tinyman392
Senior Member
- First Name
- Marcus
- Joined
- May 21, 2018
- Threads
- 14
- Messages
- 3,265
- Reaction score
- 2,082
- Location
- Illinois
- Vehicle(s)
- '18 Civic Type R (RR)
So the top speed of the car is limited by drag. Specifically this means that the negative acceleration from wind drag is balanced in an equilibrium by the forward acceleration of the wheels, and thus no more acceleration is possible.Assuming it’s on level ground, I think it’s speedo error, which is very common on all makes of cars. 2% sounds well within reason for normal street driving.
Regarding power loss from weight, I don’t think that will make a difference in top speed. The power loss that people measure is due to the difference in measured acceleration on a dyno and that certainly reflects in how the car accelerates. However, at top speed, you are no longer accelerating. Instead, you are in a static situation where the torque at the wheels is balanced against friction. At that point, the weight of the wheels doesn’t matter. Heavy or light, they’ve already accelerated to speed and are no longer influencing things.
Typically the you’ll need 4x the amount of power to get 2x the top speed of a vehicle from this perspective (which is why it’s very difficult to go faster as the power required is huge). So, if you increase the power/torque (they’re increasing one technically will increase the other for a given RPM), you will be able to accelerate just a little bit faster which means you’ll be able to “beat” more of the negative acceleration due to drag just a little bit more. This, in turn, will increase the top speed ever so slightly.
The thing is, with the loss of weight in the wheels, it’ll amount to what, 5-10 HP (if that)? That definitely won’t be enough to add 10 MPH to the top speed (maybe 1-3 MPH using similar math below). A tail wind, slight downhill, and speedometer error certainly could though.
Getting the car to 180 MPH (290 KPH) would be a total increase of 6.5% in top speed. In order to get this, you’d need 1.065^2 the amount of total power the car puts out or about a 13% increase in power. Say the stock car makes 300 WHP, then you’d need about 340WHP to get to 180 MPH with identical conditions to the original test (and a transmission/tuning that will allow the overrev to hit said speed).
Something else has to be going on to hit that 180-ish MPH mark on top of the speedometer error. A downhill grade of a tail wind certainly could help explain it.
Edit: let’s say the car is hitting 177 MPH (redline of the Type R), then it would probably need about 329 WHP to do that (again, identical situation to the original 169 MPH test).
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