Re: [AMC-list] engine rpm versus mph

[tom jennings <tomj@xxxxxxx>]

Yeah, I'm with Mark on this one. Old tachs never seem to work right. The
meter movement bearings get stiff, caps get old, circuits drift. Most of
them are crap electronic designs, even supposedly good tachs, as components
like transistors were expensive then. An old Moon tach I had had ONE
transistor in it. They assume points as the signal source. Some of the other
old tachs I had had adapter boxes just to connect to CDI ignitions etc. I'm
fairly sure modern tachs, even cheap ones, have integrated circuits with 10,
50, 100 transistors and do a good job, look for ignition timing info in a
more robust way and don't use DWELL as part of the RPM-to-voltage
calculation (which is what fouls up the old tachs).

It's a bummer cuz the old tachs looks great, and most new tachs look like
gaudy candy boxes.  SW has some decent new tachs though, I have one in my
American that looks completely vintage, 270 degree sweep and spot-on
accurate (my two DVM type tachs and the SW all agree, at least :-) I even
got a 6000 rpm job for the 195.6OHV. Got it from Summit.



On Tue, Apr 12, 2011 at 08:57, Wrambler <wrambler242@xxxxxxxxxxx> wrote:

> Sounds like the tach and dist are not getting along.
> With a tach that old I have never had one work correctly.
> I have an Autometer I bought circa 1979 and it is way wrong.
> Had a stewart warner tach that did the same thing.
> It was like they were running through molasses and then they would bounce
> up
> only to drop back down.
>
> If you've got access to a spare I'd ship it off for a thorough redo and
> calibration. Maybe have them swap in new guts? Then it would work with
>  most any ignition.
>
> As for accurate mph, most any GPS will show you that, I have two apps
> on my android that show speed and while they have a slower reaction time
> then my speedo in the Jeep they are dead on accurate.
>
> I've been meaning to send out my Sport cop speedo to have it calibrated,
> but always forget till spring is here.
> It reads high on the speed and low on the odometer! No way to calibrate
> that out since it is electronic and you
> program it off of marked 2 mile stretch.
>
> Mark Price
> Morgantown, WV 26508
> 1969 AMC Rambler, 4.0L, EFI, T-5
> 2004 Grand Cherokee Laredo, 4.7L, Quadratrac II
> "I realize that death is inevitable.
> I just don't want to be around when it happens!"
>
> ----- Original Message -----
>
> > From: "Joe Fulton" <piper_pa20@xxxxxxxxxxx>
> > To: "Rambler AMC, Nash, Jeep and family" <amc-list@xxxxxxxxxxxxxxxxx>
> > Sent: Tuesday, April 12, 2011 11:33:44 AM
> > Subject: Re: [AMC-list] engine rpm versus mph
> >
> > Another thought?  In addition to the "bouncy" tach, are you really
> > doing 70
> > mph?  Do you have the correct speedo gears?   Have you run a measured
> > mile to
> > check?
> >
> > Joe Fulton
> >
> >
> >
> > ----- Original Message ----
> > From: Sandwich Maker <adh@xxxxxxxxxxxxxxxxx>
> > To: amc-list@xxxxxxxxxxxxxxxxx
> > Sent: Tue, April 12, 2011 6:03:06 AM
> > Subject: Re: [AMC-list] engine rpm versus mph
> >
> > " From: "Armand Eshleman" <aje1@xxxxxxxxxxxxxxx>
> > "
> > " []
> > "
> > "
> > " 2,87 axle ratio = 1630 rpm
> > "
> > " []
> > "
> > " the curious thing about this information is the tach (factory in
> > dash) for
> > " the white 70 Javelin reads about 2100 rpm at 70 mph using the 2.87
> > axle and
> > " T5 in 5th gear.
> > "
> > "  Apparently there are issues with the tachometer. That becomes very
> > real
> > " when noticing the tach with bounce up 300 to 500 rpm without using
> > " additional throttle.
> > "
> > " I'm using a MSD Pro Billet ready to run distributor that needs no
> > translator
> > " box for the tach.
> >
> > possible it's not really suitable for -this- tach?
> >
> > " Makes me think the electrics for the tach are a bit unstable or the
> > system
> > " ground is intermittently or always a bit resistive.
> > "
> > " Got any ideas?
> >
> > first thought - after all this time, the tach needs recalibration.
> > components age and drift, contacts get dirty...  if it has
> > electrolytic capacitors, they don't age at all gracefully and
> > probably
> > need replacing.
> > ________________________________________________________________________
> > Andrew Hay                                  the genius nature
> > internet rambler                            is to see what all have
> > seen
> > adh@xxxxxxxxxxxxxxxxx                      and think what none
> > thought
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