Bryan;
Your experience is certainly typical. An engine with really fast throttle
response can rev faster than any tach (that I've seen on the market) can
follow. It's a matter of the tach designer's goal; if he is trying to make a
tach respond quickly, he can certainly do just that. I suspect that the
demand just hasn't been there for a fast- response tach. Speaking of slow
tachs, does anyone remember the old Jones- Motorola tach? If anyone has a
videotape of "Grand Prix", in the opening credits get a load of the tach
jumping all over the place when the throttle is being blipped. Those were
chronometric types and they were so bad at dynamic RPM indication that it's
a wonder they were used at all on Formula 1 or Formula 2 cars.
Regards, Neil Tucson, AZ
-----Original Message-----
From: Bryan A. Savage Jr [mailto:basavage@earthlink.net]
Sent: Monday, December 10, 2001 12:36 PM
To: Albaugh, Neil
Cc: List Land Speed
Subject: Re: Traction & Slipage
Neil, you have cut to the heart of the "driver - gage interface".
Without substantial "low- pass filtering (smoothing)" or dampening,
the gage is "faster than the eye can see".
This smoothing or dampening is what has generated the myth about revving
an engine without a load. I would set the rev limiter off (11,000) when
clearing/blipping the engine and the tach never went over 6,000.
How quick are electronics? Remember folks, OBD-II detects and identifies a
misfiring cylinder by a change in angular velocity in part of one
revolution. It then spends some time recording that information.
And that's cheap production electronics.
Bryan
"Albaugh, Neil" wrote:
>
> John;
>
> The problem isn't in the fact that they're "digital"-- it's the
fundamental
> design of the tach electronics & readout mechanism. Most tachs (analog or
> digital) are designed with so much low- pass filtering (smoothing) that
they
> can't follow fast engine RPM changes very well. I suspect this is done to
> make the display look more steady to the average driver. There is one
> limitation that affects some digital systems-- a fundamental limit imposed
> by sampling speed-- its Nyquist limit. There is no good reason to be
limited
> by this these days. Even very high resolution ADCs and DACs can be
obtained
> that have sampling rates approaching 100MHz. They can respond to RPM
signals
> as fast as 20 billionths of a second. I had an '85 'Vette and I agree with
> you regarding its video game display instrumentation-- flashy- looking but
> poor response.
>
> Regards, Neil Tucson, AZ
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