BRZ / FR-S Boxer Engine Dyno Powerband revealed

[serialk11r]

^^^ the virtues of a 6 speed gearbox. Actually, having a lot of pull in 6th gear isn't really a good thing IMO since an overdrive gear should be optimized for 50-80mph range efficiency, but the closer ratios in between are definitely helping.

[bw1235]

Originally Posted by Deslock

Here's a rough estimate for comparing acceleration of the BRZ and WRX. As expected, once the WRX hits boost the BRZ never matches it. Still, I was a little surprised to see that:

• The BRZ in 6th gear has more pull than the WRX in 5th until 70 MPH.
• The BRZ has more pull than the WRX when both are in 5th.
• The BRZ has more pull than the WRX when both are in 4th until 55 MPH.

This doesn't account for traction and of course it doesn't take much to increase the WRX's output. Even stock, it'll be 1.2-1.5 sec faster just to 60 MPH.

... and it doesn't account for the WRX's, um, brick-like aero =)

[Deslock]

Originally Posted by bw1235

... and it doesn't account for the WRX's, um, brick-like aero =)

Actually it does. Read the text on the bottom left of the plot.

[serialk11r]

How did you add drag in? CdA 1/2 \rho v^2?

[Homemade WRX]

also note that he is using the 4.1 final drive which we probably will not be getting here. Thank you CAFE.

[serialk11r]

Originally Posted by Homemade WRX

also note that he is using the 4.1 final drive which we probably will not be getting here. Thank you CAFE.

I doubt it, current fuel economy regulations in Europe/Japan are probably stricter than they are in the US but 2016 will be different. Higher final drive doesn't hurt city test economy much anyways. What I want to see is a crazy 0.6 6th gear with 4.100 final drive, best of both worlds. Probably something like a 10-15% fuel economy increase on the highway as it'll decrease rpms by 14% compared to the 3.727 final drive even. With a 3.727 final drive you'd be getting into like 260-270g/kwh cruise which is great but you'd lose first gear acceleration.

[WingsofWar]

Originally Posted by serialk11r

How did you add drag in? CdA 1/2 \rho v^2?

looks like only Cd was used not CdA.

[Deslock]

Originally Posted by serialk11r

How did you add drag in? CdA 1/2 \rho v^2?

1/2 * VFA * CoD * rho * V^2

rho varies depending on temp, elevation, etc but I used 0.00235 slugs/ft^3

Originally Posted by WingsofWar

looks like only Cd was used not CdA.

I used VFA of 23.68 ft^2 for the WRX (found from google) and approximated the BRZ as 20.5 ft^2. I think the VFA I initially used for the BRZ was a bit too low, so I changed it and added more notes to the plot.

I used this WRX dyno:
http://www.clubwrx.net/forums/engine...yno-pull.html:

I know nothing about rolling resistance, but I based my numbers on the formulas I found at http://www.mayfco.com/aero1.xls which was discussed at http://www.rx7club.com/showthread.php?t=541682

Other links:
http://www.miata.net/garage/tirecalc.html
http://www.mayfco.com/mazda.htm
http://www.engineeringtoolbox.com/ca...on-d_1309.html
(I used the last two linkes to make sure my numbers were in the right ball park)

[serialk11r]

Rolling resistance is a constant value, good estimate is about 0.008 of the car's weight.

[old greg]

Originally Posted by serialk11r

Rolling resistance is a constant value, good estimate is about 0.008 of the car's weight.

For force, yes. For power, it scales linearly with speed.

Oh, and LRR prius tires FTW.

[serialk11r]

The stock Prius tires are in the 0.006xx range :O Michelin Primacy tires all look to be a tad over 0.008, and there is some Bridgestone tire that is 0.00615, pretty ridiculous.

[Deslock]

Originally Posted by serialk11r

Rolling resistance is a constant value, good estimate is about 0.008 of the car's weight.

Yeah at legal speeds rolling resistance ends up being close to 0.008 * weight, but according to that spreadsheet it starts increasing around 80 MPH and it's significant above 100 MPH.

Originally Posted by old greg

For force, yes. For power, it scales linearly with speed.

Oh, and LRR prius tires FTW.

The formulas in the spreadsheets are:

fs = Speed Effect Coefficient
V = speed (mph)
Fd = force rolling drag (lbs)
fo = 0.008
W = weight (lbs)

fs = 0.00195 + 0.000024833 * (V - 150)
Fd = (fo + 3.24 * fs * (V/100)^2.5) * W

From what I've read, though the force required to offset rolling resistance isn't related to velocity, at high speed the additional heat increases the coefficient of friction. I got curious about the formula's numbers and the 5/2 exponent, so I clicked around the domain where the spreadsheet came from and found rolling drag at http://www.mayfco.com/keith.htm

DR = W * (fo + 3.24 * fs * (V * (60/88)/100)2.5 (where 60/88 is just a conversion)

It's also mentioned at
http://www.physicsforums.com/archive.../t-321017.html
http://www.ffcars.com/forums/17-fact...top-speed.html

At some point I might dig my old textbooks out of the basement to look it up.

Anyway, while putting this together last night, it also occurred to me that the FT86's "Prius" tires ought to be advantageous for this. Everyone bashes them, but if steering feel is as good as the reviews say, then I think they make sense.

[serialk11r]

Coefficient of friction doesn't matter as long as the tires don't slip...?
I'd think that at higher speeds the tire actually becomes stiffer since it experiences strong centripetal forces...

I think the reason why it apparently takes extra power at high speeds could be the fact that C_d changes a bit as speed changes. I'm not sure about this but for example a fastback whose rear windshield is angled at 20 degrees could have attached flow at 40mph, but at 120mph the flow will have significant separation and a much larger wake. With FlowIllustrator (a 2d fluid dynamics thingy) turning up the reynolds number definitely has this effect.

[old greg]

Originally Posted by Deslock

From what I've read, though the force required to offset rolling resistance isn't related to velocity, at high speed the additional heat increases the coefficient of friction.

The extra heat is not the cause, it is the effect. Rolling resistance isn't caused by friction, but rather by hysteresis in the rubber. Basically, as the tire rolls forward and a certain patch of rubber comes into contact with the pavement, that rubber gets squished and stores up energy like a spring. As the tire continues to turn and the patch of rubber starts to lift off of the pavement it pushes away from the road and releases most of the stored energy, most but no all. The difference in energy has been absorbed by the tire as heat.

If I had to make a wild guess about that exponential velocity component, I'd say it comes from the viscoelastic nature of rubber. The faster you try to squish it, the harder it is to squish, and presumably the higher its hysteresis becomes.