Flywheel weight logic?

[slamhouse]

This thread is aimed at those of you that know the parasitic losses between compression and power strokes given a different weight flywheel.

From what i've gathered thus far, diesels like heavy flywheels to bridge the gap from power to compression stroke especially in smaller diesel engines like our own.

My main goal is not power but fuel consumption at highway speeds and driveability from stops.

So far I have my eyes set on a 22lb single mass conversion.

What are your opinions on flywheel weight? heavier? lighter?

Dual mass?


Thanks for any insight,
Cory

 

[No More Buffalo]

The physics tell me that a heavier flywheel should be at least a marginal MPG loss, as that's more rotating mass you have to spin every time you rev up the engine. It also means it will slow down slightly less quickly when in overrun, so that may cancel out.

If nothing else it is more mass to haul around.

 

[slamhouse]

Yes, but the rotating mass aids the engine from the previous power stroke to the next compression stroke which is 180 crank angle degrees away. that's a long gap to bridge...

with the lighter flywheel, the engine has less rotational energy to exert to the next compression stroke resulting in a parasitic loss.


I spend 130 miles a day on the highway, this engine will rarely see starting from a stop.

 

[IndigoBlueWagon]

If you want to save some rotating mass get lighter wheels and tires. You can make a much bigger difference there and it helps acceleration, handling, and fuel economy. You can easily save 6 lbs. plus a corner. That's a lot more than you'll save with a lighter flywheel.

 

[slamhouse]

My goal is not what you elaborate on.

My clutch and dmf are going south and I am investigating my options for replacement.

 

[Barnitz]

Just my opinion with no professional knowledge... You have already stated that a heavier flywheel will assist the engine in 'bridging the gap' between cycles.

VW Spent a lot of money in R&D to determine that the stock flywheel was the best weight for the engine. So unless there's good evidence to change it (Lighter or Heavier), I would go with the stock weight.

And you might as well switch to a SMF and an upgraded clutch if you plan on doing any performance mods. You probably already know but the stock clutch in your car is basically maxed out from the factory. Mine started slipping when the only modification was a Stage II Cam.

 

[Andrew 179]

"parasitic loss"

"with the lighter flywheel, the engine has less rotational energy to exert to the next compression stroke resulting in a parasitic loss."

Where do you think the energy goes from the "parasitic loss"? Maybe to your wheels?

You seem to have 2 options:

1. Earn an engineering degree, duplicate VW's work, spec out the stock weight/mass flywheel, and buy a stock mass/weight flywheel.

2. Buy a stock weight/mass flywheel from a Fred's supportive vendor

As others have said: "drive more, worry less"

 

[IndigoBlueWagon]

I don't think flywheel weight is going to make any measurable difference on highway fuel consumption. I've had stock weight and lightened flywheels in one of my Golfs and it didn't seem to matter, at least not enough to show any consistent result.

It will affect driveability in traffic, however. The car needs a bit more pedal from stop, and revs drop much faster between gears so you have to stay alert or on the accelerator to shift smoothly. Not a big deal, but noticeable.

 

[KLXD]

Yes, but the rotating mass aids the engine from the previous power stroke to the next compression stroke which is 180 crank angle degrees away. that's a long gap to bridge...

with the lighter flywheel, the engine has less rotational energy to exert to the next compression stroke resulting in a parasitic loss.


I spend 130 miles a day on the highway, this engine will rarely see starting from a stop.

I think I understand what you are after but I think you're looking at it backwards. The parasitic losses are from friction, blowby and heat transfer. None of these are caused by the flywheel nor will they be any greater with a lighter flywheel.

Consider a frictionless adiabatic (no heat loss) cylinder/crank assembly. Release it at TDC with a given pressure above the piston and it will return to that position. Put on a heavier flywheel and it'll do the same but will take longer due to the greater mass. It won't go any farther than TDC.

Now add some friction. The flywheel will no longer make a full revolution. Adding mass to the flywheel won't help that.

The flywheel is there to smooth out the power pulses. I don't see it having any effect on mileage at constant RPM other than its weight on the tires.

 

[40X40]

You want a SINGLE MASS flywheel and it needs to be HEAVY, IE, not any lighter than stock.

Bill

 

[PondRacer]

When I replaced my clutch setup, I went with a 21lb single mass flywheel. When it came time to take the old flywheel, I thought, hmm, the new one seems a bit heavier than the stock flywheel (which is supposed to be 22 lbs). It wasn't until I compared both and realized that the reason for the apparent heaviness was actually how the weight is distributed. In the dual mass flywheel it seems that the weight is more evenly distributed throughout the flywheel; in the smf it is distributed more to the outer portion of the flywheel.

Hope this makes sense.

 

[deepseafortydog]

My car came with a VR6 DMFW. 50mpg highway.


I get the same from the ultralight (18 lb) SMFW from South Bend. If you have a choice (I didn't), go with the heavier weight. See this thread:
http://forums.tdiclub.com/showthread.php?t=345295

Also, unlike the gasser, there is no noticeable performance benefit from a lighter flywheel. The noise was really annoying.


When it's time for a new clutch and FW, I'm going back to stock weight.

 

[slamhouse]

TDIMeister?