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Posted by Mark Palmer, Utah [216.241.207.126] on Thursday, January 21, 2010 at 17:25:23 :
In Reply to: tach-o-meters posted by Dave Royal [76.182.149.93] on Monday, January 18, 2010 at 22:47:54 :
Even after reading all your comments, I was still bewildered by this question. (Please excuse my slowness.) So I worked out this little mathematical model that I offer below for the easily confused (such as I).
What is clear is that the 8-cylinder motor (m8) fires 4 times per revolution (m8 = 4/rev) and the 6-cylinder motor (m6) fires 3 times per revolution (m6 = 3/rev). So the ratio of sparks of m8 to m6 is 4/3. What is not clear, however, is if the tachometer reading for an m8 that is connected to an m6 should be multiplied by 4/3 or 3/4?
To complete the model we ask: for every (1) m8 revolution, how many (X) m6 revolutions occur (1*m8 = X*m6)? Solving for X we get: X = m8/m6. Substituting, we get: X = 4/3. So the answer is to multiply the reading on tachometer by 4/3 (or 1.333).
When the tachometer reads 800rpm, the motor is spinning at 1067rpm (which is precisely what Will (in IL) said).
Thanks for the indulgence.
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