What happens if a flywheel/damper is lightened too much?

[dtype38]

Does anyone know what the actual symptoms are if a flywheel and/or crankshaft damper have been lightened too much?

[jerryk]

very poor erratic or non existant tickover , very poor starting , harsh running at medium revs but superb pick up if you can get it going.

[dtype38]

At low speed driving, would you say the drive train sounds like somone shaking a metal bucked full of bolts?

[Tim Falce]

When you drive at a lowish speed in say 4th gear does the car tend to lurch or surge, I would call it hunting but I'm not sure if that's the correct term, or does it drive smoothly but just make a lot of noise?

[dtype38]

At 30mph in 4th it's properly lumpy, but that's basically tickover. Everything rattles like I've forgotten to do up the big end bolts and the gearbox if full of square cogs. If I touch the throttle, though, it picks up fine and as soon as I put any load on it the noises go away.

[GORDON STREETER]

A normal engine will "try" to stop at every compression stroke. The lighter the flywheel gets will only accentuate this. Obviously it can't so it will minutely "flutter" and you could probably pick this up with a strobe light at tick over.
This makes everything connected to the crankshaft also do the same, and any slight play will make a noise especially cam drive gears or chains.
This may not be your problem but my best engine with an alloy flywheel sounds like a bag of nails if it drops below 2000 rpm with no load ! in my case its the gear driven cam.

[Tim Falce]

Gordon, my engine has a 4kg flywheel and on tick over it is difficult to use the strobe light, it will flutter with up to 6 deg of variance, rev it over 2K and it smooths out.

[jerryk]

Originally Posted by dtype38

At low speed driving, would you say the drive train sounds like somone shaking a metal bucked full of bolts?

have you got oil in the gearbox?

[GORDON STREETER]

Originally Posted by falcemob

Gordon, my engine has a 4kg flywheel and on tick over it is difficult to use the strobe light, it will flutter with up to 6 deg of variance, rev it over 2K and it smooths out.

Not really the same thing Tim. That's more than likely play in the distributor and drive.
You would have to use a crank sensor to pick up this effect as we are only talking about a very small amount at low RPM.
4kg blimey, thats about the weight of my car !

[Tim Falce]

Originally Posted by GORDON STREETER

Not really the same thing Tim. That's more than likely play in the distributor and drive.
You would have to use a crank sensor to pick up this effect as we are only talking about a very small amount at low RPM.
4kg blimey, thats about the weight of my car !

My previous lightened flywheel was 11kg, I think a standard one is 18kg. No play in the drive as it's an almost new distributor and I've just had the engine apart and the dizzy drive is OK. The only thing it may be is the rotor arm although I doubt there would be that amount of play in it.

[GORDON STREETER]

Rotor arm play won't make any difference to the timing !
Play will develop in any part of the drive from a crankshaft if it is turned backwards,
gears have backlash, and chains have tensioners that don't work well when an engine goes in the opposite direction.

[Tim Falce]

Originally Posted by dtype38

At 30mph in 4th it's properly lumpy, but that's basically tickover. Everything rattles like I've forgotten to do up the big end bolts and the gearbox if full of square cogs. If I touch the throttle, though, it picks up fine and as soon as I put any load on it the noises go away.

I drove mine to get it MOTd yesterday and it sounds pretty much the same with no load on the transmission, don't forget there is a lot of free play/backlash, or whatever you want to call it, in the diff which will add to he noise. Touch the throttle and it all goes away.

Originally Posted by GORDON STREETER

Rotor arm play won't make any difference to the timing !
Play will develop in any part of the drive from a crankshaft if it is turned backwards,
gears have backlash, and chains have tensioners that don't work well when an engine goes in the opposite direction.

Aha, that may be the problem as I fitted the starter on the other side of the engine (gearbox side) due to clearance problems with my new carbs and I couldn't work out why I now have 5 reverse gears and 1 forward.

[Al Weyman]

I run very light flywheels on my small blocks one has the McCloued Magnum Force clutch with cut away aluminuim flywheel the other a 7.5 inch triple plate Tilton with a Auto trans flexiplate so they dont come much lighter. I would definitely not lighten the crankshaft damper though and make sure I have the cranks balance properly. The engine I am currently using will only tick over at 1500 rpm but that is down to a nasty cam not helping as I had the same Tilton set up on milder engine and it was happy to tick over at 1000rpm. As for starting, instant no difference whatsover, pickup is instant and gear changing much snappier you can lock the rear wheels if you get off the gas in a low gear. It does sound like a bag of bones at low speeds but the timing is steady as a rock.

[dtype38]

Originally Posted by jerryk

have you got oil in the gearbox?

Dont think I didn't check to make sure!

In fact I did think it might be a gearbox problem. I just had it rebuilt and it was possible it had been done badly, but reading the above I think its more likely to be lack of flywheel/damper weight. The crank damper on this new engine is significantly lighter than my old one, and combined with a lot more static compression than my old engine I think it's just rattling the gears etc.

Thanks guys

[greenamex2]

In my case cracks developed between the lightening holes, the flywheel started vibrating and it broke following -

Flywheel
Crank
Two rods
Clutch
Engine block
Two pistons
Most of the valves
Several cam buckets
Several cam shims
Both camshafts

And, oddly

The gearbox casing and input shaft bearings!

On the plus side having TOO HEAVY a flywheel and damper used to snap the cranks, also damaging any or all of the above!

[Al Weyman]

I cannot see how a lightened flywheel would cause that on the contary. Unless it was a self lightened effort and you actually skimmed away metal from the flange where it bolts to the crank. On that subject i would be very wary machining a stock flywheel away if you like your legs and feet the way they are!

BTW D-Type as a matter of interest why did you lighten a critical component like the front damper, is that an accepted tweak on the Jag engine?

[GORDON STREETER]

Norman Ricketts had a fair old blow up at Snetterton a while back in his Beemer.
The clutch diaphragm broke starting the grief that ended with a broken bell housing/gears/and the back of the block !
I know somebody that lightened a flywheel on a crossflow in an Escort that exploded when it was revved up in the workshop. It came out of the side of the car like a circular saw and the main part embedded itself in the Celcon block wall !

[Mike Sweeney]

Without drearily wading into the subjects of reciprocating mass and harmonics, a good rule of thumb is do not lighten secondhand flywheels...especially if they are cast as many oem production items will be. After not using correct/new fixings and tightening them correctly, it's the primary cause of flywheels exploding.

Only mention it in case anyone reading this is contemplating flywheel mods...not meaning to preach to the masses

[GORDON STREETER]

I'll second that Mike. The guy I was talking about actually asked my advice about lightening it and I told him not to and just buy the correct thing.
However I don't seem to have got through to him !

[dtype38]

Originally Posted by Al Weyman

BTW D-Type as a matter of interest why did you lighten a critical component like the front damper, is that an accepted tweak on the Jag engine?

I bought the engine from an experienced and very successful Jaguar racer. He races at a level rather above mine and some of his engine build ideas "push the envelope" a bit, but over the years I've always found his advice trustworthy. When I bought the engine it came with heavily modified damper from a different type of engine. Obviously I queried it, but he said the crank was lightenend and the damper had been calculated to have the correct inertia to suit. I have no reason to doubt him, and although I haven't come across it before it seems logical.

Oh, and my lightened flywheel was new from a reputable supplier of tuning parts for these engines.

[Al Weyman]

I wasnt questioning it was wrong D-Type just curious that all.

I see a flywheel explode at Snett in a spectacular way lifting the bonnet of the car ripping a vent in it (fortunately it was not fibreglass and the steel bonnet retained the missle of a flywheel or it could have killed someone) with a resulting fire as it severed the oil cooler lines which spurted oil on the hot exhaust. I was surprised to learn after it was a lightened standard cast flywheel, a big no no IMHO.

[Mike Sweeney]

Damper inertia?

Maybe your engine builder has learned ways to improve the Jaguar 6cyl engine, and you are reaping the benefits of this. However, from afar it sounds awry to me - not knowing much about the dynamics of that particular engine - so for the sake of a phone call or two, I'd be inclined to ask, in detail his reasons for some of the mods inside your motor, and upon what proven theories they are based. It's (presumably) your money at stake...

[GORDON STREETER]

Originally Posted by falcemob

Aha, that may be the problem as I fitted the starter on the other side of the engine (gearbox side) due to clearance problems with my new carbs and I couldn't work out why I now have 5 reverse gears and 1 forward.

Well it'l be easy to drive back to your home !

[GORDON STREETER]

Originally Posted by Al Weyman

I wasnt questioning it was wrong D-Type just curious that all.

I see a flywheel explode at Snett in a spectacular way lifting the bonnet of the car ripping a vent in it (fortunately it was not fibreglass and the steel bonnet retained the missle of a flywheel or it could have killed someone)

Like the Mustang one that embedded itself in the underneath of the wooden walkway bridge at Mallory Park !
Or worse still the Mini one that cut the drivers foot off !!! I believe it was Castle Combe.
I seem to remember the poor guy died later in hospital from complications.

[Al Weyman]

Thats terrible, read Vizards take on lightening Mini flywheels.