Lightweight flywheel input please

I’m about to buy a clutch assembly so that I can install the TKO-600 in my 351 Cleveland powered ‘69 Cougar.   Engine is a mild rebuild with TBI swap.  Car weight is probably about 3850 lbs from my best guess.  I’ve just swapped out the rear end to steeper final drive (3.70) to make best use of my new gear spread.   I’m looking hard at aluminum flywheels as part of the replacement.   I use the car for basic blasting around now and want to give CAM autocross a shot since I’ve done massive suspension, brake, and steering changes as well.  

I’ve been reading up on others experiences with lightweight flywheels and that’s a wide variety of input from “its awesome do it NOW” to “what a horrible mess that was I just tore it out”.   Have any of you had experience with a V8 powered car and a lightweight flywheel?   Can you tell me your thoughts and any other input?   Thanks!

Nope, just on lil' Mazdas, in which lightweight flywheels berkelying shine. I shall watch this with some interest.

Yeah it very much depends upon the weight of the car. Add one to a, oh, 4000-lb car and the results will be "meh, wasn't worth it, can't even tell." At the other end is doing the same in a Lotus Seven or Miata, where it'll be very noticeable and makes the engine more lively.

Another more practical way to look at it is when starting from a dead stop; in a heavy car you need a fair amount of stored rotational energy (from a heavy flywheel) to get moving. Put a crazy light flywheel in that same car and it'll be hard not to stall it. The extreme case is not having one at all, like in a motorcycle. Put that bike engine in a car and it's a real bitch to get it off the line without stalling.

You e said it well.  Light weight flywheels make the engine much more responsive at the cost of  being too easy to stall.  Especially if the engine uses a non-stock  camshaft.  
 

If the transmission selected is "close ratio"  it's even worse.  
Add taller than stock tires and you've made getting rolling a delicate task requiring a deft foot, lots of revs,  and slipping clutch. 
 

Trucks have heavy flywheels and "granny" first gears to make getting rolling easy.  
 

Race cars are the opposite  because ease of getting rolling doesn't matter. 

Think of it like cams. A torque heavy cam makes a better street car. A heavy flywheel transmits that torque better. Also, NVH can be a thing with lightened flywheels. That's my opinion, for what it's worth.

I swapped out to the LS7 flywheel on my 06 CTS V ~3900lbs.  It was a very nice improvement and something like ~25lbs of rotating mass gone (OE is a dual mass monster).  It is easier to stall but that just took a day or two to get used to.  I have put lightened flywheels in most of my cars and generally really enjoy them.  The only thing I'll mention is that they seem to highlight any clutch disk issues you may have.  I have a 4 puck unsprung race clutch in my rabbit that absolutely did not agree with the lightweight flywheel.  I went back to a nice sachs oe style clutch disk and everything is super happy.

Lightweight flywheels are easy to drive, especially with a big torque beast under the hood like you have. 

It's the twin-disc clutches that become tiresome. I'd probably do it as long as I had a good harmonic balancer up front.

Thanks for the input all!  I absolutely understand the heavier flywheel impact to driveablility.  My car, with the TKO, will have a lower first gear ratio and lower final drive ratio and slightly shorter tire height than it did when built.  I've been driving manual transmission cars and trucks for most of my driving life (40+ years) in a variety of vehicles-diesel trucks, 4 cylinder gutless wonders, V-8 coupes, flat 6 and turbocharged 4 cylinder Subaru cars-with mechanical, cable, and hydraulic mechanisms-but never anything with a lightweight flywheel.  I don't anticipate any difficulty adapting to the change but wanted to see what you all may have experienced.  Muscle car forums show mixed responses and one clutch supplier told me it would be a bad idea to use one.  Going to talk to some more today.

The only time I regret a light flywheel is with a raunchy engine.  A bridge ported 13B cruises much more nicely with a stock 30lb flywheel than a 10lb flywheel because with the 30lb, the engine braps and sounds cool.  With a 10lb, the engine braps and finds all the lash in the drivetrain with every stutter, destroying U-joints and transmissions and clutch disks.

Other than that, zero downsides all upsides.  If you are using the mass of the engine to pull from a stop instead of accelerating into the clutch engagement, you will hate it.  That is a technique problem not a car problem.  That said a 351C in a nice light Cougar with short gearing is perfect for a light flywheel.

My Supercoupe and LS1 Camaro both have aluminum flywheels. Both are extremely easy to drive on the street and I have no problems with stalling.

It depends on how light too. A bit lighter that stock will work, just takes a bit of getting used to, but is workable.

The 11 lb Quartermaster setup I had in my Camaro (CP autox car) wasn't much fun to get moving, and was real easy to stall. Co-drivers had many issues getting going, Gimp knows first hand. Of course that engine had a big by huge camshaft, which contributed to the problem greatly. But man did it rev quick.

Too many factors to really know for sure.  I have driven vehicles with flywheels that were too light, and some that were too heavy.

Flywheel too heavy:  Pretty much any Taco with a 4-banger.  The compensate for the heavy truck and tiny engine by storing a ton of energy in a heavy flywheel.  It makes starting to move easy, but dear GOD, shifting is a royal pain.  The transmission shifts to the next gear smoothly and quickly, but it takes forever for the revs to come down.  Re-engaging the clutch for the next gear is difficult to do smoothly.

Flywheel too light:  The one I drove was a car much like yours... classic car, 3600 lbs, aluminum flywheel.  It had a combination of a mild cam with an otherwise stock driveline.  Good luck not stalling it.  Rear ratio in the low 3s, 3 on the tree.  Awful combination.  It would have been perfect with an iron flywheel to store up some energy, but the aluminum was a mismatch.  In order for the aluminum to make it work just right, it would have required a LOT of other mods; rear gears and/or different trans, lighter weight, or less cam.  It was one of those situations where just adding lightness meant the rest of the combo was a tragedy.  It was also nearly impossible to get to the next gear before the RPMs returned completely to idle, so daily driving wasn't just messed up in the getting moving part, it was an active, two-foot dance just to shift without getting whiplash.

Long story short... Make sure that A) you will benefit from a lightweight flywheel, and B) that you don't cause six other problems downstream.  The real benefit from an aluminum flywheel is much like putting in a cam.  If you go too far with cam for the vehicle weight, you just add drivability problems and kill performance.

Having said that... I'm putting an aluminum flywheel in a 67 LeMans that weighs (currently) 3500 lbs-ish.  I should be easily shaving 150 lbs with the LS swap and a few other ejections.  It's also getting 4.10s (which, actually, is not that deep with a T56), but the cam is pretty mild.  I'm leaning on good CNC porting to make power, not just camming the crap out of it and killing low end torque.  This is a clean-slate driveline that is engineered together from the air filter to where the treadblocks meet the road, and I think aluminum is the right material for the flywheel.

In my opinion, just adding an aluminum flywheel and not doing anything else is something that should only be used in specific applications:  That is to say, it should only be used where the only reason the OEM didn't use one was cost/warranty/etc.  I'm sure a Miata would benefit.  Likely an MR2, S2000, BRZ, etc.  But just putting one in won't necessarily be a benefit any more than stabbing in a bigger cam.

Driving a late model with a 5 inch twin disc clutch mounted to a flex plate is lovely on an oval, horrible to drive into a trailer.  That's about the most extreme example.

Thanks again all.  The harmonic damper noted above is a good call and something I;ve been thinking about too.

Since Curtis asked, here are more car details:

Vehicle weight: 3850 (assumed, base convertible with no power options, no A/C)

Final drive: 3.70 with Auburn diff (2.75:1 stock final drive ratio)

First gear ratio: 2.87:1

Tires-245/45-17 (25.75" tall, stock tires about 25" tall E78-14)

351 Cleveland, OEM 4V iron heads

Sanderson shorty headers in to full length 2.25" dual exhaust, no cross-over (yet) with Borla mufflers

Dual plane Edelbrock Performer intake for Cleveland 4V with FiTech TB fuel injection system 14" round air filter offset and dropped base with 2" tall element.  Electric in-tank fule pump, AN-6 fuel line (delivery) with stock line used for return to tank

Mild rebuild by previous owner with less than 4k miles.  No cam details but it is mild (no idle lump)

MSD distributor, 14 base timing, no vacuum advance capability, MSD 6A CDI box, MSD wires and new stock temp range and gapped Autolite plugs

Electric cooling fans with PWM control, 192F thermostat, FlowKooler water pump

125 amp (cruise) alternator new 2GA cables

A wide variety of suspension, brake, and steering upgrades front to rear.

Not everything is perfect (distributor incapable of vac advance and no exhaust crossover for instance) but car runs well now with the three speed.

The suspect awaiting 5 speed transplant.

Thanks for the details.  You shouldn't be lacking in torque :)

What trans?  TKO600?  That's the only Ford T5 I can think of with a 2.87 first.  If so, I think you're on the edge of happy.  I'd give it a whirl.  I think that combo would be happier with 4.10s with an aluminum flywheel (high-ish first and a deep OD), but I think you're exactly where I am... should I do it?

I tend to be more conservative than most.  I would put this combo (in my head) right on the borderline, which means I wouldn't do it.  But like I said, I'm Mr. Conservative when it comes to go-fast goodies.  Which means you should probably do aluminum

Edit.... saw you mentioned TKO600 right away.

PS... you got the stance just right.  And those MINILITES.  Yes.

Thanks for the compliments Curtis.  Nice to know you're willing to live vicariously through my decision here!

Yup, TKO-600 with 0.64:1 OD.  351C 4V so plenty of cubes and revs.  I envision some longer road trips so I started off with the 3.70 final drive.   My parents have a place on Houghton Lake about 2.5 hours north of Detroit and I want to do Carlisle and Power Tour/touring events in the future.  Plus I take this to customers and all over the Detroit area all spring/summer/fall every year.  Surface streets, freeway, two lane twisty's and hopefully some CAM-T for fun.

I really am on the fence on the flywheel.  I think it would be a cool addition to the car though I'm still hesitant hence the post.  Worst that could happen is I hate it and replace it/sell it.  I'm doing the install myself.           

With a first gear that short I would want as light a rotating inertia as possible, so the engine can accelerate the car and not just itself.

Thanks Pete.  My thoughts exactly. 

I think zinging the engine up would be intoxicating but detrimental to my fuel level.  

In reply to wawazat :

I am remembering a certain Pontiac I did that had a 2.2 1st trans and a 4.10 rear, and its acceleration in 1st was asymptotic with its neutral free-rev speed.  Heavy iron flywheel.

It had no shortage of grunt, it could boil the tires in 4th gear from 1500, and it pulled to 6000 and felt like it could go much more but, well, I am nervous about spinning a Pontiac that fast.  But there is only so quick an engine can ever accelerate no matter what gears you have.

I bet it would have been great with slicks, a prepared track, and a high RPM clutch dump though!

Beautiful car!

You can probably get away with a lighter flywheel in that car but since it's primarily a street car I don't know that I'd want to give up the streetability.  Also, with the standing start of autocross you may want the heavier flywheel for the launch.

Of bigger concern to me is that you don't seem to know much about the internals of that engine.  The 351C was originally and externally balanced engine.  There are however aftermarket cranks that are internally balanced.  If you don't know which you have you're not going to be able to order the flywheel until you pull the old one off.  If it's got an aftermarket flywheel already it may be difficult to determine what the balance is.  If you have access to the previous owner then you may be able to find out what it's got which would obviously address that concern.

If I could daily drive a cammed up Swift GT with an aluminum FW I think you're going to be alright

Well after some thought I ordered the 14 lb AL flywheel today.  Discussed this with Ram and decided to go ahead with it.  Should be here Saturday along with the clutch and various other goodies I need.   
As a Plan B I picked up a Ford Motorsport billet steel flywheel last weekend so if this doesn't work as I expect I can swap it out.  I'll capture all the masses of the bits in my build thread.

I've never installed a lightweight flywheel that I didn't like, but those were all on vehicles pretty different than what you're describing (Miata, FD, E46 M3).  I've driven a friend's 351W Fox Mustang with a  lightweight aluminum flywheel and it was fine too, but again it was really a road race car pretending to be streetable.

For a heavy, mostly-street car like you mention above, I'd probably aim for a mild reduction in weight from stock.  Many flywheels have a ring you can machine off, something like that.