Why no talk about balancing?

alvin l nunley

Premium User
I very seldom hear any talk about balancing for stock appearing and open engines. Just wondering why that is. Is it a secret and were just not being told about it?
 

95 shaw

Premium User
Because of the nature of a 4 stroke engine, ie, power stroke every 2nd revolution, balancing is only correct for a very small rpm window.

Some power can be had within that window, often at the expense of power outside the window.

Knowing what the best balance factor is for your particular setup becomes expensive knowledge.
For most, using the balance as it is yields best bang for the buck.

Ymmv
 

Don K

Member
Since most speedway motors operate in a narrow rpm band balancing may be relevant. Isn't it true most speedway motors operate within a close range for a given class? I can see where a better built motor would be balanced internally perfectly.

DK
 

alvin l nunley

Premium User
A stock Briggs operates at a constant 3600 RPM. I'm assuming the crankshaft is balanced to that RPM.

The balance factor of a stock Briggs, I'm assuming, would be greater in an engine that operates between 3600 and 5500 RPM. More like 4500 RPM. How can you dispute that?

Someone should cut large holes in a Briggs stock appearing piston and see if I'm right. If you lighten the piston, it does pretty much the same thing as making the crankshaft counterweights heavier.
 

95 shaw

Premium User
Open engines operate in a wider range of rpm than stock engines, as you still have power after peak hp (not limited by valvetrain constraints). Also, because of more power, track speeds are higher, and rpm drops are also much larger.
Typical open 4 cycles can run to 10k plus rpm, depending on cam choice and needed longevity.
Typically use a longer rod than stock. Thus, shorter forged piston, with 2 rings instead of 3 like stocker. So, yes, weights change.
Valve train also adds to imbalance.
I'm not saying that balancing for a 2-300 rpm range isn't beneficial, just less cost effective for most.

I'm sure you can ask any builder to balance your rotating assembly to whatever factor you choose.
Don't expect to see any large gains.
 
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95 shaw

Premium User
A stock Briggs operates at a constant 3600 RPM. I'm assuming the crankshaft is balanced to that RPM.

The balance factor of a stock Briggs, I'm assuming, would be greater in an engine that operates between 3600 and 5500 RPM. More like 4500 RPM. How can you dispute that?

Someone should cut large holes in a Briggs stock appearing piston and see if I'm right. If you lighten the piston, it does pretty much the same thing as making the crankshaft counterweights heavier.
How much would you change the balance factor, just changing peak rpm from 3600 to, say, 8500 rpm, changing nothing else?
What would you expect the power gains to be?
 

alvin l nunley

Premium User
Would you change it from 80% to 60% ?
I am not schooled in balancing percentages. I don't know what a good number would be. I just know that something heavier would be better.

In 1974, when I built my Mac 91 open, Horstman was selling tungsten counterweights for the Mac crank, that's the most experience I've had personally. A couple of guys have drilled KT100 crankshaft for adding weight, that seem to work. Unfortunately, When it was discovered, people lost first Place trophies that IKF events, Both Sprint and enduero
 

flattop1

Dawg 89
Here is something to ruminate on .
 

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Chris Cooper

Premium User
I am not schooled in balancing percentages. I don't know what a good number would be. I just know that something heavier would be better.

In 1974, when I built my Mac 91 open, Horstman was selling tungsten counterweights for the Mac crank, that's the most experience I've had personally. A couple of guys have drilled KT100 crankshaft for adding weight, that seem to work. Unfortunately, When it was discovered, people lost first Place trophies that IKF events, Both Sprint and enduero
Was that a builder in mid Cali ?
 

Pete_Muller

Moderator
I believe the page from Jockey Journal is dead on...

- A single cylinder engine can not be perfectly balanced without a crank-driven (timed) balancer shaft.

- On an engine where the bore is vertical, low 50% to low 60% seems to be the range that historically worked well, in my experience.

- On a single cylinder engine, changing balance factor percentage will shift imbalance between horizontal and vertical directions, as the article states.

Since there is no magic "correct" value, the best way to find what works for a particular situation/application (say a 4-cycle running between 3500-5500rpm) is to make a significant change and test. My own approach would be to balance one engine at 50% and one at 60%, and test the engines back to back.

Probably best to also mention that (in my opinion) the stiffness of the motor mount -- in a horizontal (fore/aft) direction, in the vertical direction, and also in the "roll" direction (tipping left/right) -- can change the way an engine runs and feels on a kart. It's a difficult thing to quantify, but I *have* tested with different motor mount designs on a 2-cycle road race kart and had a very measurable difference in performance (on a stopwatch).

PM
 

Bumpy

Member
A single cylinder engine can never be balanced as say a flat four or V8 can.
It is always out of balance except at TDC and BDC. All the add-on sliding
weights and countershafts do is stop fore and aft shaking.
It's what Flattop posted above.
 
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