PDA

View Full Version : Anyone have a suggestion to verify tach accuracy?



JayZ
06-10-2013, 11:14 AM
...I want to start prop testing an thought it would be wise to verify rpms. I am spinning the viper 21 up to just shy of 5000 and I would have expected more rpms from the 502 and 1.43 ratio King.

gcarter
06-10-2013, 11:29 AM
I don't know Jay. That sounds good to me considering you're spinning the propeller about 5% faster than if you had a 1.5:1 ratio like everyone else has.

Greg K
06-10-2013, 12:12 PM
...I want to start prop testing an thought it would be wise to verify rpms. I am spinning the viper 21 up to just shy of 5000 and I would have expected more rpms from the 502 and 1.43 ratio King.

Do you have a timing light with tach display to compare at different rpm's? There are tach testing meters out there, might be able to get from auto store for rent.

After I dropped a 502 in mine, my original '93 tach was off quite a bit, not so much at idle but as engine speed increased, error increased. I had a spare digital tach I was able to hook up to number one spark plug wire which had a max function to determine what I was turning.
I ended up with a Turbo 1 at 25" as my best prop, turning 5150 rpm and speed just over 75 mph. To get there, it was trimmed out pretty good.

Be careful and good luck.

gcarter
06-11-2013, 09:30 AM
But, back to your original question.....a good way to verify your tach is to use a Fluke model 78 automotive multimeter.
It can read engine RPM plus a bunch of other stuff.
There're several for sale on eBay currently. I think I paid about $100.00 for mine.

SanDogDewey
06-11-2013, 12:34 PM
I've got one of these from Harbor Freight. $39

http://www.harborfreight.com/automotive-motorcycle/diagnostics/digital-photo-sensor-tachometer-66632.html

Put a piece of reflective tape on your harmonic balancer and it works really well.