Rod Bolt Strength / Requirements

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BBO Omega
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Rod Bolt Strength / Requirements

Post by BBO Omega » Fri Oct 13, 2017 1:58 pm

I see articles that discuss the need to upgrade Rod Bolts due to the increased HP of a new engine combination [quite often due to forced induction in LS applications]. If the increase in HP occurs at a relatively similar RPM, why the need for the upgrade? Other than RPM, Rod, and Piston weight, what else dictates a need for a stronger Rod Bolt?
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Re: Rod Bolt Strength / Requirements

Post by pamotorman » Fri Oct 13, 2017 2:28 pm

any time I bought Carrillo rods they asked me the HP per cylinder I was expecting.

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Re: Rod Bolt Strength / Requirements

Post by David Redszus » Fri Oct 13, 2017 6:19 pm

BBO Omega wrote:
Fri Oct 13, 2017 1:58 pm
I see articles that discuss the need to upgrade Rod Bolts due to the increased HP of a new engine combination [quite often due to forced induction in LS applications]. If the increase in HP occurs at a relatively similar RPM, why the need for the upgrade? Other than RPM, Rod, and Piston weight, what else dictates a need for a stronger Rod Bolt?
The loading on a conrod is determined by several parameters: stroke, rod length, rpm, and piston mass; each contributing differently.

Assuming a change in each of the above parameters of 10%, listed below is the change in force seen by the bearing.

+10% stroke = +13.4%
+10% rod len = -3.1%
+10% rpm = +21%
+10% piston mass = +10%
Obviously, rpm and stroke will have the greatest impact on resulting forces.

Changing all the above at one time will increase force by +46%.

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Re: Rod Bolt Strength / Requirements

Post by joe 90 » Fri Oct 13, 2017 6:53 pm

HP doesn't have any affect on rod bolts.

Common sense?
Maybe not common.
BBO Omega wrote:
Fri Oct 13, 2017 1:58 pm
, what else dictates a need for a stronger Rod Bolt?

Everyone knows that the more money you throw at it makes it better.
You get what you pay for.




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Re: Rod Bolt Strength / Requirements

Post by MadBill » Fri Oct 13, 2017 7:56 pm

BBO Omega wrote:
Fri Oct 13, 2017 1:58 pm
Other than RPM, Rod, and Piston weight, what else dictates a need for a stronger Rod Bolt?
One often overlooked factor is throttle opening. At WOT, compression opposes inertia approaching TDC every other stroke, but not on decel. Driving cycle, as opposed to just RPM, is therefore a factor.
(Inline six cylinder racing Jaguars in the fifties were renown for blowing up on lift-off at the end of the straight...)
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Re: Rod Bolt Strength / Requirements

Post by digger » Fri Oct 13, 2017 9:39 pm

even under load a failure would likly occur during exhausting when the cylinder pressure at TDC can be below atmospheric so can be slightly additive if there is no pan vac

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Re: Rod Bolt Strength / Requirements

Post by MadBill » Sat Oct 14, 2017 12:42 am

digger wrote:
Fri Oct 13, 2017 9:39 pm
even under load a failure would likly occur during exhausting when the cylinder pressure at TDC can be below atmospheric so can be slightly additive if there is no pan vac
Right, but with closed throttle there are twice as many fatigue failure-inducing high tensile cycles and the previous compression ones are now 25" or more of vacuum.
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Re: Rod Bolt Strength / Requirements

Post by Diodedog » Sat Oct 14, 2017 1:00 am

David Redszus wrote:
Fri Oct 13, 2017 6:19 pm
BBO Omega wrote:
Fri Oct 13, 2017 1:58 pm
I see articles that discuss the need to upgrade Rod Bolts due to the increased HP of a new engine combination [quite often due to forced induction in LS applications]. If the increase in HP occurs at a relatively similar RPM, why the need for the upgrade? Other than RPM, Rod, and Piston weight, what else dictates a need for a stronger Rod Bolt?
The loading on a conrod is determined by several parameters: stroke, rod length, rpm, and piston mass; each contributing differently.

Assuming a change in each of the above parameters of 10%, listed below is the change in force seen by the bearing.

+10% stroke = +13.4%
+10% rod len = -3.1%
+10% rpm = +21%
+10% piston mass = +10%
Obviously, rpm and stroke will have the greatest impact on resulting forces.

Changing all the above at one time will increase force by +46%.
41.3 using those factors, an increase in rod length reduces the force.

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