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Old 12-12-2010, 09:01 PM
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External Balancing

I have an externally balanced 383 chevy rotating assembly. Question- would you feel safe spinning it to 6500. Bolts are ARP throughout. If not, why so.

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Old 12-12-2010, 09:48 PM
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Without knowing the rest of the parts list i.e. cast...forged? how it was built and who built it I would say no. Now If a knowedgeable person built it with the right combination of parts and it was balanced and tuned properly then yes it would be safe to 6500.
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Old 12-13-2010, 01:09 AM
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Put together with properly clearanced parts and with ARP hardware... 7500 rpm wouldn't really bother me, lots of folks have done it including myself and friends for drag race or street/strip use. Circle track use would make me think twice but 6500 rpm still wouldn't bother me there.

Internal balance of "normally" external balance assemblies has been a little over-emphasised the last 5 years or so, 100,000's of engines were built external balance before this trend became so in vogue, and we didn't worry about it back then, and stuff still ran good and stayed together.

I don't see it being a necessity until you get over the 7800-8000 rpm drag level or 7400-ish rpm circle track level.
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Old 12-13-2010, 10:32 AM
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Quote:
Originally Posted by 803Paul
I have an externally balanced 383 chevy rotating assembly. Question- would you feel safe spinning it to 6500. Bolts are ARP throughout. If not, why so.
Depends on the crank. Cast no, Forged yes. One has to keep in mind that while the whole assembly is in balance, there are local differences that want to cause the shaft in those areas to run other than in its machined centers. This results in local bending moments which castings being more brittle really don't like. A high quality damper is a must as these local distortions run in and out they put vibrations into the shaft that if they gang up can become quite destructive.

This is fine for a drag or short track engine, if you're thinking of long term survival at these revs on a road course, an internally balanced crank would be a better way to go, especially with a production block as these aren't as beefy as Bow-Tie or aftermarket components which better tolerate externally balanced cranks.
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