Brain teaser Alternator Pulley ratios to RPM

[geoleo]

I put a 2.5" dia pulley alternator on my perkins prima 50 M50 engine in place of the stock alterator which had a 3" dia pulley. So the alernator makes the RPMS on the gage read fast I believe. I think it is either 18% or 20% faster. (So if the gage is reading 2000 rpm its actually running at 1600? )Who knows the correct number?

[SailingHarmonie]

Originally Posted by geoleo

I put a 2.5" dia pulley alternator on my perkins prima 50 M50 engine in place of the stock alterator which had a 3" dia pulley. So the alernator makes the RPMS on the gage read fast I believe. I think it is either 18% or 20% faster. (So if the gage is reading 2000 rpm its actually running at 1600? )Who knows the correct number?

It is correct that you will be reading faster on the tach than you would with the stock pulley. To actually calculate the difference we would need to have the diameter of the drive pulley as well.

Most tachometers I am familiar with have a calibration on the back that allows you to fix this. Get an inexpensive laser tach, measure actual engine RPM and tweak the calibration screw until the dial matches.

[Bowdrie]

Assuming the 2.5" and the 3" are actual pitch diameters the speed of the engine with the new pulley is ~.833 times the speed that the tach says.
Be that as it may, inconsistencies in belt manufacturing as well as belt wear and the tension applied all work to change the reading an alternator driven tach will show.

[Wotname]

Originally Posted by SailingHarmonie

......

Most tachometers I am familiar with have a calibration on the back that allows you to fix this. Get an inexpensive laser tach, measure actual engine RPM and tweak the calibration screw until the dial matches.

^^ THIS

Even cheaper than an inexpensive tach is a free tach app for your smartphone. There are a few around.