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

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

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...?

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

Yes.

Thank you, sir.

paulie

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

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.

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

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

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

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.