Forged engine: rework counterweights?
 

I'm currently building a turbo engine with forged rods and supertech pistons. The parts AR weight-matched within 0.3 g.

however, the forged rods are 50 g each lighter than the stock ones. So should I have the machinist remove 4x50 g from the counterweights on the crank? Or should I just have the crank fine-balanced and call it a day?

Have it balanced as a rotating assembly.

i just had this done for an eninge. pistons, rods and crank balanced together. Cost $130 to get done. Well worth the price for getting it done right in my opinion. However i put manley rods in my engine without doing any more than making the rods all the same weight. so far mine seems ok but i do wish i would have taken the time to get it balanced.

When I got my engine built, they asked for everything between the pressure plate to the harmonic damper. It was balanced as a rotating assembly. Though I think the bling billet crank 949 offers is already zero balanced.

The counterweight doesn't balance the rod. It's more of a damping effect on the reciprocating assembly. Any force applied to the crankshaft from the rod itself on #1/#4 is matched by an opposite force acting on #2/#3.

Crankshafts are balanced independently. Sometimes people will add the damper/flywheel/PP, although since the clutch disc will naturally change its position on every shift, I see less benefit in doing this to a manual-trans motor (different on a torque converter auto where everything remains static). The reciprocating parts are then balanced as three unique sets (big end, small end, pistons). This gets you a complete balance.

Removing weight from the couterweight of a BP crank is asking for trouble. IMO, it's not advisable under any circumstance.

So the "counterweight" isn't designed to "counter the weight" of the rod/piston assembly? You know, to balance the forces acting on that area of the crank. Are you freaking serious? That big steel crankshaft is a damper????This is very incorrect.

Crankshafts are balanced with a presumed bobweight when they are made, and any decent shop that builds engines will balance the crankshaft with the correct bobweights clamped to the journals.

Oops, yes. I'm a little confused on it, clearly.

OP, your guy isn't just going to remove 50g from the crank. Read this for more:

Understanding Crankshaft Balancing - Engine Builder Magazine

Thank you Pat.

FWIW I built like 5 motors using ebay rods on stock pistons, balanced nothing, and it worked. Yeah it vibrated more than a stock motor, but other than that it worked. My current motor was balanced correctly, and it's as smooth as a stock miata with stock motor mounts, and I have solid motor mounts. The reduction in vibration was very nice, and that's gotta help engine life in some way. If I blew the engine I have now, my new motor would get balanced even if I cheap out on some other things to save money. As Sav mentioned a shop will balance the individual pistons/rods to each other, then they calculate a bob weight, install that on the crank, and balance the crank.

We are talking about Miatas here right? Inline four cylinder engines?

You guys need to get your facts straight.

No bob weights needed for balancing an inline four. NONE. Yes, the rods and pistons require balancing, but have not relation to crank balancing on a Miata motor.

Crankshaft & Engine Balancing : MA,CT.RI,NH,ME,NY,NJ

you're talking about first order vibrations, right? But don't forget second order harmonics.

Or simply put: why did Mazda and all other inline-4 engine makers use counterweights in the first place?

@ Pats, Savington: My plan is to have the crank balanced, the flywheel balanced and then crank+flywheel+harmonic balancer. The Pistons and rods are already balanced. FWIW, my engine builder said I wouldn't need to have the counterweights matched to the lighter piston/rod combo. But then again he admitted he didn't have the machines anyway...

I won't pretend that I design engines or crankshafts for a living. I don't. And such I don't know everything about crankshaft design.

As you mention, first order vs second order is of course one reason why counterweights are a certain weight, which minimizes vibration due to the throw across from the counterweight, and rod/piston slinging around opposite of the counterweight. Beyond that there's other things they do as well that changes the torsional rigidity and probably some other things as well. I asked my machinist about balancing the crank when they built my bottom end, and he said they only took a little off the counter weights. My rods were lighter than stock, but the pistons heavier than stock. I don't know what the numbers were for any of it so can't say what exactly was done. It runs way smoother than anything I ever built, I know that for sure.

My understanding is that you only need to 'balance' a L4 crank for centripetal forces so that the center of mass is inline with the centerline of the bearings. You don't need to factor in the weight of rods, pistons etc because there are 2 on each side at equal spacings from the centerline. An out of balance crank will put increasing sideways loading via vibration on the bearings as revs increase.

So this means you can remove the counterweights entirely right? Well you can if the forces acting on the crank are equal, but the pistons apply load at different points along the length of the crank at different times which causes harmonic flex. This is basically causing the crank to bend like a guitar string at multiple orders of harmonics at once all combining on top of each other like waves in an ocean. At certain frequencies they all add up to cause really high stresses. It's these frequencies that the harmonic balancer is attempting to dampen. Harmonic flex causes the crank to twist, and potentially fail outright... however more likely is that the main bearings fail first due to rotational misalignment between the crank and the bearing surfaces (causing contact or leaking oil pressure and then contact).

The counterweights are there to try and put mass on the opposite side of crank to the piston forces to try and slow the amount of deflection with time.... but the more counterweight mass the more the crank can deflect once it gets into a harmonic frequency. So it's an engineering challenge to balance all the various forces so that the crank doesn't outright fail or the cranks don't deflect enough to cause bearing failure.

You can't really balance out harmonics as it depends on lots of factors, like crank weight distribution, piston and rod weight, horsepower, revs etc etc. Plus you really need to spin the crank at ludicrous speeds to test it. F1 teams run sensors on the crank bearings to measure deflection at really high data rates which is really the best way to test for flex.

My experimental lightweight crank was made because I know in a L4 design that counterweights 2 & 3 and 6 & 7 are less critical than the others for harmonic balance. Plus there are lots of racing cranks like Cosworth and even the SOHC Protege crank that have these counterweights removed and survive high revs. There is a reason that I haven't run it yet in my race engine as I'm not sure on the outcome. I have to build up enough courage to drop it in first.

https://cimg9.ibsrv.net/gimg/www.mia...db895d65ca.jpg

please drop it in

that crank is pure sex :makeout:

madjak,

So after your mods don't you essentially have a 180 degree/single plane crank?

I had Bryant build me a single plane crank for an experimental race project...Each counterweight was much smaller than normal, but they were all exactly the same size... balancing both sides of each crankshaft journal if you will.
I see where you are going with this, but for it to work properly everything else in the engine has to work toward those goals?
The crank made a much (MUCH) rev happier, extremely explosive package, but the piston bore, rod length, camshaft, firing order, and head/ports were designed around it....and not by me! :)
Therein lay a problem with the BP...none of the supporting mods are available....Not to mention dealing with the factory rod design!

It is possible that you might sleeve the block, go to a larger bore, cut the deck just above the water, single plane crank, and get a better rod out of the deal (maybe a K20 rod and piston?).....Then you have oiling issues to contend with.....Big project....I will shut up now :)
Just throwing ideas out there!

Or...Maybe just go to a K20 ? :)

Aren't all inline 4 cranks single plane cranks?

yeah, I suppose so....I am new to 4 bangers.
I was more referring to a high performance single plane crank.
If you look at a Ferrari or other high performance single plane cranks they look like there is almost no counterweights. Pretty strange the first time you see one!

Trying to get a BP to behave like a NA Ferrari would be difficult, but it could be cool!

No not necessarily. Yamaha has had a crossplane i4 in production since around 2009 in the R1. They sound like sex.
https://cimg6.ibsrv.net/gimg/www.mia...7c2e67a94a.jpg

The F1 cranks have no counterweights or harmonic balancers. They can manage this because they run such short stroke rods and large diameter bearings / crank so it's extremely rigid. Also the harmonic forces build up over time at set resonant frequencies within the rev reange and since F1's are changing revs so rapidly they don't sit at a single rev for long enough to generate peak forces. Nascar engines run massive counterweights and large dampers as they sit at a constant speed for longer, plus rapid engine speed changes aren't so critical.

To achieve something in a BP would be basically impossible given the stroke length. You'd need to drop the stroke length considerably, up the bore size to try and retain some cc, and then you'd need to rev it past 10k to see reasonable power. It's far easier to slap a blower or turbo onto a built engine and call it a day!

Here is something cool that is kind of related. You can actually hear some harmonics in the below video as a warble every gear change... It's not quite the same thing we are discussing above but it's a nice example. In these old F1 engines, there is such low crank mass with no damper or flywheel that the entire rotating assembly is vibrating in rotation. I don't think it's crank flex, but it affects the note of the exhaust.