backplate, flywheel and gearbox.

[RogerRust]

I have just taken the gearbox, flywheel and back plate off of the engine I took out of Sophie.

The Gearbox is a recon early sprite box. The clutch is brand new (or was 25 years ago) and never used the release bearing is unused as well.
So I intend to reuse the gearbox and clutch.
The back plate I have just removed is a pressed steel one and thinner than the one off the replacement engine.
The Flywheel has new ring gear, but is thicker and heavier than the one off my newer engine.

Two questions

Should I fit the thin back plate and thick flywheel as a pair?

Is there any advantage of a heavier or lighter flywheel?

Later Edit.

I think I have worked it out the flywheel and backplate from the 2nd engine were from a 1098 and would have had the larger clutch. So I guess I'm right I will have to use the 948 kit to mate up with my 948 gearbox.

Roger.

[HarryMango]

Hi Roger

Yes I think you're correct - If you recall. I was originally sold that engine as a 1098 - so when the prior owner broke up the car it was probably fitted to a 1098 gearbox hence the confusion.

Rog

[RogerRust]

ahh very clever.

[JimK]

A lighter flywheel allow the engine revs to change faster. In fact, weight removed from the flywheel has a much greater effect on acceleration than the same weight removed from the rest of the car (including slower-rotating parts of the transmission).

You probably know that lightening the flywheel is a fairly common thing to do aming the tuning brigade. Makes it easier to stall the engine though and I guess it would make for more lump idling. Tends to be done on engines that spend most their time screaming rather than idling...

[Kevin]

Makes it easier to stall the engine though and I guess it would make for more lump idling.

Why ?
Surely a lightened flywheel allows the engine to run smoother as there is less weight hanging of the end of the crank.

[JimK]

Makes it easier to stall the engine though and I guess it would make for more lump idling.

Why ?
Surely a lightened flywheel allows the engine to run smoother as there is less weight hanging of the end of the crank.

Think about what the flywheel is for: to keep the engine turning during those times when a cylinder is not firing. When a cylinder doesn't fire the angular momentum of the flywheel enables the engine to compress the next charge of fuel.

If the flywheel is heavier, it has more angular momentum. Because of this, it slows down less between cylinder power strokes and thus the engine speed varies less.

The static weight of the flywheel has an almost negligable effect compared to its rotating momentum.

[Kevin]

A correctly lightened and balanced flywheel I cannot see that this makes a car stall easier and lumpy running would only be caused by a superlight flywheel most standard flywheel can benifit from lightening especially on cars like our Moggies where things tend to be over engineered whether the cost and effort involved is worth it is a different matter, but if you have an engine rebuild underway to have it balanced will make it run smoother, but to quote from a specialist,

Most standard flywheels are substantially over engineered and have surplus metal which can be removed safely to reduce the weight and moment of inertia. The less inertia the engine has to accelerate the more is left over for accelerating the car. Material should be removed from the outside of the flywheel i.e. as far from the centre as possible. Weight removed from the centre has little effect on reducing the inertia and can weaken the flywheel. Doing it properly requires experience and engineering know how. The inertia of the average flywheel can be reduced by 25% to 30% which has a significant effect on the acceleration of the car and correctly balanced will caused no problems.

[JimK]

A correctly lightened and balanced flywheel I cannot see that this makes a car stall easier and lumpy running would only be caused by a superlight flywheel most standard flywheel can benifit from lightening especially on cars like our Moggies where things tend to be over engineered whether the cost and effort involved is worth it is a different matter, but if you have an engine rebuild underway to have it balanced will make it run smoother, but to quote from a specialist,

<mentally inserts a little punctuation >

I didn't say it would stall or run badly, just that the lighter the flywheel the more likely it is to do both of those things. The flywheel's entire purpose in life is to even out the power pulses coming from the detonation in each cylinder, and so by definition: if the flywheel is lighter it will do that job less well. As you correctly surmise, this will only be a noticable problem if the flywheel is too light for the application.

I am certainly giving some thought to this sort of work on my car.

[RogerRust]

Do you hink it is case that the heavier flywheel contributes to the smooth running of the 948? I always thought it was because it was a shorter stroke. I think I'll leave it alone and fit it as it is coz I'm fast overrunning my budget.

[bmcecosse]

Light flywheel is a dream - but of course you just need to be a little more careful with the clutch when starting off - and give it a bit of revs ! The lighter fly also moves the crititical revs speed further up the range - less likely to break the crank.

[JimK]

The lighter fly also moves the crititical revs speed further up the range - less likely to break the crank.

Sounds interesting, but can you explain this in more detail, please? "Less likely to break the crank" sounds good, but less likely than what, and why is it less likely? What is the "critical revs speed"?

Ta.

[bmcecosse]

There appears to be a 'resonance' or 'critical frequency' at about 6000 rpm - if you run the engine continuously at these revs (more so with 1098 engine) it soon snaps the crank. Lighter flywheel (in conjunction with crank damper on the nose) seems to move this critical frequency upwards - to a rev range you are less likely to run continuously. I know when I had modified 948 engines (many moons ago now) I broke two cranks - then had the flywheel lightened (mate at college) and fitted crank damper scrounged from a very early Morris 1100 East-West engine in a scrappy - and no more broken cranks. Mr V confirms all this too in his excellent book.

[chrisd87]

How long do you mean by continuously? On the road I'd only ever use 6000rpm+ for less than 10 seconds probably.

[bmcecosse]

I think it applies more to race cars - or low geared road cars where 6000 rpm could be = 90 mph. Your 10 seconds won't do too much harm.