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Dyno curve physics - Results and a generic spreadsheet for everyone



On 9/28/05, GGehrke <ggehrke@gmail.com> wrote:
>
> Nate & all,
>
> I have come to the conclusion that you should shift from 1-2 at
> redline, 2-3 at 55 (nearly redline), 3-4 at about 75 mph (6500rpm) and
> 4-5 at 95 mph (6100rpm) for best acceleration.
>
> Disclaimer: The professor just taught us how the concept works. Other
> factors, particularly the time it takes to shift, will affect this
> too.
>
> The idea behind all this is that from the dyno graph of Torque vs.
> RPM, with the car's other information like weight, tire size and gear
> ratios, you can get acceleration. If, for each gear, you find the
> acceleration for each data point in the dyno curve and the
> corresponding speed for that engine speed, you can get a set of
> acceleration curves from about 10mph to wherever speed your gearing
> would run out at. These 5 (or 4 or 6) curves can all be plotted on
> the same graph, and where they intersect are your best shifting
> points. If you shift too soon, what you'll usually see is that you
> are accelerating at, say, 0.8g in 2nd gear and shift (at the hp or
> torque curve peak) to 3rd where you accelerate at 0.6gs at that same
> speed. If you held 2nd a little longer, the acceleration dwindles,
> but you're still doing better than the acceleration curve for the next
> gear. Where the two meet, you can make a smooth trasition and keep
> your acceleration up as high as possible. Another way to think about
> it for the math geek types is that the area under an acceleration
> curve (integral) would be speed. You want speed so you want area
> under the curve. Therefore, you always want to be in the gear that
> would have the highest acceleration associated with it.
>
> So that was just a mush of ideas. If you care, it should give you all
> something to think about.
>
> We are going to keep talking about this in class for the next few
> weeks so when I keep getting more info, I'll probably put together a
> technote of some sort with pictures and downloads and everything.
> Until then, if you want the spreadsheet that I used to compute Nate's
> data, drop me an email and I'll send it as an attachment. All you
> have to do is fill in weight, tire size, gear ratios and data points
> for the torque curve from a dyno graph. Magically, a graph of your
> acceleration curves will appear. Unfortunately you do need a dyno
> graph for this to work.
> What's fun is that you can play with different settings such as seeing
> what would happen if you changed your transmission or differential
> gearing or dropped weight from the car. For example, if nothing else
> changed but Nate could get his car down to 1700 lbs, he could hit 1g
> of acceleration in 1st gear. It's interesting stuff to dork around
> with, and it could potentially make you faster through the gears.
>
> -Grant-


Grant sent this out, but it seems the attachment made it only came to me. He
said he was going to retry without the attachment (the excel sheet he
mentions) but I'm impatient and don't want to wait for that. Anyway, the
excel sheet he has is pretty cool. From playing around with it I've learned
that if I can get my car down to 500 lbs, it'll be really friggin fast ;)
But in all seriousness, this spreadsheet is a really cool thing to check out
for your car. I'm sure a lot of you have dynos for your cars, and if you do
this will give you a great tool to not only see where you're at as far as
power in all gears, but also where you could be if you made some changes. If
you're not a math kind of guy/girl, then what Grant says about where you
should shift is true (where the graphs for each gear cross, unless you want
to get really anal, when you should account for shift times and therefore
shift a little bit after the optimum shift time... how much after depends on
how fast you can shift, but I'd think for the average person it would only
be a couple hundred rpms after the optimum shift point).
Anyway, good stuff Grant, it gives me the leeway to play with my torque
curve to see what I could get with a flatter curve (upon my return to the
east coast I found a vaccume leak that was causing some running problems,
I'm hoping that this was a contributor to my torque curve not being very
flat.)
Anyway, lets all post our optimum shift points so we can all compare.