BRZ / FR-S Boxer Engine Dyno Powerband revealed

[Ryephile]

Good stuff. So, basically 25% less accelerative force most of the legal-speed-limit time in the BRZ vs. WRX. That's fine.

[Deslock]

Thanks for the interesting comments serialk11r and old greg, especially regarding the tires' hysteresis.

GTI

[DSPographer]

Originally Posted by serialk11r

Yea, I almost have them memorized already rofl. 4.1f/d (or 3.727 non-lsd), 3.626, 2.188, 1.541, 1.213 (had to look these two up), 1.000, 0.767. Wheel is 24.6 inches I think. Okay 24.61 or 625mm.

The ETRTO rolling circumference for a 215/45R17 tire is 1907mm so the effective tire diameter in inches is 1907/25.4/pi or 23.9".

[ZDan]

Any toe whatsoever and the effective rolling resistance goes up significantly. ~.012 is going to be more realistic than .006-.008 for a real-world RR figure including a small amount of toe and less than ideal surface.

Regarding Cd and Reynold's number effects, you can assume constant Cd for cars at realistic real-world speeds.

[serialk11r]

Are you sure? If you take Cd and compute how much power it theoretically takes to reach top speed vs. the actual top speed at which cars top out, I think it's usually off by a bit. For example, a GTR tops out at 193mph, which going by the formula would take far less than 400hp to do. Especially for 200mph+ cars, it looks like they have way more power than they need, but that's not the case.

[ZDan]

I'm sure, at 200mph Cd hasn't changed.
There's more going on than aero drag and rolling resistance, btw. There's also loss to tire slip, not insignificant at 200mph. Should be accounted for in wheel hp.
Gearing will also play a HUGE role.

[Deslock]

I created one vs the 2004 WRX, which made me doubt the validity of the comparison:



It's not that I don't think the BRZ will be faster than the 2.0L WRX... I expect the BRZ to have better top-end, and if launched abusively might even almost match it in the 1/4 mile. I'm just surprised how much of an advantage that plot gave the BRZ.

I think part of the problem is that published power curves often show more low-end than dynos. So I plotted again, but this time vs the BRZ dyno screencap. Because the scale of the dyno screencap is unknown, and the curve doesn't quite match the published dyno, some guesswork is needed. Here's what a couple users speculated in the screencap thread:



Because it has significantly more midrange output than the published curve, I'm dismissing it as unrealistic.

Next up are plots using a more conservative estimate of the screencap scale. It's also questionable because some runs appear to go way past 7500 RPM (but I didn't use them):



This is more like what I've been expecting all along, at least in the mid and high RPM range. This as probably the worst case scenario, and I won't be surprised if it's in-between this and the other plots.

Again, all of this is speculative.

Here are plots with the conservative dyno scale vs the current GTI and WRX:

[OrbitalEllipses]

170whp is an appropriate estimation of the 2L WRX. They dyno between 160whp-180whp depending on transmission and other factors.

[serialk11r]

Originally Posted by ZDan

I'm sure, at 200mph Cd hasn't changed.
There's more going on than aero drag and rolling resistance, btw. There's also loss to tire slip, not insignificant at 200mph. Should be accounted for in wheel hp.
Gearing will also play a HUGE role.

200mph vs. 60 is a 11 fold difference in Re. Let's consider airplanes which will start to have separated flow when the upper surface of the aerofoil is at just a little over 10 degrees. Most cars have rear windshields that will slope a bit more than that, and I could see the case being that at 60 they can get away with it but bump the speed up to say 150 and the flow can no longer stay attached. (not saying this is correct, just possible) And since supercar top speed is a major advertising point you'd think they care about getting 6th gear to hit the top speed. Are you sure about the tire slip? Seems unlikely to me...

Not saying you're wrong necessarily, it's just hard to believe these things.

[ZDan]

Changes in Reynold's number are quantified on a log scale. 10x difference isn't really that great once you're in turbulent flow and well below sonic.
But anyway, Reynold's number at 200 is only a 200/60 or 3.33x increase. Not a big deal.

Re = density * velocity * characteristic length/viscosity
changes *linearly* with velocity, not with the square.

You don't start seeing significant drag rise until you're approaching transonic velocities. 200mph is definitely in the same flow regime as 60mph or even 30mph.

[serialk11r]

Hmmm okay thanks for explaining..
But where does the extra drag come from then?

[ZDan]

Didja look into GEARING like I mentioned? I'm guessing not...

[serialk11r]

So are you saying the GTR could do 220mph if they geared it right?

[ZDan]

Originally Posted by serialk11r

So are you saying the GTR could do 220mph if they geared it right?

WHAT?! I don't see how anyone could have read my posts and come up with that conclusion!