in the shop yesterday just fiddling around, had a mechanical tach drive magneto in the distributor machine and thought I'd spin the three tachs I had laying around, I knew they all worked but thought I'd see how they compared to each other and the tach on the machine itself. I was surprised at the results..
Hilarious...welcome to my life, over the past few years. If you're gonna run an old mechanical tach, build a stout engine.
I was wondering the same thing. I expected the needle bounce at idle, always kinda liked that, it's kinda like blower surge. I was told when I was a young guy that was cable whip, and it only happened at steady rpm, no idea if that's true or an old racers tale. But I was SHOCKED that the second tach tested read 1000 rpm low at 7000. Man, I am putting an original late fifties Corvette mechanical tach in my T Tudor with the 283, was planning on running it to 7000, if the Corvette tach reads a 1000 rpm low, I'll be cleaning it up with a broom!! I also have one like the first SW tested, and a moroso like that for my FE.
The shop I cut my teeth in did speedo repairs (he NEVER rolled any back ). He explained to me that all speedos and tachs were not accurate due to torsionial flex of the wound cable. Just sayin what he told me. Pretty interesting Oldmics
I was just in the shower thinking about this and thought I had observed something about the second tach. Came back and watched the video again and confirmed it. The second tach doesn't show any needle bounce to speak of at low rpm, which in my experience most mechanical tachs do. That, combined with the fact that it reads so low, makes me wonder if there's a buildup of gunk in the cable, or possibly in the gear drive at the back of the tach. Just a suggestion, but I might repeat the test and check for heat buildup either in the cable or the gear drive at the back of the tach. Does it feel stiffer than the other two when you rotate the end of the cable by hand? If it is full of gunk, and that's exerting a drag on the cable or gears, one of two things could happen if you run it on a vehicle. Either it will loosen up, or it will probably snap the drive cable in the first 10-20 miles...
the first tach is as found and has a new cable the second tach was billed as restored, it also has a new cable the third tach and cable are as found the tach in the machine is also as found.
very interesting. was the same cable used on all the tachs tested? i have found that having the cable as straight as possible reduces needle bounce. i also clean the cable and housing then use graphite before re***emble. i wonder how the test would go using a single fresh cable. this video makes me want to test mine in a similar manner.
all three tachs have their own unique cables. the head ends are specific to each tach and are not interchangeable.
Even a slight kink in the cable will cause bounce. And the cables are NOT making the tachs read wrong... SW tachs are magnetic, like a speedometer. The Moroso (Jones Motrola) is centrifugal. The SW tach I have from the 80s usually works pretty well, but I left the cable a tiny bit loose at the tach end once, and I think it messed up the bearing. Also if the center part of the cable is a tad too long, it will eat up the tach. The Jones tach I have got sticky...it usually works ok if RPM changes frequently, but on a long drive, it will stick in one spot. Not good. It would be fun to do a similar comparison with half a dozen electronic tachs, of varying ages.
I think I have a few electronic tachs, at least two SW and one Dixco and was thinking of running them, just to see what they do..
my experience with most older electric techs is that they " overshoot " when the rpm climbs quickly, that won't show in this style of test.
Well this is interesting, I had been planning on running a cable tach on my roadster. Any recommendations or advice before I purchase? I had been looking at used SW or a Jones. Sent from my iPhone using The H.A.M.B. mobile app
buy several old mechanical tachs, and find a buddy with a distributor machine, or get a good name brand, new, electronic tach also. Do your own comparison testing. Expect to spend money. Sell the tachs that don't read properly on ebay "as is" "for display use"
I was mesmerized by it all, as well as surprised. I'd love to see how an electric 60's Sun tach with an EB-9A sender would compare. Thanks for taking the time to do that.
Dad ran a bunch of different electrical tachs on his distributor machine and found about the same thing as with the mechanicals. Not very many of them read the same. The top of the line Mallory's back in the early 80's read right on with his calibrated tach, we had three of them for different cars and all three read dead on. The off brands were all over the place. If you rig a way to hold the tach in the machine and short cable drive, you could compare the actual tach heads, by removing the gear train and long cables from the picture. This would confirm its the cables, gear train, or tach head thats causing the fluctuations.
ah HA--- so my 11,500 red tail tell needle tach drive was in all actuality only 9,500 oh well It all MAKE's FOR A GQQD STORY...LOL
I thought about that, but didn't see an easy way to get the bezel back on after I probably destroy it during removal.
I had a sun Super Tach in my old V-8 pinto. I used to be able to make extra gas money by betting guys it would "rev past 8000",total ********, it was more like 7200 and 1000 rpm of overshoot, but most guys don't know that.
The Jalopy Journal
