I realize that there is a never ending debate in progress on the pros and cons of using longer than stock rods.
So I'll try to keep this as simple as possible.
I'm just wondering if there is any documented info out there regarding compromises in longevity when using a 6" rod, and the accompanying .150" shorter piston in a 350 engine?

 

dont think you are going to have any problems with either... the longer rod doesnt actauly take any more abuse than the shorter rod, and in fact, you might even be able to say it takes less. The whole avantage to the longer rod is when the piston is at TDC and the gas begins to burn, the longer rod is closer to a vertical postition then the shorter rod is. The fact that the rod is closer to vertical allows it to transfer more froce directly into the crankshaft. Torque is all about the angle it is being applied at. apply a torque at a 90* angle and you get MAX torque... apply it at an 80* angle and you get sin(80) or 98.4% as much... may not seem like alot, but 1.5 percent makes a decent amount when you are talking about 320 horsepower to begin with.

 

My main concern is with the piston itself, being shorter than the stock rod length counterpart and possibly being more prone to rocking in the bore.
Would there be a significant difference in wear there?

Thanks

 

the pistons themselves are not shorter, the pin is moved up inside the piston... The skirt on the piston should be the same length, if not very close... pistons rock in the bore either way, but there is no real movement they can really achieve. Every engine has a small amount of piston slap, and in fact when the pistons are set up with the pins towards the major thrust side of the motor, this "piston slap" along with the longer rods produces even more power by making that whole rod angle EVEN closer to 90* People have been using 6 inch rods for a while with no real problems, I say go for it, it is a nice litte bump in performance for all the same price

oh... just cuaght this... psitons should never really wear, or cause wear on or due to, coming in contact with the cylinder wall. The only time piston wear really happens is when clearances are too tight or from overheating

 

Thanks for the good advice.
It makes sense that the skirt length of a piston wouldn't need to be changed to accept a change in the pin height, but for some reason the pistons that I see in most of the site photos show a shorter piston to fit a longer rod.
And that is apparently supported by the fact that the 6" rod pistons are also lighter than the 5.7" rod pistons.
Any ideas what the reason for that might be?

http://kb-silvolite.com/performance....tails&P_id=157
http://kb-silvolite.com/performance....etails&P_id=58

 

one thing that is interesting to note is the cut in the third groove, that can account for some of that weight loss. IN all honesty, it does look as iff the skirt is a slightly shorter, but I can almost gurantee that the lengths of both skirts are the same from the center of the pin, does that make sense?

 

It sounds like you're saying they just shorten the area above the pin, and the piston might be shorter overall, but the skirt length stays the same.
Raising the pin height probably gives the piston makers an opportunity to remove some weight, and they do that, instead of keeping the piston the original size and changing the pin location alone.
Talking about this stuff without pictures can lead to gnashing of teeth. I think we're understanding each other tho.
I'll take your advice and go with the 6" rods and accompanying pistons, and accept the 90g weight savings as a bonus.

Thanks

 

with 90 less grams if weight on the pistons, not only are you going to have a more powerful motor because of the longer rob, buy you are also going to have an engine that revs a little quicker and easier... One thing I would not do hower is put an aluminum flywheel on a motor like that. you are esential lowering the rotational inertia of the motor with the subtraction of weight, and if you were to combine that with an aluminum flywheel, your RPMs may drop TOO fast in between shifts, and may leed to slight idle problems.

one thing I forgot to mention before too as far as torque transfer goes, not only are you getting the angle closer to 90 degrees, but you are also increasing the length of the 'lever arm' applying the force, so that 1.5% power increase I menioned before, is actualy slightly higher. Good luck with motor, I am actualy in the process of buying up parts to make a long ron 383... right now I have a short rod, but I want H-Beam rods anyway, so it is an excuse to upgrade :-D

 

I love it when I have an excuse to upgrade.
I can't afford a $1500 rotating assy all at once, but I'm well past the point of no return on this project.
It should be exciting to compare the two motors. They'll be the same except one will have a lighter rotating assy with 6" rods.

Yammer... Yammer... Yammer...

Good luck with yours too.

 

By putting the pin closer to the face of the piston, there's less side load placed on the piston. The skirt will not press as hard against the thrust-side surface of the cylinder. This frees up a little power. And (as FstBrd said) it gives the piston more leverage on the crank.

Think of it as YOU pushing down on the piston while it's not in the bore. With the pin closer to the top, the piston is less likely to rock while you're trying to push down. Plus, if you can get more toward a 90* angle on the crank, the easier it is to apply your weight (or force) on the crankshaft. Same principles apply.

Of course the skirt is going to have to be shorter. If it was the same height as a piston for a 5.7" rod, it would hit the crank throws as they passed by. That would be ikky. That's why they weight less to. But.....
Keep in mind that whatever weight is "saved" from the lighter piston is going to be offset, and overtaken by the added weight of the rod. After all, steel does weigh more than aluminum.

 

This is one of the few discussions on the advantages of longer rods that hasn't turned into an argument.
An interesting point you presented AJ_92. Here's another interesting point:
Weight diff between 6" rod piston and 5.7" rod piston = about 90g.
Weight diff between 6" rod and 5.7" rod = about 20g.

http://www.holley.com/HiOctn/ProdLin...ECCR/Race.html

Scroll back 6 posts for additional links/data.

No flame intended, just thought I'd point that out.
It seems like there's a definite advantage there to the 6" rod over the 5.7".

 

I don't see where you're finding the piston weights. Gotta link to that? Are you using the pin weight as reference? I'm confused.

Not saying your wrong or right. I can't say either since you're not supplying sufficient information.

 

Scroll back on this thread to the 5th post, there's a link there to the KB site.

 

Sorry. I usually ignore anything that says KB on it.

I guess I was wrong. The second time hurts more than the first.

 

At 70g weight savings per cyl that = 560g or 1-1/4 lb total.
Something worth keeping in mind for the next time there's a long vs short rod debate.

 

90 grams difference, out of a 400 g piston more or less, just from moving the pin boss up .300", doesn't pass the smell test.

So, FWIW, I went and looked up a few typical parts....

An Eagle bushed I-beam 5.7" rod = 535 grams
Same rod, 6" = 590 grams
Difference = 55 grams
I had to rely on eBay spec listings since Eagle doesn't put those on their web site.

A JE piston, "2-barrel" (400 HP rated) piston for 5.7" rod = 373 grams
Same piston for 6" rod = 334 grams
Difference = 39 grams
http://www.jepistons.com/pdf/2002-je4-13.pdf

Looks to me like you incur a 16 gram penalty per cyl with the longer rods using these common parts.

You can't generalize something like that. Each mfr's part is different.

 

That could be true with the Eagle I beams, but with the following mfgrs
http://www.holley.com/HiOctn/ProdLin...ECCR/Race.html
http://www.flatlanderracing.com/index.html (Ref Manley Rods)
there was no more than a 20g difference in weight between the 6" rod and the 5.7" rod. And in some designs the dif was as little as 4g.
It's true, you could say that you can't make the generalization that a 6" rod/piston assy is lighter or heavier.
But you could also say that if you're looking for a weight savings, it can be found in a long rod assy, more often than not.
The more examination you give it, the more valid the point becomes.

 

hey one question for you now... you going to do pressed in pins or are you going to go with the full floating pins? also, you assembling this yourself? or having someone else do it for you?

 

It'll be floating pins for me this time around.
I'll assemble it myself too. Unless I have some extra $$ when the time comes, then I might pay someone to do it for me.
I always like to do a mock up assembly on my own tho, just to check that everything is as it should be.

 

ok, yea I am using floating pins as well, I like the ease of assembly and relative ease of "swapping" parts. one thing you have to make sure of, that most manufacturers dont put in the directions is the the lock for the pins are two sided... they have a rounded side and a flat side from being punched... you absolutely have to make sure the flat side is facing the outside of the piston or you risk the chance of popping a lock and losing alot more then 1 cent piece of metal.

 

Well, whoever it is that ends up assembling my engine, I hope they remember that.
It seems like I remember reading that somewhere else too, it's one of the small details that can be easily overlooked.
I haven't even bought pistons or rods for this project, hopefully I'll be able to afford them this summer.
Too bad sitting here BSing about it doesn't bring the project any closer to completion.
For a stroker, floating pins are a must, IMO. Cause you have to preassemble to check for clearance, and it just makes it more of a pain if you have to disassemble your rods/pistons if they're pressed on.
There's a guy on Ebay selling a plastic pin that fits into your rod top end, and it holds the rod in position for stroker preassembly. So you don't have to mount the pistons.
Seen one of those?

 

I have a set of pins that were turned down .010" so that they'll do the same thing, and they were free as we used old pins.

 

is the clearance in the pin suposed to be so great?? aluminum alloys expand alot faster then a steel pin would, I would think that the clearance created might even be slightly too large in that case?

 

I'm pretty sure that he was referring to a "checking pin".
That's a pin that slides through the wristpin bore (bushed or not) and holds the rod in position for checking clearances on a stroker.
At least I hope that's what he was talking about.
You could use a wooden dowell or a piece of PVC for the same purpose.

 

Correction:
Yesterday, I cited as an example, the weight difference between two KB pistons, that ended up having a spread of 90g.
More careful research has shown me, that is not typical. Typical would be more like 35g.
I originally posted two pistons from the KB site as an illustration of skirt length, then in haste used those same two as an example of weight difference.
How unprofessional of me, I'm Fired.

 

Just thought I'd post a pic of what's in my engine. Eagle 6" rod (625 grams I think) and Wiseco pistons, 408 grams. From what I understand, for the higher reving engines the longer rod is easier on parts. That's one of the main reasons for me buying them. I'm spinning this engine to 7k.

 

wow, I love how those two parts look together... too bad we cant get blocks made out of some super-exotic clear allow to show off the interior parts lol. I will prolly stick with SRP pistons, like the ones I have right now, quality pieces, but I am def going for the long rods, so if ya know anyone looking for a 383 rotatinf assembly with less then 5K on it send em to me lol.

 

EvilCartman, what type of crank are you running? Stock? Forged? Just wanted to know. Sounds like a nice set-up you've got going.

 

Yeah, if I had a set of rods and slugs like that I'd photograph em and post the pics too.
I changed horses in the middle of the stream on my project too. I originally bought a cast crank then decided to get a forged.
I can't find a buyer for the cast one, I might have to build 2 strokers...
Fast Brd 6.3, Does that mean you're selling the pistons+rods only, and keeping the crank you have?

 

I got a GM steel crank and had a little extra work done to it. Doesn't run as tight as a stock built engine would so it stays happy at the higher rpms