Tach Recalibrating

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Tach Recalibrating

Postby h-bot » Sun Mar 23, 2003 4:46 am

From: img width=12 height=12 border=0 src="http://opi.yahoo.com/online?u=dempsey2995&m=g&t=0" alt=""_REMOVE_929445_THIS_@yahoo.com

Is there any way I can recalibrate me tach myself? or is something I
should send in and have done? It's reading 500 PRM's to high on the
low end. The tach seems to be working ok with no other problems. I
have checked it out with a hand held tach. On top end Im also 500
RPM's to 700 RPM's to high as far as I can tell. Has any body had any
experience on recalibrating a tach?

Thanks for any help.

David




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Tach Recalibrating

Postby h-bot » Sun Mar 23, 2003 7:13 am

From: img width=12 height=12 border=0 src="http://opi.yahoo.com/online?u=doctorduke&m=g&t=0" alt=""_REMOVE_941062_THIS_@yahoo.com



CV tachs typically read high, particularly at the top end. A
competent speedometer shop should be able to improve the tach
calibration. They do this by removing the pointer and repositioning
it on the pin. I had this done a few years ago. I knew my tach was
accurate at the bottom end, but became increasingly more optimistic as
revs increased. I asked the speedomenter shop to make it dead on at
7000, which they did, and now it reads about 100 revs low in the mid
range and 200 revs low at idle.

Speedometer accuracy can also be improved for the shorter than OEM
tires we typically have nowadays. Short tires cause a "gain error"
that is a constant percentage throughout the range. A speedometer
shop can change the magnetic flux of the head magnet to eliminate gain
error, and any "offset error", which is a constant MPH error
throughout the range can be corrected by repositioning the needle on
the pin. Such recalibration only affects the speedometer. The
odometer cannot be corrected, so it will typically read 5 percent high
with 205/60R-13 tires, which roll about 920 revs per mile versus 873
for the OEM BR70R-13 tires.

Gain error in the tach cannot be corrected as with a speedometer.
Only an offset error can be corrected by repositioning the pin. Once
you have a accurate calibration table for the speedometer in its
present state, you can evaluate it and instuct the shop to relocate
the pin, and I suggest that you shoot for best accuracy at the top end
for obvious reasons.

Duke




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Tach Recalibrating

Postby h-bot » Mon Mar 24, 2003 3:24 am

From: img width=12 height=12 border=0 src="http://opi.yahoo.com/online?u=dempsey2995&m=g&t=0" alt=""_REMOVE_111948_THIS_@yahoo.com

Thanks for the reply Duke. I will see if I can find a speedometer
shop to calibrate the tach for me. I could live with a tach reading
200 revs low at idle, but 500 revs high at idle is a bit much for me.
As far as my speedo. Im running 205/60/R-13 tires with an 5 Speed. I
have the speedo dead on by using the Stewart Warner adapter with
a .9705 gear ratio installed. I had to fabricate a speedo cable so I
could install the adapter in line. By installing the adapter to the
trans it made the speedo cable make a to tight of a turn as it was
hitting the transmission tunnel.

David




--- In <a href="/group/cosworthvegas/post?protectID=219233066105193209050199029077192253163098100009128121188190230091025019053061151110147">cosworthvegas@yahoogroups.com</a>, doctorduke <<a href="/group/cosworthvegas/post?protectID=091233212180056219138097203245129208071">no_reply@y...</a>>
wrote:
>
>
> CV tachs typically read high, particularly at the top end. A
> competent speedometer shop should be able to improve the tach
> calibration. They do this by removing the pointer and
repositioning
> it on the pin. I had this done a few years ago. I knew my tach
was
> accurate at the bottom end, but became increasingly more optimistic
as
> revs increased. I asked the speedomenter shop to make it dead on
at
> 7000, which they did, and now it reads about 100 revs low in the
mid
> range and 200 revs low at idle.
>
> Speedometer accuracy can also be improved for the shorter than OEM
> tires we typically have nowadays. Short tires cause a "gain error"
> that is a constant percentage throughout the range. A speedometer
> shop can change the magnetic flux of the head magnet to eliminate
gain
> error, and any "offset error", which is a constant MPH error
> throughout the range can be corrected by repositioning the needle
on
> the pin. Such recalibration only affects the speedometer. The
> odometer cannot be corrected, so it will typically read 5 percent
high
> with 205/60R-13 tires, which roll about 920 revs per mile versus
873
> for the OEM BR70R-13 tires.
>
> Gain error in the tach cannot be corrected as with a speedometer.
> Only an offset error can be corrected by repositioning the pin.
Once
> you have a accurate calibration table for the speedometer in its
> present state, you can evaluate it and instuct the shop to relocate
> the pin, and I suggest that you shoot for best accuracy at the top
end
> for obvious reasons.
>
> Duke




[This is message #10710 by user img width=12 height=12 border=0 src="http://opi.yahoo.com/online?u=dempsey2995&m=g&t=0" alt="" on Yahoo! Group Cosworth Vegas: http://groups.yahoo.com/group/cosworthvegas ]
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Tach Recalibrating

Postby h-bot » Tue Mar 25, 2003 2:48 am

From: img width=12 height=12 border=0 src="http://opi.yahoo.com/online?u=vegaman76&m=g&t=0" alt=""_REMOVE_128051_THIS_@yahoo.com

--- In <a href="/group/cosworthvegas/post?protectID=219233066105193209050199029077192253163098100009128121188190230091025019053061151110147">cosworthvegas@yahoogroups.com</a>, dempsey2995 <<a href="/group/cosworthvegas/post?protectID=091233212180056219138097203245129208071">no_reply@y...</a>>
wrote:
> Is there any way I can recalibrate me tach myself? or is something
I
> should send in and have done? It's reading 500 PRM's to high on the
> low end. The tach seems to be working ok with no other problems. I
> have checked it out with a hand held tach. On top end Im also 500
> RPM's to 700 RPM's to high as far as I can tell. Has any body had
any
> experience on recalibrating a tach?
>
> Thanks for any help.
>
> David

Hi There David- There is a place in denver that works on tachs and
speedometers I'f I rember it's called deluxe speedo and tach I can't
rember the phone number and address for them but I had to send my
tach there to have it recalibrated for my 2.8 v6 in my 76 gt and it
works great if you want the address to it email me at
(<a href="/group/cosworthvegas/post?protectID=081056178237038031172026097248244063018152196241226130252055210">vegaman77@h...</a>) I i'll give it to you
Travis




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Posts: 53214
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