just swapped engines in my WHB310 While doing this I discovered that the A14/15 factory flywheel is about 5mm thicker than the A12 flywheel fitted to the new motor. The thinner A12 flywheel resulted in the release bearing being too far away from the clutch. Problem solved by retaining the A14 thick flywheel. While I had both flywheels off , I weighed them and found the A14 thick flywheel was only 500g heavier than the A12 flywheel.

My question is : on a stock motor, is there any advantage or disadvantage in using the slightly lighter flywheel?

Good work. The lighter flywheel makes for quicker revs. On the other hand, the heavier flywheel gives smoother low rpm running. To mix-and-match flywhwels, use the corresponding release bearing sleeve.

A12 flywheel is great on A15, abit lighter is even better.
A lighter flywheel is great (within reason).
For example this one is a pig for street driving, up for sale soon 2.6kgs.

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Try driving it in stop-n-go traffic, and you'll appreciate a heavy flywheel. It smooths things out.

"better" does not mean better all around.

You are so right DD, Mildly lightened A12 flywheel was great with my A15 but the one above would be nasty in traffic.
Depends what you want I suppose smoothness or responsiveness, best go for the in between.

Yes, flywheel weight is all about compromise. I have one of the super light flywheels like Levey showed above in my dedicated autocross car that occasionally gets street driven. While it's entertaining for short periods it's a pain in the a** after awhile.

The other 1200 I drive is a stocker with the full weight flywheel. Way too far in the other direction...

The best compromise for me is a stock flyweel lightened to the Datsun comp specs. They come in around 12 pounds and work well. Light enough to be fun but not a pain in the a**.

I'm using the 14lb flywheel on my car, it revs well and is still user friendly. I used to rev the A12 to 8600 and the A15 goes to 8200, never felt the 14lb one was to heavy. Even for my race only coupe I wouldn't got lighter than 9-10lbs.

Tom

Hey Levey - how much you looking for for that flywheel (pm me if you'd prefer)??

David Vizard spelt out the maths about light flywheels in one of his books about either minis or mini/bmc A-series engines. Basically with such small and relatively underpowered engines, esp with really outrageous gearing stuff approaching 5:1 diff gears with smaller tyre diameter than a datto, and that combined with the multiplication ratio of 1st gear as well and you're actually to the point that in first gear (and to a much lesser extent 2nd gear, and almost negligible in the higher gears) that flywheel weight actually 'appears' to the engine at full throttle to be a relevant weight/mass to be accelerated, since the torque multiplication of the diff/1st gear combined makes the engine 'think' the car is a lot lighter (sort of) vs if it had to accelerate it with 1:1 gearing and say a 3.7 diff. So in that first gear and with very low diff gears (higher numerical ratio) you can actually gain something like (traction willing) perhaps a 5% acceleration advantage with a lighter flywheel. I foget the exact figure quoted and obviously it would be different with different engines/cars and their specific gear/diff gear ratios, but the point is, traction willing it can have an effect in first gear.

But keep in mind just how little time the car will be in first gear at all before it hits the redline and the advantage is less than that '5%' approximate figure might otherwise suggest. Still, if you were in a very hotly contested and tightly restricted racing class and flywheel weight wasn't restricted to the lightest factory stock flywheel, well every little bit helps.

As already mentioned, it'd be an absolute abomination in gridlock traffic, also a little more 'fussy' if you were leaving from a standstill, when parked uphill somewhere and wanted to take off at a normal/sedate pace (if it's a racing takeoff, hillclimb, well you can give it all the rpms you want I guess. I'd probably suggest that for a street driven car, a pretty decent compromise would be running an a14/15 with an a12 flywheel. Personally I'm a bit of a masochist, and have always loved race cammed engines (don't even get me started on highly modified 2 stroke bikes, I think there's nothing more enjoyable to ride) and the lighter flywheel and all that. In a strange way the more difficult they are to get into the groove with and drive optimally the more satisfaction that comes from mastering their idiosynchrasies.

NOt suggesting I'd take that sort of car to work and back, but would have no reservations about street driving it on weekends in that state of tune :) .

I am also interested in that flywheel levey if JMac passes. Cheers