is there an advantage to either one over the other? im building a 396 so any info would be appreciated. thanks guys
Rob

 

A longer rod may give you a better rod ratio. Generally, the longer the rod the longer the piston dwells at tdc. Also there is less side loading of the piston toward the cylinder wall and there is consequently less scuffing. If we are giving suggestions, I vote for 6" olivers. I have never seen better made products. But just my humble opinion.

 

The only positive I have heard about the shorter rods is for nitrous. I guess that the taller pistons stand up to it better, and you can have the rings lower or something. This is just something I read.

Ben

 

You hit that one on the head, if you're gonna be spraying or puushing boost run a shorter rod, for na you can go with a 6".

 

I was looking for my thread, but I found yours. This should apply as well. I consulted a buddy of mine who's been doing mopar's for a while. here's a good link for the rod discussion.

http://victorylibrary.com/mopar/rod-tech-c.htm

 

well i will be using nitrous but i was just gonna buy nitrous pistons. if i buy those then i guess the rod length doesnt really matter? id like to go with the 5.850 rod if theres no real advantage to the 6 in.

 

If its a NA motor that will be spray every once in a while go with 6". Its an N20 motor that will be sprayed all the time go with 5.85.

The generall consensus is that this debate is made more complicated than it really is.

Do a search in the Advanced section. A TON of threads will show up.

 

I don't see why a piston with a higher wrist pin would hold up better under Nitrous. Unless the dome has to be made thinner to clear the small end of the rod. Ring placement should not be effected by wrist pin location. Increased crevice volume, because of a lower top ring, is usually desirable for nitrous. Go long.

 

Its a matter of only having so much room to work with. The longer the rod, the higher the pin location.... we both agree on that. With nitrous or anything that's gonna add a bunch of heat to the combustion, you want the top ring away from the action. Now, you have a higher wrist pin (long rod) and a lower top ring..... that's no good, because you have too little area between the top and second ring. Good formula for top ring unseating problems..... better to use a shorter rod, move the top ring down and keep as much volume there as possible.

Lots of nitrous (150+) go to the shorter 5.85 rod or a 5.7.

-Mindgame

 

If you are running a power adder, I'd go 5.85 or better yet, 5.7" for the extra strength.

 

I have always thought that longer rods helped in higher RPM apps. With a street driven, 7000 rpm PCM limitation, you need to make power down low and fast. Now if your going to be going out the back door @ 8000 rpm, then you start seeing 6-1/8" + rods.

 

MINDGAME-Now, you have a higher wrist pin (long rod) and a lower top ring..... that's no good, because you have too little area between the top and second ring. Good formula for top ring unseating problems..... better to use a shorter rod, move the top ring down and keep as much volume there as possible.
thats about the best reason i've heard yet! that truly makes sence now. the nitrous pistons im thinking about running have the top ring lowered .250" and with a highr wrist pin they would really be slammed together. thanks a lot for the info. with my 396 id say i should still be able to go with a 5.850 rod with no probs. i'll be running a 150 shot at the biggest and no more.

 

There's another more subtle reason for the adage about long rods for NA and short for a SC or nitrous setup besides the greater freedom for locating the rings package with the shorter rod. A shorter rod dwells less near TDC and this reduces peak cylinder pressures, which can be extremely high with nitrous or a SC.

OTOH, you need to keep in mind that these are subtle diffrences, not worth losing sleep over. I use a 5.7" rod in my blower+nitrous combo.

Rich Krause

 

When I made my post I was considering that not all piston blanks are made for all rod lengths. Many manufactures simply offset the piston pin in the piston pin boss in the piston. If that is the case with the people who machined your pistons dome thickness will not be effected by rod length because the same blank is used. Not all piston "manufactures" make their own blanks. The blanks are purchased from, say, Mahle. Then the piston pin hole is drilled to match the rod length and ring grooves are cut to specification. If this is the case with your piston manufacture I would stay with my go long recommendation. Ask the manufacture what the difference is between the piston for the 5.7 and the 6.0 rod. If the only thing is the piston pin location go long. If the dome is thinnner with the 6.0 rod stay with the 5.7.

 

Your post is a bit confusing. If all they did was "offset" the wrist pin bore then you'd need a 1.87" offset to maintain a 1.25" compression height in a piston made for a 5.7" long rod.... that's to make that same piston for a 6.0" rod.

Do the trig... draw a piston with "zero" at the piston top, now draw the pin centerline 1.25" from the top of that piston. That's what you're saying we maintain right? We're just going to "offset" the pin towards the cylinder wall. Let's see what kind of offset you'd need to make a 5.7 rod piston work with a 6.0 rod. So the short leg of the triangle is 5.7" and the hypotenuse is 6.0", now we find the shortest leg (sin) and that is our offset. 5.7/6.0 = -cosine .095 = 18.20 degrees, sin 18.20 degrees*5.7= 1.87".
So there's the length of the short side of the triangle and everything is set so we maintain 1.25" from the top of the piston.

Seems a little flawed to me cause most pistons aren't offset more than maybe .03-.04.

Hopefully you can clarify this for me.

edit: this example as I layed it out would be impossible anyway because the trig would need to be calculated based on the new arc and it's radius being 6.0". That'd lower the pin location and move the offset but anyways... the pin would still be way off in "X" (pin bore normal to you)

-Mindgame