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5 Posts
Discussion Starter #1
Hi all!

Used to be a member of this forum way back when I was at university (2011!) when I had a 156 V6. Now, I've finally got back into Alfa world with this 75 :)


It's a little ways off the project starting, but I've been doing some research into various improvements that I can make as it goes back together (there's a 24v 3.0l and various conversion bits that come with it).

One of the main things I can see is a bit of a compromise is the drivetrain setup from the flywheel backwards being quite heavy. A standard Triumph 2.5l flywheel is ~12kg and I thought that was a bit heavy so I've got a ~6kg one for my Spitfire. The stock 75 setup includes a 11kg front flywheel, a 4.4kg rear flywheel and whatever rotational weight the propshaft and guibos have. Probably not too far off having the equivalent of a 20kg flywheel! No wonder the synchros have a tough time of things.

You can lighten the front flywheel down to about 5.5kg, and the rear down to about 2.7 (from this thread), or more conservatively 9kg front and 4kg rear. Either that or sort an alloy front flywheel which apparenty weigh in about 4.7kg. That's still between 13kg and 7.4kg of flywheel weight, plus whatever inertia the propshaft has (it'll be less as the weight isn't as far out as the flywheel's, but stll significant).

7.4kg + prop is pretty good, but I was wondering if it might be possible to do better. Might it be possible to get rid of the front flywheel (almost) entirely?

Here's what I was thinking:

1. Find a small-diameter ring gear from another car's flywheel (lots of cars with dinky little flywheels). Machine the rear flywheel's outer lip to accept the ring gear, and scallop the gearbox casing to allow a starter to fit at the rear (with suitable mounts tigged on). Getting the OD of the ring gear right will be critical so it doesn't foul the housing.
Rear flywheel lightening 1.jpg

2. Remove (potentially) the entire outer mass of the front flywheel, leaving just the mounting lugs in the middle for the propshaft guibo. Either using the OEM flywheel, or getting something machined out of billet (like the below lightweight flywheel).

3. If you wanted, you could leave the counterweight in place when removing the rest of the front flywheel (probably best to use the billet steel variety), or you could use a separate crankshaft counterweight to balance the V6 crankshaft like the ones used on Ford smallblocks.

That way, you could have something around a 4ish kg front 'flywheel' with **** near all the weight right in the centre, coupled with perhaps a 4kg rear flywheel (lightened, but with an additional ring gear). Although the total weight wouldn't be enormously different from the alloy/lightened setup, the difference in rotational inertia should be huge as the front 'flywheel' now has a centre of mass that's ~2" from centreline rather than ~6".

If you're making up a billet adapter to mount to the propshaft, you could take the opportunity to change the propshaft arrangement as well. The E46 330d has a very clever (and beefy) propshaft guibo that has some built-in give to it which should damp torsional vibrations quite nicely (the little gaps each side of the mounting sleeves allow a progressive feed of torque into the propshaft), and the rest of the propshaft joints are a UJ and a CV allowing for better tolerance for the misalignment required in the Alfa setup. As far as I can tell, these guys are still happy after they've been remapped to 600lb-ft+.


I'll need to take some measurements from the bits at home to work out the rough rotational inertia of things and make sure you wouldn't end up too light, and it'll probably be a while before I start on the Alfa in earnest (I'm turbodiesel-swapping an XJ40 beforehand), but I'm quite excited to see what I can do on these cars on an enthusiast's budget :)

What do people reckon? Might the idea have legs? Anyone done it before? Any glaring issues that I've completely missed?

Premium Member
844 Posts

I think you would do better to lose the rear flywheel entirely and keep the front flywheel.

The front: It's my understanding that the flywheel, clutch disc, pressure plate & throwout bearing from a 164 V6 engine will all fit inside the GTV6/Milano bell housing. You would then need to fabricate some kind of crossplate inside the bellhousing to hold a shaft connecting the clutch disc on one side of the plate to a guibo tripod on the other side. Presumably you could use the transmision input shaft from the 164 transmission as the start of this process.

The Middle: The OEM driveshaft, the front section might have to be shortened or lengthened.

The Back: The bell housing at the front of the transaxle where the clutch normally lives would be empty. You could use the nosepiece of the clutch unit and machine a coupler to mate that to the splines of the transaxle input shaft.

I'm very interested in engine flywheels and why they are needed.

If you follow the link below you can see the torque output of various engine types, single cylinder, V6, V8 (flat crank) and V12.

ET-X-6 Cyl.gif

You'll that the even fire V6 drops into negative torque terrority three times every revolution. I think that without a flywheel it would stall.

It's my understanding that a flywheel acts as a inertia bank for an engine. When the engine is in positive torque, it deposits inertia into the flywheel, and when it is in negative torque, it relies upon the flywheel's inertia to keep the crankshaft turning until the the combustion stroke.

Only the V8 (flat crank) and V12 are in positive torque all the time and only barely for the V8. I think a flywheel is also necessary for starting from a stop.
ET-X-8 Cyl.gif

ET-X-12 Cyl.gif

If you moved the clutch function to the front, you would achieve your laudable goal of reducing rotating inertia, and still have an engine mounted flywheel.

The reason why the engine mounted flywheel is preferable is diameter. The engine flywheel has a bigger diameter than the clutch one.

The general equation for inertia is mass times the square of the radius. So a flywheel with twice the diameter will have four times the inertia. Since the front flywheel has a bigger diameter that's the one to keep.

BTW, in one of your pictures you show a removable crankshaft throw weight spined to a keyway. What engine is that from?

Hope this was useful to you

5 Posts
Discussion Starter #3
Fascinating. I knew engines need flywheels to keep running properly, but had no idea of the reasons for it. Very interesting that they run into negative torque (presumably when there's a period where pistons are only being compressed and there's no combustion happening). I can see why things like dual-mass flywheels and rubber driveshaft couplings are desirable to smooth out the shock-loadings.

I've come across a formula for moment of inertia here that represents objects as cylinders which is useful for working out the propshaft: Inertia = 1/2 Mass(outer radius squared + inner radius squared). That should let me estimate the entire rotational inertia of the two flywheels and the propshaft as well (which seems to be left out of the picture when working these out). I'll then compare that to the rotational inertia of a lightweight Triumph 2.5l I6 flywheel I have which is 6.3kg with big cutouts around the outside (and a 4-bearing crank). Should be pretty comparable as the firing pulses for an I6 are the same for a 60-deg V6.

If we work that out, we could size a (probably much smaller) front flywheel appropriately to match that inertia.

The only issue I can see is the effect of the guibos on the transfer of the rotational inertia from the rear flywheel to the crankshaft. As you dip into negative torque they'd wind up a bit before transferring the inertia into the crankshaft, meaning you might need a bit more weight than you'd expect to keep the idle smooth.

Clutch up front would be good. It would work quite well for the 24v I have as there's plenty of 156 V6s on this side of the pond I can pinch a solid flywheel from. However, the clutch might have a tough life having to spin up the driveshaft, although it would be better without the rear flywheel. Transaxle cars like the 944 with the clutch up front get away with it because their torque tube setup lets them have a very narrow diameter propshaft which has bugger all inertia.

Would be an interesting comparison to make though. Will try and do some calculations of various bits and see what I come up with!

129 Posts
If it helps, I have removed the front counterweight from the crank pulley. A disc of Mallory metal was added to the front counterweight to compensate.

5 Posts
Discussion Starter #5
Right! I've done some rough measuring and some rougher maths on the bits of drivetrain I've got lying around (and tried to work out bits like the rear flywheel that are still in the car). It's all very rough as I've had to just assume all the bits are simple cylinders with evenly distributed masses, but it should give some good ballpark figures. Oh, and there's no clutch baskets in all this as I don't have any of those about (but for argument's sake, let's assume they're all equal even though they're not).

For comparison, I've estimated some flywheels from different cars:
  • The standard TR6 2.5l flywheel weighs 12.3kg, and has an estimated rotational inertia of 138g/m². The general consensus is that these are pretty heavy flywheels for the application.
  • The lightweight TR6 2.5l flywheel weighs 6.3kg, and has an estimated rotational inertia of 71g/m².
  • The 24v 3.0l FWD solid flywheel weighs 7.9kg, and has an estimated rotational inertia of 89g/m². This accounts for the added inertia from the offset counterweight (offset weights have more inertia than evenly distributed ones).
We'll use these as baselines for what to expect for a high, mid and low flywheel weight for 6-cyl even fire engines.

Added up the various bits in the Alfa drivetrain and came to this:
  • Front flywheel weighs 11.1kg, and has an estimated rotational inertia of 124g/m².
  • Propshaft combined weighs 12.1kg, and has an inertia of 13g/m². This one will be less accurate as there was a lot of estimation in the weighing and calculating of the propshaft bits (although I did break it down into sections). Should only be out by a little though.
  • Rear flywheel weighs 5.9kg (I think), and has an inertia of 52g/m².
  • So, combined, the Alfa flywheel and prop setup weighs 29.1kg and has an inertia of 189g/m². Quite a heavy rotating assembly! No wonder the synchros struggle. Estimated that this would be equivalent to having a 17.2kg standard 24v flywheel on a FWD V6.
Next step was to work out some of the lightweighting options. Lots found from this thread, and others around the web.
  • One conservatively machined lightweight front flywheel (no holes drilled) weighs 9kg, and has an estimated rotational inertia of 101g/m². This will be a bit out as most of the weight was taken from the outside edge, but it's a decent indication.
  • Another standard flywheel with holes poked in it weighs 5.45kg, and has an inertia of 63g/m².
  • An alloy front flywheel from performatek weighs 4.7kg, and has an inertia of 54g/m².
  • One conservatively machined rear flywheel weighs 4kg, and has an inertia of 35g/m².
  • A more aggressively lightweighted one weighs 2.7kg, and has an inertia of 24g/m².
  • A lightweight 24v flywheel I found was 6.5kg, and has an inertia of 74g/m².
So, adding them into the equation in various combinations gave this:
  • Standard rotating assembly: 189g/m².
  • Conservative lightweight front flywheel: 166g/m².
  • Conservative lightweight front and rear flywheels: 149g/m².
  • Alloy front flywheel and conservative lightened rear flywheel: 102g/m².
  • Deleted rear flywheel and standard 24v front flywheel: 102g/m².
  • Deleted rear flywheel and lightweight 24v front flywheel: 87g/m².
  • Deleted front flywheel and standard rear: 69g/m².

So, I think this confirms a few things that seem to be accepted in 75-land.
  1. The standard rotating assembly has a lot of inertia, which hinders the synchros working well.
  2. Machining the stock front flywheel is useful, the rear one is less useful but still worth it, but this still leaves everything a bit heavy.
  3. Expensive alloy flywheels and lightened rears start to bring things back to a reasonable level, but still not what I'd call lightweight (or even what a standard FWD flywheel is).
  4. If you add in an even more expensive carbon prop then you can probably drop down into respectable figures, but even if it completely eliminated propshaft inertia you'd equal the standard 24v flywheel (although realistically you'd be slightly over).
However, it does look like there might be some legs in trying to delete either the front or rear flywheel. If we aim for the standard 24v flywheel weight as a good conservative figure then we can get there by deleting the rear flywheel and using a lightweight 24v front-mounted flywheel. Or, we can get there by having a minimal front flywheel and keeping the rear flywheel standard (plus a ring gear).

Next question I suppose is whether it's possible to do all of that for a cheaper price than an alloy front flywheel, carbon prop and machining of the rear flywheel, and making sure that the engine still runs alright through its power pulses.

If anyone wants to beat me to it then by all means! If not, then it'll have to wait for the current project to finish for the 75 to start :) still, seems exciting to me, so I'll get to it sooner or later...
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