Re: How many grams change before rebalance??
[Re: DemonDust]
#1868443
07/11/15 09:54 PM
07/11/15 09:54 PM
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Joined: Jun 2003
Posts: 52,972 Romeo MI
MR_P_BODY
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Master
Joined: Jun 2003
Posts: 52,972
Romeo MI
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The amount that is going to be removed is probably only going to be a few grams.
But the high rpms is what I'm worried about. 9400+ You better rebalance.. and get it close... some of these guys that dont balance dont give a damn or just have junk... I balance every engine... yes Chrysler was the worst when it came to balancing and in a lot of cases it showed.. unless it was just stock junk... I normally turned 8200 but I did turn 9600 on a few runs.. smooth as silk... see what happens at 9500+ with a crap balance... you sure wont like it ![wave wave](/ubbthreads/images/graemlins/custom/wave.gif)
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Re: How many grams change before rebalance??
[Re: DemonDust]
#1868480
07/11/15 10:56 PM
07/11/15 10:56 PM
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Joined: Nov 2003
Posts: 10,384 Upstate NY
Bigcube
I Live Here
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I Live Here
Joined: Nov 2003
Posts: 10,384
Upstate NY
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I went with a piston that is 14 grams lighter when I converted to pump gas. No re-balance and ran just fine on the dyno. No issues on the street either. I don't turn 9400 RPM ![shock shock](/ubbthreads/images/graemlins/custom/shock.gif) On the dyno we went to 7200.
Jim
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Re: How many grams change before rebalance??
[Re: MR_P_BODY]
#1868697
07/12/15 11:11 AM
07/12/15 11:11 AM
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Joined: Jan 2003
Posts: 32,394
Quicktree
I Win
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I Win
Joined: Jan 2003
Posts: 32,394
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The amount that is going to be removed is probably only going to be a few grams.
But the high rpms is what I'm worried about. 9400+ You better rebalance.. and get it close... some of these guys that dont balance dont give a damn or just have junk... I balance every engine... yes Chrysler was the worst when it came to balancing and in a lot of cases it showed.. unless it was just stock junk... I normally turned 8200 but I did turn 9600 on a few runs.. smooth as silk... see what happens at 9500+ with a crap balance... you sure wont like it ![iagree iagree](/ubbthreads/images/graemlins/custom/iagree.gif) stock application no problem. high rpm race motor balance it, shouldn't even be a conversation about it.
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Re: How many grams change before rebalance??
[Re: DemonDust]
#1868714
07/12/15 11:58 AM
07/12/15 11:58 AM
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Joined: Oct 2003
Posts: 7,506 Az
Crizila
master
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master
Joined: Oct 2003
Posts: 7,506
Az
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In that case, there really shouldn't even be a question about it - at any rpm.
Fastest 300
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Re: How many grams change before rebalance??
[Re: DemonDust]
#1868727
07/12/15 12:24 PM
07/12/15 12:24 PM
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Joined: Oct 2007
Posts: 4,213 New York
polyspheric
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master
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New York
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Vizard's latest BBC book strongly suggests that closely matching all reciprocating weight (1 gram) is far more important than making a specific percentage number.
Boffin Emeritus
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Re: How many grams change before rebalance??
[Re: DemonDust]
#1868743
07/12/15 12:52 PM
07/12/15 12:52 PM
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Joined: Jan 2003
Posts: 1,237 North Central, Indiana
Roughbird72
pro stock
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pro stock
Joined: Jan 2003
Posts: 1,237
North Central, Indiana
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I just want everyone to know we're talking maybe 3-4 grams the amount of oil on the rotating assembly could vary more than that, i think you'll be fine.
72 Plymouth Roadrunner 11.08@123.25
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Re: How many grams change before rebalance??
[Re: Crizila]
#1868758
07/12/15 01:18 PM
07/12/15 01:18 PM
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Joined: Jan 2003
Posts: 32,394
Quicktree
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I Win
Joined: Jan 2003
Posts: 32,394
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In that case, there really shouldn't even be a question about it - at any rpm. how many 9600 rpm motors have you tested that theory on?
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Re: How many grams change before rebalance??
[Re: Quicktree]
#1868804
07/12/15 02:51 PM
07/12/15 02:51 PM
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Joined: Oct 2003
Posts: 7,506 Az
Crizila
master
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master
Joined: Oct 2003
Posts: 7,506
Az
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In that case, there really shouldn't even be a question about it - at any rpm. how many 9600 rpm motors have you tested that theory on? None of course. Don't think there are too many out there spinning that #. Mine has lived 16 -19 grams piston heavy @ 7K for some time now - which is the extent of my experience. I pretty much relied on my engine machinist for the decision not to rebalance. Could blow up tomorrow ![shruggy shruggy](/ubbthreads/images/graemlins/custom/shruggy.gif) . ![beer beer](/ubbthreads/images/graemlins/custom/beer.gif)
Fastest 300
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Re: How many grams change before rebalance??
[Re: Crizila]
#1868883
07/12/15 05:18 PM
07/12/15 05:18 PM
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Joined: Nov 2004
Posts: 4,330 Lynchburg, VA
Leon441
master
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master
Joined: Nov 2004
Posts: 4,330
Lynchburg, VA
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The engine is probably overbalance already. Small changes in pistons are tolerable even pins. You change rods that a different weight on big end, rattle bad.
Career best 8.02 @ 169 at 3050# and 10" tires small block power.
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Re: How many grams change before rebalance??
[Re: dogdays]
#1868892
07/12/15 05:32 PM
07/12/15 05:32 PM
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Joined: Jan 2003
Posts: 26,000 Rio Linda, CA
John_Kunkel
Too Many Posts
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Too Many Posts
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Posts: 26,000
Rio Linda, CA
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Now for a real life example. A board member reports that he changed 591 gram 318 pistons with 507 gram KB pistons with no noticeable change in how the engine ran. There was no attempt to rebalance the engine.
Some people won't notice a subtle vibration, others will.
The INTERNET, the MISinformation superhighway
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Re: How many grams change before rebalance??
[Re: justinp61]
#1870205
07/14/15 05:37 PM
07/14/15 05:37 PM
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Joined: Jan 2003
Posts: 16,376
dogdays
I Live Here
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I Live Here
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That's pretty much the responses I expected. "How Dare You question my religion? If you don't do it exactly as I say, the sky is going to fall and a giant 9600rpm Kraken will come out of the sea and eat you!"
The plain and simple truth is the V8 balancing formula isn't mathematically derived. It's just what seems to work the best. The ESTIMATE on oil weight is in that same ballpark, it is true.
To amplify, I think matching component weights is simply good engine building practice, and also for an initial build, balancing by the formula is a good place to start. I also believe that Vizard has a pretty good handle on things. He's also done a ton of dyno work.
I remember years ago when Grumpy Jenkins was being paid by Chevrolet to develop one of their V6s for racing he complained that the counterweights were in the wrong places. It blew my mind. Factory engineers had sweated the details on the engine, but Bill Jenkins was no dope. Sometimes the conventional wisdom isn't wisdom, just conventional.
The simple fact that engine builders are fudging the percentages on the magic formula tells me that it is at best a very good guess of what will work, and logically a difference of 2-3 grams per piston which is in the order of 0.2% of the bobweight will make absolutely no difference, even at 10,000 rpm.
Now, what other sacred cows are ready to be speared?
R.
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Re: How many grams change before rebalance??
[Re: DemonDust]
#1870229
07/14/15 06:16 PM
07/14/15 06:16 PM
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Joined: Jun 2003
Posts: 52,972 Romeo MI
MR_P_BODY
Master
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Master
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Posts: 52,972
Romeo MI
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I have a question, everyone throws a few grams in for oil sticking to everything. My engine is dry sump and pulling 17 in/hg, with that being said everything is in a vacuum wouldn't that affect how much oil sticks to the bottom end? What counts even more is if your using a crank scraper.. but Dog knows that.. I'm sure he runs engines into the high 9500+ range ... why not have it balanced as close as possible and save your mind and a engine ![wave wave](/ubbthreads/images/graemlins/custom/wave.gif)
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Re: How many grams change before rebalance??
[Re: DemonDust]
#1870237
07/14/15 06:31 PM
07/14/15 06:31 PM
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Joined: Feb 2011
Posts: 4,302 Nebraska
72Swinger
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master
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Nebraska
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My first drive in my Gen III it had some weird harmonics. It was balanced locally and has an ATI "blem" balancer on it. After a few hard runs it smoothed out and is smooth now? Has anyone else ever experienced this?
Mopar to the bone!!!
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Re: How many grams change before rebalance??
[Re: Oyvind Mopar]
#1870245
07/14/15 06:43 PM
07/14/15 06:43 PM
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Joined: Jun 2003
Posts: 52,972 Romeo MI
MR_P_BODY
Master
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Master
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Posts: 52,972
Romeo MI
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The oil factor is just a guess. I am not sure it is a factor at all, more of a fixed value. It should not be related to weight, but surface/windage/shearforces in the crankcase. In practice it is a slight overbalance, 0.5-1%, obviously something that works well. When you add the oilfactor to all sorts of engines it just shows that the formula could be improved upon, as dogdays mentions. The engine builders I have used have never asked what weight oil I plan on running... be it 10 weight or 50 weight and I'm sure that weight will be different ![wave wave](/ubbthreads/images/graemlins/custom/wave.gif)
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Re: How many grams change before rebalance??
[Re: MR_P_BODY]
#1870296
07/14/15 07:58 PM
07/14/15 07:58 PM
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Joined: Aug 2003
Posts: 43,311 Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,311
Bend,OR USA
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The oil factor is just a guess. I am not sure it is a factor at all, more of a fixed value. It should not be related to weight, but surface/windage/shearforces in the crankcase. In practice it is a slight overbalance, 0.5-1%, obviously something that works well. When you add the oilfactor to all sorts of engines it just shows that the formula could be improved upon, as dogdays mentions. The engine builders I have used have never asked what weight oil I plan on running... be it 10 weight or 50 weight and I'm sure that weight will be different ![iagree iagree](/ubbthreads/images/graemlins/custom/iagree.gif) Same deal on bearing and lifter bore clearances on how much oil leaks down into the rotating assembly ![work work](/ubbthreads/images/graemlins/custom/work.gif) A guy can drive himself crazy worrying about some things ![shruggy shruggy](/ubbthreads/images/graemlins/custom/shruggy.gif) Not me though, put it together properly and beat the dog doodoo out of it ![devil devil](/ubbthreads/images/graemlins/custom/devil.gif)
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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