Balancing P/C only

[rbraun]

I'd like to change the pistons, jugs and rings on my 1600 SP, but don't want to split the case to get to the rods. Is there a best way to distribute the weight evenly without taking the weight of the rods/pins into account, other than weigh each P/C and factor in the firing order?

[woodsbuggy1]

Is the rest of your rotating assembly balanced?
If not, I wouldn't be to worried about it. The rods can be removed and replaced without splitting the case if you are careful.
How close are the pistons in weight(the last set of Mahle pistons I used were within 1 gram from heaviest to lightest)?
Be sure to check and adjust your ring gaps.
Good Luck
Kenric

[Marc]

Boxer (horizontally-opposed) engines do not need to have bobweight considered when balancing. The rotating assembly (crank/pulley/flywheel/clutch cover) should be spin-balanced; the rods should be matched for overall weight and end-to-end weight, which can be done statically.
The piston assemblies (pistons/pins/clips/rings) merely need to be weight-matched - and all four don't have to be incredibly close, so long as each opposed pair is matched. Often there's enough variation in the wrist pin weights that you can come up with two closely-matched pairs by merely juggling the pins around.
What sort of RPM range will this engine see? The factory spec for a stocker is no more than 5 grams from lightest to heaviest (for both the rods and the pistons, so in the worst-case scenario the reciprocating weight might have a 10 gram range) which isn't usually too hard to achieve, and is "good enough" at sub-4500 RPM speeds...but as the RPM goes up the importance of the balance escalates dramatically.

[rbraun]

Thanks for the quick responses. The weights, including the pins are: 509.5, 509.2, 508.3, and 507.6 making the highest to lowest difference of 1.9 grams. Could I use the two heaviest for cyl. 1 and 4, and the 508 for cyl. #3? Not sure just how to "balance" - just thinking about the firing order.

[Marc]

They're close enough that there'd be no "wrong" way, but it's not the firing order that's of concern, it's the opposed pairs.
I'd put the ~509's on #1 and 3 and the ~508s on #2 and 4 and call it done.

[rbraun]

Thanks - will do.

[Phil69]

They're close enough that there'd be no "wrong" way, but it's not the firing order that's of concern, it's the opposed pairs.
I'd put the ~509's on #1 and 3 and the ~508s on #2 and 4 and call it done.

I agree.
Unless you are planing to rev the engine beyond 5000rpm's there is no real point in balancing the rotating mass.
As for counter weight cranks having any real effect on a flat four they don't.

Check out JPM on the cal-look lounge. This guy built a 1603cc air cooled engine producing a streetable 194.9hp at 8200rpm and over 160ftlbs of tourque at 6000rpm.
This engine is still around today and I believe it is in a car in the uk.

[Marc]

...As for counter weight cranks having any real effect on a flat four they don't...

I don't know what you mean by "real effect" but it's an undeniable fact that the stock crankshaft is susceptible to flexing when pushed beyond its intended use. With only three significant main bearings and a light-alloy case, it lacks the support provided in most engines with more mains and stiffer blocks. Run a stock crank at over ~5500 RPM and the center main bearing will suffer premature wear. Adding counterweights extends the operating range significantly. IMO "full-circle" counterweights such as those proffered by Bernie Bergmann are a complete waste of time and money, all they do is interfere with crankcase windage while providing minimal benefit.

The rotating mass of any engine matters at any speed, but its importance increases dramatically at higher RPMs. Fortunately the factory balancing tends to be better on a typical German engine than on Detroit iron, so if you're dealing with an OEM German crank & flywheel there's probably more imbalance in the clutch cover. Once you start using aftermarket parts, all bets are off and you'd better plan on having a dynamic balance done. Even on a stock engine there's sometimes a little room for improvement, but I agree that it's probably not particularly cost-effective for most mild motors.
Other engine configurations lack the "self-cancelling" effect offered by the boxer design, where the opposed pistons are travelling at identical speeds throughout the rotation of the crank. Pistons travel faster approaching & departing TDC than they do near BDC, so most engines can never be inherently balanced like the boxer.

[Phil69]

...As for counter weight cranks having any real effect on a flat four they don't...

I don't know what you mean by "real effect" but it's an undeniable fact that the stock crankshaft is susceptible to flexing when pushed beyond its intended use. With only three significant main bearings and a light-alloy case, it lacks the support provided in most engines with more mains and stiffer blocks. Run a stock crank at over ~5500 RPM and the center main bearing will suffer premature wear. Adding counterweights extends the operating range significantly. IMO "full-circle" counterweights such as those proffered by Bernie Bergmann are a complete waste of time and money, all they do is interfere with crankcase windage while providing minimal benefit.

The rotating mass of any engine matters at any speed, but its importance increases dramatically at higher RPMs. Fortunately the factory balancing tends to be better on a typical German engine than on Detroit iron, so if you're dealing with an OEM German crank & flywheel there's probably more imbalance in the clutch cover. Once you start using aftermarket parts, all bets are off and you'd better plan on having a dynamic balance done. Even on a stock engine there's sometimes a little room for improvement, but I agree that it's probably not particularly cost-effective for most mild motors.
Other engine configurations lack the "self-cancelling" effect offered by the boxer design, where the opposed pistons are travelling at identical speeds throughout the rotation of the crank. Pistons travel faster approaching & departing TDC than they do near BDC, so most engines can never be inherently balanced like the boxer.

The effective weight of the counter weights are not large enough or far enough away from the centre line of the crank in relationship to the distance of the rods and pistons (and there collective weight and distance from the centre line) to have an effect as the pistons and rods as you quite rightly understand reciprocate in opposition to one another.
This is why it is more important to match the rods and pistons weight (and rod length) as accurately as possible as they effectively become each others counter weights.
Also it is the weight of the pistons and rods which determine the rpm limit of the crank the lighter the rods and pistons the less stress and twisting.
Yes it can expensive,time consuming and difficult to achieve but it can be done with mostly stock parts
I don't know anything about V8's.

[Marc]

Spin a stock, uncounterweighted crank BARE (no rods, no pistons) to 6000 RPM and it will start flexing. Take it past 7000 or so and there's a very good chance it will permanently bend. Adding reciprocating mass to it cannot change the fact that the crank itself lacks sufficient rigidity and support to retain straightness at high speed.
The reciprocating mass is inherently balanced, yes....but the counterweights are needed to prevent the crank itself from whipping out of line due to the offset masses of the crank throws. The need for counterweights on a high-RPM VW engine was established science 45+ years ago.

[Phil69]

Spin a stock, uncounterweighted crank BARE (no rods, no pistons) to 6000 RPM and it will start flexing. Take it past 7000 or so and there's a very good chance it will permanently bend. Adding reciprocating mass to it cannot change the fact that the crank itself lacks sufficient rigidity and support to retain straightness at high speed.
The reciprocating mass is inherently balanced, yes....but the counterweights are needed to prevent the crank itself from whipping out of line due to the offset masses of the crank throws. The need for counterweights on a high-RPM VW engine was established science 45+ years ago.

Please read this article it may help you change your mind.
http://cal-look.no/lounge/index.php?PHP ... ic=3257.30
This engine is still running in a uk street car I believe.

[Phil69]

To save you reading through the whole article which I really hope you do.
The crank in the "MM" is a bone stock one, only 8 dowelled and carefully balanced.
Forces created to the crank is from the weight of pistons and con rods, longer stroke and high rpm creates higher G-forces that pulls and bends the crank, and this is no good for the case.
The counterweights helps the crank to be straight.
The pistons and rods in the "MM" is much lighter than stock which means that the need for counterweights are less, I do not say that you do not need them but you defenetly need them less than with 94mm piston and long stroke.

I will show you an example on force created to the crank with a light and heavy piston at the same rpm,stroke and con rod length.

Ex:69mm stroke,8500rpm,5.4"rod and 565g piston(std) F=19276N
Ex:69mm stroke,8500rpm,5.4"rod and 375g piston(JE) F=12793N

As you can see the created force is 50% more with the heavy std piston.To reach 12793N with the 565g piston the rpm is aprox 6800.

Keeping the weight down will save your engine and make it last longer with more performance.

[Max Welton]

Just in case rbraun is not familiar with SI units, 19276N is ~4333 lbs of force.

Max

[Steve Arndt]

The wbx crank has thicker cheeks and webs to brace and stiffen it in the same way that welded counter weights stiffen the stock type 1 crank.

[Marc]

I'll try one more time. The reason an ACVW crank, even a short-stroke one, benefits from the addition of the counterweights is not because of balance per se, it's because the two center crankshaft throws are offset to the same side of centerline. If there were two more main journals/bearings this would not be an issue, but since there are not it's only natural for the crank to flex like a jump-rope once the inertial forces overcome the stiffness of the forging.