Virtual Dyno HP, Cd, frontal area and Top Speed

[Siestarider]

Fooling around with datalogs today, I wondered if I could calculate how much top end gained by tuning and HP mods since I was stock.

Started with an old log with OEM IC (already had Mountune intake) with top end recorded two ways, average 138.5 mph calculated on max rpm times gear ratios.

VDyno 3rd gear run (smoothing 5) around same time showed 161 HP.

Found Cd of 0.33, CdA of 7.57 in Wiki Ecomodding

Picked relevant easy formulas from here: http://craig.backfire.ca/pages/autos/drag. Ran calcs and concluded either I have 133 HP at drag-limited speed of 138.5 mph and VDyno is less accurate than I thought; or CdA is wrong in Wiki.

I checked Wiki again and found their assumed 0.84 reduction factor used for calculating CdA was not appropriate for this calculation. Using Cd 0.33 with A=27.3 sf, at 138.5 mph, HP calculated to 158, very close to the VDyno # of 161 HP.

With current Cobb S3 tune, 2J DP and recent logs, VDyno says 181 HP, and top end now calculates to 144.8 MPH. Sounds right to me. Butt dyno says I am at least 5 mph faster.

Just wanted to share some calculation fun, and lend some backhand credibility to VDyno. Data and analytic tools are wonderful these days.

 

[dyn085]

161 hp seems low for some reason, what was your hp spread between smoothing 1-6?

I took a brief look at the link and then realized that I was going to need to have some coffee and quiet to wrap my head around the math. I don't always maths good until I've done the calculations about 4k times.

 

[Siestarider]

Smoothing set on 3 shows peak HP 162.

VDyno allows three "dyno" settings, most folks use Dynojet setting, shows higher numbers. I use the VDyno setting, the 161 HP log in the Dynojet option shows 176 HP, about what I have seen posted for stock on Dynojets. Mustang option setting is lower still, 154 HP.

Just have to be careful what you are looking at when folks post VDyno HP.

In theory and in the math I used, if you lower your car 1" it will reduce frontal area by 1.7%, not that it matters much since velocity cubed is the big variable. I measured mine and its 57" tall, not 58" as shown in Wiki.

Just kidding about butt dyno BTW, mine lies like a rug.

 

[dyn085]

Ok, that completely makes sense, I'm just not used to anyone using anything but DynoJet numbers with V-Dyno. Not that there's anything wrong with that, just me making an incorrect assumption. I only ever use the DynoJet setting, but that's just me and it's only ever a general reference to compare data legitimacy and to watch for trends. Definitely not something I would quote in a signature, but still very simple math that can be used as a tuning tool.

For those that aren't familiar, this is a decent read on getting accurate V-Dyno readings-
http://www.evolutionm.net/forums/evo-dyno-tuning-results/619146-how-use-virtual-dyno-properly.html
Some day I would like to take my cars to get an actual dyno to compare, but it suits me perfectly for my needs currently. Whether I'm using V-Dyno or Excel, math is math.

Fwiw, my butt dyno is also highly calibrated-hence my constant quest for data...