Fiat 2 litre 16V Crankshaft lightening

[Kovacevic]

I'm currently building a 2 litre 16V engine for rally use, and having built a couple of engines previously, one with a standard crankshaft that was nitrided, and the next one radically lightened (knife-edged and nitrided). I'm in a quandry as to which way to go as there are two schools of thought here in Australia with Fiat crankshafts.
The first is that as much reciprocating weight as possible should be removed to allow the engine to spin up quickly, and the crankshaft knife-edged to reduce drag.
The second is that the counterweights smooth out the torque pulses and should be left alone, especially with a high-revving engine, unless you plan on regularly crack testing the crankshaft.
I'd be really keen to hear some informed opinion on where I should be going with this.

thank you,
Alex

[Guy Croft]

My response in light of exp of doing these things from the 9500rpm NHRA unit I developed in the 90s is you can pretty-well do what you like to the 2 liter crank! Provided you don't grind them and/or mess up the journal fillet radii & balance properly with FW they are fantastically strong, as a forging way more so than a machined billet version.

If you ever break one I should like to know because I never have.

G

[tricky]

Good question,

Guy, how is a crankshafts individual counter weight 'sized' or matched in terms of roting mass according to the weight of the piston and rod assembly ? Are there other factors other than just weight ?

For instance;

If we assumed that the counter weight is appropriately sized by Fiat in conjuction with the OE cast piston and con rod, if we as race engine builders then fit lighter forged pistons and rods typicly saving say 150 grams off the rod and 100 grams off the piston. Is it reasonable to assume one could remove 250 grams from each counter weight ?

[WhizzMan]

It's not that straightforward, unfortunately. There are several ways to approach this, but the most common way is to weigh two things. You balance for all of the weight of the big end side of the con rod added to the total weight of the rod and piston assembly. Weighing the con rod big end side is usually done by hanging it horizontally by the little end and putting the big end side on the scale. Once you've weighed the original setup and your new setup, you can now calculate how much weight you compensate for. The weight you get now, is usually replicated in "clamps" that get clamped on the big end side of the crank shaft. Remember, this is all very dynamic and just cutting off the equal weight "somewhere on the crank" will most likely not give you the balance you thought it would. It will require careful measurement and balancing to get a crank balanced decently.

[TomLouwrier]

hi guys,

There's more to it than that. The crank/rod system knows a lot of acceleration forces from the rotation and the oscillation of masses.

If you consider a 1 cylinder engine, you might be tempted to add a counterweight that has a mass equal to that of the piston and con rod assembly. This will even out inertia in both upper and lower dead centre, but as the weight swings through the horizontal plane, it pulls the engine to and fro. The part of the con rod that is considered rotating mass (about two thirds, centred in the big end) only goes so far to cancel out this horizontal force.
You will end up with a counter weight that is half of the resulting imbalance, so near 0,5 x (m_piston + 0,3 x m_rod). This will leave an imbalance force that is rotating at crank speed and has more or less equal value all the time. The engine vibrates a bit, but it doesn't jump or bump.

When you add a second cylinder you can logically give it a phase of 360 degrees. That will get you a nice even ignition interval (once every revolution of the crank) but leaves the issue of balancing untackled. In fact it gets twice as big, since there are now two identical imbalances working together. This is the classic Euro parallel twin bike from the 50's - 80's (Triumph, Norton, Laverda, etc). Bouncy bouncy.
So what if we place the cranks at 180 degrees? The ignition interval will be irregular, giving a bit less torque low down and idling that is less smooth. Oh dear.
The balance however is much better. The one piston going up will pretty much cancel out the one coming down. Only problem is that they are some distance apart, so the engine will want to rock left-right. To get rid of this effect you add a counter weight on the both outer webs (you want them wide apart, this gives more resulting couple with smaller weights). You end up with a rotating imbalance much like the single cylinder engine, but much smaller and the engine (rather: the crank) will try to make a movement like a diabolo.

Now we double up the 180 degree twin: two pairs of pistons/cranks that cancel each other nicely out but leave a rocking force (couple). By hooking them up we get a system that not only cancels out the forces at upper and bottom dead centre, but also cancels out the rocking resulting from the distance between the paired cylinders.
It has good end-to-end balance. Most pleasant effect: no more need for all that weight on the crank.

OK, so strictly speaking an inline four does not need counterweights at all!
Then why do factories put them on? The main reason they are there is that the crank does not turn just by itself, it get whacked by the force of the combustion every time. These pulses result in downward forces (logically) and in a piston going down faster that it did go up. But another blow on anther piston will see to that...
To dampen out these two sources of vibration we still need to add some mass to the crank.
Well... when you're out to get rid of all unneeded weight you may get those heavy counterweights off. Be prepared for an engine that scores very high in the NVH (noise vibration harshness) department!

In a V-engines things can be different. A racy Euro flat plane V8 may be considered as 2 4-pot engines sharing one crank. It even sounds like two. This engine can indeed run without counterweights. A regular US cross plane V8 does not have the inherent end-to-end balance, it must use counterweights on the webs. The reason they make these cranks is that it now acts as 4 pairs of V2's with each pair heavily balanced like the 100% balanced single above. Thing is: when the couterweight goes through 90' ('horizontal' in my single engine example), the second piston/rod on the same crank cancels out the force that would be pulling the engine to and fro. All this tilted 45' of course, it's the 90' between banks that matters.
This gives a very smooth engine, running at low speeds. Just what the American market wants from their family saloons. And everyone knows that burbling sound they make. It's the irregular firing interval that you hear.


Mind you: I'm not considering here things like secondary inertia forces due to the con rod being shorter than 'indefinite length' which makes the movement of the piston more like a sinus with the bottom half flattened.
In case you wondered (I did for years): no, the piston does not go down at the same speed as it goes up. Reason being that when going down the rod is tilting (big end goes sideways away from the cylinder centre line) adding to the downward acceleration. When nearing the bottom dead centre the rod gets vertical again, adding to the deceleration. When the crank goes through bottom dead centre and the piston starts to rise, the rod again tilts, taking some speed out of the upwards movement. Once the big end is past horizontal, the rod come straight, pushing the piston up faster that it would if only for the crank. You see the extra acceleration twice every stroke.
This is not a a very regular movement, and thus it has considerable forces (= vibrations) at twice the engine speed. The shorter the rod, the stronger this effect is.

Neither am I discussing engines with 3 or 6 cylinders that have better behaviour in this respect; you can imagine a straight 6 being a paired triple just like the straight 4 being two twins. It's the inherent primary and secondary crank balance of the 6 that makes these engines so smooooooth, helped by the overlapping of combustion strokes on anything with more than 4 pots. Of course this makes a V12 (that's two sixes!) nearly perfect. I want one.

And in boxers things are a bit different still. But that too would take us all night.

If you want to know more on this, do some Googling and/or Wiki on "engine balance" or "flat plane" or "cross plane". There are some very good and well illustrated pages out there.

Have a good weekend.

regards
Tom

[WhizzMan]

Tom, that's a very good summary. However, I'd like to add that there are reasons to still balance each individual cylinder on a straight for (or any other multi cylinder engine for that matter). If you're going to rev high, you will not only have the unbalance you described rocking the engine, but also twisting of the crank itself. At high power and high revs, this can be enough to accelerate wear on the main bearing journals, or even break the crank. You may in fact cut away a lot of weight off cranks that are strong enough, but the per cylinder balancing weights do help against vibrations and resonance in the crank. I am in favor of honoring this balance, even with large weight reduction performed. It saves your crank from breaking up and it will let you cut away more metal without much of the NVH factor you are mentioning. Sure, there will be more torque related harshness, since there is less rotating mass dampening the pulses, but that's about it, nothing more.

[TomLouwrier]

hi Homme

It seems we agree on this subject; I did not mention the twisting as such, but did indicate the combustion forces as reason for the counterweights on an inline 4.

Then why do factories put them on? The main reason they are there is that the crank does not turn just by itself, it get whacked by the force of the combustion every time. These pulses result in downward forces (logically) and in a piston going down faster that it did go up. But another blow on anther piston will see to that...
To dampen out these two sources of vibration we still need to add some mass to the crank.

In theory you could dampen out these vibration with the flywheel and/or a damper on the crank nose, but the distance between the rod and the damper wil indeed lead to torsional vibration and fatigue breaks in the crank. Hence the counterweights.
Thanks for adding that.

regards
Tom

[Kovacevic]

Thanks gentlemen for the informed responses - I remain a little confused, but on digesting it all I think I will lighten the crank and have a harmonic balancer made for the crankshaft pulley. Any further comment is more than welcome.

[WhizzMan]

Thanks gentlemen for the informed responses - I remain a little confused, but on digesting it all I think I will lighten the crank and have a harmonic balancer made for the crankshaft pulley. Any further comment is more than welcome.

If your engine guy can do it, have him balance the individual crank pins for the 1+0.5 weight. Simply put 3 counter weights on and measure the unbalance of the pin that isn't loaded. It should give a correction weight of the 1+0.5 at half the stroke from the center of the crank, at the same angle as the crank pin. Do all four and you should have each pin individually balanced for as little vibration as possible. This does require an expensive dynamic balancing rig, I think. I've never seen anyone even try it without one, so I wouldn't know any DIY trick to do it yourself on a budget. If anyone here knows how to do it, I'm sure a lot of people would want to know.

Ik believe Guy put some excellent advice on harmonic balancers on here already. I'd read that before ordering/purchasing one.

[Guy Croft]

I don't have a digital photo of the crank that I used in the GC 2 liter Fiat 8V NHRA motor but here is an 1800 one by Abarth - a unique forging with thicker webs than the production 1800 crank and actually exactly the same weight. The latter is quite a similar shape to the 2 liter variant.

You can mill a fair bit off the webs - as you can see they have parallel sides - but 'hoping' it will be 'balanceable' for reasons stated in this thread - is a bit 'hit and miss'..

G

[Kovacevic]

Thanks everyone for your help and advice - really appreciated.
Alex

[torkil]

Hi Thank you for a wery interesting artickels about lightning cranks, Its a lot to lern,

I can not coment on the teories shown here, but i have a lot off practical experience on other type off engines.

In my racing times ,we have tried a lot when it coms to two stroke engines for racing, we have tryed to make wery light cranks by Hoeckle in Germany . They had small counter weight . and some had so mutch viberations that they disintegrated the engine , or was riped off the medium section off the race outboard engine engine.This was 3 cyl 85o cc and chaft power about 120 hp. engines. from OMC .
operating revs obout 11000 rpm.
I have been racing Selva 2cyl race engines witch i did set a official world speed record in 1976. 750 cc sport class.
The Selva had a nasty habit of breaking the crank whenn we throttled down . due off wibrations on about 5ooo rpm.
The Factory in Italy had trubble to find the problems.
The Lycoming aircraft engines piston flat 4 boxer engines has variable weights on the crank .
I have been looking in to Rolls and Benz and aircraft engines all have counterweights,
My opinion is that you relly have to know what you are doing when you are lightning a chrank or modifeing it ,theoreticaly and practically, The counter weights has something to doo with wibrations.


Sorry for bringing in a off topic ,it wasent relly the case you was discussing.!
Also my Englich is bad , i have never had it att scool.

Regards Ove Thorkildsen Oslo Norway

[Guy Croft]

Thanks for that interesting post Ove.

No need to apologise for your English - it is easy to understand and I know you are not fluent. I do know also that you are highly regarded in Norway for your engineering skill - a much more important thing.

GC