Heavier or lighter Flywheel to improve FE?

[Frank Lee]

I think a heavy flywheel would allow me to short shift, skip shift, and lug smoothly at much lower rpms. I think it would also be beneficial if one was to gear higher. Cruise = no advantage.

[Arragonis]

I dumped my DMF and went for a lightened SMF about 18 months ago, this was in prep for a remap (not done) as the standard clutch can't handle it - this is a TDi 130, remap is claimed to be 180 but it probably nearer 160-170. With the high gearing the clutch is the weak link in the TDi after remaps.

The local taxis do this for reliability over higher mileages - the DMF has random failure issues and is expensive.

It made very little difference to FE, although at the time I wasn't monitoring it as closely as I am now. In terms of driving the really noticeable difference is the faster pickup in all gears, not just the lower ones, overtaking is slightly more sprightly than it was before.

Shifting is exactly the same, although I have a heavier clutch pedal because of the higher clutch spring rates.

There may be some other changes relating to coasting in gear etc. but to be honest they aren't significant enough for me to notice them compared to a standard car.

I do have a rattle or chatter at speeds under 1500 which is a pain at idle or when crawling in traffic. It was diagnosed as a gearbox issue but after a rebuild (spendy), and then another rebuild (free) to try and fix it, the rattle remains. So now I turn the radio up and ignore it.

In total the SMF, clutch kit, required bolts, fitting, gearbox rebuild and labour all came to about £2k. But it should be good for a few hundred K miles but I might not keep the car that long.

I know loads of people have DMFs which have been to mars and back, and some which have failed in under 20K. Totally random it seems.

[ShadeTreeMech]

So in summary.....

if you're planning on luggin in the lower rpms, heavier flywheels are better. If the engine needs to change speed frequently, lighter is better.

With a lightweight flywheel I would expect quicker engine speed changes with more vibration being transmitted to the drivetrain, especially at lower rpms.

With standard flywheel, I would expect a balance between light and heavy.

With a heavy flywheel I would expect a smoother idle and low range, a bit quicker starting, but delayed shifting with a manual and it feeling a bit less perky and a bit more powerful. The feel of torque would be higher, the quick accelerations slower.

A tractor would have a heavy flywheel because of the low rpms used, and the generally steady engine speed. Most tractors don't have a gas pedal but instead have a throttle selector similar to a lawnmower.

For city driving I would want a lightweight, for steady state driving or driving in hilly country or driving with a modified transmission with a higher top gear I would want a heavier flywheel.

So one is not better than the other, the standard flywheel is a compromise, and it all depends on your driving needs.

Or you can go electric and thumb your nose at the ICE.

[Arragonis]

There seems to be a reasonable write up here.

Flywheel lightening & performance gains from lighter flywheels

I can't say I have noticed the effect of the lighter flywheel on hills as mentioned (I assume it means coming down hill, the engine compression is slowing the engine faster as there is less weight to keep the momentum up) but the engine is a bit of a lugger anyway.

[ShadeTreeMech]

Originally Posted by Arragonis

I can't say I have noticed the effect of the lighter flywheel on hills as mentioned (I assume it means coming down hill, the engine compression is slowing the engine faster as there is less weight to keep the momentum up) but the engine is a bit of a lugger anyway.

Actually, I believe they stated one disadvantage of a lighter flywheel is losing power going up hills. The engine would have a harder time lugging in the lower rpms without losing rpms, requiring a downshift when simply letting the engine lug would be the norm with a standard flywheel.

It is possible as well the flywheel you have is not as light as the lightweight flywheels mentioned in the article. A DMF is more complex and a bit heavier, while a standard flywheel is as basic as they come, but they are still heavy.

[Arragonis]

If there is an effect on climbing I haven't seen it, or maybe not noticed it since the swap. But I have 250 lb/ft in a car smaller than a Golf so its not an issue

[ShadeTreeMech]

Are you able to lug it in the lower rev range as well?

250 lb/ft is indeed impressive. I've actually considered getting a diesel from a wrecked VW to replace the 4L gasser in my Explorer and be glad for the increase in power

Originally Posted by Arragonis

I do have a rattle or chatter at speeds under 1500 which is a pain at idle or when crawling in traffic. It was diagnosed as a gearbox issue but after a rebuild (spendy), and then another rebuild (free) to try and fix it, the rattle remains. So now I turn the radio up and ignore it.

question: Did you have this rattle with the DMF?

Theory (assuming answer is no): maybe the heavier flywheel helped to eliminate the rattle with its weight?

[Arragonis]

Originally Posted by ShadeTreeMech

Are you able to lug it in the lower rev range as well?

It lugs at lower speeds quite well. It vibrates more than before but that is another issue to do with badly fitted cup bushes which are going to be fixed in December. I can go from stop through the gears to 6th and cruise along with no throttle on the flat or idle my way up a hill in traffic in 3rd or 4th.

The lighter flywheel may make a difference but the car is light (smaller than a golf) so I don't think it made much difference overall.

Originally Posted by ShadeTreeMech

250 lb/ft is indeed impressive. I've actually considered getting a diesel from a wrecked VW to replace the 4L gasser in my Explorer and be glad for the increase in power

You mean more torque. power = (torque * rpm) / 5252

You would have to account for the different gearing though. My redline is 4K and my governer cuts in about 4.5K. I assume a truck would be low geared for towing and pulling loads. You may end up like the Diesel Suzuki Vitara conversion I once had for a day, it hit the governer at about 60.

My friend's now sold chipped and tuned one had over 300 lb/ft. Its a bit wild though, spins its wheels in 5th in the wet and 3rd in the dry as the power comes in like a sledgehammer. There is a guy on youtube called TDIFurby who has nearly 300hp in his

The Fabia is not blessed with good handling though.

Originally Posted by ShadeTreeMech

question: Did you have this rattle with the DMF?

Theory (assuming answer is no): maybe the heavier flywheel helped to eliminate the rattle with its weight?

It didn't rattle beforehand. Originally just after the clutch was fitted I was told it was because the box was worn, there is an issue about the casing changing shape slightly in use. It wasn't urgent so I left it until I had saved the pennies for the first rebuild, which made no difference. The second rebuild also made no difference but did cure a synchromesh issue (I couldn't get 4th without slipping the lever into 3rd beforehand).

I think it is something wrong with the way the clutch has been fitted but curing it will mean removing it again which is tricky in the space available.

I wish I had a tree I could spanner in the shade of, but I don't have the space, skills or time to do it myself.

So I'll live with it for now.

[ShadeTreeMech]

For giggle, I pulled up some info on a VW diesel with exactly 1/2 the displacement.

From Wikipedia
2.0 R4 16v TDI CR

132 kilowatts (179 PS; 177 bhp) @ 4,000 rpm; 400 newton metres (295 ft·lbf) @ 1,500-2,000 rpm — Volkswagen Transporter (T5) GP biturbo

The engine in my Explorer is a 4.0L V6 with 155 hp and 220 lb/ft torque.

So I would halve the engine displacement, add 22 hp and 75 ft/lb torque, plus increased fuel economy. And if I kept the transmission stock, it wouldn't be an issue, as I rarely revved to 4000 rpm with the gasser, and it redlined at 5500 rpm anyways.

I'd likely never find the money to do this ideal setup, but it illustrates how different a low tech gasser is compared to a modern turbo charged diesel.

And FYI while I may call the 4.0L low tech, it had direct ignition (instead of a distributor) and multiport fuel injection in a day when the distributor and throttle body injection were still in common usage.

[Arragonis]

That 170 unit is pretty nice, Audi sell it in the TT here in Europe. Why bother with a Porsche hybrid ?