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narrowing rods

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xspiriment View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote xspiriment Quote  Post ReplyReply Direct Link To This Post Topic: narrowing rods
    Posted: Jun/15/2009 at 9:28am
hello AMC'ers:  I'am questioning about how to narrow a rod big eng. I guess its a question for a engine macninist out there. What i want to do is run jeep 4.0l rods in a destroked 360. im using a 343 crank. My plans a to build a long rod engine, like the old 340 mopar (those engines ran great) using the 4.0l rods gives me the length i want but need to be narrowed on the big end to fit the 343 crank.  I know there is also the rod center line to deal with when doing this. So what is the correct way to do this? Hope your answers dont push me away from doing this project. Thanks for reading and giving me your opinons.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2009 at 2:19pm
Take them to a machine shop and tell them what you want to do. The 258 rods are the same casting as 360 rods, the factory machined them narrower on the big end to fit the 360. 4.0L rods are pretty much the same as 258 except for the length, I see no reason why they can' be narrowed. It can be done in a mill easy enough. I'd take the stock V-8 rods along (well, at least one of them, assuming they all look the same) so they can measure how much to narrow.  I don't recall if they are thicker on one side than the other or not (on the V-8)/


Edited by farna - Jun/15/2009 at 2:20pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Steve_P Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2009 at 2:42pm
I would assume a surface grinder would be the best/easiest way.  But why bother doing this?  I understand the theoretical benefits of a long rod engine, people seem obsessed with "long rods" LOL, and wanting to build something "custom", but the back to back dyno tests I have seen on this show the power gains to be marginal- like 10 HP on a 600 HP SBC when undergoing a sizeable change in lengths.  For the trouble and expense this doesn't seem worth it to me unless you have plenty of $ to spend and are going for every last HP.  IMO it even makes less sense using stock rods.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2009 at 3:13pm
I think you'd better read this before you go spending money.....
You may change your mind.
Unless you also make OTHER changes, you may actually go backwards and increase pumping losses, etc.

http://www.stahlheaders.com/Lit_Rod%20Length.htm


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Post Options Post Options   Thanks (0) Thanks(0)   Quote purple72Gremlin Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2009 at 3:18pm
Only advantage is more torque, but is is enough for how much money you are going to spend?????   I would research it more.............much more..
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2009 at 4:32pm
Is that an advantage?
Here's from the papers or reports I've seen:
(quote)
>>A short connecting rod achieves this right angle condition sooner than a long rod. Therefore from a "time" perspective, a short rod would always be the choice for maximum torque. The shorter rod achieves the right angle position sooner and it does so with the piston slightly farther up in the bore. This means that the cyl pressure (or force on the piston) in the cylinder is slightly higher in the short rod engine compared to the long rod engine (relative to time).<<

Just to clarify - this is longer rods, not longer stroke.
Longer stroke leads to a slower more "torquey" engine. (tractor engines or industrial engines requiring torque and not so much speed are typically long-stroke engines)


Edited by billd - Jun/15/2009 at 4:51pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Class Guy Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2009 at 4:52pm
If you want to do it, then do it.  Most of us have never "really" built an engine trying stuff like that, but have opinions based on what somebody else says that may or may not be true.  It seems that the more recognizable the name, the more "written in stone" that opinion is.  Rarely have I seen real world info to back up claims.  Most are just projections off one engine experience.  Not to knock Stahl, because his is a legendary name, but when was the last time one of his engines won a race?  There are way too many variables and you can build an engine to be efficient for any number of factors.  If there were nothing to the long rod phenomenon, then the high-buck NASCAR guys would not be using 6.2" and 6.3" rods in their engines.  Granted they are carburetor limited and run at high rpm, but that alone should give you some clue as to the characteristics of the long rods. 
 
I personally am building a 350 CI engine using Ford 300 6 cyl rods (6.21" long), 400 Chev pistons, a 304 crank in a 360 block.  This is going into an offroad Jeep using iron heads, EFI and E85.  We are working on increasing cylinder pressure over a longer period of rotation to produce torque at low to mid RPM.  We are still talking camshafts at this time, but the short block is fitted.  Other than the time spent on fitting the parts, the cost of the short block is about the same as a regular rebuild.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ken_Parkman Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2009 at 5:04pm
I've used a small surface grinder with a magnetic table to narrow rods - very easy.
 
I know you asked us not to try to push you away, but rod length is essentially meaningless for performance. A lot of successful EMC engines have used very short rods, and they were all about torque and power both. But for sure the destroke will kill power and torque roughly linearly with the cubic inche loss. Having shorter pushrods is more important for performance, one of the reasons race engines have gone to shorter deck heights and therefore shorter connecting rods and rod ratios. Absolutely everything else is more imprtant than connecting rod length.
 
BTW those 340 Mopars did work amazingly well for such a miserable design. An AMC is a vastly better engine and will absolutely destroy a small block Mopar.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote billd Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2009 at 5:09pm
Here's another - is it worth less than 2% in this fellow's studies?
http://www.rustpuppy.org/rodstudy.htm
Study his report and the final comments at the end.

Everything on gains and losses is as class guy said, anecdotal at best since you must have a baseline engine study first, then change ONLY what's required to change the rods, then dyno the engine again. In that respect, I must agree with c g.

Go for it, but know - you may see no or little gain - but you may indeed gain. Other factors come into play. Say that your engine otherwise - head flow, carb, cam, etc. are better for running longer rods - you'd gain.
Changing the rod length without considering other factors could backfire - OR you could come out way ahead if your other parts just aren't ideal for a shorter rod engine.
Of course, unless you do this build, then go back to "shorter rods" and don't change cam or head or any other pieces (other than pistons) how can you say you gained or lost since there's no baseline to compare to?

How many people REALLY study the whole, instead of buying parts because so and so said this was a good cam. Good for him and his build, his heads, his flow characteristics, timing curve, etc. Good for you and your parts? Maybe - maybe not.

If every member of the orchestra isn't watching the same baton, it will sound like crap no matter how good the individual players.

Do it and let us know! Maybe even do a write-up yourself...........
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 74Bubblefender Quote  Post ReplyReply Direct Link To This Post Posted: Jun/15/2009 at 5:55pm
Sure you can cut them down! When we cut rods we turn them down to about .008 in width and surface grind from there. Depending on how much you turn them down you may want to put the chamfer back on the ID. Keep in mind that the notch for the bearing is relative to ONE SIDE and if your not carefull you will be adding width to the notch of god forbid cutting a new one :D :)  Good luck and good speed!
We are just about to forge new AMC V8 crankshafts.. please check here
http://www.bulltear.com/forums/showthread.php?19564
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