Re: Bob weight and RPM
[Re: emarine01]
#585024
01/17/10 06:32 AM
01/17/10 06:32 AM
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Joined: Mar 2006
Posts: 9,910 Eighty Four, PA
B G Racing
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Quote:
Is there a way of knowing the effective results of a lighter bob weight, Example// A engine with a 1800g bob weight turns 7500 vs the same engine with a 1600g bob weight turns 8500/ before failure
Lighter weights free up RPMs.Before failure is is relative as to what fails.High RPMs will find the weak componants.Quality of parts and quality of machine work and assembly are more a determining factor at the bobweights mentioned.Check with the manufacture for any test data that supports their claims.For instance Oliver Rods has someone do distructive and non-distructive test on their various design of rods and well as metallergical studys on failed componants.
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Re: Bob weight and RPM *DELETED*
[Re: emarine01]
#585026
01/17/10 01:51 PM
01/17/10 01:51 PM
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Joined: Oct 2007
Posts: 4,219 New York
polyspheric
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Post deleted by polyspheric
Boffin Emeritus
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Re: Bob weight and RPM
[Re: emarine01]
#585029
01/17/10 03:32 PM
01/17/10 03:32 PM
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Joined: Jun 2003
Posts: 52,972 Romeo MI
MR_P_BODY
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Quote:
Given the engine with the lighter bob weight has proper balancing for the weight change, is there a way of measuring the lighter rotating assembly and express it in max rpm change?
I dont know about the rpm change BUT if you were to move the weight inward on the crank to balance it (cut the counter weights down) you do 2 things, you move the weight in tighter to the c/l and you take that big sail out of the oil pan reducing the windage which in turn allows it to accelerate faster... JMO
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Re: Bob weight and RPM
[Re: MR_P_BODY]
#585031
01/17/10 04:44 PM
01/17/10 04:44 PM
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Joined: Jan 2007
Posts: 3,695 nc
emarine01
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Thankx Mike, that makes perfect sense but this doesn't, small block chevy 434, 650 hp with a cast steel crank and a bad rod ratio, alu rods with light pistons, crank cut down to 40 lbs, this motor sees 8k for the last 4 years, I have drove this rail and seen the trap rpm so I know he is not full of it, and the crazy part is it was a used crank when he started, This goes against everything I have read about crank shafts, He has also built several engines for other racers the same way and they have not failed , Is this just dumb luck or is there more to bob weight than meets the eye? I read the cast stroker post the other day and have seen all the failures and opinions from the guys on here and tend to agree with the reasoning that cast cranks don't belong in race engines, But it seems to work in chevies?
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Re: Bob weight and RPM
[Re: emarine01]
#585032
01/17/10 04:49 PM
01/17/10 04:49 PM
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Joined: Jan 2003
Posts: 6,591 Canton, Ohio
Sport440
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Quote:
Given the engine with the lighter bob weight has proper balancing for the weight change, is there a way of measuring the lighter rotating assembly and express it in max rpm change?
The answer is yes. If you know the weak link capacity of either the rod or bolts.
With any given balanced assembly/bobweight you could calculate a max rpm based on the calculated forces. Example, if your rod bolts are good for 74,000# You could calculate a RPM that could be reached before you surpass that 74,000# limit.
Some engine programs will calculate that automatically for you. Boggs had one that would give the numbers.
Sorry, I dont have the Formula to figure it out at hand.
I bet Jeff, Polyspheric would have it. mike
Last edited by Sport440; 01/17/10 04:51 PM.
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Re: Bob weight and RPM
[Re: emarine01]
#585035
01/17/10 11:13 PM
01/17/10 11:13 PM
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Joined: Nov 2003
Posts: 6,293 Rock Springs
Bob_Coomer
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572" Hemi has a 2389 Bob weight..it will see 7000-7200 rpms..
My 557" wedge has a Bob weight of right under 2300 grams..I turned it 8K once or twice...LOL
[color:"red"]65 Hemi Belvedere coming soon [/color] [color:"#00FF00"]557" Indy engine 1.07 60ft 144mph in the 8th 2100 lbs package [/color]
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