To calculate power, all it needs to know is the speed your car reached
and how long it took to reach that speed. Theoretically, it doesn't even
need to know the engine RPM if it knows the speed of the car.
But since speedometers typically have 5% of error and are harder to plug
into, reading RPM via an inductive pickup off the spark plug is the
easiest way. The only reason it needs to know the gear ratios and tire
sizes is so it can compute the car's speed from the engine RPM.
And yes, accurate weight is essential. But even if you know the exact
weight, the power number you get will be lower than what your car would
measure on a dyno for rwhp -- because of drag. There's probably just a
percentage they would use as a rough estimate, I would guess in the
neighborhood of 4%. That is, if you measure 200 hp with this thing, you
probably would measure 208 on a chassis dyno and you probably have 240
at the crankshaft.
It would be quite useful to get repeatable comparisons from one run to
another. Then you can test whether that K&N filter really did anything,
or whatever else you might have changed.
But keep in mind that the engine's power output will vary depending on
atmospheric conditions. Having done this before, I've seen differences
of 5-10% in power output from a cold morning to a hot afternoon (234 hp
vs. 263 hp). This is more than enough to throw off any comparison you
might want to make, so the only way to get comparable measurements is to
make them back to back at the same time on the same day in the same
place.
> -----Original Message-----
> From: owner-ba-autox@autox.team.net
> [mailto:owner-ba-autox@autox.team.net] On Behalf Of Jerry Mouton
> Sent: Wednesday, March 27, 2002 18:34
> To: Bay_Area_Autocross_List
> Subject: Fw: [mrpca] dyno software & equipment (fwd)
>
>
> Intriguing... This could be made to work, I think...?
> Very accurate RPM at every two revolutions from 1000
> RPM to redline, plus weight and gearing and air density...
> I think you could calculate HP and torque from that, no?
> Accurate weight would be necessary, though. And drag
> would only be approximate, if you figured that in.
>
> Jerry
>
> Jerry Mouton mailto:jerry@moutons.org Laissez les
> bons temps rouler!
>
> ----- Original Message -----
> From: <creif@att.net>
> To: <jerry@moutons.org>
> Sent: Wednesday, March 27, 2002 4:44 PM
> Subject: [mrpca] dyno software & equipment (fwd)
>
>
> > FYI...
> > ---------------------- Forwarded Message: ---------------------
> > From: "Brian Scudder" <brian.scudder@mindspring.com>
> > To: "Mrpca" <mrpca@yahoogroups.com>
> > Subject: [mrpca] dyno software & equipment
> > Date: Wed, 27 Mar 2002 10:49:54 -0600
> >
> > Y'all,
> > I ran across this link. They claim it is a dyno. You
> > hook the inductive lead to a spark plug, do a 2nd gear
> > 1000 rpm-redline run, download the data, enter a few
> > things about your car (weight, gear ratios...) and
> > weather. Then it will print out your torque and hp
> > curves.
> >
> >
> > http://www.charm.net/~mchaney/homedyno/dynokit.htm
> >
> >
> > This is certainly alot cheaper and convienent than a real
> > chassis dyno. You could do a baseline dyno run, change
> > sparkplugs then do another, just to see if the new plugs
> > help. I wonder if it is as accurate as they claim. Any thoughts?
> >
> > Thanks,
> > Brian
> >
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