Thinking about lightening your flywheel
Discussion
grahambell said:
I've run fast road engines with lightened cast flywheels with no problems.
Congratulations. Many people do.
Unfortunately, it's not all about machining - cast iron is not the most consistent material in the world and if you are working with the O.E. flywheel for a car expected by its manufacturer to rev to 6K, machined down and revved to 8K+, you're playing Russian roulette.
The vast majority of people (including you, thankfully) are lucky. As the stories and photos on this thread show, every now and then you get a chamber with a bullet in it.
Do you feel lucky, punk?
grahambell said:
...it's safe enough and it gives much better throttle response.Less engine power is required to spin up a lighter flywheel, meaning more engine power is available to accelerate the car.
Safe enough? 530dtphil's mate, and mine, and the people who owned the cars in the photographs on this thread, would probably disagree with you, but if you like that sort of gamble, good luck to you.
As to 'people who know what they're doing', if you can point me in the direction of a machine shop that runs FEA stress analysis on their designs before machining the flywheel and then x-ray/dye-pens the results afterwards to ensure that the remaining material is free from faults, then I may be prepared to accept your assertion.
In reality, the people who 'know what they are doing' are just slightly more skilled, slightly more cautious empirical engineers, for the most part.
Throttle response? Yes, it's much improved when revving the engine under no load. Very impressive when you're blipping the throttle at traffic lights.
The Oselli figures are a gross exageration, unless the flywheel was hugely overweight in the first place. I suspect that they may have been talking about something as agricultural as the old MGB engines in which they used to specialise.
In practice, I doubt you could consistently time the difference between a standard and lightened flywheel, with no other mods.
Like you say, the results are more noticeable in bottom gear, anyway, which is exactly why the most noticeable effect on a road car is to make clutch control trickier and the engine easier to stall when pulling away. It's not a massive disadvantage, unless you've gone overboard with the lightening, but then the advantages are even less pronounced, so why bother?
grahambell said:
So even for a fast road car it's well worth getting the flywheel lightened - just as long as it's done right.
Each to their own. I have a lightened flywheel on one of my current cars (built for racing - and it's a proper steel one, balanced along with the crank, rods and pistons), and I'd certainly include a proper steel one as part of the package of improvements on any Full Race engine build, but I'd urge anyone thinking about lightening a standard cast iron item to carefully consider the well documented evidence that they DO occasionally explode, and balance the very unpleasant consequences when it happens, against the advantages of saving yourself a couple of hundred quid.
The usual argument against cheap crash helmets is 'do you have a £200 head?'.
The analogous argument for using a lightened CI flywheel instead of a pukka steel one is: 'would I like to gamble £200 on being eviscerated and having my genitals shredded by shrapnel?' For me? Not even at very long odds, thankyou!
Sam_68 said:
In reality, the people who 'know what they are doing' are just slightly more skilled, slightly more cautious empirical engineers, for the most part.
You're probably right there, but that's the thing here - knowing how much metal you can safely remove and where. Obviously the more cautious types will play safe and leave more metal instead of trying to shave off every last gramme.
Regarding the Oselli figures I mentioned, I think they just quoted the 10lb as a nice round figure to demonstrate the point rather than suggesting you could actually take that much off the flywheel.
As for revving to over 8k (
)with a lightened iron flywheel being like playing Russian Roulette - well you could be right there but I've never revved any of my engines that high. Those revs are beyond my definition of a 'fast road' engine (VTEC etc excepted) and I'd agree you wouldn't want to take chances with that amount of energy. But for my 6-7k momentarily through the gears type driving I still reckon a sensibly lightened standard flywheel is good enough. As you say, it might not make a huge difference to performance, but as Tesco say, every little helps.
Sam_68 said:
As to 'people who know what they're doing', if you can point me in the direction of a machine shop that runs FEA stress analysis on their designs before machining the flywheel and then x-ray/dye-pens the results afterwards to ensure that the remaining material is free from faults, then I may be prepared to accept your assertion.
Yeah, I could do that, and more to ensure the accuracy of the component, but then the bill would be in four figures.
Can people really not afford aftermarket flywheels?
They're not what you'd call expensive.
Sam_68 said:
I have a lightened flywheel on one of my current cars (built for racing - and it's a proper steel one
Ok, so you're saying that an item like the Fidanza I have is ok then?
I had the Fidanza fitted to the Corrado at the same time the cams, high compression head gasket, variable inlet manifold, lower final drive and rolling road remap were done. So I have no way of comparing before and after. I'd like to think that it made some difference, but even if it didn't at least it was only £200 for the parts (God bless the weak dollar at the moment) and the labour was essentially free as there was so much other engine work being done.
sultanbrown said:
Yeah, I could do that, and more to ensure the accuracy of the component, but then the bill would be in four figures.
Can people really not afford aftermarket flywheels?
They're not what you'd call expensive.
exactly... if you're willing to spend the money necessary to be confident that a CI flywheel is safe, you might as well have bought one in a better material, anyway! JonRB said:
Ok, so you're saying that an item like the Fidanza I have is ok then?
I'm not familiar with the Fidanza brand, but if it's steel, then yes, there's no reason it shouldn't be perfecty safe.
The risk with flywheels is that at high revs the centrifugal force is basically trying to tear the flywheel apart under its own weight. Cast iron doesn't particularly like tensile loads and can also be quite brittle if subjected to any vibrational imbalance (though it depends on the exact metallurgy - some CI is quite ductile).
At the end of the day, steel is just cast iron with a bit of extra carbon added, but it has substantially better tensile strength and tends to be of more consistent quality. It's enough to increase the safe rev limit to comfortably more than most fast road engines are capable of, though.
The other obvious consideration is engine configuration... if you have a longitudinal mid-engine, you probably aren't too worried about the flyweel letting go, since it's a long way behind you. A longitudinal front engine isn't too bad, either - it will just cut your legs off at the knees.
The Mini is the worst possible arrangement, because the tangent to the flywheel is basically in line with the centreline of the driver, so if fragments go backwards, they are pretty much guaranteed to hit something vital.
grahambell said:
As for revving to over 8k ()... those revs are beyond my definition of a 'fast road' engine (VTEC etc excepted)
...for my 6-7k momentarily through the gears type driving I still reckon a sensibly lightened standard flywheel is good enough
)... those revs are beyond my definition of a 'fast road' engine (VTEC etc excepted) ...for my 6-7k momentarily through the gears type driving I still reckon a sensibly lightened standard flywheel is good enough
That's probably why our views differ! I agree that cast iron is pretty safe at 6-7k revs.
Possibly as a result of early experience with Imp racing engines (10.5-11K rev limit), I didn't consider 7K+ to be anything exceptional... out of deference to its age and fragility, I stick to 7,250rpm on my Elan (on a CI flywheel), but the Crossflow in my Sylva (quite hot for a fast road spec, but certainly not 'full race' at circa 155bhp) has the rev limiter set at 7,800rpm and even my bog standard workhorse estate car regularly sees 7K (redlined at 6.5K on the tacho, but I haven't found the rev limiter yet!
). And although I'm not an A-series expert myself, I know from friends who are that 8K is not regarded as an extreme limit for a properly modified engine, despite the relatively long stroke and crude head design, hence the propensity for flywheels on Minis to detonate every now and then! At such modest revs, though, I'd question the need for a lightweight flywheel even more - what's the point of a modification intended to make an engine rev more freely if it runs out of breathing at 6-7K?!
Sam_68 said:
grahambell said:
As for revving to over 8k ()... those revs are beyond my definition of a 'fast road' engine (VTEC etc excepted)
...for my 6-7k momentarily through the gears type driving I still reckon a sensibly lightened standard flywheel is good enough
)... those revs are beyond my definition of a 'fast road' engine (VTEC etc excepted) ...for my 6-7k momentarily through the gears type driving I still reckon a sensibly lightened standard flywheel is good enough
That's probably why our views differ! I agree that cast iron is pretty safe at 6-7k revs.
Possibly as a result of early experience with Imp racing engines (10.5-11K rev limit), I didn't consider 7K+ to be anything exceptional
Given that the standard Imp bottom end with no modifications is good for 8k, 7k+ is not anything exceptional on a road car IMO.
Mine in the Davrian theoretically is safe to 9500, not owning a rev limiter I have discovered that the valves bounce at 10200 though! oooops!
Ok here goes....
Late reply but feel compelled to dismiss some myths.
1) Cast iron is not the same as cast steel.
2) Cast iron is not for lightening, throw it in the bin
3) Cast steel is fine as long as you know where to remove material to get the best results.
4) if you design it for 8k rpm then it is either only to be used upto 6k rpm or tested to 11k rpm.
5) Standard Mini flywheel can weigh 30lbs and a super lightweight TI equivalent can weigh 3lbs the difference IS noticeable on the road and is a massive improvement especially if run on a low CR turbo engine (think about this one guys as it offers benefits beyond most peoples imagination even so called experts struggle)
6) All steel billets are cast, but they are CAST STEEL ! cast iron goes in the bin !
7) Design MUST be covered by FEA and or lots of testing above the useage threshold, ie refer to point no4.
8) If you are modifying your car and you fit a lightened flywheel it will react differently than your heavy standard item, however cams, exhaust etc etc will also have a dramatic change on the engines characteristics
9) So if you want a smooth as silk quiet peaceful easy to to drive car keep it standard.
10) Not my website but I agree with their figures... http://www.uucmotorwerks.com/flywheel/how_a_lightw...
11) try and lose the equivalent by removing trim etc....
Late reply but feel compelled to dismiss some myths.
1) Cast iron is not the same as cast steel.
2) Cast iron is not for lightening, throw it in the bin
3) Cast steel is fine as long as you know where to remove material to get the best results.
4) if you design it for 8k rpm then it is either only to be used upto 6k rpm or tested to 11k rpm.
5) Standard Mini flywheel can weigh 30lbs and a super lightweight TI equivalent can weigh 3lbs the difference IS noticeable on the road and is a massive improvement especially if run on a low CR turbo engine (think about this one guys as it offers benefits beyond most peoples imagination even so called experts struggle)
6) All steel billets are cast, but they are CAST STEEL ! cast iron goes in the bin !
7) Design MUST be covered by FEA and or lots of testing above the useage threshold, ie refer to point no4.
8) If you are modifying your car and you fit a lightened flywheel it will react differently than your heavy standard item, however cams, exhaust etc etc will also have a dramatic change on the engines characteristics
9) So if you want a smooth as silk quiet peaceful easy to to drive car keep it standard.
10) Not my website but I agree with their figures... http://www.uucmotorwerks.com/flywheel/how_a_lightw...
11) try and lose the equivalent by removing trim etc....
KMPE2011 said:
Ok here goes....
Late reply but feel compelled to dismiss some myths.
1) Cast iron is not the same as cast steel.
2) Cast iron is not for lightening, throw it in the bin
3) Cast steel is fine as long as you know where to remove material to get the best results.
4) if you design it for 8k rpm then it is either only to be used upto 6k rpm or tested to 11k rpm.
5) Standard Mini flywheel can weigh 30lbs and a super lightweight TI equivalent can weigh 3lbs the difference IS noticeable on the road and is a massive improvement especially if run on a low CR turbo engine (think about this one guys as it offers benefits beyond most peoples imagination even so called experts struggle)
6) All steel billets are cast, but they are CAST STEEL ! cast iron goes in the bin !
7) Design MUST be covered by FEA and or lots of testing above the useage threshold, ie refer to point no4.
8) If you are modifying your car and you fit a lightened flywheel it will react differently than your heavy standard item, however cams, exhaust etc etc will also have a dramatic change on the engines characteristics
9) So if you want a smooth as silk quiet peaceful easy to to drive car keep it standard.
10) Not my website but I agree with their figures... http://www.uucmotorwerks.com/flywheel/how_a_lightw...
11) try and lose the equivalent by removing trim etc....
21mths to respond to a 10yr old thread Late reply but feel compelled to dismiss some myths.
1) Cast iron is not the same as cast steel.
2) Cast iron is not for lightening, throw it in the bin
3) Cast steel is fine as long as you know where to remove material to get the best results.
4) if you design it for 8k rpm then it is either only to be used upto 6k rpm or tested to 11k rpm.
5) Standard Mini flywheel can weigh 30lbs and a super lightweight TI equivalent can weigh 3lbs the difference IS noticeable on the road and is a massive improvement especially if run on a low CR turbo engine (think about this one guys as it offers benefits beyond most peoples imagination even so called experts struggle)
6) All steel billets are cast, but they are CAST STEEL ! cast iron goes in the bin !
7) Design MUST be covered by FEA and or lots of testing above the useage threshold, ie refer to point no4.
8) If you are modifying your car and you fit a lightened flywheel it will react differently than your heavy standard item, however cams, exhaust etc etc will also have a dramatic change on the engines characteristics
9) So if you want a smooth as silk quiet peaceful easy to to drive car keep it standard.
10) Not my website but I agree with their figures... http://www.uucmotorwerks.com/flywheel/how_a_lightw...
11) try and lose the equivalent by removing trim etc....
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