Race Engine Forum UK

Dear Guy,
Is there a rule of thumb regarding the lowest weight a flywheel can have before top speed starts to suffer? I believe in most circumstances the stock flywheel is the limitor - as only so much weight can be removed.
But for a normal street driven car which has to have a reasonable idle can you go to half the weight without penalties?
My car is a 1200cc Lancia Fulvia angine where the stock flywheel with cover weighs 12000 grams. It has been lightened to 8400 grams and even though throttle responce is very good (not quite motorbike like but close) I believe motorway pulling-power is down a bit.

The only really that is affected by the lightness of the fw is the throttle response. The throttle response is quicker with a lighter rotating mass, in every case. There is no good argument that I know of for having the thing heavier than it needs to be, the determining factor being whether, if overly-light, it makes the engine rather subsceptible to stalling on take-off.

And yes, I have read a lot about the necessity of a heavy flywheel being needed on drag-race engines but I don't want to put my comments re into print here.

There is a relationship between power at output shaft from the engine and fw mass, in that the mechanical losses are less with a lighter itme, but I have never actually measured it.

As regards how light the fw should be on that motor for good throttle response and reliability (ie: not so thin that it may crack) I have no experience, but if it's a vee engine (pardon my complete ignorance of the Fulvia units!) it may one of those setups where the crank depends on (external) balance via masses on the fw - in which case lightening must be done with great care, don't know to be truthful, but Andrew Cliffe from Omicron and Simon Hall from Richard Thorne Classic Cars are both members here and have plenty of robust experience - and may like to help.

GC

You may be noticing the lack of inertia from the heavier flywheel. I changed my stock Fiat flywheel and clutch to a unit weighing 12 lbs (5443 grams) and while it revvs quickly, it doesn't free wheel as well on the highway.

Good point Denise,

you did a good post on fw, could you possibly trace it and copy the link to this thread please?

The job of a fw is to store energy between firing strokes, using energy from a cylinder that is on that phase to push the other cylinders up on their compression and exhaust phases. The phenomenon Denise refers to may be the engine struggling to do that when the fw is too light.

GC

12kg standard is a real heavy flywheel esp for a 1200cc engine. Is it really that heavy?

GC

Guy Croft wrote:Good point Denise,

you did a good post on fw, could you possibly trace it and copy the link to this thread please?
GC

Here's the link:
viewtopic.php?t=559