There has been a lot of confused people on this forum lately due to not knowing very basic operating principals or people "conveniently forgetting" the basics when it interferes with the point they are trying to make.

So let's try and clear up the confusion in this thread.

First there are basically two different types of balancing an engine (yea yea I know there may be more exotic ones hiding out there but this is a BASIC guide not advanced knowledge).

First one may ask why does an engine need to be balanced in the first place?

Good question grasshopper.

Anything that is spun in a circle is subject to forces that want to make the heavier part of the object (and there always is a heavier part) move away from the center but then that heavy spot is moved around the center as it spins.

This movement of the heavy spot moving around the center makes the part want to wobble which sets up vibrations.

So what the heck does this have to do with your Fit engine?

The stock fit has an INTERNALLY BALANCED ENGINE that is a fact.

So what the heck is an internally balanced engine and why is that different?

Internal balancing is the first method of engine balancing we are going to discuss today.

All that means is that the spinning parts are designed by the engineers to be dynamically balanced (that means balanced while spinning just like your tires and wheels) at the engine factory WITHOUT ANY EXTERNAL PARTS ON THE ENGINE like let's just say the flywheel and crank pulley.

What happens is the pistons, piston rings, piston pins and retainers,rods and rod bolts are balanced statically (not spinning).

That just means they are weighed on scales and then metal is removed from the heavier parts (compared to the other matching parts) so they all weight as close as the same weight as you want them to be. (you can do much finer work for more money like in racing engines).

Once you know the weight of the spinning parts "bob weights" or chunks of metal the same weight but built to be added and removed easily with a shape that can be spun and not flop all around like the rods would do are bolted on the crankshaft.

The bob weights are dynamic balanced on their own so as not to throw off the crankshaft when it is spun.

The next step is to spin the crankshaft with the bob weights attached (with no flywheel and crank pulley) and using a balancing machine, with the same principal as a tire and wheel dynamic balancing machine, the machine then determines where the heavier and lighter portions of the assembly are located on the spinning assembly so one can add or subtract weight from the crankshaft to balance it.

Now the next step this is the very important DIFFERENCE between internally balanced engines and externally balanced engines.

With an internally balanced engine like the FIT weight is added or subtracted ONLY FROM THE INTERNAL PARTS, IN THIS CASE THE CRANKSHAFT, to change the heavy or light spots.

Just like the balance weights on your wheels weights in the form of "heavy Metal" (No not like the bands) can be added into drilled holes in the crankshaft but this is an expensive process so MOST factory balance jobs use the cheaper method of grinding off parts of the crankshaft weight to reduce the heavy spots.

So here we are with a freshly internally balanced engine lets move on to the other method EXTERNALLY BALANCED and we will find out why the difference is critical when changing parts on the Fit engine.

Externally balanced is almost the same but with one CRITICAL difference.

You start with the same processes BUT WITH THE EXTERNAL PARTS ATTACHED SPECIFICALLY THE FLYWHEEL AND CRANK PULLEY.

So we are at the point of spinning the components but his time the flywheel and crank pulleys are on the spinning crank.

So the process is the same it is spun on the machine and heavier and lighter parts are found.

Now here is the big difference WEIGHT IS REMOVED OR ADDED NOT TO THE CRANKSHAFT BUT TO THE EXTERNAL PARTS LIKE THE FLYWHEEL AND OR CRANK PULLEY.

What difference does adding or removing the weight from the crank or EXTERNAL PARTS you ask?

Think about it like this if you buy a set of rims and they come dynamically balanced ON THEIR OWN WITH NO TIRES and then you add new tires what happens?

What happens is now when you balance them TOGETHER you are really only balancing the TIRES as the rims are already balanced.

This is where some people are losing it.

It's the same with the lightweight flywheel problem THE FIT ENGINE IS ALREADY BALANCED ..... ANYTHING you add to the engine HAS TO BE DYNAMICALLY BALANCED ON IT'S OWN AND IF IT'S NOT BALANCED CORRECTLY IT WILL NEGATE THE BALANCE OF THE ENGINE IT IS BEING ADDED TO.

It's not like tires you can't now add a new flywheel THEN balance them together it has to be done before adding it to the crankshaft.

So this was a BASIC explanation of balancing methods.

Most car manufacturers DO NOT balance each and every engine (some very expensive makes do but not the FIT) they produce their engine parts to a known weight and the crankshaft is balanced without the parts which will be added later because their known weight is within the specs the factory determined is best for them.

There will ALWAYS be some imbalance in every engine but factories determine what amount of vibration is acceptable to THEM and they produce the parts to that weight and just go from there as it is MUCH CHEAPER.

The bottom line on the Fit engine, or ANY internally balanced internal combustion engine, is that anything you change externally like the crank pulley has to be NEUTRALLY DYNAMICALLY BALANCED (the part itself being balanced) or you MAY introduce vibrations COMING FROM THAT PART.


So why did some manufacturers use the externally balanced engine... like EVERYTHING IT'S CHEAPER. You can add a mishmash of spinning parts like rods and pistons etc. then add weight OUTSIDE THE ENGINE TO PARTS THAT ARE EASILY REMOVED AND REPLACED like the flywheel and crank pulley and not worry too much about the weights of the internal parts that are harder to remove and replace.

So why did the vast majority of modern brands change and go to internally balanced engines?

Because internally balancing is more accurate and modern customers demanded smoother running less noisy more economical engines so the factories decided it was worth the extra money to make smoother running more gas efficient engines using internally balance engines

Here ends the lesson for today.

I'm waiting for the contrarians and the professor to show up.

 

The majority of people do not give a hoot how it's balanced.

 

What Next?

 

They might just be interested if they attempt to change parts LOL.

And some people may want to learn about things they don't know about.

And the answer for those that don't give a hoot is don't click on the thread.

 

What do you want to learn about? There are plenty of subjects we can discuss.

 

When building an engine for lots of boost what kind of compression is ideal? without the engine being super boggy

 

Good HARD question. To keep things so people can find them I will be making a new thread with the answer so people can find it easier.

Here is the link to your answer dewthedrew

https://www.fitfreak.net/forums/fit-...ml#post1191336

 

Please provide an example of a "bob" weight on a Fit engine. I'm assuming that at this stage the crankshaft has already been balanced (along with the rods and pistons) and the only other attachment to a crankshaft (aside from the flywheel and crank pulley) might be a pilot bearing.

 

why not talk about the benefits or effects of running a harmonic fluid dampener?