While completing an overdrive transmission conversion on our 1973 Triumph Spitfire–look for the full story in a future issue–we took the opportunity to make an upgrade, one you’ll never spot on the showfield.
While the transmission was out, we traded the stock steel flywhee…
The issue that alloy flywheels raise is the thin steel friction surface attached by screws to the alloy - they can warp or come unglued/unpegged.
I have almost always gone the way that doesn't have that possible issue and usually costs less anyway - lightening the stock steel flywheel. Same result, less expense, no risk in regard to the friction surface. Check around - sometimes the factory wheel used on a slightly different (sportier?) model is interchangeable ( early MGB are lighter and interchangeabke with MGA for instance).
You can get pretty close to alloy wheel weights if desired, by windowing the periphery of the flywheel. This is an MGB race wheel.
Another issue with an alloy wheel can be the steel ring gear coming adrift while in use - we routinely braze the ring gears to the flywheel on lightened steel wheels before balancing so that can't happen, but you can't do that with an alloy wheel.
A lighter flywheel can have a profound effect - an Austin Healey 100 used a stock flywheel that weighed just on 40 lbs. Talk about agricultural - they took the engine straight out of a commerical vehicle without lightening it.
Alloy flywheels have been used in racing for many decades - I ran one on one of my MG engines that came from an Elva Courier - they had homologated it for Le Mans - and I'm not saying that they are necessarily risky, just that there are other ways of getting there for a street car.
On both my 1600 Cortina's I took a lot of weight off the flywheels w/o any issues.Just don't take off too much I guess.
While my 1997 Jetta is not a classic, I did have the following conversation with the local VW machine shop when I had the flywheel resurfaced.
Shop: you want this lightened also?
Me: no, I just need it resurfaced.
Shop: you don't want to lighten it?
Me: no. Just resurface it. It is stock. It is a daily driver.
Shop: so don't lighten it. Only resurface it.
Me: correct.
The Shop seemed really confused that I didn't want that done.
I put that same Fidanza in my TR6. 'about 9 or 10 pounds i think. The stock one was about 22lbs? It revs up and down much quicker but you have to feather the clutch much more from a stop.
A Fidanza flywheel is working great on my TR6. It's fine for a fast street car and makes the engine much more willing to rev. Since I had a stack of stock flywheels, I did consider having one of those lightened. But getting it anywhere near 10 lbs. would be difficult without removing too much material. I previously used an aluminum flywheel in an autocross TR6 that was run hard for over a decade without incident.
Add one more to the list with a Fidanza on my TR6. I like it and have never had any issues with it.
Went from 29lbs on my stock flywheel to 11lb Fidanza on my MGA 1500. Also a MGB competition pressure plate and Volvo P1800 clutch disk 
Engine revs faster and pulls quicker through 1st and 2nd gear which is perfect for stop and go traffic and Autocross
It does increase vibration though - especially with new competition engine and trans mounts. I can feel engine impulses through the gearshift and at certain RPM through the wheel and seat from harmonics. Very, very minor and minimal, but there.
In reply to RobMason :
Every performance modification has trade offs. I would expect your mounts are primarily to blame here.
I recently had a Fidanza flywheel installed on my MGB rebuilt engine before it went back in the car. It starts much quicker than the original and it sounds good with a new original type starter. It runs great. No problems
My first experience with lightened flywheels was on The World's Crappiest Lotus-Cortina, which I sold immediately before Lo-Cort prices tripled.
The previous owner subscribed to the philosophy that if a little was good, then. lot must be better so Too Much must be Just Right.
The flywheel in that car, along with the rest of the engine, was said to come from a Formula B race car. It was VERY difficult to drive in a parking lot, for example, stalling frequently if the right rev-to-slip ratio wasn't used. And of course because the wiring was as wonky as everything else on this car, the starter wouldn't always engage. (My favorite wonky bit: when I went to sell the car, the potential buyer pulled up the green fake turf in the trunk to find a pair of steel Castrol signs that had been cut and pop-riveted to cover the rust holes.)
My next experience with a lightened flywheel was when the clutch needed replacing on my 1996 Miata. I decided to go just one step lighter than stock, to achieve pretty much exactly what your goals are for the Spitfire: let the engine spin up just a little quicker without the Lo-Cort's "whoops, I did it again" tendency to stall backing out of a parking lot.
The shop recommended an ACT Street-Lite flywheel, which proved to be a great example of why the sweet spot is often more important than the maximum. The engine felt lighter (still not quite like a 1.6L but with the 1.8L torque) but it was totally docile in the grocery store parking lot, my kids' high-school parking lot, etc. AND it was a blast on track at Laguna Seca and PIR, plus forest roads for rallies and tours.
When I replaced that Miata with another '96, that one an M Edition in Starlight Blue Mica, I was pleased to note that the Street-Lite flywheel had already been installed.
May you have the same balance of additional liveliness and streetability that I enjoyed on those two cars.
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