<all> home dyno kit??

[Roger Heflin]

On Fri, 29 Jan 1999, Mike wrote:

> >
> >That is mostly what the home dyno kit does.   You collect the
> >information off of a spark plug, and then use that to calculate
> >acceleartion based on the rpm changes.  The program is setup to record
> >the data as a audio signal and then later process that audio signal on
> >a computer to get the info out.
> >
> 
> That'll teach me to read (more accurately "peruse") only a few posts on a
> thread then post a follow-up...
> 
> Still, I'd have thought that doing this measurement in the "digital" domain
> with a timer resolution of 500nS or better would produce better results than
> in the audio domain with the associated noise and distortion etc.
> 
> After looking at the web page though I see the results look pretty good
> though so things can't be that bad in audio-land. Neat.
> 
> 

I would guess that the trigger point or the center of a given spark
(measured in the audio-domain) is probably centered pretty
consistantly at the same point, and since you don't have to do things
real time you can take some time to analyze and determine exactly were
each pulse was centered.  And even if they varied a  bit, the average
should be pretty good.  I did some quick calcs, and 3000 rpm the pulse
spacing is 20 ms, and 3025 rpm is 19.83 ms (DIS), so if you have .1 ms
resolution you would be able to get better than 25 rpm accuracy at
this point.   And ao long as you have enough bandwidth to be able to
determine each pulse you should be able to do it.  My guess is that a
basic cheap take recorder should give you enough data to do the job.
A 6000 rpm pulse is 100 pps (DIS) or 50 pps (fire every other rpm), so
to get accurate and distinct centers you would not require that much
bandwidth (ie 1-2khz looks to be adequate)

				Roger