What is the best way to determine rod length and why....

A friend of mine is building a 355 LT1 and is trying to decide what rods to use. He is going with a set of Eagle Forged H-beam rods but doesn't know which ones the 6.0 or the 5.7's to use.

A lot of people we talk to even the guy at the parts house say to use the 6.0 rods, but our machinist prefers the 5.7's.....Instead of telling him which ones to use, I want to help teach him why ........Don't just use something cause everyone says so is my point....

I guess what we are looking for is not really opinions on which rod to use but want to know how to calculate the correct rod length and why it is needed...

Just for the record, I am running the 5.7's in my 355ci without any problems

Thanks

 

You can't tell the differance in either,on the dyno,at the track,or SOP meter.
A rod is a good way to connect the pistons to the crank.
Ya can use the RR to influence the head flow and detionation,but that's another book.
Do a search as this topic has been beat to death.

 

The long rod thing started with Smokey Yunick. a legend among engine builders. Keep in mind that these were high rpm, restricted, carbureted race motors.

I agree with LR, it's been beat to death and not worth rehashing.

Rich

 

Pick pistons first, then rod length based on the pistons.

 

My personal theory, based on Smokey; is to run the longest rod you can that doesn't cause you issues in other areas.

FOR INSTANCE:
On an N/A engine, you may as well go 6". On a nitrous engine, go 5.85" so you don't have the pin intruding into the ring pack. On a forced induction engine you should probably go with a 5.7" rod to keep the dwell time near TDC to a minimum. Unless of course it's an alcohol engine, in which case the added dwell time would probably help.

HERE'S THE IMPORTANT PART:
Those suggestions are only for an engine you are rebuilding anyway and for which you need to buy new parts anyway. I would NOT suggest rebuilding an engine just to change the rod length. That would be silly.

 

How come nobody has mentioned piston mass?

 

i was under the impression that piston mass really only affected the terminal velocity of the rotating assembly, but if there's more to it than that i'd really like to know.

 

I don't understand how piston mass relates to the issue of rod length.

I mean, I understand how it relates to performance, just not to this question.

 

Because pistons for short rods are lighter. The rods are a little heavier, but the total mass of the rod+piston is lower for the long rod combo. And the mass out at the small end (the piston) is the most important. Look at a piston and a rod catalog and pay attention to the weights for the different rods and pistons in the same family.

Rich

 

I run 6.125 rods in my 350 LT1. Longer rods put less side loading on the piston bores and rings. The will also increase dynamic compression because of long dwell times and higher piston speeds. The down side is the small compression height, many times the oil ring will be in the wrist pin. Making a easy target for detonation! I dont know if the longer rods or gapless rings have helped any for performance, but I do think they benfit driveablilty somewhat. I'm running a 24*/25* on a sub 110 and still consider it streetable, it may be all in the tune.

 

Most factory performance engines have rod/stroke ratios between 1.6/1.8.
These are engines that need to accellerate.

289 Ford 1.796
302 Ford 1.696
Boss 302 1.718
351 C 1.651
351 W 1.711
427 Ford 1.716

340 Mopar1.859
440 Mopar1.630
426 Hemi 1.829

350 Olds 1.772
455 Olds 1.584

455 Buick 1.694

389 Pontiac 1.766

472 Cadillac 1.662

409 Chevy 1.646
283/302 1.900
327 1.753
350 1.637
427 1.631

Modern 500in NHRA ProStock 1.555-1.611

I think the Ford combinations are the most interesting as the Ford engineers designed many engines with a clean piece of paper, in contrast to the small Chevy that used the same rod length from 262-350 and shared deck heights, cylinder heads etc. If you figure that all of these engines were developed by smart guys in a competitive environment at a time where drag racing was more popular, or at least as much so than oval track racing I belive you will come to the conclusion that there seems to be a concensous about rod/stroke ratio, with the Mopars being the odd *****. (of couse the hemi rules s/s racing because of the cylinder head design, but the 427 Chevy is pretty close behind...)

 

Damn, you're right. I wasn't even THINKING about that.

 

Rich, You may want to verify your typing....I think you meant pistons for "long" rods are lighter, because the pistons are shorter in overall height.

 

DOH. Of course trhat's what I meant.

Rich

 

Just for comparison, my '02 Acura RSX-S has a rod length of 5.472" and a stroke of 3.386".

This makes for a rod/stroke ratio of ~1.62 .

Honda is known for making the most out of what they have to work with so I believe that there are more variables involved than just the ratio.