Tachometer Drive Ratio

[darrin bob]

Can someone out there advise what the ratio is between engine RPM and the tachometer drive cable RPM.  Thanks.

[DTort96646]

Are you asking what the ratio is for a normal setup or specifically for a Kaiser Darrin? I'm not being a wise guy, but the Darrin tachometer is nothing more than a speedometer with a tachometer faceplate. The ratio is different. For a four stroke engine the distributor turns at 1/2 crankshaft or engine speed. Aftermarket or I should say regular distributor ratios are 2 to 1 when using a mechanical tachometer specifically made as a tachometer. I have an AMC/Jeep V8 in my Darrin, and the 2 to 1 ratio tachometer drive distributor is the wrong speed for the Darrin tachometer. I have had to add a reduction gear drive adapter to change the ratio to correctly get the stock Darrin tachometer to read correctly. What are you trying to do?

[darrin bob]

I have a '55 Oldsmobile 324 engine in my Darrin with similar vintage Corvette generator which has a mechanical tachometer drive.  I need to know the correct original Darrin ratio between the engine RPM and the drive cable RPM, so that I can figure out the ratio of the reduction gear adapter that I will need to hook up the generator tachometer drive to the stock Darrin tachometer and obtain a reasonable tachometer reading.     

[DTort96646]

The easiest way to figure out the ratio you need will be to use an electric dwell/tachometer or even an old electric tachometer to determine actual engine rpm. Compare the difference with the rpm your current mechanical setup is showing. If the mechanical setup is lower than the electrical reading, then you need to increase the ratio. If it's higher, decrease the ratio. I used a speedometer ratio adapter, generally used on trucks to change mine. Example: if the electric reading is 1200 rpm and the mechanical setup reads 1000 rpm, you need to up the ratio 1.2 to 1. If the opposite is true, it's .8 to 1. The speedo adapter uses gears inside to get to ratio you need. It's all old school technology that's been around forever. Adapters are online, Ebay, or from some speed shops. Hope this helps.

[darrin bob]

Yup, that's the easy way. BUT, at this point I have a restored running chassis (that's good), the body is on a dolly, the tachometer is seized, and I don't have an electric tachometer (that's all not so good), so I'm back to ratios.  Just the ratio of the distributor shaft to the  tach drive will help plan what will be needed.  At this point, I'm just trying to get in the ball game.  Will probably wait till I get the tach (and the speedo) rebuilt, to actually purchase the ratio adapter, if needed.  Then I can do the final check "the easy way"!

The ratio will be helpful.

A recommendation on a good speedometer shop to rebuild the tach and speedo would be helpful.

Thanks Bob   

[DTort96646]

I don't remember my ratio, but it probably would be best to wait until everything is together and running. Then you can get a better idea what you will need for a ratio. The distributor turns at 1/2 engine speed. If you are having you tachometer rebuilt, it can probably be set correctly as a tachometer not a speedometer. The magnets in the head will have to be replaced and calibrated to a tachometer. The engine runs at 1000 rpm and the distributor will turn at 500 rpm and should indicate 1000 rpm on the meter.

[darrin bob]

Thank's for your input.  It was certainly helpful.

If anyone out there has a distributor on the bench and would turn it a couple of revolutions  and count how many revolutions the tach drive turns, I would still be very interested in that information.

Thanks again.  Bob

[Terry T]

it's   1 t0 1

[darrin bob]

Terry:   Thank you very much.   Bob

[darrin502]

I have a 1954 331 Cadillac with in my Darrin with corvette generator with a tach drive, I took my tach to a spedo shop and he calibrated it with no problem.
the cable from a Corvette is not the same as a Darrin tach It will not screw on, I took the original nut cut a slot in it just wide enough to get off the Darrin cable and slid it on the GM cable and it works fine there is enough thread to hold.
In my other Darrin I had the original Darrin tach converted to electric they were able to put in an electric movement in the stock tach.
Lee


 

[darrin bob]

Lee, thanks for the additional information.

Bob