I was curious how to know my engine’s revs at idle. I tried a HF “photo” tach but was disappointed in ease of use and accuracy so I calculated my own numbers with simple math.
I measured the outside circumference of the crank pulley where the belt runs, the outside circumference of the fan belt and found the drive ratio. In my case the engine turned exactly 2.71 times for one full revolution of the belt.
Then I painted a little white mark on the belt…
Belt mark.jpg
and I count how many revs it makes in one minute (or, for simplicity, count 30 seconds and double). It’s a lot easier to count the belt revs since it runs about 2 ¾ times slower than the crank.
In my case here are the engine revs (rounded off), YMMV.
Belt Engine
140 380
145 395
150 410
155 420