Lightened Flywheel - pros & cons?

[funkaholik]

I talked to my local machine shop about lightening the flywheel for my 1.6TD.  He says that the heavier rotating mass in important for a small diesel engine, and feels that torque will be less with a lighter flywheel.  He says that light flywheels are great for high revving hp in gas engines, but not on a stockish, low-revving turbo diesel.

Can anyone weigh in on this?

Oh yeah - and what is the recommended weight to lighten to?  Stock is 13 lbs.

Thanks!

[jtanguay]

i can see having a big flywheel on larger diesels to smooth out the heavy pulses, but this is a relatively small diesel engine.  i've only heard of positive results from people lightening their flywheels.  to me, it would be comparable to going with carbon fiber driveshafts.

5 cyl diesels have a counterweight (sort of like a flywheel) on the injection pump to help smooth it out.  while it does take energy to spin it up, it takes relatively the same energy to keep it spinning once up to speed.

for quicker acceleration and better acceleration response, go for it.

[maxfax]

There may be a slight loss of torque, especially at lower RPM's however these little buggers make enough torque you probably won;t miss it when it comes to being able to accelerate faster..  I don;t think you would have a noticable increas in vibration as long as your engine is in good repair and tune.. Possibly some more when it's cold and idling...  The decreas in rotational inertica should even help with mpg's somewhat as long as you keep the right foot under control..     I remeber reading somewhere (It may have been a post here actually) that 6-8 pounds seems to be the ideal weight without taking too much off the flywheel and weakening it...


Been considering trying this myself on one of the cars...  I'm able to remove the weight on my lave but I haven't found a machine shop that has a mandrel large enough to re balance it...  And if it isn't properly balanced that will cause all sorts of vibration, and long term is hard on the engine and trans...

[theman53]

I could be wrong, but this is how I think of it. The engine makes the power, the flywheel stores power. If the flywheel is heavy the engine will be slower to accelerate/decelerate, but if it is lighter the engine will accelerate/decelerate faster. The engine should be relatively the same in power, but the heavier should have a little more torque. The lighter should have more horse since it revs faster. An old timer explained it to me and used this example...like I said I could be wrong, but it makes sense to me.
If you have a train made of aluminum and an identical powered dementioned train made of cast iron traveling in a circle. The trains being equal in size, but completely different in weight the aluminum train should accelerate faster and stop faster than the cast iron one. But if you flip the track switch and run them head on into eachother the heavier one will probably knock the lighter one around more. It has more kenetic energy.
    Back to the engines the same principle is in my friends John Deere B type. You would never get his started if the flywheel was the same size as it is, but weighed 1/2 as much...the thing about kills you anyway.

[maxfax]

theman, you hit the nail on the head...  Back in the 60's Buick made thair first v6 the 225ci..   In 1964 Buick sold the tooling to Jeep and they produced the engine for CJs and Commandos..  Nothing changed about the engine except the flywheel.. Jeep used a flywheel that was about 9lbs heavier.. Hence when Buick produced the engine it had a lower torque rating than when Jeep produced it..

[jtanguay]

theman, you hit the nail on the head...  Back in the 60's Buick made thair first v6 the 225ci..   In 1964 Buick sold the tooling to Jeep and they produced the engine for CJs and Commandos..  Nothing changed about the engine except the flywheel.. Jeep used a flywheel that was about 9lbs heavier.. Hence when Buick produced the engine it had a lower torque rating than when Jeep produced it..

was the HP the same as the Buick though???  i wanna see dyno proven results that say a heavy flywheel will give more torque.  like i said up top these aren't very large motors... maybe if the flywheel was about 50-100 lbs and the size of a tire, i could see it be able to churp the tires real good (rev to 1500rpm and then dump clutch- some buses make use of large flywheels to store energy, but they're specially designed), but in an automotive application that is just not feasible.  if there is any kind of torque gain, it would be relatively low, and possibly proportionate to the amount of material removed from the flywheel...  

like Andrew stated above, you would need to spin that flywheel up pretty good to create that 'fake' torque  :wink: but the problem is that from a standstill the flywheel will still be brought back down to lower rpms and all you'll be doing is stressing parts like the clutch etc... just imagine the extra responsiveness you'd get from less rotating mass??!   i would imagine a zippier car off the line.

now what about those carbon fiber driveshafts??!!  :lol:

[maxfax]

AS per the 1965 Motor repair book..

1965 Buick   155hp @ 4400   225lb/ft @ 2400
1966 Jeep    150hp @ 4200   235lb/ft @ 2400

But keep in mind these are gross ratings...  AS in not installed in the car, measurments at the......FLYWHEEL..  Note the HP loss, and that the peak hp is at a lower RPM.. This probably worked out okay in the jeeps since most of them were geared low and the engine was spinning at the peak torque RPM when cruising up and down mountains...

Anyways, lightening the thing up I feel is only gonna make things better... THe only time it may even be slightly noticable is when you are pulling out form a dead stop.. Feather the clutch a bit more, it's good from there...

[jtanguay]

almost seems like they've just messed with the timing...

[maxfax]

Same Base timing, advance curve, camshaft,compression ratio, same Rochester 2bbl with the same jetting.. Cam timing is even the same which is where I suspected the difference to be... The only diferences are the flywheel was about 9lbs heavier, and if mind serves me right the driver side manifold was different in that they just used two passenger side manifolds so that the exhaust would exit at the front of the engine as to clear the steering box...  On the dyno, though, I'd bet the false torque from the flywheel showed up... On that line of thinking, I wonder if Buick used a manual flywheel, or an automatic flex plate when testing......

Maybe they used different oil when they tested the things.... International did that on one of their tractors year back...  Instead of using gear oil in the trans and diff they used hydraulic oil... Took people a while to find out where they were getting the additional power...

[zukgod1]

I installed a lightened flywheel on my engine and I saw zero difference.
I didn't install it because I was looking for one either, it came with a Sachs Super Clutch pressure plate and Clutch net disc I bought.

[clbanman]

As already stated, the heavier flywheel doesn't increase torque, it affects momentum of the crank assembly.   If you let off on the fuel pedal, the engine doesn't slow down as quickly, but when you accelerate again, it takes a bit longer to build revs.  This works great on something like a tractor where you have your engine at a constant speed, and may hit a stone while plowing.  The extra mass helps pull through the increased resistance without adding throttle, as long as the extra resistance is of short duration.  Heavy flywheels smooth engines out, but reduce throttle response.  If your normal driving style is to look far ahead, back off throttle early, and accelerate slowly, the heavier flywheel should help fuel mileage.  If you drive the opposite way, a heavier flywheel increases load on your brakes (slightly) and adds load (slightly) while accelerating, so would reduce your fuel mileage.

[dillenger1]

Ive heard if it to light there may be problems with the clutch.

[maxfax]

I would imagine if you took too much material off the thing it would weaken it...   Just taking it off the out edge within reason shouldn;t hurt anything...

[maxfax]

At steady speed/load operation, the flywheel will not have any bearing on the numbers at all.
Andrew

Except maybe the hp that would be true.. Turning that extra weight still may eat some hp..  Those are also the advertised specs I was using..  Most of the time the advertised specs are lower than what they acutally get on the dyno.. Mainly to avoid issues like Ford had in '99 with the Cobra Mustang... (320hp advertised, 290 actual... Oh the lawsuits..)

[saurkraut]

The weight of the flywheel may also serve as a torsion damper and shield the trans from the fireing pulses.  If you go too light, you might start puking transmissions.