SBC Connecting Rod Length

General engine tech -- Drag Racing to Circle Track

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SBC Connecting Rod Length

Post by GotSomeRust » Tue May 19, 2009 7:59 pm

I was told that I should use a 6 inches connecting rod in my Chevy 350 instead of the stock length of 5.7 inches. What are the advantages/disadvantages of using a 6 inch? Thanks in advance for the info...
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Post by stealth » Tue May 19, 2009 9:10 pm

Oh No.... :shock:

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Post by bigjoe1 » Tue May 19, 2009 9:26 pm

The piston is very much lighter in weight-about 100 grams

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Post by GotSomeRust » Tue May 19, 2009 10:14 pm

bigjoe1 wrote:The piston is very much lighter in weight-about 100 grams

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Huh?
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Post by bigjoe1 » Tue May 19, 2009 10:36 pm

What I am saying is that the rod length is very UNIMPORTANT- but the piston for the 6 inch rod will be much lighter---GET IT ????

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Post by 1989TransAm » Tue May 19, 2009 10:39 pm

Right. Usally the longer the rod the lighter the piston and the rotating assembly.

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Post by BGriff » Tue May 19, 2009 11:03 pm

Longer rod, longer dwell at TDC, better for high RPM HP.
Short rod, better for low end torque, more side loading at piston skirt.
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Post by GotSomeRust » Tue May 19, 2009 11:39 pm

bigjoe1 wrote:What I am saying is that the rod length is very UNIMPORTANT- but the piston for the 6 inch rod will be much lighter---GET IT ????

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Yes, thank you.
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rods

Post by k-star » Wed May 20, 2009 7:37 am

BGriff wrote:Longer rod, longer dwell at TDC, better for high RPM HP.
Short rod, better for low end torque, more side loading at piston skirt.
Can you share some of your testing data to prove this??

Like how much tq increase did you see in A-B-A testing with the shorter rod??

Please list all the info of each combo and the actual TQ increase numbers.


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Post by CNC BLOCKS » Wed May 20, 2009 7:52 am

We see a big differance on the 2 bararrel circle track engines going from a 5.7 to 6.250 rods and from what I have seen they male peak power about the same RPM but the 6.250 rod will carry alot longer then the short rod engines.

The cam will be different for a longer rod engines as well.
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Post by 3V Performance » Wed May 20, 2009 8:09 am

CNC BLOCKS wrote:We see a big differance on the 2 bararrel circle track engines going from a 5.7 to 6.250 rods and from what I have seen they male peak power about the same RPM but the 6.250 rod will carry alot longer then the short rod engines.

The cam will be different for a longer rod engines as well.

Seen the same here..
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Post by k-star » Wed May 20, 2009 8:16 am

Carl or 3V, can you give some actual numbers??? When you say carry it longer I assume you mean the peak HP father up the RPM range. How much farther???

Is this something that is only found with the 2bbl engines???

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Post by GotSomeRust » Wed May 20, 2009 9:33 am

Sorry to be dumb, but (referring to CNC Blocks post) what needs to be different about the cam when using 6 inch connecting rods? That doesn't make sense to my little brain... Thanks
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Post by bigjoe1 » Wed May 20, 2009 10:24 am

What I have seen with the longer rod is that the engine does not nose over as bad above the HP peak-- say the peak is at 7000, with a longer rod , it can carry the better power way uo to 8000. On the other hand, with a shorter rod, even when the peak is at 7000, by 8000, the power will drop off 30 to 40 HP-- The differance might be 25 more HP with the loger rod at 8000


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Post by williamsmotowerx » Wed May 20, 2009 11:20 am

Friction kind of "squares" with RPM. Exactly what BigJoe said... doesn't nose over as hard.

I'd guess a faster ramp rate cam for longer rod, plus a tighter LCA

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