[Healeys] Lightened flywheels

Roger Grace roggrace at telus.net
Wed Sep 8 11:26:24 MDT 2021

I am with M on this one. There is no doubt that where the metal mass is
removed is important. The moment of inertia is proportional to the square
of the radius. I removed 6.8 lbs from my BJ8 one mostly around the
periphery. This is a 23% reduction in weight but must be a higher %
reduction in inertia - perhaps 30% - who knows !
Next issue being discussed is the potential improvements (if any) with this
mod. No doubt  when in neutral the engine will accelerate faster as less
inertia to get moving. Total inertia is engine plus flywheel (ignoring
drive train) inertias. So the relevant fact is the CHANGE/REDUCTION in
total inertia. I have no idea what the engine inertia is.
When the engine torque accelerates the flywheel, and the engine mechanical
components, energy is added to the flywheel. This is proportional to change
in angular velocity (RPM) and Inertia.
So, with a lightened flywheel, less engine torque (energy) is used to
accelerate the flywheel to a new speed and thus more energy (engine output)
is available to accelerate the car.
Very difficult to estimate. - may or may not be insignificant.
So until someone does some bench tests to measure these inertias we will
just have to rely on anecdotes !
On my car the engine now shuts off pronto and no fear of the dreaded

On Tue, Sep 7, 2021 at 6:10 PM Michael Salter <michaelsalter at gmail.com>

> Perhaps another way to look at this question is with respect tp engine
> power.
> It takes a certain amount of horsepower to accelerate a mass (car) at a
> certain rate.
> Taking weight off the flywheel decreases the amount of horsepower required
> to accelerate the car and for that matter the engine's speed of rotation
> but the change in the amount of horsepower required is very small.
> Sure engines with light flywheels sound racey but they aren't more
> powerful.
> M
> On Tue., Sep. 7, 2021, 6:22 p.m. richard mayor, <boyracer466 at gmail.com>
> wrote:
>> I should add  one more thing for clarification.  The general rule of
>> thumb for comparing rotational weight to other weight (sprung or unsprung)
>> is 10 to 1.  So, using that rule, 5 pounds of weight removed from
>> rotational mass would equal 50 pounds of fuel. As Michael pointed out.
>> But, it doesn't alway work out that way.  If you removed one pound from
>> the flywheel in the area where the bolt pattern is then the 10 to 1 rule
>> may apply.  But if you remove one pound of material out near the ring gear
>> then it is more like a factor of 30 to 1.  Any rotational weight removed
>> further away from its axis will have a much greater effect. And when you
>> lighten a Healey flywheel the place where the meat comes off first is out
>> near the ring gear.  Removing 3 or 4 pounds has a big effect on
>> acceleration.
>> On Tue, Sep 7, 2021 at 2:24 PM Michael Salter <michaelsalter at gmail.com>
>> wrote:
>>> The total mass of the flywheel is not completely relevant insofar as
>>> engine moment of inertia is concerned.
>>> What is important is the mass removed times the square of the radius, so
>>> just how much weight has been removed per se is not important.
>>> Secondly removing rotational inertia is not always the best thing to do,
>>> if it was you can be assured that manufacturers would be all over it as a
>>> way to save money.
>>> The rotational mass is often designed in to protect the drive train, in
>>> particular the gearbox.
>>> Read this:
>>> https://www.netbug.net/blogmichael/2017/01/03/austin-healey-100-crank-failures/
>>> Thirdly lightening a flywheel in of itself has very little effect on
>>> acceleration.  An engine with a lightened flywheel may accelerate to 6000
>>> RPM 1 - 2 seconds more quickly but that doesn't necessarily equate to 1-2
>>> seconds faster to 60 MPH. You may be able to shift marginally faster if the
>>> gearbox can stand the extra loads and the 4 to 5 pounds off vehicle weight
>>> is really only the equivalent of half a tank of fuel. (Who ever notices
>>> that their car accelerates more quickly with a half full gas tank).
>>> Just sayin'
>>> M
>>> On Tue., Sep. 7, 2021, 4:17 p.m. Earl Kagna via Healeys, <
>>> healeys at autox.team.net> wrote:
>>>> I have Bill Bolton lightened flywheels in both my Healeys.  According
>>>> to Kent Lambert, who took over Bills Healey business awhile back, the stock
>>>> weight (6 cyl) is 28.5 pounds, and Bills machining program reduced them to
>>>> 24.
>>>> Both of my cars run just fine, no problems at all in the 20 + or so
>>>> years that I’ve had them installed.
>>>> Earl Kagna
>>>> Victoria BC
>>>> BJ8, BT7 tri-carb
>>>> *From:* Healeys <healeys-bounces at autox.team.net> *On Behalf Of *richard
>>>> mayor
>>>> *Sent:* Tuesday, September 7, 2021 10:23 AM
>>>> *To:* healeys <healeys at autox.team.net>
>>>> *Subject:* [Healeys] Lightened flywheels
>>>> 6 cylinder flywheels vary slightly but are all generally in the 29
>>>> pound range. Very aggressive machine work can get the weight down to the
>>>> 22-23 pound range.
>>>> How much weight you take off will affect your driving style. To
>>>> maintain the feel of a stock Healey while still improving the engines
>>>> response, I suggest removing no more than 3 pounds.
>>>> Richard
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