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Discussion Starter #1 (Edited)
I am assuming that your question is about "Why would you want 6" rods in a 327 ?"

First, I must say I am not an engineer, so I'm not schooled on all the science of it.

Here is a quote I found:



There are many articles on the web, to explain the reasons. Do a google search with the words : 6" rod vs 5.7" rod

This picture shows what compression height is.



Here is a chart from Ross pistons, showing the differences with different rods etc.

Notice that when you go from a 5.7" Rod to a 6" Rod, your Compression height (CH) also changes by that .300" .



So, why a longer rod ? My understanding is that the angle of the rod to the crankshaft is different, so the actual turning of the crankshaft is easier there by making more H.P.

Correct me here guys ! Let's hear your comments...
 

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I "think" the biggest difference if the amount of time (in degrees of crankshaft rotation) that the piston "hangs around" top dead center. This lets the fire get moving without lighting it so soon, you can run less advance.

There are also differences in "leverage" on the crank, and piston speed.

here is a recommended ratio of rod length to stroke, though I can't remember what exactly it is, 1.7:1 or so??
 

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Yes, like Jim said the number of degrees of rotation that the piston "hangs around" TDC is longer with longer rods. But for a street motor it probably would only be a few HP difference at peak HP RPM.

The way I like to think of it is: The piston is near TDC and the fuel/air charge is ignited and burns. Then the hot, high-pressure cloud of combustion gas is "pressing hard" against the piston. The longer the piston can stay near TDC the longer the gas can "press hard".

I've seen some near identical engine builds with long and normal 5.7 rods, and they were within a few HP.

Sorry for the rambling.
 

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Well, Ive have seen identical engines except for different rid lengths be on opposite ends of the RPM hp/trq range.
Longer rods were more torq down low and shorter rods were more HP up hi.

The piston speed is the reason I like longer rods. IIRC, I used a 6" rod and a 283 piston in a 230ci 6cylinder with an OFFY and 2 2bl carbs and it was awesome.
 

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As said, the two reasons are the better angle, read straighter alignment, of the rod to the crankshaft journal, for transfer of force and less piston scuffing/friction against the cylinder wall.

Your Butt Meter would have to be finely calibrated to feel the difference. Longer rod is great for building a torque monster, but with the added weight of longer rod, rpm is kind of limited, all else being the same, meaning a shorter rod engine will rpm higher.
 

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We have found in our cirlc e track engines going from a 5.7 to a 6.250 rod they both make peak power at about the same RPM per say 6500 example but the longer rod engine at 7500 will make about 30 horse over the short rod engines 2 barrel engines, The longer rods will carry ithe HP longer.

We do use different cam profiles with the longer rods as they doi breath different!!
 
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