harmonic balancers

[blykins]

I completely understand what you're saying....just wanted to make sure you weren't buying into a lot of wives tales that get passed around the forums though....

I hear this quite often from customers...."Well, I'd rather have the 4.125" stroke instead of the 4.250" because the longer stroke engines just won't rev."  Horse poo-poo.   I think a lot of guys have seen truck engines with 150 hp, no compression, and 40 lb flywheels, and automatically make a correlation that it's a longer stroke, so it's lazy...

Jay could probably tell you about the response of some of his longer stroke SOHC and wedge engines....they are not slouches. 

[turbohunter]

I understand inertia but I've never heard of "moment of inertia".
Would that be defined (in an internal combustion engine) as the "moment" the piston hits TDC and reverses course?
And if that is so, how does detonation play into (I assume) reducing (?) inertia?
Also since there are 7 other piston/rod/crank combos going through the same thing, is there a "moment"?
I'm guessing this is part of the reason why Brent cautioned about being general.

[blykins]

Inertia is a straight-line measurement... think of when you're driving in your car and you tap your brakes....your body wants to continue in the same direction it was headed. 

The moment of inertia is not really a time based unit, but it's a measurement of an objects resistance to changing rotational motion.  In physics/engineering, a moment is a tangential force which makes a torque. 

So in a rotating assembly, the crankshaft would have a moment of inertia, which would affect its ability to accelerate or decelerate quickly. 

What many people leave out when thinking of rotating assemblies and their abilities to accelerate/decelerate quickly is the amount of force acting upon them.  If you think of a figure skater, when she draws her arms and legs in close to her body, she rotates quicker.  That's what most people think of.  However, when you add a tangential force to that figure skater, you could essentially make her buzz up even with her arms and legs spread out a little. 

That's what I was referring to when I was talking about making general statements.  There are lots of variables in how quickly a rotating assembly can accelerate or decelerate.

Now, my caveat is that I'm a degreed Mechanical Engineer, but it's been 13-14 years since I've had to crack a physics/statics/dynamics book....so I'm probably a little rusty.

[fekbmax]

Easy fellas, lol.
So I'm running a kinda odd ball stroke/rod combo I guess. I suppose it would be a medium stroke. I do use a 4.125 steel crank and I'm using a 6.535 aluminum rod with venolia 4.140 pistons. Would I still benefit from a heavier balancer ?

[blykins]

Yes. 

If you're doing nothing but drag racing, I would aim for an ATI.

[cjshaker]

However, when you add a tangential force to that figure skater, you could essentially make her buzz up even with her arms and legs spread out a little. 

I want to hear more about THIS

[jayb]

Reading through this I have a few comments, and can offer a little more insight on moment of inertia.

First, it is not correct to simply say that an engine with a higher moment of inertia will accelerate more slowly than an engine with a lower moment of inertia.  It would be correct to say that at a specified horsepower level, an engine with a higher moment of inertia will accelerate more slowly.

Second, I think it is true that a longer stroke engine will not be able to rev as high as a shorter stroke engine.  At some point, right around 6300 feet per minute for piston speed, you hit a number where you just can't spin it any higher without damaging parts.  The longer the stroke, the higher the piston speed at any given RPM, so you can always RPM higher with a smaller stroke, given equivalent components.

My big SOHC engine revs faster than any engine I have ever had, and it is 585 cubic inches.  I'd never turn it past about 7800 RPM, but it gets there way, way faster than any other engine I've ever had.  Saying that a big engine won't rev fast is just flat wrong; you can rev really fast with lots of power, no matter what the moment of inertia is.  BTW, piston speed at 7800 for my 585" SOHC (4.6" stroke) is 5980 feet per minute.  That's as close as I ever want to get to the limit.  A super stock 427 FE can rev to 9500 RPM, and it will have the same piston speed as my big SOHC.

One other comment is that I don't think it is precise to say that moment of inertia is why you need a heavier harmonic balancer.  If the engine is a perfectly balanced flat plane engine, I don't think that a heavier harmonic balancer would help with a larger stroke, even with a higher moment of inertia.  Because FEs are V8 engines, there are some balance harmonics that can't be totally balanced out, and the heavier harmonic balancer helps to dampen the vibrations caused by those harmonics.  So I think that it is more a compensation for the basic layout of a V8 engine than strictly a function of moment of inertia.

A moment of inertia calculation basically takes the mass of a reciprocating component, and then multiplies that mass by the distance from the center of rotation to the center of gravity of the component.  So, taking a crankpin as an example, if you knew the weight of the crankpin, you could then assume that the center of the crankpin was its center of gravity, and multiply its weight by the distance from the center of rotation to the center of the crankpin.  This is just half the stroke.  You would then have to do this for all the crankpins, all the crankshaft counterweights, each connecting rod and piston, plus the harmonic balancer, flywheel, and any other reciprocating components. 

I took a stab at this once with a spreadsheet, reprinted below.  A lot of the stuff is a guess, because I just wanted to get an idea of how the moment of inertia affected the power production of the engine.  The units are kind of bizarre in the English system, slug-feet (!).  In the metric system it is kilogram-meters.  Inches are all converted to feet by dividing by 12, and pounds are all converted to slugs by dividing by 32.2 ft/second squared.  At the bottom of the third sheet is the calculated result, which tells you how much torque is required to accelerate the engine at a certain rate.

Just a little food for thought, fellas...   

[Autoholic]

Jay, I'm sorry but I have to disagree with you on this part... "It would be correct to say that at a specified horsepower level, an engine with a higher moment of inertia will accelerate more slowly."

If everything is equal except for the stroke, the longer stroke will accelerate slower than the shorter stroke. It's apples to oranges once you try and say this engine accelerated faster than this other engine even though it had a longer stroke AS WELL AS a bunch of other differences. With everything being equal, there is no way that the longer stroke can physically match the acceleration and rpm level of the shorter stroke. It doesn't matter what horsepower or torque is being made, it comes down to a shorter distance for the piston to travel. Shorter the distance, the quicker the piston will go from A to B. In reality it's damn near impossible to have everything equal except for the stroke BUT if you did do this, you would find that the shorter stroke engine made its peak power higher in the RPM band than the longer stroke. You would also find that the shorter stroke accelerated quicker. It has nothing to do with how much power is being made, this boils down to introductory physics.

Now if we were to build 2 engines that were very different, 1 with a long stroke and 1 with a short stroke and made sure that they were balanced as much as humanly possible. Both made to rev as high as they possibly can. Longer stroke has the higher moment of inertia, still can't rev as high or as fast as the shorter stroke. The longer stroke will end up reaching the max piston speed sooner.

It all comes down to how close to the maximum you build the engine. Doesn't matter what power is being made, for any given piston speed, the lower moment of inertia will rev higher and faster. Is the lower moment of inertia due to the stroke? When everything else is at the bleeding edge of what is possible, yes. If you build a light weight, big stroke engine and compared it to a rather heavy short stroke engine, then which ever one actually had the smaller moment of inertia would win.

[blykins]

Thanks Jay, you echoed my thoughts exactly, but with more eloquence....sometimes my brain works but my fingers/mouth don't. 

"It would be correct to say that at a specified horsepower level, an engine with a higher moment of inertia will accelerate more slowly."  That sentence pretty much sums it up. 

[turbohunter]

Not the first time on this forum I had to turn to the ole inter web to look up a definition.

[Nightmist66]

I'm sure rod ratio plays an important role in all of this too.

[Autoholic]

The stroke to bore ratio plays into this as part as part of the corrected piston speed. The longer the stroke, the faster the piston will have to travel and at some point you cannot go any faster due to the speed of combustion is maxed out. The lower your moment of inertia, the quicker the engine will accelerate for any piston speed and this is due force. Think of trying to push a cart and then push a car, at the same speed. You'll be able to reach 5 mph pushing a cart faster than a car. Now imagine having to bring both to a stop, and change directions. Oh and brakes don't exist in this example, for all you clever persons. The shorter the stroke, the higher the engine will be revving for any piston speed and this is due to distance. I should say that when I am talking piston speed, I'm looking at mean piston speed which does not deal with the stroke to bore ratio. The corrected piston speed is used more for determining the longevity of the engine.

Jay, I tried understanding your spread sheet but I can't make heads or tails of it without looking at it personally. There are a bunch of numbers that either should be left alone (like a weight) or have been put into some calculation that cannot be seen. There should also be more inputs. If this was an attempt to see how much torque is needed to rotate and engine at a given RPM, the lower the moment of inertia the less torque is needed with everything else being equal. I'm sure you know personally that this is an over simplified view of the matter though, friction comes into play and that is largely driven by your tolerances. I would recommend that if you want to play with this math in the future, screw the imperial system and just use metric units. So much easier to work with.

[blykins]

It doesn't matter what horsepower or torque is being made

Horsepower does indeed matter and in most scenarios, it matters more than MOI.   I believe you are focused too much on distance/position and not on focused enough on acceleration. 

[Autoholic]

It doesn't matter what horsepower or torque is being made

Horsepower does indeed matter and in most scenarios, it matters more than MOI.   I believe you are focused too much on distance/position and not on focused enough on acceleration.

I am focused primarily on the acceleration. The moment of inertia plays directly into the acceleration. The distance plays into the acceleration, as well. The moment of inertia in statics plays into the force. That force is used to determine the acceleration of the specified mass. The longer the piston has to travel, the higher the piston speed due to that acceleration being constant until it suddenly changes direction. At TDC and BDC, when the piston changes direction it exerts a force, equal to the moment of inertia times the acceleration. It doesn't matter how much power is being made because when you are looking at different strokes and different moments of inertia times the acceleration, you don't know much power (work) is being made (done) and you assume that for the comparison, they make the same amount of power. As such, they cancel each other out. This allows you to focus on the moment of inertia and how the stroke will play into it. The force comes from the fuel exploding and the faster, cleaner it burns the more force it exerts. To figure that out correctly, you need the dynamic compression ratio. But for any specific dynamic compression ratio, a specific amount of force is available upon combustion. That force is broken down as f= moment of inertia * acceleration. The smaller the moment of inertia, the faster the engine accelerates. When you decrease the stroke, the moment of inertia goes down. This results in the acceleration increasing. Due to the distance being shortened, the mean piston speed falls. The increase in the acceleration decreaing the time for the piston to go from top to bottom, increasing the rpm's. Due to the mean piston speed being decreased, the higher the engine can theoretically turn before reaching what is possible.

Having gone through all this, I must now apologize to Jay. His statement about specific power was correct. I had a problem of trying to focus on the power output to dictate what a specific moment of inertia can spin at. If you were to really get into the math, it could be said that the moment of inertia and the dynamic compression ratio is what will determine how much work the engine produces. 

[HolmanMoodyStroppeGang]

Love the data Jay and the engineering conversation, we did a lot of this back when and of course along the way.

Now I am going to refine the question a little for fun.  This is a very broad question.  I am building a full race FE

To me, I also want some bones on the engine, car, clutch, trans, gear,tire, goal and weight, anticipated launch RPM, and if she has a tube chassis, 4 link, and all the combo info. I also wonder, race gas? alky?

Reason why is because I am thinking about ET/cost and hundredths on the time slip too.

I like a heavy Damper on a long stroke engine but at first, wondered the compression, cam, heads, and she has aluminum rods, so....

Clearly I am turning the question on it's head to round out the analysis a little

Is this a 10 second car, goal wise?  950? 900?    850?    And also, how heavy a flywheel do you run, or is it a auto?

If it is a 10 second car, the mass difference will have an effect, on your ET data, but probably very small.  So you might think it all through, chose a quality SFI piece, and maybe say, my next build will be even bigger and faster, and higher revving, so I think I will get something that I can use on future builds too.

No doubt the power level is key, and stroke, as you guys see a lot, a blown car that is full race let's say, tosses the damper, and lets the flange, hub, blowere belt and blower smooth away huge resonant frequencies for that 4,5,6 second blast

Power level is huge, but so is the goal.  The old NASCAR type builds had to live 3,4,5 hours, and our OffRoad FE's had to live, 12,18,24 hours.   

So I would refine the question too, and think horsepower/ET per dollar too

And I want to know, how hard this car likes, or needs, to hit the tire?

Here is an old fact.  Remember Dyno Don's 71 Maverick with a SOHC?  The Pro Stock car that won so much? 

That car liked a low gear, and the tires,chassis, and leave RPM, liked more then more mass.

The 60ft and the ET, picked up, by adding heavier then heavier flywheels, and a bigger dampener

It ran best, because every race car and engine, trans, is, unique to some degree, it worked best with a custom, super duper heavy Flywheel. You guys would laugh if I told you

Reason was, he left sky high, before rev limiters, if he was on a good track, in good air and for the win.  That was a 2 disc car, and the entire combo got tested and tested many ways, so having about twice the weight of a regular flywheel helped him, hit the tire very hard, get the car moving fast, and power shift real high to allow that increased rotating mass to slip the clutch on a gear shift and stay higher in the power band.  Kind of a, try 10 things, and what works works thing

Man, this is such a fun thread by many smart guys.

Race cars deserve race parts, it is nice to just get the best you can budget. ATI and the others will get you to the other end and she sounds fast so NICE WORK man

Loved that chart Jay, thanks to all of you

Just rounding out the issue in case it helps him get that new dampener to the balance shop and spinning

A SOHC can run low 9s with a tricked out stock dampener, not legal, but they got 'pinned'