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Tom Deutsch

RPM measurement

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Tom Deutsch
How do I fairly accurately measure the rpm on a single or twin cylinder engine? I have an old needle gauge, but it is only set up for 4,6 or 8 cylinder engines. Do I just hook it up, set it for the 4 cylinder then divide the rpms by 4? that would be fine for idle, but I want to test to make sure my engine is turning at the 3600 rpm at full throttle -- I doubt the gauge is accurate up to 14,400 rpm! Is my math all wet? Is there a way for me to do this without spending big bucks on an instrument?

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PhanDad
Tom, If you meter would work on a single or twin, I believe you'd have to MULTIPLY the reading by 4 or 2 to get the correct rpm. At 3600 rpm, a 4 cylinder, 4 stroke engine would fire 7200 times (3600/2*4). A single cylinder engine fires 1800 times at 3600 rpm, so the meter, counting the fire rate, reads low. My old dwell tach wouldn't work on a single cylinder engine with magneto, so I bought one of the "Tiny Tach" type of tach/hourmeter to determine rpm. Found a deal on one for $25 a while back.

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HubbardRA
Bill is correct, that you multiply by 2 if the tach is set on 4 cyl. I actually had a friend set up one of those tachs to run on a 1 cylinder engine. All he had to change was a resistor. I use one of the vibratach units that I purchased from a Tecumseh site on the internet. Just can't find the listing at the moment. I think Briggs & Stratton may also sell them through their service tools area. I think they only cost about $15 and are accurate for both 1 and 2 cylinder engines. All you do is hold the vibratach on top of the engine, or even the tractor hood, turn the knob till you see the maximum vibration of the wire, then read the rpm off the scale on the tach. It is really that simple and they appear to be very accurate.

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firefoxz1
One correction to PhanDad's post is that a single cylinder (or most) fires every RPM and not every other RPM as most ignition run off the crank not the cam. If you have one like I use that reads the plug wire you have to divide by 2.

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HubbardRA
Firefoxz1, It really is multiply by 2. A four cylinder engine fires only time per cylinder for each two revolutions, which means 4/2x2=2 times per revolution. A one cylinder, as you said, fires only one time per revolution. This means a one cylinder fires half as many times as a four cylinder at the same speed, so it would produce half the reading. This would require the tach reading to be multiplied by two to give the correct speed (rpm).

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MikeES
I have found that the Tiny Tach for the B&S magneto or Kohler electronic ignition engines work great and are very accurate. Using the same tach on a Kohler battery ignition (K-engines) does NOT read 1/2 of the rpm. I cannot find a correlation to actual rpm (measured with digital mechanical tach) and the Tiny Tach readiing, and it is not repeatable.

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HubbardRA
The two cylinder engines, with the double coil, also fire only once per revolution, but fire both legs of the coil. This means they would hook up same as a one cylinder.

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