[parish8]

i ordered a scale that goes to 5000grams with a resolution of .01 grams.

there is a bit of a debate about the "drop ins" and if they are ok. my motor is appart and I have some stock parts and the cp drop in stuff sitting here to weigh.

before the scale shows up what is an acceptable tolerance to be considered effectively the same weight? is this just a total weight of the entire rod, rod bolts, piston, pin, retainers, rings, rod bearings? or do we do a "bob weight" technique where I weigh the big end/small end... and do some math?

[parish8]

https://www.enginebuildermag.com/201...alancing-work/

"Realistically, an engine doesn’t have to be that close to be good enough. Most stock street engines will run smooth enough if they are balanced to 1 oz. (28 grams), although many late model engines have much tighter factory specifications (as little as 4 to 6 grams). For a high-revving performance application, aim for the recommended 2 grams or less."

[DorkMissile]

I am in the midst of a similar situation. The problem with these articles is that they are too general, and are trying to capture a wide audience.

According to my math using my stock parts, stock bob weight is 1919gr, recip weight of the parts is 935gr, rotational is 980gr. Supposedly "balanced" is taking half of the bobweight (960gr) and comparing it to the reciprocating weight. The difference would be 25grams, with the rotational weight being heavier (referred to as over-balanced). This is what I deduct the "stock" balance is.

I haver been able to get a lot of info on what the "drop in" kits weigh, and they are all over the map on rotational and reciprocating weight. Some companies even offer "drop ins" that have different piston and rod combinations that vary these numbers also. There is one train of thought out there, that when it comes to engine balance, that as long as the rotational weight is more than the reciprocating weight (unless it is horribly offset) is that it will be fine and run smooth.

I have a setup that calculates to a bobweight of 1876, recip of 874gr, rotational of 998gr. The "balance" would be half of 1876 (938gr) compared to a recip of 874, which comes to a crank being 64gr overbalanced vs the reciprocating weight.

I have the data into my machine shop to ask them if it needs balanced.

In the end, I have dropped reciprocating weight without dropping hardly any rotating weight.

[parish8]

I have some rough data. someone tell me what I am doing wrong. I should probably take the rod off to measure the big end weight. I will try that later.

cp drop in

total 1407gr
big rod end 527gr
remainder of 880gr for the rest


stock stuff

total 1410gr
big end 493gr
remainder 917

right off the bat they are pretty close in total weight. according to that article above you should calc the bob weight and then balance the crank with that bob weight. to do that you take 100% of the big end and 50% of the rest then x 2 because there are 2 piston/rod per journal.

for the cp that is (527+(880/2)) x 2 = 1934grams

for the stock that is (493+(917/2)) x 2 = 1903grams

that looks to be 31grams in bob weight difference between the 2 sets. that seems like a lot to me and less than ideal without balancing. am I messing up my math?

[parish8]

for the record my car was making a ton of power with the drop ins and would still be if I hadn't dropped an exhaust valve. motor felt smooth to me. I want to spin my next motor up a little higher so I will probably get it balanced.

[oldman]

Big end needs to be screwed together and needs to be on a wedge at the split.

[oldman]

Pistons and rods need to be 1g of each other, since you have a scale. Also I should note that my rods are longer than stock.

[parish8]

Originally Posted by oldman

Big end needs to be screwed together and needs to be on a wedge at the split.

I will fix that and adjust the numbers.

[TJay74]

Originally Posted by oldman

Pistons and rods need to be 1g of each other, since you have a scale. Also I should note that my rods are longer than stock.

I think Oldman is spot on here. If it was me I would take the lightest rod and piston of the 8 and try to get the other 7 piston and rods to within 1g of the lightest piston and rod in the set.

I would also make sure the ring stacks for each cylinder together all weigh close as possible to each other as well.

The less variance you have in each setup for each cylinder the easier it will be for the shop to get it balanced. As with anything that has rotational mass, the less it has the easier it will be to rotate it and the less HP that is required to rotate the mass.

The closer your drop in stuff is to each of the OEM stuff or if possible lighter than the OEM stuff the better off you are going to be. I personally wouldnt be looking for aftermarket drop in stuff that was heavier than the OEM stuff, I would hope it would come in lighter and stronger than the OEM stock stuff in this day and age.

[oldman]

usually rods have a mill pad on the bottom to balance, my rods wich are Callies Compstars did not and there is a warning not to change balance on them, as well as a warning about being surfaced hardened and should not be touched, "cohesive stress".

[DorkMissile]

Parish - your math is in the ballpark of mine and my machine shop. The stock bobweight is 1913gr (+/- gr or so).

That being said, when weighing the big and little ends - you have to be spot on with the location of how you hold the rod. You have to do everything you can to get the rod resting level horizontally and pushing down on the scale from the centerline of the diameter. with how your rig is setup, if the rod is touching the scale 1/8" of an inch ahead of or behind the center of the center of the crank journal, you will be off 15-20gr.

Easy way to find out if you are getting good data or not is that your small end and your big end weights should add up to the same weight as the rod on the scale measuring it's whole weight. If it is off by more than a gram or two, you have friction somewhere.

I doubled down and double checked. Added all 8 bigs together & divided by 8. Then added all 8 smalls together and divided by 8. Added the two averages together and I was spot on what one rod weighed. Mine were all 643gr on my scale.

That being said - spoke to my machine shop today (Tesar Engineering - Long Lake, MN), he double checked my math on the bobweight. I was getting 1914, he came up with 1917gr. Now with my setup it calculates to 1870gr, a difference of 36gr, and he wants to rebalance.

In his mind, when going from a stock bobweight to one that needs a lighter crank like mine, he is fine with about 20gr of change as long as the engine doesn't go over 7500rpm, but being I am up to about 36-37gr, he would like to rebalance. Particularly since it is one that is going to be easy. We just need some small holes in the crank counterbalances, he will have it done in a day or two, and it's going to be less than $200.

He says that if the bobweight calculation needed to go up from stock, regardless of RPM or how much weight, it needs to rebalance. With a calculated bobweight under stock, you can get away with some. How much is determined by how far off it is, and how many RPM it will turn.

[parish8]

That is some great info. I will work on getting better numbers. The big end of the rod feels heavier in my hands.

I don’t know if this is true but I have been told the crank flexing and wiping out the mains is something that happens at higher hp’s. Seems like having it balanced might get you a little farther before failure.

[parish8]

got some new numbers. lets see what that does to the calculated bob weight

stock

piston assembly 730.7
rod 679.6
big end 476.5
small end 203.4

bob weight = (((730.7+203.4)/2)+476.5) x 2 = 1887.1gr

cp stuff

piston assembly 720.0
rod 686.2
big end rod 499.6
small end 186.1

calculated bob weight = (((720.0+186.1)/2)+499.6) x 2 = 1905.3gr

a difference of 18.2grams heavier with the drop in stuff. that is a lot less than before. this doesn't add in any for oil but that should be the same for either set up.

is 18.2 grams heavier bob weight cause for concern?

[DorkMissile]

Well I am by far not someone who makes a living doing this, but this is how I look at it:

There is "Balance" (where all the individual parts weight the same - pistons, rods, etc.....) and then there is "Balance" when the rotational weight and the reciprocating weights are the same. The first is easy to figure out, the second not so much.

I made a spread sheet to calculate all of this for my build - and I put your data into it on both setups to see what it spit out. When I double checked my numbers to my machine shop's calculations, we were nearly dead on to each other.

On the stock setup - you have a 1889gr bobweight, with a ROT Weight of 952gr, and a RECIP of 933gr. These are the two forces that are fighting against each other as the engines run, and the farther from each other they are, the more opportunity for harmonics to happen.

On the new setup - you have a 1910gr bobweight, but since the new rod is heavier on the big end, and lighter on the small end vs stock (and the stock crank counterweights), you have a ROT that is higher by almost 50gr and a RECIP that is lighter.

In effect, you have gone from a indifference of 11gr on the stock setup, to a indifference of 49gr. Which is 2 ounces - one tire weight. Once you consider that the balance is off that much, on 4 crank journals, and it is fighting against each other 2 times a revolution (TDC and BDC) you can see how it makes harmonics. My math says you will have 930 "events" per second fighting to tear everything apart at 7000rpm.

Its not a lot of money to get it balanced, and it looks to me that since your ROT weight is higher than the RECIP, they will just need to remove some weight on the crank, not add too it (which is more expensive)

Here is my math: