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6.135 or 6.385 rod length?

Old 03-07-2005, 08:36 PM
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Default 6.135 or 6.385 rod length?

I want to build a 454 to 496 BB Chev. When going to a 4.250 stroke some crank suppliers say to use a 6.135 rod, others say to use a 6.385 rod length. What gives, cant I use my stock dimple rods?
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Old 03-07-2005, 09:10 PM
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Default Re: 6.135 or 6.385 rod length?

Sure if you want a horrible rod to stroke ratio.
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Old 03-07-2005, 09:19 PM
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Default Re: 6.135 or 6.385 rod length?

Why would a longer rod be a better choice when the total length of rod and piston for both set up is the same?
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Old 03-07-2005, 09:27 PM
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Default Re: 6.135 or 6.385 rod length?

The longer rod gives you less rod angle on the crankshaft.
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Old 03-07-2005, 09:33 PM
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Default Re: 6.135 or 6.385 rod length?

longer rod shorter piston, the crank isn't trying to push the rod out through the side of the block, as bad. Get out your calculator, 6.135 divide by 4= 1.53.rod to stroke ratio. 6.135 divide by 4.25 = 1.44 not good on the rings. Probably hard to balance. A lot of frictional loss.
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Old 03-07-2005, 09:52 PM
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Default Re: 6.135 or 6.385 rod length?

Originally Posted by GPM
longer rod shorter piston, the crank isn't trying to push the rod out through the side of the block, as bad. Get out your calculator, 6.135 divide by 4= 1.53.rod to stroke ratio. 6.135 divide by 4.25 = 1.44 not good on the rings. Probably hard to balance. A lot of frictional loss.
I agree !00%.......In simple terms the longer rod stays at a more straight up and down angle as the stroke moves up and down.
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Old 03-07-2005, 10:26 PM
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Default Re: 6.135 or 6.385 rod length?

i have done them both ways do the long rod you wont be sorry everyone above is right the rod angle is better i have even done with a 400 long rod with a 4.25 stroke but you need a tall block to make that work
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Old 03-08-2005, 12:00 AM
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Default Re: 6.135 or 6.385 rod length?

I have built many 632 CI combo's with 6.535 rods with HP ratings between 850 and 1050 with no probs. I usually turn these motors 7000 rpm. This combo has a rod ratio of 1.37 which is baDDDDD!, but still lives. Having a strong aftermarket block also helps. Not saying everyone should use bad rod ratios but you see my point.
The shorter the rod ratio the faster the piston will leave TDC and this fact alone will reduce the chance of detonation, but there is a line you don't want to cross.
In a blown boat app. the less chance of det. the better.
The sweet spot is to have the rod roughly 2 inches longer than the stroke, and in this case that would be 6.250 which they don't make cheap so I would say either would be ok.

Last edited by NOBODY; 03-08-2005 at 09:45 PM.
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Old 03-08-2005, 08:35 AM
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Default Re: 6.135 or 6.385 rod length?

. . .and mock up the assembly. I can't remember how many customers have had guys bring in a "good" deal on a rotating kit, have them balance it and then take it home only to be surprised that the counterweights hit the pistons. Their complaint then falls on deaf ears from who they bought it from because, "sorry, you have had it machined therefore we can't take it back". Your know out around $600 for balance work x 2.

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That 2" deal I agree. That's the number among the top engine builders.
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Old 03-08-2005, 08:45 AM
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Default Re: 6.135 or 6.385 rod length?

Is that drag race or marine ?
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