Tachometer calibration needed

[Cliff Beer]

I installed a SBF 347 in my old 1988 Lotus Esprit a few years ago (I did take the Lotus engine out first...). The conversion took a while to say the least.

The car runs well, pretty much as planned/expected. Ignition and fueling is by way of an 8 stack downdraft system using FAST 2.0 ECU, FAST duel sync distributor and FAST E6 CD ignition box. I'm ironing out a few details to get the car finally "completed." One of these details is that the Lotus tachometer reads exactly twice as fast as it should (no, that's not 8,000 rpm.....). I'm pretty sure this is because the Lotus tachometer was calibrated for a 4 cylinder engine...while the tach signal wire out of the E6 ignition box is delivering an 8 cylinder signal. In other words, twice as many voltage pulses per XX rotations. The Lotus tachometer is the standard VDO 80's tachometer.

I'm wondering what my best options are....? Get a VDO tachometer from a later V8 Esprit (2004 or later)? Somehow adapt/calibrate the tach signal to cut the signal in half? I don't believe there's any adjustment or calibration function built into the ignition box or ECU.

Anyone have any experience with this kind of thing before?

Thanks in advance.

 

[Neil]

I installed a SBF 347 in my old 1988 Lotus Esprit a few years ago (I did take the Lotus engine out first...). The conversion took a while to say the least.

The car runs well, pretty much as planned/expected. Ignition and fueling is by way of an 8 stack downdraft system using FAST 2.0 ECU, FAST duel sync distributor and FAST E6 CD ignition box. I'm ironing out a few details to get the car finally "completed." One of these details is that the Lotus tachometer reads exactly twice as fast as it should (no, that's not 8,000 rpm.....). I'm pretty sure this is because the Lotus tachometer was calibrated for a 4 cylinder engine...while the tach signal wire out of the E6 ignition box is delivering an 8 cylinder signal. In other words, twice as many voltage pulses per XX rotations. The Lotus tachometer is the standard VDO 80's tachometer.

I'm wondering what my best options are....? Get a VDO tachometer from a later V8 Esprit (2004 or later)? Somehow adapt/calibrate the tach signal to cut the signal in half? I don't believe there's any adjustment or calibration function built into the ignition box or ECU.

Anyone have any experience with this kind of thing before?

Thanks in advance.

You could use a flip-flop to divide the pulse rate in half. Old versions of CMOS logic will work on 12 volts.

 

[Cliff Beer]

Hi Neil,

Thank you.

Do what? Huh? I think I get the concept, but what's a flip flop?

 

[Neil]

It is a digital circuit that can exist in only two states- flip or flop. Seriously.

 

[David mono]

One of my aftermarket tachs has a 4cyl 6cyl and a 8cyl switch , maybe of some use if you find one to wire in.

 

[Bob Putnam]

If you're in the US, Nisonger might be able to help.

Nisonger Instruments :: the Exclusive U.S. Distributor, Warranty & Service Center for Smiths & British Jaeger Instruments since 1949,

Nisonger Instruments is a full-service supplier of Smiths Instruments gauges and high performance components and trim items for Shelby Cobras and other high-performance applications.

nisonger.com

 

[Ian Anderson]

Google suggested this

UNIVERSAL TACH ADAPTER / CONVERTER, COP-1-2-3-4-5-6-8-10 cyl to any 4-6-8 Tach | eBay

This sturdy device is perfectly comfortable living on top of a COP coil or on the bottom of a lawnmower. The number of pulses per revolution is all the info you'll need. In most COP applications, 3 of the 4 wires can be tapped straight off the handiest coil, with the adaptor mounted right on top.

www.ebay.co.uk