DIY Crank balancing (difference between 3.5l and 3.9l crank)

[Frank.de.Kleuver]

Hi All,

I'm new to this forum so please be gentle :-) My excuses for some possible bad englisch.

A DIY Dynamic crank balancing device is something that crossed my mind multiple times in the past so I started to do some research.

Does somebody know if Rover did balance their 3.5l and 3.9l with the same Bobweights during balancing or are these different?

A crank for a 3.5l or a 3.9l has the same ordernumber so that would suggest that they are balanced somewhere in the middle or so. The pistons of a 3.9l are somewhat of 60-80grams heavier than those of a 3.5l. This means that all four bobweights (individually) for dynamic balancing would be 60-80 grams heavier also. In mine opinion too much to just leave alone when using a 3.5l crank in a 3.9l engine build.

I've read that Rover used to produce all rotating parts (conrod, piston, pin, rings) within +/- 4 grams that would be matched just by selection. If for some reason they could not produce within their accuracy then the engine was dry spun with an electric motor and a couple of strategic placed sensors would tell where to add some metall to the front pulley or if some bolts should be added to the flywheel. But that would in my humble opinion not be enough to correct a mismatch of 4 times 60-80 grams piston weight.

So, in a nutshell:
- Rover did balance their 3.5l and 3.9l the same somewhere in the middle
- Rover did balance their 3.5l and 3.9l the same based upon the 3.5l weights and didn't mind for some unbalance in the 3.9l
- Non of the above ...........

Any help or insight is much appreciated.

Greatings,

Frank de Kleuver

[Bill Young]

Frank, I am no expert, but it seems to me that the extra weight of the larger pistons for the 3.9 was probably compensated by the shorter connecting rod to a large extent, making the resulting assembly very close to that of the 3.5. I don't know the cost of having the rotating assembly balanced in the Netherlands , but here in the US it's not very expensive and well worth the cost for a really smooth running and long lasting engine. I'd just take the crank and other parts to a good automotive machine shop and let them balance the assembly for you and then you're ready to use any front balancer or flywheel you want without worry.

[MGBV8]

The 3.5, 3.9, & 4.2 all used the same rod.

[Frank.de.Kleuver]

Bill,

Here in the Netherlands it's not cheap to dynamic balance you crank. It would cost me approx 420 euros (incl VAT). This is roughly 550 dollars. In the UK it's much cheaper (200/250 pounds = 300 dollars) but I have to send it back and forth. But it's also the fun of building and understanding these kind of techniques.

Today I did a 2nd run with my newly designed bob weights and found on a standard 3.5l crank I get the best balance with a bob weight of approx 1623 grams. I expected a bit less like 1558grams based upon some old worn parts I had laying around.

It my looks like it that the 3.5/3.9l crank is balanced somewhere in the middle since Rover did balance the whole engine again after assembly. So I was told. But this all is based upon my preliminary data.

Kind regards,

Frank

[Bill Young]

Frank, I am impressed with your fixture. Amazing ingenuity you've displayed. I would never have dreamed that anyone could balance a crank assemlby themselves, but I think you may well do it.

[roverman]

Frank, Very creative, hence the term "If it ain't Dutch it ain't much" lol. If your going for extreme rpm., not likely, you might consider "overbalance" to counter inertia forces, (mostly drag motors). Welcome to our forum, roverman.

[Frank.de.Kleuver]

Hi Roverman,

I've read something about overbalancing but I couldn't find any numbers to calculate how many grams or gramscm to add. Are these just a couple of grams or hole pounds :-) I don't have any feeling for the amount they add. Have you know some numbers? I image it will depend on the rpm the engine is mainly used in. Are these overbalancing additions for the forces produced by bangs of the combustion chambers?

If included an extra image of the last balance data of the 1623 grams bob weights. My blue printed piston/conrod set would need a calculated bob weight of 1633 grams so it looks this one would be ok when just put together.

Kind regards,

Frank
1580grams

1623grams

Link to youtube movie
http://www.youtube.com/watch?v=kRzBvqfcUQ8

[roverman]

Frank, I've "heard" of a Top Fuel Funny Car having a total of 1,600 lbs, total dynamic overbalance at 8k rpm.(not verified).Under certain conditions, an engine can have higher "extensive" loads than compressive. I suspect a Formula 1 engine would be such an example, 20k + rpm .? Cheers, roverman.