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Old 01-17-2006, 10:13 AM
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5.7" vs. 6"

Would I see much loss in using 5.7 I beam rods vs. I beam rods in a 350?

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Old 01-17-2006, 10:21 AM
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Loss of what?
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Old 01-17-2006, 10:53 AM
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Loss of power?
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Old 01-17-2006, 11:14 AM
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rods

none!!!!!!!!
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Old 01-17-2006, 01:41 PM
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i didn't quite get the question the way it was written.
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Old 01-17-2006, 01:50 PM
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From what I understand using 6" rods cuts down on the amount of side loading on the piston skirts. Is this right?
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Old 01-17-2006, 02:10 PM
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Here's a study with some some numbers:

http://www.rustpuppy.org/rodstudy.htm
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Old 01-17-2006, 02:15 PM
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I wish this topic would become a sticky it seems to come up every week or two. With all the "hype" out there, people seem to be confused for good reason. The bottom line is you will not see any difference at all. From a 5.565, 5.7, 6.0 rod the differences are so minute you will never see or feel any difference. The whole side loading, more time at TDC etc is not worth wasting time on. The differences between them are so minor/tiny, do a search on rod length and see what comes up on this site. A lot of people have "opinions" about this, but very few have "facts". Find the facts and decide for yourself.

Buy the best rod you can afford or can get the best deal on, that will work for your application.

I guess that's just my opinion as well, huh? LOL

Royce

EDIT: The study above is one of the very informative (fact) things I was talking about.
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Old 01-17-2006, 02:29 PM
How fast is fast enough?
 
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There is a increase in durability with a 5.7" rod- and for that reason I think they are best for street engines taht are ment to last more than 15 passes.
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Old 01-17-2006, 03:36 PM
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you have to use what you can afford but you know what smoky unick always said,,use the longest rod you can.from the different builds i have done i can tell the difference.
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Old 01-17-2006, 05:10 PM
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Quote:
Originally Posted by ap72
There is a increase in durability with a 5.7" rod- and for that reason I think they are best for street engines taht are ment to last more than 15 passes.
I know of stock rod 400's that were raced for years with no unusual problems caused by the rod length.
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Old 01-17-2006, 07:18 PM
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That is an informative study. The only part I don't follow is his reference to "Dwell time". He measures dwell time as a number of degrees to TDC with the piston in the hole a quarter inch. This measurement would seem to have little to do with dwell time as I understand it. The correct dwell time comparison/measurement would be the number of degrees (before and after) TDC it takes to make the piston move a predetermined amount (like .050 for example). He seems to desregard the fact of the slower moving piston with the longer rod while it is in this area of TDC building cylinder pressure. He does mention that the short rod made more power from 2000-4000 rpm which I am having difficulty understanding. It seems that the piston falling away relatively faster would reduce cylinder pressure not increase it. This presumed reduced cylinder pressure coupled with a slightly reduced mechanical advantage of the piston over the crank pin should lower torque from 2000-4,000 rpm and beyond. Its too bad this guy didnt ask Smokey for his dyno data on the long rod theory.

At any rate, I think that when the Smoke said to use the longest rod you can fit in there, he was talking about all out racing anyway. I don't think you could really know the difference in a street motor anyway but if you gotta buy rods and pistons, you might as well buy long ones if it makes you happy. As long as they all cost the same that is.
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Old 01-17-2006, 08:29 PM
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This study has nothing to do with dyno data, it is a study in the actual geometry involved. The angles aren't somebody's theory, they are calculated using simple math.

I can't see any difference between his definition of 'dwell' and yours. You use .050" as your dwell standard, he uses .250" as his.

If you calculate the piston displacement at 15 from TDC you will find:
A 3.48" stroke with a 6" rod has the piston 0.0762" down in the bore.
A 3.48" stroke with a 5.7" rod has the piston 0.0771" down in the bore.
Note that there is less than .001" difference in piston displacement.

A short rod has better leverage on the crank which helps it produce more torque at lower rpm's. The same effect that causes more cylinder wall loading, also causes more crankshaft loading. Note that a shorter rod has a mathematically longer effective stroke.
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Old 01-17-2006, 09:57 PM
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I just don't get the myths that are out there. My old 385ci 5.565" rod engine is still running to this day. It has been together for over 6 years and has been ran pretty hard. I used to shift it between 6300 and 6500RPM. My buddy bought it from me and now it's in his Nova, he has a bad habit of over reving the engine (stepped up from a 327 and still has not got the hang of how fast the 385 revs).

I bought into the hype and pulled that engine to install a 6" rod 383", guess what, same heads, headers, car, gears, etc.. ran the exact same ET. As far as 6" rods only lasting 15 passes, that is hog wash. I am still running the 6" rods (only because they were bought and paid for) in my current 388 with a blower on it. Lots of street miles and everything is still happy.

Moral of the story "Don't believe the hype". Use a good quality rod that will fit your engine and worry about things that really make a difference.
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Old 01-17-2006, 10:25 PM
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once again, i'll say this, con rod length is unimportant. use what fits, and leave it at that.............here's part of a quote from reher-morrison that i posted before:

These are much more important considerations than the rod-to-stroke ratio. There's no magic - a rod's function is to connect the piston to the crankshaft. Period.
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