Question for you technical buffs
 

I recently found out that only not all 6 cylinders fire in one 360 revolution of the flywheel. Good ole wikipedia...

So my question is, for the RPM reading on the tacho, is it counting the revolution of the flywheel or the revolution of the entire engine per minute?

thanks,

Engine rpm is the same as the flywheel rpm. In a 4 stroke engine, each cylinder fires the compressed mixture on one revolution and expels the exhaust on the next revolution. Only 3 cylinders fire during one revolution. The piston goes down on the intake stroke, up on the compression stroke(ignition occurs here), down on the combustion stroke and up on the exhaust stroke completing 2 revolutions.

I'm not sure if the tachometer is reading via the crankshaft position sensor, or what it's driven off of, but I do believe it would be measuring one rotation of the crankshaft (or flywheel) and noting that as a revolution.. not cutting it in half for each firing.

If you think about it, the engine is still doing a full revolution, there are just explosions in 3 cylinders at a time, but the parts are still moving, doing their job expelling the exhaust gases..

Good on ya mate, makes sense.

In a four cylinder engine would it be 2 cylinders firing in one rev then woudln't it? Explains why the v6 is so much smoother and why the v8's like butter...

You are correct that the number of cylinders has a lot to do with how smooth an engine is. The configuration of the cylinders has a lot to do with it, too. An inline 4 cylinder is just a very naturally unbalanced configuration. So is a 60º V6. The V6 is definitely smoother simply because you have overlapping power strokes. But if you look at a boxer 4 cylinder, it's still a 4 cylinder, but it's naturally balanced, just by how the cylinders are laid out and a lot smoother. It really takes some effort by auto manufacturers to make a smooth running V6...counterweights and ballancing and such.

The only inherently balanced engines are inline 6's, flat 4's and 6's, and V8's...at least as far as common engine types are concerned.

Late,
Trav

gotta love that boxer motor! :p

I believe that the ECU gets its tach signal off of the flywheel, and uses the cam sensors to determine which cycle a given cylinder is in.