With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?

plovett July 14th 2012, 9:46 am

Say your offset grinding a factory 460 crank to get good rods and increase the stroke.

Choice A) "Mopar" 6.76" rods with a 2.375" journal and a 3.95" stroke.

Choice B) "BB" 6.8" rods with a 2.2" journal and a 4.14" stroke.

Choice B would have lighter pistons and could make the same power at slightly lower rpm.

Would the bigger 2.375" rod journal of choice A make a difference in crank strength/longevity? Is there a very rough rule of thumb like, "above 600 hp and/or 7000 rpm it's better to keep the rod journal bigger than 2.2"?

thanks,

paulie

rmcomprandy July 14th 2012, 10:18 am

plovett wrote:Say your offset grinding a factory 460 crank to get good rods and increase the stroke.

Choice A) "Mopar" 6.76" rods with a 2.375" journal and a 3.95" stroke.

Choice B) "BB" 6.8" rods with a 2.2" journal and a 4.14" stroke.

Choice B would have lighter pistons and could make the same power at slightly lower rpm.

Would the bigger 2.375" rod journal of choice A make a difference in crank strength/longevity? Is there a very rough rule of thumb like, "above 600 hp and/or 7000 rpm it's better to keep the rod journal bigger than 2.2"?

thanks,

paulie

Just by bulk the 2.200" journal will be 8% weaker then a 2.375" journal. Filet Radius' will have VERY much to do with the actual load carrying capability of the journal in question.
In a crankshaft the journal/main overlap has a lot to do with the resistance to flexing and the 2.375" journal will have about .085" more overlap when having the same stroke; (depending upon the main journal size). Using a shorter stroke and a larger journal diameter, the overlap gets to be even more.

The question shouldn't be , "which is stronger" but, will the finished product be strong enough...?

plovett July 14th 2012, 11:11 am

Thanks!

Would you say for a 625 hp 7000 rpm street strip motor, a factory crank with a 2.2" rod journal and a 4.14" stroke would be strong enough for long term reliabililty? Assume good machine work with a generous filet radius.

paulie

dfree383 July 14th 2012, 12:03 pm

plovett July 14th 2012, 12:14 pm

Thank you, sir.

paulie

plovett July 14th 2012, 3:04 pm

Is there a hp level and/or rpm that you would start to feel uneasy with, using a factory crank offset ground to 2.2"? I don't imagine it would be a black and white thing, where at a certain hp your okay, then at 10 more hp all of a sudden it's no good. Just looking for a generalization.

paulie

rmcomprandy July 14th 2012, 3:43 pm

plovett wrote:Is there a hp level and/or rpm that you would start to feel uneasy with, using a factory crank offset ground to 2.2"? I don't imagine it would be a black and white thing, where at a certain hp your okay, then at 10 more hp all of a sudden it's no good. Just looking for a generalization.

paulie

Without any detonation involved ...? At 750 lb/ft of torque would probably get a little suspect.
When addressing strength, horsepower really means nothing; (other than that it is what people seem to want to hear).
Actual TWISTINGFORCE or reciprocating weight at a given RPM is what needs to be looked at.

plovett July 14th 2012, 3:59 pm

Yes, assuming a good tune, no detonation. Also assuming a typical non-exotic (not super light) rotating assemby.

Is there an approximate rpm level at which you might tell you customer it's not a good idea?

thanks,

paulie

jasonf July 14th 2012, 6:44 pm

With today's availability of good aftermarket Parts is it even cost effective to go this route?

plovett July 15th 2012, 6:57 am

Oh heck no. But the factory crank is apparently plenty strong. And I like to use American made parts whenever I can. And I like combinations that aren't "cookie cutter" assemblies. And I like different.

paulie

BOSS 429 July 15th 2012, 10:38 am

for years there were 750 and 850 engines with a 2200 off set ground, but now its just as cheap to buy a crank forged,or cast.

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With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?

With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?

Re: With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?

Re: With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?

Re: With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?

Re: With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?

Re: With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?

Re: With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?

Re: With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?

Re: With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?

Re: With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?

Re: With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?

Re: With an offset ground factory 460 crank, is there a significant strength difference between a 2.375" journal and a 2.2" journal?