Being Picky, but how to measure a "rough idle"

[john8662]

Dr. Lee,

Just out of curiousity, how would one technically measure vibrations in a vehicle? The reason being is I'd like to compare how rough and engine runs from one to another. Just to know a baseline of what's "good" and what can be considered bad.

I know it's a picky thing, but I can't think of any inexpensive instrumentation that could be used for the task.

This could also be used to compare changes made to the engine that either improved or made the engine run rougher (timing, etc.).

Perhaps you can also ask the CleviteKid, since he might know of a method as well!

J

[Robyn]

OK
I have done engine ballancing many times on street rigs as well as for race engines turning Mega RPM's
Obviously the race engine needs to be ballanced very close to allow it to venture into the arena of very high RPM for which the designers cringe at the very thought.
For the street driver we do basically the same thing other than we most likely are not going to take the factor quit as tight for a low RPM streeter.
All reciprocating parts are weighed on a scale and pollished,ground, cut or whatever to make them all the same. Pistons, rods, everything that is part of the rotating mass.
Then a set of wieghts that can be bolted to the crank journals are mounted representing the rods and piston assemblies. The ballancer and flexplate/flywheel are afixed too. Now we spin this thing in a set of bearings with measuring devices to tell us where it shakes (Vibrates)
We either remove metal from the counter weights or in some case add mallory or heavy metal to correct the errors.
Factory ballance is a ball park shot and is usually fine for the grocery getter.
Now the actual vibration you feel when the engine is running can be a whole different bag of worms. We are dealing with combustion processes that can vary due to slight variations in the internals of the engine. Air flow through the valves, and combustion chamber ect. Fuel delivery in our case the pump and injectors. The amount of slight differences some off which are very hard to measure can all influence the way and engine runs and the slight vabrations or wiggles you see and feel in the rig. The engine mounts, are they new or used, the chasis application and how stiff it is. This is not an easy explanation or an easy diagnosis either.
No two engines built with exactly the same parts are going to run the same either. The running tollerance variations all play a part in the big picture.
I guess what is important here is that unless there is something obviously amiss and the thing is rattling the doors off you take what you get.
My 94 sub that I just rebuilt is rock solid at idel, the other one with all its miles and Im sure may slight needs has some shake to it.
Just the slight variations in pop pressure and spray pattern in a new set of squirters can contribute to how smooth your little beastie runs and idles.
There is really no easy way to measure it in the rig either due to so many influencing things.
I suspect this is not what you wanted to hear but its pretty much how it is.
Take 5 trucks all the same from GM you will have one rocket ship one slug and the rest will be scattered between. One will be smooth one rougher than a cob and the rest just ok.
I had a Power stroke that must have been the Monday morning after truck and it had all the bad issues you could get on a rig. It shook, ran poorly got lousy mileage and was always broke down and in on warranty.
But the factory said it was all to spec and was fine. HMMMMMMMMMMm go figure.
In closing, if you want perfection you will need to take some time and massage every little piece of the beasties engine to make sure its perfect and then there is no guarantee.