Good Eve,

I need some advice. I picked up a cherry Vintage Sun Super Tach model SST-802 at the Bowling Green Hot Rod reunion.

After looking up the model number and reading the back (8 cyl) Can this tach work on my 1966 inline 250? or do I need to sell this and go for a re-pop with a 4,6,8 cyl switch?

Thanks in advance!

Shad

Hi Boucher . . .

Technically speaking the SST-802 is too fast/high-freq for a 6 cylinder application. But that is not to say it is entirely unusable.

Transistorized tachometers are not very complicated gizmos in principle – essentially they ‘count’ ignition pulses – and display that count on a ‘scaled’ sweep-face. The SST-802 sweeps 0 – 8000 RPM assuming 8 cylinders.

Think about a 4-cycle engine in operation. With 8 cylinders a 4-cycle engine will produce 8000 TDC events at 1000 RPM – 4000 of those will be spark events. This is what a transistorized tach is counting. So your SST-802 Tach head will display 1000 RPM when it detects 4000 sparks per minute and will sweep to the max at 32000 sparks.

But a 6 cylinder engine has two less plugs. So 1000 RRM is just 3000 spark events. If you attach that SST-802 to your 250 it will read 750 RPM when you are actually turning 1000. You could get a new decal for the sweep face that reads 0 – 10000 and the tach would read essentially true . . . but half the dial would go unused on a 250 – and it would be hard to read the difference between 1200 and 1500 on the face.

A 6 cylinder tach head is designed and scaled for lower frequency of spark events – typically sweeping 0 – 5000 or 6000 RPM - from 15 to 18000 spark events.

Regards,
stock49

Stock 49,

thank you for the clear response! That totally makes sense and I will shelf this one for the future.

Boucher