Happy 46R Posted April 16, 2023 Report Posted April 16, 2023 I purchased a 6 volt positive ground tachometer for my 230 cu in flat six. Car is all original and the reason for the tach is to see what my engine revs are at highway speeds when I can't hear the engine. While attaching the tach wires to the coil I noted that the Tach diagram is different than the cars actual wiring. On my car the single wire to the distributor goes from the negative terminal on the coil and the dual wires that go to the ignition system are on the positive terminal. The Tach diagram Is reversed. I cannot determine the correct orientation as neither my wiring diagram nor shop manual show the terminal markings on the coil. Another concern was that when I pulled the coil out of the car I found the manufacturers markings on the coil were on the bottom of the coil where they could not be seen so was the coil rotated 180 degrees at some point? I have not looked at the wiring in my car since purchase and wonder if it is possible the coil was hooked up backwards or is the tach diagram wrong. Any help is appreciated. 1 Quote
Sam Buchanan Posted April 16, 2023 Report Posted April 16, 2023 A good way to see this is to remember the wire going to the distributor gets "grounded" to the engine block when the points close. Since you have a positive ground car the positive terminal on the coil should be connected to the distributor. However, the engine will run either way, but wire it correctly just because. 3 Quote
Happy 46R Posted April 16, 2023 Author Report Posted April 16, 2023 Thanks Sam, great answer made it understandable right away. Followed your advice and wired coil properly which matched the tach companies instructions and car runs smooth with 675 RPM at idle. Thank you again. 1 Quote
Saskwatch Posted April 17, 2023 Report Posted April 17, 2023 Hey there Happy 46R ! Good to hear from you & that you’re back working on your Coupe …… would we be able to see a picture of that Tachometer & maybe give us the inside track on where you found that little treasure? …. I was under the impression that there was no such thing for our flatheads ?? ??♂️? 1 Quote
Sniper Posted April 17, 2023 Report Posted April 17, 2023 https://www.ebay.com/itm/195241938676 https://www.westach.com/product-page/3ct43-6v 2 Quote
D35 Torpedo Posted April 17, 2023 Report Posted April 17, 2023 2 hours ago, Sniper said: https://www.ebay.com/itm/195241938676 https://www.westach.com/product-page/3ct43-6v I like the westach. They obviously ranged it properly for our cars. Unlike the other ones on ebay that look like modified 12volt units....that rev to 8000.... 2 Quote
Sam Buchanan Posted April 17, 2023 Report Posted April 17, 2023 4 hours ago, Happy 46R said: I purchased a 6 volt positive ground tachometer for my 230 cu in flat six. Car is all original and the reason for the tach is to see what my engine revs are at highway speeds when I can't hear the engine. Another method for finding highway rpm is with a gear ratio calculator. https://spicerparts.com/calculators/transmission-ratio-rpm-calculator 1 Quote
Happy 46R Posted April 17, 2023 Author Report Posted April 17, 2023 It is just the right size to view through the spokes of the steering wheel and to not block the speedometer or control knobs. I could not find any markings on the differential housing and have as yet not had to re & re rear seals, knocking on wood as I type, so I wasn't able to calculate the engine speed as mentioned by Sam. My next best option was to touch base with P15-D24 member Loren who had mentioned one in her post and from there it was dropping big hints to my children with Christmas coming. We settled on WESTACH.com and Pete was really great in helping my daughter get the right unit. Nice thing about it is it is a simple spring clamp to install and two wires to hook up. Dave 2 Quote
Solution Sam Buchanan Posted April 17, 2023 Solution Report Posted April 17, 2023 Your service manual will have the transmission and final drive ratios for your car. If it is the cousin of the P15 that I have third gear is 1:1 and the rear end has a 3.91 ratio. Only other thing you need is to measure the diameter of the tires and rpm can be calculated for any speed. As a datapoint, in my P15 (no overdrive) 2548rpm is 55mph, 2780 rpm is 60mph and 78mph would be 3614 rpm which is where rated power is achieved (but that isn't a recommended cruise speed!). 1 Quote
Sniper Posted April 17, 2023 Report Posted April 17, 2023 I was reading up on the Australian Hemi 6 the other day and one of the quotes from it was that during durability testing which is basically wide open throttle at different RPM levels they used the coldest plug they could get. So if you're going to run 78 miles an hour in our flat head and I want to go with a colder plug Quote
Kilgore47 Posted April 18, 2023 Report Posted April 18, 2023 On 4/16/2023 at 5:57 PM, Happy 46R said: Shouldn't the ignition switch be between the battery and the coil - not the coil and distributor. Quote
Sam Buchanan Posted April 18, 2023 Report Posted April 18, 2023 26 minutes ago, Kilgore47 said: Shouldn't the ignition switch be between the battery and the coil - not the coil and distributor. Yep, I saw that, too. But since the original post doesn't involve the switch I didn't mention it. Quote
Kilgore47 Posted April 18, 2023 Report Posted April 18, 2023 2 hours ago, Sam Buchanan said: Yep, I saw that, too. But since the original post doesn't involve the switch I didn't mention it. I tried to ignore it but I just couldn't 1 Quote
kencombs Posted April 18, 2023 Report Posted April 18, 2023 4 hours ago, Sam Buchanan said: Yep, I saw that, too. But since the original post doesn't involve the switch I didn't mention it. I ignored it too. But a mistake like that causes me to wonder about the vendor quality. 2 Quote
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