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Discussion Starter #1
We are going back together with basically the same combo after our rod "mishap". Same pistons and crank, but a new set of rods (Callies Compstars we think). The new rods total weight are within a gram of our old ones. However, the small end is 10 grams lighter and the big end is 10 grams heavier. Seems with the piston/pin/small end total weight around 1170 grams (not counting rings/spiro locks), 10 grams differance isn't much in a 7000 RPM combo... What do ya'll think...
 

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We are going back together with basically the same combo after our rod "mishap". Same pistons and crank, but a new set of rods (Callies Compstars we think). The new rods total weight are within a gram of our old ones. However, the small end is 10 grams lighter and the big end is 10 grams heavier. Seems with the piston/pin/small end total weight around 1170 grams (not counting rings/spiro locks), 10 grams differance isn't much in a 7000 RPM combo... What do ya'll think...
I personally would have the rods balanced if that is all that has changed. An average balance job gets the whole reciprocating mass within a quarter gram and normally closer. Its apart why take it apart again because of an odd vibration. CHEAP INSURANCE. IMO. M
 

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Well, since you take 100 percent of the rotating weight (big end) and 50 percent of the reciprocating weight..you do the math. 10 grams per rod=20 g per rod throw. Now, subtract the 10g for the reciprocating end (5g per rod). (remember 50 percent reciprocating)
Will this rattle your teeth. Hell no.
Is it right? Hell no. (not huge either)
Are you planning on running 8000 rpm?
Just food for thought.
Wags
Oh, and don't forget the black magic of balancing. Some guys like to use 51 percent or 52 percent of reciprocating. Some guys use 3 or 4 g of oil allowance, some use 6 or 7.
 

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not sure i follow your math.when i balance pistion ,pin,rings,oil 3grms,1 small end,2 big ends and 2 sets brg and keepers if apply add up,divide by 2,bob wt is 2200 each half is 1100,sometimes a customer will 1 percent over balance on recricating only high rpm.now v6 with 6 crank pins use 36.6 percent nothing is doubled.iv seen stock rods be 15 grams off and balenced to 25 grams.i get mine down to 5 and depends on the engine and mood iam in sometimes under 1 gram.
 

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Discussion Starter #7
OK, it took a bit longer to get to the point Steve and Vami are bringing up than I thougt it would; calculating the bobweight. Seems everyone does it differently, yet no one seems to be able to explain which way is best/why. That's where the magic in crank balancing seems to be.... And I hate magic.....
In my day job/career, I've done lot's of balancing of large plant equipment. And there are accepted standards for residual imbalance of different types of equipment. If a client tells me he ground 10 grams off a rotor of some type, I can do some basic calcs and tell him how much vibration that will result in. Given that there are millions of engines out there, seems there should be standards for calculating the bob weight as well.... meaning no magic to it... Maybe I'm wrong.
Finally, FWIW, this engine should never see over 7500 RPM...
 

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steelcomp was here
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If you trust your machine shop to do the balance up to your own standards, then I would guess whatever method they use would be acceptable. I don't see why you wouldn't re-balance unless you have time or financial constraints that won't allow it. Remember, weather it's actually measurable or not, the more imbalance, the more parasitic drag and HP loss. FWIW.
 

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Discussion Starter #9
......I don't see why you wouldn't re-balance unless you have time or financial constraints that won't allow it. ......
Man steel, when you put it like that, I guess it seems kinda dumb not to.... :)bulb
 

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B1 Racing
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I would re balance. Doesnt hurt to double check things.

Hope to see ya in Sept. David.
 

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I would re balance. Doesnt hurt to double check things.

Hope to see ya in Sept. David.
You don't need to rebalance whole motor. Crank, f/wheel and dampner are balanced separate from the rods. As are the pistons. Have the rods balanced and save a buck. Once the pin end and journal end weight the same it's all good. IMO. M
 

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steelcomp was here
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You don't need to rebalance whole motor. Crank, f/wheel and dampner are balanced separate from the rods. As are the pistons. Have the rods balanced and save a buck. Once the pin end and journal end weight the same it's all good. IMO. M
Exactly right.
 

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not sure i follow your math.when i balance pistion ,pin,rings,oil 3grms,1 small end,2 big ends and 2 sets brg and keepers if apply add up,divide by 2,bob wt is 2200 each half is 1100,sometimes a customer will 1 percent over balance on recricating only high rpm.now v6 with 6 crank pins use 36.6 percent nothing is doubled.iv seen stock rods be 15 grams off and balenced to 25 grams.i get mine down to 5 and depends on the engine and mood iam in sometimes under 1 gram.
Yes...50 percent per throw (rod throw, not per cylinder) of reprocatring. That is what you are doing. Tanking two big ends (100%) rotating per rod throw. 1 piston, 1 pin, 3g oil, 1 set of rings (this is 50% of ther total weight on that rod throw).

I think we are..no, if I read your post correctly, I know we are on the same page.
Wags
 

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What are you gonna save 50 bucks get the whole motor rebalanced as cs said no reason not to double check everything you might as well thats just doesnt make since.
 

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Just stating my opinion. If I had hurt my last motor (for an unknown reason) and was putting the same combination back togethor using alot of the same parts I'd be double (maybe triple) checking everything so i sleep good at night. Everyone does things different.
 

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Well, since you take 100 percent of the rotating weight (big end) and 50 percent of the reciprocating weight..you do the math. 10 grams per rod=20 g per rod throw. Now, subtract the 10g for the reciprocating end (5g per rod). (remember 50 percent reciprocating)
Will this rattle your teeth. Hell no.
Is it right? Hell no. (not huge either)
Are you planning on running 8000 rpm?
Just food for thought.
Wags
Oh, and don't forget the black magic of balancing. Some guys like to use 51 percent or 52 percent of reciprocating. Some guys use 3 or 4 g of oil allowance, some use 6 or 7.
Damn glad I don't go to your'e machine shop. Factory is closer than that. M
 

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steelcomp was here
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What are you gonna save 50 bucks get the whole motor rebalanced as cs said no reason not to double check everything you might as well thats just doesnt make since.
It's not the savings. A complete re-balance job just isn't necessary if you're confident that it was done right the first time. Balance jobs are funny...you can balance and balance till the cows come home, chasing the light, and never get it "perfect".
Having said that, and what no one is mentioning here (and I just realized) is that there's no way you're going to make the small end of the new rod the same as the old ones. If you make the big end the same, then your rods will be 10g lighter (over all) than before. Something to consider.
 

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Discussion Starter #20
I would re balance. Doesnt hurt to double check things.

Hope to see ya in Sept. David.
Chris, that's kinda what we decided. We are going to double check the whole engine balance. This time I'm going to hang out at the shop and watch just because we had an unusual failure. My machinist is pretty cool. We are working hard on being at Bakersfiled in September... My wife has already OK'ed buying another engine if necessary. Man she's cool....

You don't need to rebalance whole motor. Crank, f/wheel and dampner are balanced separate from the rods. As are the pistons. Have the rods balanced and save a buck. Once the pin end and journal end weight the same it's all good. IMO. M
With the small end light by 10 grams, and the big end heavy by the same amount compared to the old rods, this doesn't seem feasible. I don't mind taking a gram or two off a rod if one end or the other is off, but not 10.

What are you gonna save 50 bucks get the whole motor rebalanced as cs said no reason not to double check everything you might as well thats just doesnt make since.
You must not live here in Cali... $50 does'nt buy the shop coffee around here. A complete race balance starts at $325, not counting heavy metal. We had $120 of heavy metal installed when we first balanced the combo. I get a "racer deal" but it's still way more than $50. These people are friends and I can't ask them to work for free...
 
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