Well Stu, you know how long winded I am, I figured I'd written enough last night. I mean I could go on forever but I do gotta sleep once in a while! But now that ya woke me up here goes.
Yep, the trigger signal doesn't seem to make a whole bunch of difference. 4ontheFloor predicted the results and pretty much the direction that I took earlier on page one. I’m not gonna repeat all of it again here. I already mentioned that rather than mess with trying to find a 6v bulb so I could control it with the dash lights that I just wired it in with the 12v hot wire that drives the tach so it’s on whenever I have the tach switched on. Oh and I believe that it wants about 500ma to power this up.
So how do you check the calibration of your new tach??? Well just off hand I figured about all I could do was go to a calculator (this is one of the best I’ve found)
http://www.angelfire.com/fl/procrastination/rear.html and using the rolling rear tire diameter I discussed a few weeks ago, (that be 29.68”) make a prediction chart up.
I started at 20mph in high gear with five mph graduations up to 55mph. The tach seems fairly close all the way up. One thing that you have to conceder is that we’re working with a cheap ($30) plastic tach. It may say SunPro on the face but this isn’t a Sun instrument by any means. Also this is a tiny tach, and where the ads may say 2 5/8” however that’s to the outside of the bezel. The face is actually 1 7/8” across so the resolution is not really high enough to give terribly accurate readings. That’s one of the reasons that you always see those 3 1/2” dials on most real tachometers, the larger circumference gives you a higher resolution. Since I’m not going to leave this in permanently I probably should have spent the extra $10 and gone with the larger face even though it would have blocked out some of the instruments.
As I’ve been doing for 50 years I’ve just been shifting by sound and feel. In this truck that equates to 15mph for number two gear, 30mph in number three and I try to hold her down to 45 or 50 in fourth. According to the tach the shift points correspond to roughly around 2500rpm and I’m running about 2400 to 2600 in high. My idle reads right around 500rpm. I did take it up to 3000 in second and third and that seemed to sound right. Where my predicted rpm was 2850 at 55mph when I ran her up there (and shes a screamin’ at 55) my tach reading was low by a couple of hundred rpm, again this may just be the low resolution of the small dial face. Something else to consider is the update time on a cheap tach, which can affect the readings.
All in all it was an interesting little experiment, and in retrospect I should have bought the bigger face if I wanted more accurate readings.
Denny Graham
Sandwich, IL
