[v12-engine] Lightweight flywheel or not

V12-Engine
1

As I have my V12 out for gearbox repair, I’m wondering if I
should replace the original flywheel with a lightweight
version.
Ideally I’d like to do a cam and carb modification as well, but
that will have to wait or not happen at all.
Apart from a quicker reving engine, what are the pros and cons
of a lightweight flywheel ? Less smooth idle ? Vibration and
therefore more wear on various parts ?–
Ole-XKE1974
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2

In reply to a message from Ole-XKE1974 sent Sat 17 Oct 2015:

In an E-type, I would guess this to be a good move.

In my XJS, I run an aluminum flywheel, and the only real
downside I can see is that I do have to be a bit careful
engaging the clutch- slipping a bit to get moving.

This was essential when I ran the 2.88 diff (3.27 first
gear), but is even needed (but to a far lesser degree) with
the 3.54 diff I am running now. I attribute this in part
to the lighter flywheel, figuring a heavier steel unit
would likely make clutch engagement a bit more insensitive.

As for vibration, I would think a careful balance of the
rotating assembly would do the trick- don’t miss the prop
shaft, either, in this process. And, I also have an
acquaintance who was able to take the crank (he was using
an HE V12 in an E type, as I recall) in with the flywheel,
clutch and pressure plate for balancing, and match
marking. I would guess it would be tough to get a smoother
assembly than that! Also, aluminum should be more
effective at dampening vibration relative to steel, if you
do have any residual.

-M–
Mike, '90 5.3 XJS Conv., 5-spd+3.54, SE-ECU+TT F/R bars
Lakewood, OH, United States
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3

Apart from a quicker reving engine, what are the pros and cons
of a lightweight flywheel ?

On a V12 carb engine, almost no cons. Primary pros are quicker
revving and therefore shifting, and better acceleration in low gear.
The OEM flywheel is stupid heavy, so this is a good area for
improvement.

Less smooth idle ?

You’ll never see a difference on a V12.

Vibration and
therefore more wear on various parts ?

No, vibration won’t increase. Stress on the driveline may be
reduced.

The biggest downside will be that it’ll be easier to overheat your
clutch. If you tend to ride it a lot, you’ll want to change your
ways. Just driving with the clutch fully engaged doesn’t generate
heat, though, and neither does sitting at a light with the clutch
fully disengaged. Only slipping the clutch, as when trying to first
get rolling in 1st gear, generates heat in the clutch. Since the V12
is very difficult to stall or bog, there’s really no reason to slip
it much unless you’re in the habit of using it to pull stumps or
something. IMHO, Jaguar’s decision to use such a massive flywheel on
this engine is inexplicable.

Don’t throw the old flywheel away. Lots of people with A/T’s would
love to have it for a M/T conversion. In fact, you can probably sell
it for most of what the lightweight flywheel will cost you.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 17 Oct 2015 at 3:55, Ole-XKE1974 wrote:

4

Kirby you specificall mentioned carbs. Are there any negatives in a fuel injected version?
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5

Kirby you specificall mentioned carbs. Are there any negatives in a
fuel injected version?

There are issues with going with a M/T with a fuel-injected engine.
First off, the fuelling maps built into the ECU may not be optimized
for performance below the stall speed of the torque converter, since
it presumably never has to perform in that region. Second, the ECU
has an overrun cutoff feature which works fine with an A/T but can
cause driveline shunting with a M/T, and that shunting can be
affected by flywheel mass – although I’d expect the lighter flywheel
to be better in this respect.

The biggest issue, though, especially with early (1980’s) EFI
systems, is that the EFI system may not work very smoothly off-idle.
An EFI fuelling map often uses a considerable portion of its data
trying to optimize smoothness in the first coupla hundred RPM above
idle, but the early EFI’s don’t have that much data to work with.
With some early EFI cars, putting the car in 1st and trying to drive
around a parking lot at walking speed can result in some very weird
effects, the car lurching like a bucking bronco with very slight
changes in throttle. Having a heavier flywheel might help smooth
such lurching – but a better option would probably be to ship the
ECU off to AJ6 Engineering with instructions to modify it for M/T
use. If you have a 6CU, you might also opt to upgrade to a later
16CU which is pin-compatible (plug-in upgrade) and has a better
processor and more extensive fuelling map.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 17 Oct 2015 at 12:23, John wrote:

6

In reply to a message from Kirbert sent Sat 17 Oct 2015:

I would save my money. I can say from personal experience in
my 71 V12 E-Type, it was a dog. The V12 is low in torque at
low rpm to start with, and if you drive it mostly in town,
you most likely won’t like it. After doing the 5 speed
conversion, and aluminum flywheel, I wish I had spent the
money elsewhere. 99% of my driving was in town. I feel the
light flywheel completely changed the driving of the car.
The 5th gear didn’t help much, as I did little driving on
the highway. The heavy steel flywheel would allow me to pull
off literally without touching the throttle, but the
aluminum flywheel had to be ‘‘feathered’’ to get a smooth pull
off. Pretty much like Kirbert said, you will possibly
overheat the clutch. Mine didn’'t last long, had to pull it
all out again. Syupidly, I got rid of all of it right after
I did it, and then wished I hadn’t.–
89 XJS convertible Marelli ignition
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7

In reply to a message from Kirbert sent Sat 17 Oct 2015:

The V12 is very difficult to bog down for sure.

But you can easily stall it on roll out if your gearing is
tall enough. Kirby makes his comment from the experience
of a 4.11 diff ratio, and what is likely a 2.9 to 3.3 first
gear.

I guarantee you if you run a 2.88 and any readily available
box (meaning a first gear of around 2.9 to 3.3) you WILL
have to slip the clutch on roll out, or otherwise will
likely stall the engine more than 50% of the time. You
will even have to do it with a 3.54. I speak from direct
experience on this.

The only thing that might modify my opinion is that your E
type GVW is some 500 lb less than my XJS’s.

Aluminum wheels make sense, though, regardless. I will
take the lowered weight and faster spool every time.

-M–
The original message included these comments:

get rolling in 1st gear, generates heat in the clutch. Since the V12
is very difficult to stall or bog, there’s really no reason to slip

–
Mike, '90 5.3 XJS Conv., 5-spd+3.54, SE-ECU+TT F/R bars
Lakewood, OH, United States
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8

In reply to a message from Gene Holtzclaw sent Sat 17 Oct 2015:

Had roughly the same situation with my etype. The clutch
lasted roughly 20k miles. Swapped back to the original
flywheel and did not really realize how much the
lightweight flywheel had changed the driving
characteristics.

Save you money…

Jeff Smith
Atlanta, GA
74’etype OTS
57’ Mk1 MOD–
The original message included these comments:

In reply to a message from Kirbert sent Sat 17 Oct 2015:
I would save my money. I can say from personal experience in
my 71 V12 E-Type, it was a dog. The V12 is low in torque at
low rpm to start with, and if you drive it mostly in town,
you most likely won’t like it. After doing the 5 speed
conversion, and aluminum flywheel, I wish I had spent the
money elsewhere. 99% of my driving was in town. I feel the
light flywheel completely changed the driving of the car.
The 5th gear didn’t help much, as I did little driving on
the highway. The heavy steel flywheel would allow me to pull
off literally without touching the throttle, but the

–
Jeff Smith
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9

Kirby makes his comment from the experience
of a 4.11 diff ratio…
I guarantee you if you run a 2.88 and any readily available
box (meaning a first gear of around 2.9 to 3.3) you WILL
have to slip the clutch on roll out…

To be clear, I drove my 5-speed with a 2.88 for months before I got
around to swapping the diff out for a 4.11. Didn’t really HAVE to
slip the clutch much at all with either, as the V12 will pull from a
standstill without even using the clutch if you so choose. I
normally did slip the clutch a bit on takeoff, but never enough to
overheat an aluminum flywheel. Those takeoffs are never enough to
cause problems there. Where you run into problems is where you’re
really abusing the clutch, like trying to get the car unstuck from
deep sand or snow, or trying to pull a stump, or trying to remain
motionless at a stop sign on a hill using the clutch rather than the
brake. Or trying to do burnouts by holding the brake and slipping
the clutch waiting for the light to turn green.

Like I said, providing a V12 engine with such a massive flywheel just
makes no sense to me. My 289 Mustang with a 3-speed stick had a V8
rather than a V12 and nearly the same displacement, and it came from
Ford with a flywheel that probably weighed less than half what the
Jaguar V12 flywheel weighs.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 18 Oct 2015 at 5:29, mike90 wrote:

10

In reply to a message from Kirbert sent Sun 18 Oct 2015:

You keep referring to a massive flywheel Kirby.

I have a stack of manual transmissions, as Ole will
attest, and I cannot tell a 5.3 flywheel from a 2.4, 2.8,
3.4, 3.8 or 4.2 flywheel at first glance. The V-12
flywheel is not ‘massive’ at all, it’s normal for pretty
much any Jag and the E-type and XJ-S cars Jaguar fitted it
to - neither of which is a featherweight vehicle or
designed for street racing.–
1E75339 66 D, 1R27190 70 FHC, 79 S2 XJ12L
Gaithersburg, Maryland, United States
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11

The V-12
flywheel is not ‘massive’ at all, it’s normal for pretty
much any Jag…

Then I guess Jaguar had a penchant for massive flywheels. I simply
couldn’t believe how overweight that thing was when I opened the
package. It’s, like, an inch thick all the way out to the rim. The
Ford 289 flywheel I replaced once was the same thickness as the ring
gear out at the rim, making it a very light, quick-spinup flywheel
indeed – and that engine was a dog, a low-performance V8 coupled to
a garbage 3-speed stick.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 19 Oct 2015 at 2:09, PeterCrespin wrote:

12

I’ve had both behind the 6.0 in my 1977 XJ12. Perhaps the 6.0 has more off-the-line grunt, but I find the lightweight flywheel to be perfectly streetable. It does give quicker throttle response when free-revving the engine, but whether this translates into meaningful performance gains is not clear. The flywheel was done is in the midst of a number of other weight-loss upgrades so I will never know how much the flywheel alone contributed to increased performance.

FWIW, I’m running a Toyota R154 (3.25 1st gear) with the 3.31 rear end. It drives just like any other car. Certainly it’s smother off the line then the wife’s Subaru.

Perhaps more important then the weight of the flywheel is the type of disc installed. I prefer a very mild street disc as I like the smooth buttery launch gives and really the V12 doesn’t make THAT much power that would require a super grabby clutch assembly.

-Gary

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13

In reply to a message from Gary Evans sent Mon 19 Oct 2015:

I have a aluminum flywheel(FW) behind my 5.3 along with a
TKO 600 5sp. 2.87 1st & 3.54 diff. The only down sides I
notice is it does stall easier than any other stick I ever
drove with a steel FW the other is city driving, but that’s
part of any standard trans vs automatic . I never had a
steel one in the XJ-S so I really can’t compare the two.
Larry
91 XJ-S 12cyl 5sp
95 VDP 6cyl–
Larry Hartman
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14

It does give quicker throttle response when
free-revving the engine, but whether this translates into meaningful
performance gains is not clear.

The question is whether the inertia of the flywheel is a significant
portion of the overall inertia of accelerating the car. In top gear,
that’s a no. However, after I installed the 4.11 rear end in my '83,
I found that the car would accelerate faster in 2nd than in 1st; the
performance in 1st seemed rather anemic, but the instant you grabbed
2nd it’d pin you back in the seat. In 1st gear, the car accelerated
from standstill to 6000 rpm barely any slower than the engine
accelerated from idle to 6000 rpm in neutral – and that wasn’t fast,
it’d take a couple of seconds to wind up, which means it took a
couple of seconds to get from standstill to about 40 mph when it was
time to shift to 2nd. Anemic. That means that the flywheel was a
HUGE factor in the acceleration in 1st gear. If I had gone with a
lighter flywheel, I’d have had tremendously greater performance in
1st, moderately better performance in 2nd, and diminishing
improvements all the way up to 5th.

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 19 Oct 2015 at 11:27, Gary Evans wrote:

15

In reply to a message from Kirbert sent Tue 20 Oct 2015:

I thank you all for the inputs.
Both pros and cons with the majotity on the pro side.
I’ll put the Lightened flywheel on my Christmas wish list
and see what Santa brings this year.–
Ole-XKE1974
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16

True. The quicker the engine changes RPM, the more torque will be consumed increasing the kinetic energy of the flywheel. Thus the impact is greatest in 1st gear. But just how much?

Assume we want to rev from 1000 to 2000 RPM in one second
Assume the flywheel is 30 lbs (14 kg)
Assume the flywheel diameter is 14” (.36m)
Assume the flywheel mass is distributed evenly as a disc.

Torque = moment of inertia x angular acceleration. (Just like F=ma)
Moment of inertia of a disc is 1/2 mass x radius^2

For a 14kg flywheel of .36m diameter, that gives moment of inertia = .23 kg m^2
Angular acceleration from 1000 RPM to 2000 RPM in 1 sec = 104 rad/s^2

Using these numbers, we can calculate that the torque consumed spinning up the flywheel is 104x.23 = 23.9 N.m

23.58 N.m is about 17.6 ft-lb

So cutting the flywheel weight in half will free up another ~9 ft-lbs to the input shaft of the transmission. Definitely noticeable in instrumented testing. But maybe not HUGE. Bear in mind that there are a lot of other rotating pieces in motion. The crankshaft alone weighs about 70 lbs. As does the accessory drive full of cast iron pulleys.

I have gone so far on mine as to install a lightweight flywheel, carbon fiber drive shaft, electric fan, simplified accessory drive, aluminum water pump pulley, electric power steering and lightweight wheels. All in an effort to pull rotating mass out of the system. Purely an exercise in “what if” since the advances in automotive technology since I bought the car 15 years ago have soundly obliterated my feeble efforts. What was once really good performance will now be laughed off by any number of hot-rod luxury sedans and even a few SUVs

Still, it’s the journey not the destination, and as a mechanical engineer I love discussions like this.

-Gary> On Oct 20, 2015, at 3:47 AM, Kirbert palmk@nettally.com wrote:

On 19 Oct 2015 at 11:27, Gary Evans wrote:

It does give quicker throttle response when
free-revving the engine, but whether this translates into meaningful
performance gains is not clear.

The question is whether the inertia of the flywheel is a significant
portion of the overall inertia of accelerating the car. In top gear,
that’s a no. However, after I installed the 4.11 rear end in my '83,
I found that the car would accelerate faster in 2nd than in 1st; the
performance in 1st seemed rather anemic, but the instant you grabbed
2nd it’d pin you back in the seat. In 1st gear, the car accelerated
from standstill to 6000 rpm barely any slower than the engine
accelerated from idle to 6000 rpm in neutral – and that wasn’t fast,
it’d take a couple of seconds to wind up, which means it took a
couple of seconds to get from standstill to about 40 mph when it was
time to shift to 2nd. Anemic. That means that the flywheel was a
HUGE factor in the acceleration in 1st gear. If I had gone with a
lighter flywheel, I’d have had tremendously greater performance in
1st, moderately better performance in 2nd, and diminishing
improvements all the way up to 5th.

– Kirbert

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17

In reply to a message from Kirbert sent Mon 19 Oct 2015:

No Jag flywheel is 1’’ thick all the way. They have a rim
that size for the outer 1.5’’ of radius (approx). Jag made
a light version for the 3.8 E-type that only has avery
thin rim near the ring gear. All other XK or 5.3 flywheels
are pretty similar. The Getrag box flywheels on AJ6
engines are noticeably lighter/thinner and more uniform
section. The twin mass flywheels fitted to AJ-6 and
probably post 1990 3.2/4.0L AJ6 are incredibly heavy.

Pete–
The original message included these comments:

package. It’s, like, an inch thick all the way out to the rim. The

–
1E75339 66 D, 1R27190 70 FHC, 79 S2 XJ12L
Gaithersburg, Maryland, United States
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18

Assume we want to rev from 1000 to 2000 RPM in one second

Oooooh! Numbers! I was too lazy. Thanks!

Now, note that 1000 to 2000 in one second is too slow. That’d be
five seconds just to get wound out in 1st gear and we haven’t even
gotten to second yet. So, for an example, I’d presume a spin-up rate
about double that, 1000 to 2000 in a half second.

Assume the flywheel is 30 lbs (14 kg)

I think this flywheel is heavier than that. Somehow I recall 44
lbs as being the number, but I may be thinking of something else.

Assume the flywheel mass is distributed evenly as a disc.

It’s close, except the inner five inches or so is considerably
thinner and lighter, which means a higher percentage of the mass is
farther out.

So cutting the flywheel weight in half will free up another ~9 ft-lbs
to the input shaft of the transmission.

There’s a good question: Is going to an aluminum flywheel really
cutting the weight in half? That’s probably about right, I think,
although it’s possible that it removes a bit more weight from the
outermost portion, presuming the steel face plate isn’t the full
diameter of the flywheel.

For a while, I was considering just sending my OEM steel flywheel out
to have half the mass machined off of it. It’d actually be fairly
easy to do. Rather than the back side being flat, I’d machine most
of it away leaving the plate less than 1/2" thick and leaving a
flange to hold the ring gear. I’d drill out the clutch plate
mounting holes and mount it with bolts with nuts instead of threaded
holes in the flywheel. I’d also consider scalloping the OD between
the clutch plate mounting holes, since all that outboard mass is good
for nothing. This was all before someone explained to me how you
make an aluminum flywheel with a steel facing. I suppose you could
actually do both, making an aluminum flywheel and making it a bunch
less bulky than the OEM steel flywheel. Could even drill holes in
it, leaving the steel face plate undrilled.

Bear in mind that there are
a lot of other rotating pieces in motion. The crankshaft alone weighs
about 70 lbs. As does the accessory drive full of cast iron pulleys.

True, but all of those items are considerably smaller diameter than
the flywheel, so their moments of inertia are less significant. The
biggie, really, is the pressure plate, but I don’t think there’s any
good way of reducing the moment of inertia there without going to
esoteric racing hardware.

I have gone so far on mine as to install a lightweight flywheel,
carbon fiber drive shaft, electric fan, simplified accessory drive,
aluminum water pump pulley, electric power steering and lightweight
wheels. All in an effort to pull rotating mass out of the system.

Wow.

There is another factor at work, of course, and that is the EFI
system. As I understand it, it doesn’t respond well to snap accels.
It was never designed to; it was supposed to be used with an A/T.
So, you’re at idle, you stomp it, and the engine is at 5000 rpm while
the MAP sensor – at the end of ten feet of tubing – is still
thinking the engine is at full throttle at idle. The fuel/air ratio
may be all messed up. It’s this sort of thing that Bywater addresses
with his Super Enhanced ECU’s.

Still, it´s the journey not the destination, and as a mechanical
engineer I love discussions like this.

I’m glad we can keep you entertained!

– Kirbert

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !On 20 Oct 2015 at 10:22, Gary Evans wrote:

19

Assume we want to rev from 1000 to 2000 RPM in one second

Oooooh! Numbers! I was too lazy. Thanks!

Now, note that 1000 to 2000 in one second is too slow. That’d be
five seconds just to get wound out in 1st gear and we haven’t even
gotten to second yet. So, for an example, I’d presume a spin-up rate
about double that, 1000 to 2000 in a half second.

Probably correct. Just picking easy numbers for the example.

Assume the flywheel is 30 lbs (14 kg)

I think this flywheel is heavier than that. Somehow I recall 44
lbs as being the number, but I may be thinking of something else.

Wow, that is heavy.

Assume the flywheel mass is distributed evenly as a disc.

It’s close, except the inner five inches or so is considerably
thinner and lighter, which means a higher percentage of the mass is
farther out.

True.

So cutting the flywheel weight in half will free up another ~9 ft-lbs
to the input shaft of the transmission.

There’s a good question: Is going to an aluminum flywheel really
cutting the weight in half? That’s probably about right, I think,
although it’s possible that it removes a bit more weight from the
outermost portion, presuming the steel face plate isn’t the full
diameter of the flywheel.

The steel plate is pretty thin. And you still have the steel ring gear at the extreme OD. FWIW, The Fidanza aluminum flywheel is 13 lbs.

Bear in mind that there are
a lot of other rotating pieces in motion. The crankshaft alone weighs
about 70 lbs. As does the accessory drive full of cast iron pulleys.

True, but all of those items are considerably smaller diameter than
the flywheel, so their moments of inertia are less significant. The
biggie, really, is the pressure plate, but I don’t think there’s any
good way of reducing the moment of inertia there without going to
esoteric racing hardware.

Indeed.

There is another factor at work, of course, and that is the EFI
system. As I understand it, it doesn’t respond well to snap accels.
It was never designed to; it was supposed to be used with an A/T.
So, you’re at idle, you stomp it, and the engine is at 5000 rpm while
the MAP sensor – at the end of ten feet of tubing – is still
thinking the engine is at full throttle at idle. The fuel/air ratio
may be all messed up. It’s this sort of thing that Bywater addresses
with his Super Enhanced ECU’s

This is not unique to EFI. Its the exact reason the carburetors have accelerator pumps. The XJS system is somewhat unique in having the MAP sensor mounted in another time zone. Ideally the AE (acceleration enrichment) map would be programmed to cover this condition. On top of that, WOT condition brings power enrichment into play, so there should be more than enough fuel pouring in when you ’stomp it’. But perhaps Jaguar did not bother to tune it at all, or kept it very lean for fuel economy reasons.
It would be simple enough to monitor the oxygen sensor output and see if it goes lean under rapid acceleration.

-Gary

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !> On Oct 20, 2015, at 1:59 PM, Kirbert palmk@nettally.com wrote:

On 20 Oct 2015 at 10:22, Gary Evans wrote:

20

Interesting discussion.

A rough model of a Jag flywheel has moment of inertia of 0.2 kg/m^2 while
the Fidanza alloy flywheel has inertia of 0.076 kg/m^2.

If you assume acceleration rates of twice those proposed (ie 1000 - 3000 rpm
in 1 second) then you free up about 26 Nm by switching to the lighter
flywheel.

At those kinds of engine speeds the basic Jag V12 is developing around
340Nm. So you have freed up about 7.5% “extra” grunt. Or equivalent to
having an extra cylinder (V13 anyone). I would think this would be quite
noticeable.

However during this time, the engine has only turned 33 revolutions. That is
16 power strokes or hardly more than 1 ignition event per cylinder. Given
the batch fired nature of the injection and the crudity of the ignition, is
it any wonder there is “nobody home” when you floor the throttle and is it
any wonder that NO modern engine uses a similar arrangement?? This is why
sequential injection makes sense. And why transient response is so critical
in modern algorithms.

(If someone was to suggest to the Japanese engineers at Honda that their new
NSX (fastest accelerating engine on the market) should use batch fired
injection they’d be giggling into their saké)

(Somebody check my maths, but I think my numbers are right!)

By the time you grab 2nd gear, the engine is likely spinning at least twice
as fast (3000 - 5000 rpm) hence twice as many injection and ignition events
and twice as quick to respond to changing fuelling demands. So most probably
much better responses could be expected in 2nd.

It could probably be argued that a switch to sequential injection might well
improve performance by an equivalent amount, without the flywheel change. Or
doing both would help prevent the engine “bogging down” on launch.

Rgds
Mark

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