CraZ-28

I'm doing research for my stroker motor and have come uppon the question of:

"What size rods do I need/want to get?"

I've seen kits out there with either 5.7 inch rods or 6 inch rods. My question is what are the pros and cons of using either of them. Is one a better choice than the other?

Also, if your wondering about the engine, I'm looking at building a 383 or possibly a 406 or higher (depending on if I get my 400 block) and I'm looking at keeping the compression ratio just below 11.0-1. It will also have aluminum heads.

I tried doing a search and came up with nothing on a comparison between the two. Thanks for any input.

Lonestar

The 6 inch rods give you a little more stroke, but you have to machine the block for clearance.

Oops, shoud have said torque...not stroke, sorry

Mkos1980

iTrader: (8)

I went with 5.7 rods in my 383 with no problems

Red Devil

As compared to what? A 383 is a 383 regardless of which road you choose with regard to rod length. How does a 6 in rod make one 383 bigger than a 383 running 5.7 rods????


The difference is in the load exerted on the sidewall. With the longer rod there is less sidewall loading. The piston speed (as well as dwell time) changes with rod length.

A good overview:
Ros and rod to stroke ratio

and if you need a primer on 383's:
383 FAQ

Also if you use our apparently popular SEARCH feature you can come up with some interesting stuff:

https://www.thirdgen.org/techbb2/sho...rod+length+383

https://www.thirdgen.org/techbb2/sho...rod+length+383

https://www.thirdgen.org/techbb2/sho...rod+length+383

https://www.thirdgen.org/techbb2/sho...rod+length+383

https://www.thirdgen.org/techbb2/sho...rod+length+383

etc., etc., etc.

Ricktpi

The crank gives you the stroke, not the rod.

Generally a longer rod gives slightly better performance due to a longer dwell time at TDC.

ede

well since everyone else jumped on him i might as well too, rod length has nothing to do with stoke as already pointed out. stroke is a function of the crank.

Lonestar

Length of rod does affect stroke. Same as a longer ratchet gives more torque than a shorter one.

edit: no edit done, clicked edit by mistake

Ricktpi

The arm is the crank, not the rod. The rod only connects the piston to the crank arm & determines the distance between the two. It has no effect whatsoever on the length of the stroke.

ede

lonestar think about it, or try to explain it to us and i think you'll see the length of the rod could be .001" or 10.00" and the stroke wouldn't change. what might change is where the rod is when it moves, but not how much it moves.

Stekman

Lonestar, you are confusing stroke with piston height in the hole at TDC and overall piston travel.

Streetiron85

CraZ28 is gonna have to start a new thread to get his question answered now, since everyone had to jump on lonestar for saying 6" rods increase stroke.

RB83L69

Changing the rod length does not change the stroke. The stroke is determined by the crank, specifically by the offset of the rod journals from the crank centerline. Since stroke is exactly equal to twice that offset, rod length doesn't have anything to do with it.

What rod length does do, is change the relative velocity of the piston as it travels through its cycle. The longer the rod, the longer the piston hangs around TDC and BDC, and the faster it moves as it passes through peak velocity at the center of the stroke. Having more dwell at TDC increases the "thermal efficiency" (the engine's ability to convert heat energy into mechanical motion) by a small amount.

It also changes the angle that the rod makes with the cylinder wall. However, if you work out the angles for 5.7" rods and 6" rods, you'll find that for a 3.75" stroke, increasing the rod length only changes the maximum angle from 18.2 to 17.4 degrees; not a whole lot really.

For an engine with a given stroke and a given deck height, if you increase the rod length, then you must also move the piston pin bore up closer to the top of the piston. This leaves less room for rings above the pin bore.

The reason the factory used short rods in the 400 was because they did not want to compromise the ring package. At the time, they felt that the 350 "compression height" of 1.56" was the least space that would allow for an adequate ring system. So, if you take half the 400's stroke, which is half of 3.75" or 1.875"; plus 1.56" for the pin height; and subtract that from the nominal rotating assembly "height" of 9.000" as determined by the production block deck height, you will find that stock 400 rods need to be 5.565" long from center to center.

However, with a rod that short, a serious problem showed up. Since the crank counterweights are exactly opposite the rod journals on the crank, then when the piston is at BDC (fully down in the bore, closest to the crank), the CW is at "TDC", right under the bttom of the piston. with the short rods, there isn't enough space between there for a large enough CW to completely balance the rod jpurnal, on the front and rear throws; so those 2 must be externally balanced, with weight on the flywheel and the balancer.

That's the other advantage of longer rods; they allow internally balanced cranks and external parts to be used.

The down side is that they compromise the rings. But ring technology has improved to the point that their longevity isn't near as much of an issue as it was when the 400 was designed in the late 60s.

IMHO the angularity issue is the least significant of the things to consider. The improvement in thermal efficiency is more important, but still not a real big deal. The big payoff is in the practical aspect of using neutral-balanced parts. And of course there is the risk of relatively short life of the rings if used in street duty, because they will be narrower and closer together.

Meanwhile, if you don't know what you're talking about, please don't confuse people by repeating a bunch of McDonalds parking lot monkey-spank and BS. Go get an education in math and science, and do a little research.

CraZ-28

Holy smokes RB83L69, now that's the type of info that I was looking for. I appreciate all the replys that everybody has given, even if it was completly wrong. At least now I have more to go off of and I can do more research for my new motor.

If anybody else has more info, please feel free to post. I need all the info I can get.

Red Devil

That is why I've gone to pointing them in the righ tdirection and letting them read. The amount of BS on this board lately is kind of depressing. I just can't bring myself to spend a lot of time giving them an answer that they could have spent 30 seconds on google researching. (and since when has your opinion been humble?!? )

Craz, I still would prefer to have you read the links I gave you. The same stuff RB said is there, but you also get to pick up some general idea into the building of one of these motors. Also, if you use our Search feature I know this has gone into some very lengthy discussions.

CraZ-28

As my first post mentioned, I did a search and found nothing but garbage. I did look at your links though and there is some good info there. I'm just looking for as much as possible.

Lonestar

Ok, all of you. If you look, I edited my post as I was in a hurry. Since I am an experienced and educated mechanical engineer I could definately explain this to you, but I don't think it will make a difference anyway.

When you use a longer bar (rod) with the same force applied, the force is multiplied by the length of the arm. Do your math.

Lonestar

Here is some info: increasing the length of the rod does 3 things; it increases the dwell of the piston (the time that it is a tdc), it also creates a positive tangential angle on the crank, therefore increasing torque, and it increases leverage at certain angles on the crank, therefore increasing torque. Depending on the engine combination you are using, you may have to change to a different piston to avoid valve clearance issues.

Stekman

Ok, now explain: How does that increase the stroke itself?

Lonestar

ok, look at the original post prior to sticking your foot in your mouth. I mis-worded due to a rush.

Stekman

Sorry, i scolled up and saw a post 'edited' by ede. My fault.

And yes, i agree, the longer rods to produce more torque by reducing the angle on the crank.

ede

it doesn't pay to screw up around here. even if you know what you wanted to say. between being a dumbass and lacking any sort of typing or spelling skills it's happened to me once or twice.

Lonestar

I cannot agree more, unfortunately while responding to the post, I was also in the middle of checking the structural integrity on 2 different multi-million dollar projects that we are going to build. I find it unfortunate that people are so quick to jump on a typo instead of dealing with the logic of the question itself. Not to mention I don't like to pussyfoot around mincing words, and I have an extremely lousy sense of humor. People have a tendency to make mistakes once in a while, but others need to disect the whole statement and use corrective logic to deduce the proper meaning. If something in a sentence does not make sense, then prior to jumping all over it, they need to look at the statement as a whole to get the proper understanding. I have to deal with inept people who want crazy, impossible things to happen with their buildings all the time, and I have to make sense out of it without offending them as that would not be good business tactics.

Now, as far as rod length goes, the 6" rod will benefit the engine in overall torque over the 5.7", but you do need to check the reciprocating assembly for proper clearance at the lifter galley and at the oil pan mating surface. I have seen the clearance be too small before, nescessitating some grinding to remedy.

RB83L69

Lonestar, you screwed up and told a big whopper. All of this "use corrective logic" is a liar's and an egotist's smokescreen for a cover-up.

What's so tough about just apologizing? Can I give you a lesson?

Lonestar: "I had my facts wrong. I did some research and found out where I was wrong. I'm sorry. In the future I'll avoid saying things that I don't really know anything about."

Got it? Try it.

Lonestar

Again, look at my original post where I used the term "sorry".
That is what is known in most circles as an "apology".

Lonestar

I design engines with an extremely reputable company, and we do work with even the military, so no comment.

Red Devil

All right, I'm in a playful mood and I'm going to apply my "corrective logic" to this ok?


I don't care and am unimpressed.



I waited about a half-hour. Longer than usual mind, but still you want me to wait a day or two? If it were my question, I would appreciate a <i>correct</i> answer, not a 'typo'.



Neither do I. And my sense of humor is, well, humorous.


Yep, they do, that's why there are certain words, which exist in the English language, that coalesce in a coherent formulae that goes something like 'I made a mistake'. I'm sure you have heard and used that combination before, yes? If not I'm sure we can teach it to you.



Dissect. No, they don't. They ask a question, you give an answer. There's this thing on the bottom of the reply screen that says "Preview Reply'. This thing is a button. Learn to use it.
Now using my deductive reasoning I assumed you were literate (you could type and spell- more than I can say for myself, yet I know I am literate) and knew the difference between 'stroke' and 'torque' (the jury is still out on this with regard to the period of time prior to the correction of you original post).

Additionally, it would not have entered my mind that you would have done a typo this disastrous. A typo is this 'typi'. As for the 'corrected' statement to be true, you would need a lot more info in order to determine whether his choice would yield more torque with q 6 in. rod. I could see 'The 6 in. rod has the <i>potential</i> to make more torque due to the longer length of the rod'. See this is a tad different than 'The 6 inch rods give you a little more stroke' ..er.. 'torque'.

Furthermore, the second part of your answer has absolutely nothing to suggest that you meant anything other than 'stroke'. Why would you mention clearances if you were discussing torque output?!?

Rather, you seemed to be under the popular fallacy that the 5.7 in. 383 required no clearancing and that the 6.0 in. 383 required clearancing. They both do.

So yes, I did, as well as generally do, try and absorb the meaning of any statement(s) made to which I must reply or contradict. You gave no indication in your statement that would suggest ambiguity in your answer in order for me to come to the conclusion that you had a disparate direction of thought.



The customer is always right. Watch 'Clerks', you would probably like it. Like most people in the service field, you have customers. Be glad, they pay your paycheck.



Lifter galley? Is this another thing I need to turn my deductive logic to? I'm too tired, so I'll ignore it. But just so you know, the lifter galley, is generally the area of the motor referred to that sits under the intake in which you access the lifters from. If you have any encroachment of the reciprocating assembly into this area, you have fuc*ed something up royally and are, no surprise, fuc*ed by the time encroachment has commenced.

<i>Generally</i> (this is a good word some people here need to learn) a stroker motor needs to be clearanced. The rod bolts ground down, the oil pan notched, small base circle cam used, the cylinder sleeves, etc., etc. Generally not what you would want to build as a first motor, but fun to drive.


Main Entry: 1com·ment
Pronunciation: 'kä-"ment
Function: noun
Etymology: Middle English, from Late Latin commentum, from Latin, invention, from neuter of commentus, past participle of comminisci to invent, from com- + -minisci (akin to ment-, mens mind) -- more at MIND
1 : COMMENTARY
2 : a note explaining, illustrating, or criticizing the meaning of a writing
3 a : an observation or remark expressing an opinion or attitude b : a judgment expressed indirectly
Function: verb
transitive senses : to make a comment on <the discovery... is hardly commented by the press -- Nation>
intransitive senses : to explain or interpret something by comment <commenting on recent developments>

1. You just did.
2. You are going to try to use the military in order to prove your competence in engine design?
3. No comment.




This ends our public service announcement brought to you by the Darkside. Please join us overhere on the darkside where everything is, well, dark. Your regularly scheduled program will resume posthaste.

shaggy56

Your all fired. Thats why I paid a machinist with many years experience to figure it out.

RB83L69

Glad to be able to help.... usually it's not too hard to filter out the real stuff from the noise and drivel. I hope the guy that doesn't understand engines and posted some just blatantly wrong stuff didn't confuse too many people.

Incidentally, I made a mistake in my post; I apologize for it, and will try to be more rigorous with the details next time. I'd like to invite those who design engines for a living and others of like technical ability to find it.

ede

glad i don't design engines for a living RB. only thing i found i thought might be wrong was where i think your rounded a number. however i've never claimed to be an expert at anything other than expert dumbass. TGO is very forunate to you and a few others here to keep us all supplied with the correct info. "now was that sucking up or what?" to quote john force

Vader

Generally ,commonly, frequently, usually, often, typically ...

If you read many of my replies carefully, you'll notice that those kinds of modifiers are often incorporated into my responses. Having been slapped down for offering potentially erroneous feedback on more than one occasion from others on the Dark Side such as GhoSSt and Red Devil, I've learned to incorporate that into my responses as a safety net. There is almost always an exception to every rule that cannot be anticipated, and such careful selection of terms is encouraged.

Kevin G

LoneStar got OWNED! :lala:

Vader

I do not design engines as a means of providing financial wherewithal. My technical "ability" is frequently challenged to its limits by the axiom "Lefty-Loosey, Righty-Tighty", frequently causing protracted work stoppages while the ramifications of that theory are contemplated (or I spit out my gum and return to Single-Tasking Mode).

Those admissions being stated, in my layman's untrained opinion, I was under the impression that at the 90° rod journal position, linear piston velocity is solely a function of rod journal centerline to crank centerline differential (stroke) and the RPM, regardless of the length of the connecting member. Angular piston rod velocity (from its' moment center) changes relative to the length of the rod, but not linear piston velocity.

In the region encompassing the 90° journal position, just prior to and just after the 90° position, however, linear piston velocity is affected by the length of the rod. Just not exactly at the 90° position.

I think we can let it slide, since you've duly apologized - That, and the fact that you were able to help out the JPL guys in solving that pesky oxygen injector freezing problem on the J-4s.

Vader

I'm not sure anyone was "owned". In haste to offer aid or assistance, anyone can make omissions and errors. I'm sure he knew what he meant to say. The process of transcribing that thought through the keyboard may have had some inefficiencies, however.

The more important thing is that the eventual result is a better understanding by everyone, including the person posting the original question and even those who might have responded. It sometimes takes the collective efforts of the entire board community to make that happen effectively. That's why there is more than one member here, unlike the "Bob is the Oil Guy" site, where no one else can offer an alternate opinion.

ede

that's why this site isn't called "LoneStar is the engine guy.org"

Kevin G

Sure there was OWNAGE here... Owned in the sense of how you deal with "omissions and errors"

It is all on how you explain it.. Some people do it in the wrong way and the responses follow. This post is a prime example..


Anyway I have taken up enough space in this post .. Carry on.

CraZ-28

Man, did this topic go to hell in a hand basket fast or what?!

Streetiron85

Yeah... whenever someone asks the 5.7"vs 6"rod question, it usually results in a firestorm.
This one's a trophy winner tho.

JesasaurusRex

iTrader: (3)

Anybody ever tell you that by having a longer or shorter rod they do not affect the stroke, the crank shaft does that

Red Devil

We only work with what we're given within our own abilities. Some have greater abilities than others and some have lesser abilities than others. And then there are the egocentrists like me.

dave89gta

I'm not an engine designer, never even built one, but I'll give it a stab...

The counterweights cant really be directly opposite the rod journals. If they were, they would hit the rods during the stroke. (...I think).

Anyway, I have a question RB. How did you come up with the angles that you did? I worked out the geometry for both a 3.48" and 3.75" stroke, and couldnt get your measurements, or even their complements. I kept coming up with angles around 40*. Not saying yours are wrong, I just want to figure this out.


No matter the rod length, the average linear piston velocity for a given stroke will be the same. However, as stated above, with the longer rod the piston dwells at TDC and BDC a little longer. This means that it must travel the rest of the stroke in a shorter period of time. That means that the piston velocity in the middle of the stroke is greater than the shorter rod.

Sure hope I didnt make any typis in this post.

MikeS

For a 3.75" stroke, I get angles of 19.2 deg for a 5.7" rod and 18.2 deg for a 6" rod. Calculated as

angle=inverse sine of (.5*stroke/rod length)

typis....I like that!

dave89gta

yeah, I definitely forgot to take 1/2 the stroke. Damn me.


--Dave

BadSS

Yep and I'm tempted to join in the fun but don't have the time to play. Hopefully these links will prove helpful and clear up any mistakes made in this post.

http://www.iskycams.com/techinfo_index.html (click on tech tips, then rod length)
http://www.stahlheaders.com/Lit_Rod%20Length.htm
http://www.epi-eng.com/ET-PstnMotion.htm
http://www.epi-eng.com/ET-PistnVelAccel.htm
http://www.rustpuppy.org/rodstudy.htm

RB83L69

Actually, MikeS gets the cookie!!! He found my mistake.

I got in a hurry and let the rod length be the adjacent side, rather than the hypotenuse; so I accidentally used the inverse tangent of the ratio instead of the inverse sine. That was egregious and inexcusable, even worse than a typi.

I hate it when I do stoooopid stuff like that.

But, as you can see, the whole rod angularity thing is more than a little bit exaggerated as being the catastrophe that won't let you rev the motor past 5000 and all of that Chicken Little type of stuff.

Vader

Typis notwithstanding, it is understood that a longer connecting member will require a few more degrees (or minutes) rotation of the arm (crank) to facilitate an equal amount of linear motion at the ends of travel as compared to an assembly with a shorter connecting member. Obviously, most of us understand that that point of physics. As a result, most of us understand that the acceleration and deceleration rates of the linear member must be greater from the ends of its travel to the point of peak velocity (at the point of perpendicularity of the rotational axis to the linear axis).

However, this point of perpendicularity of the arm to the linear axis (90° crank position) defines the peak rate of velocity, and is a solely a function of the length of the arm (½ stroke) and its rotational rate (RPM). The only effect that the length of the connecting member will have is in the acceleration from its static points at the ends to its peak point at 90° arm travel.

Restated, a long rod assembly will create longer dwell at the ends and facilitate a faster acceleration and faster deceleration to and from its peak velocity at 90°. If we construct a theoretical model set with even an impossibly short rod and compare it to an extremely long rod, that precise point of 90° travel of the arm demands the same rate linear motion in either model.

Please excuse my typis and grammatoretical errors. (Thought I'd throw in a "Jesse-ism" to keep you all awake. That supposed relationship of language to longer and shorter members is beyond the scope of this discussion - don't go there.)

Just for that, Slitherin house gets 50 points.

Joshua

How do you figure that maximum piston velocity is at 90 degrees?

Vader

Presuming a constant rate of crank rotation (RPM), 90° is the point where one degree of rotation makes the greatest difference in linear motion of the piston. All points below and above 90° are simply either accelerating from a reversal of motion or decelerating toward a reversal of motion. The rate of that change is dependent upon rod length at a give RPM and stroke.

Joshua

When I work it out for a 5.7 rod and 3.484 stroke I get ~74.25 degrees to be where maximum piston velocity occurs. The only time I see the maximum occuring at 90 is when rod length approaches infinity.

Edit: or when the stroke approaches zero

dave89gta

Thought about it some more, and I agree with Vader 100% (likely because he is right). When the crank arm is at exactly 90* position to the cylinder bore, every inch (actually immeasureably small) of arm movement results in the same amount of linear movement of the piston in the bore. At every other position of rotation, the rotating crank also moves the rod from side to side.

Think about it this way, when does the piston rod stop rotating one way and start rotating the other way (with respect to the engine block, not crankshaft)? The answer is at the 90* position, which is where it reverses, as Vader said. This is the only position (2 per revolution) that the crankshaft rotation contributes nothing to the sideways motion of the connecting rod.

--Dave

Joshua

If we were to use a 5.7 inch rod and 3.484 stroke as our example you would see that no matter the RPM the distance traveled between let's say 2 degrees to 5 degrees would always be the same. You also know that if the RPM is constant then it takes the same amount of time to travel from 2 to 5 degrees as it would take to travel from 5 to 8 degrees. At higher RPMs the amount of time to travel 1 degree would lessen but being in our case that it's constant it would always be the same amount of time for every degree.

Knowing this we can find the change in distance from 74 to 75 degrees and not have to worry about RPM affecting anything.

When that is done you find the change in distance for that example is .81 mm.

If you do the same for 90-91 deg you find the change in distance to be .77 mm.

In both cases theta changed one degree thus time is equal. Since more distance was covered in the same amount of time going from 74-75 degrees you can conclude that at that point velocity is higher than 90-91.