will increase the friction between the tires & the road (thats why its there in the first place). That increased rolling resistance might help (if the tires were in danger of spinning) or it might hurt (if the car already has enough grip to prevent the tires from spinning). But hurting high-speed acceleration because of apparent weight? No.

Lets go back to Newton. Net (or sum or total) force = mass x acceleration

In the case of a car, our mass is the car + everything in it. For forces, lets break this down into horizontal & vertical. In the vertical direction we have the cars weight + any aerodynamic downforce pointing down. This is met by the normal force of the earth pushing back up resulting in 0 vertical forces (otherwise the car would either lift off the ground or bury itself).

Along the horizontal, you've got the force of the tires (driven by the engine) pushing the car forward. You have the friction from the air (drag) & the road (rolling resistance) holding it back. You should notice that downforce does not factor into the horizontal acceleration, not directly. It increases both forms of drag as a side effect, but it does nothing by itself with regard to forward acceleration. Subtract the frictional forces from the propulsive force & you get the net force thats trying to accelerate the car (or truck or train or boat or plane or anything really).

The major impact will be felt in increased drag. I'm not sure on the ratios for lip spoilers, but on wing-types its usually a few pounds of downforce for every pound of drag.

Although it has been a while since I took physics, I think there are two flaws with this. First, there are opposing forces to the downforce that must be accounted for. Cars a generally flat on the bottom and rounded on the top, which creates lift (like a wing). Additionally, the suspension (especially the coils) and tires provide greater opposing forces as they are compressed. That is why acceleration is near zero. It is also why they use manometers to measure pressure differentials in the wind tunnels. These differentials are extrapolated across all the surface area experiencing lift (upward and downward) to get the total lift. Downforce is just negative lift. I could be wrong, but that's how I remember it.

[Denis]

I would kill for any of those cars haha.

And that Z sounds so good.

[96MrBass]

Originally Posted by ilirg

Nope not the same car, this is the first time I've seen a stock a8 z06 vs an a8 hellcat. All the haters are getting awfully quiet now.

He can't help it lol.

[DGthe3]

Originally Posted by GretchenGotGrowl

Although it has been a while since I took physics, I think there are two flaws with this. First, there are opposing forces to the downforce that must be accounted for. Cars a generally flat on the bottom and rounded on the top, which creates lift (like a wing).

Yes, I ignored that -but with good reason. When a car is said to be producing downforce, the number quoted is the net result of the lift and downforce being generated. In theory, the body shape of the Z06 might be producing say 100 lbs of lift, while the various aerodynamic devices tacked on produce a sum total of 300 lbs of downforce, for a net result of 200 lbs downforce.

Additionally, the suspension (especially the coils) and tires provide greater opposing forces as they are compressed. That is why acceleration is near zero.

Yes, this is expressed in the increased rolling resistance. And as far as the tires & suspension are concerned, it doesn't really matter if its extra mass or aerodynamic downforce thats pressing down on them. So in that respect, you were right before when you said that aero & extra weight were the same.

But at speeds where aerodynamics become important, rolling resistance plays a fairly small role. I'm not 100% certain, but I think rolling res is a constant (based on the normal force at the tire). Aerodynamic drag is proportional to speed squared. So as speed increases, drag will quickly come to dominate over rolling resistance.

It is also why they use manometers to measure pressure differentials in the wind tunnels. These differentials are extrapolated across all the surface area experiencing lift (upward and downward) to get the total lift. Downforce is just negative lift. I could be wrong, but that's how I remember it.

Thats right. Create a pressure map, integrate over the surface area and you get a net force either up or down, for lift or downforce respectively.

[GretchenGotGrowl]

Originally Posted by DGthe3

Yes, I ignored that -but with good reason. When a car is said to be producing downforce, the number quoted is the net result of the lift and downforce being generated. In theory, the body shape of the Z06 might be producing say 100 lbs of lift, while the various aerodynamic devices tacked on produce a sum total of 300 lbs of downforce, for a net result of 200 lbs downforce.

Yes, this is expressed in the increased rolling resistance. And as far as the tires & suspension are concerned, it doesn't really matter if its extra mass or aerodynamic downforce thats pressing down on them. So in that respect, you were right before when you said that aero & extra weight were the same.

But at speeds where aerodynamics become important, rolling resistance plays a fairly small role. I'm not 100% certain, but I think rolling res is a constant (based on the normal force at the tire). Aerodynamic drag is proportional to speed squared. So as speed increases, drag will quickly come to dominate over rolling resistance.

Thats right. Create a pressure map, integrate over the surface area and you get a net force either up or down, for lift or downforce respectively.

Maybe I misunderstand, but it sounds like we are saying the same thing. Adding a person imparts a downward force (mass of person * acceleration of gravity) and negative lift imparts a downward force (pressure differential). Both can be expressed in terms of Newtons (1kg m/s2 = 1 Pa = 1/6,894.8 psi). Both increase grip by the same principle, but negative lift also increase drag but adding a person doesn't really (unless you strap them to the roof ) Does that sound right?

[DGthe3]

Originally Posted by GretchenGotGrowl

Maybe I misunderstand, but it sounds like we are saying the same thing. Adding a person imparts a downward force (mass of person * acceleration of gravity) and negative lift imparts a downward force (pressure differential). Both can be expressed in terms of Newtons (1kg m/s2 = 1 Pa = 1/6,894.8 psi). Both increase grip by the same principle, but negative lift also increase drag but adding a person doesn't really (unless you strap them to the roof ) Does that sound right?

Yes. However, you were originally talking about straight line acceleration. And in that situation, there is a difference. For the car with the passenger, you have increased the mass by about 90 kilos (200 lbs). In the downforce car, you've probably added something between 90 and 300 newtons of drag (translating to 20-70 lbs), depending on the aerodynamic efficiency of the added aero-bits. For reference, at ~170 mph, most cars have something like 400-600 lbs of aerodynamic drag. I have no clue how much propulsive force a 2015 Z06 generates at the speed where it might be generating 200 lbs (net) of downforce. Which means I am as far as I can go with guesstimating numbers.

[GretchenGotGrowl]

Originally Posted by GretchenGotGrowl

Different color calipers, so I am guessing they are different cars. GM did say the A8 would be faster than the M7 and that the Z07 produced more downforce than any car they had ever tested. If the Z07 package has 200 lbs of downforce (who knows what it really is) at 100 MPH, that's like having another full-size male occupant...not to mention the drag that must go along with it. That can significantly impact acceleration. So, maybe what we are seeing is that A8s with less aero accelerate faster. They won't handle the twisties as well, but they may be really good in a straight line.

Originally Posted by DGthe3

Yes. However, you were originally talking about straight line acceleration. And in that situation, there is a difference. For the car with the passenger, you have increased the mass by about 90 kilos (200 lbs). In the downforce car, you've probably added something between 90 and 300 newtons of drag (translating to 20-70 lbs), depending on the aerodynamic efficiency of the added aero-bits. For reference, at ~170 mph, most cars have something like 400-600 lbs of aerodynamic drag. I have no clue how much propulsive force a 2015 Z06 generates at the speed where it might be generating 200 lbs (net) of downforce. Which means I am as far as I can go with guesstimating numbers.

I agree, we don't know what the downforce or drag are. I will say that the Porsche GT3 is supposed to have about 700 lbs of downforce at 186 MPH. Given GM's penchant for comparing the Corvette to Porsches, the Z06/Z07 may be higher than that. Of course, the drag will be higher as well. So, you expect slower acceleration given similar power-to-weight ratios. Which goes back to the point I was trying to make to Newmoon. He should expect a difference between a M7 with Z07 package and a A8 without. It is all guess work until official coefficients and downforce numbers are released for the different aero packages.

[Denis]

really at what point does the down force really effect speed? these dudes in the video are only getting to about 120-130. I mean most hits seem like they're around 30 or 40. The runs from digs would mean there is zero aerodynamic influence on the launch.