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5.7" vs. 6"

5K views 30 replies 15 participants last post by  xntrik 
#1 ·
Would I see much loss in using 5.7 I beam rods vs. I beam rods in a 350?
 
#8 ·
I wish this topic would become a sticky it seems to come up every week or two. With all the "hype" out there, people seem to be confused for good reason. The bottom line is you will not see any difference at all. From a 5.565, 5.7, 6.0 rod the differences are so minute you will never see or feel any difference. The whole side loading, more time at TDC etc is not worth wasting time on. The differences between them are so minor/tiny, do a search on rod length and see what comes up on this site. A lot of people have "opinions" about this, but very few have "facts". Find the facts and decide for yourself.

Buy the best rod you can afford or can get the best deal on, that will work for your application.

I guess that's just my opinion as well, huh? LOL

Royce

EDIT: The study above is one of the very informative (fact) things I was talking about.
 
#12 ·
That is an informative study. The only part I don't follow is his reference to "Dwell time". He measures dwell time as a number of degrees to TDC with the piston in the hole a quarter inch. This measurement would seem to have little to do with dwell time as I understand it. The correct dwell time comparison/measurement would be the number of degrees (before and after) TDC it takes to make the piston move a predetermined amount (like .050 for example). He seems to desregard the fact of the slower moving piston with the longer rod while it is in this area of TDC building cylinder pressure. He does mention that the short rod made more power from 2000-4000 rpm which I am having difficulty understanding. It seems that the piston falling away relatively faster would reduce cylinder pressure not increase it. This presumed reduced cylinder pressure coupled with a slightly reduced mechanical advantage of the piston over the crank pin should lower torque from 2000-4,000 rpm and beyond. Its too bad this guy didnt ask Smokey for his dyno data on the long rod theory.

At any rate, I think that when the Smoke said to use the longest rod you can fit in there, he was talking about all out racing anyway. I don't think you could really know the difference in a street motor anyway but if you gotta buy rods and pistons, you might as well buy long ones if it makes you happy. As long as they all cost the same that is.
 
#13 ·
This study has nothing to do with dyno data, it is a study in the actual geometry involved. The angles aren't somebody's theory, they are calculated using simple math.

I can't see any difference between his definition of 'dwell' and yours. You use .050" as your dwell standard, he uses .250" as his.

If you calculate the piston displacement at 15° from TDC you will find:
A 3.48" stroke with a 6" rod has the piston 0.0762" down in the bore.
A 3.48" stroke with a 5.7" rod has the piston 0.0771" down in the bore.
Note that there is less than .001" difference in piston displacement.

A short rod has better leverage on the crank which helps it produce more torque at lower rpm's. The same effect that causes more cylinder wall loading, also causes more crankshaft loading. Note that a shorter rod has a mathematically longer effective stroke.
 
#14 ·
I just don't get the myths that are out there. My old 385ci 5.565" rod engine is still running to this day. It has been together for over 6 years and has been ran pretty hard. I used to shift it between 6300 and 6500RPM. My buddy bought it from me and now it's in his Nova, he has a bad habit of over reving the engine (stepped up from a 327 and still has not got the hang of how fast the 385 revs).

I bought into the hype and pulled that engine to install a 6" rod 383", guess what, same heads, headers, car, gears, etc.. ran the exact same ET. As far as 6" rods only lasting 15 passes, that is hog wash. I am still running the 6" rods (only because they were bought and paid for) in my current 388 with a blower on it. Lots of street miles and everything is still happy.

Moral of the story "Don't believe the hype". Use a good quality rod that will fit your engine and worry about things that really make a difference.
 
#15 ·
once again, i'll say this, con rod length is unimportant. use what fits, and leave it at that.............here's part of a quote from reher-morrison that i posted before:

These are much more important considerations than the rod-to-stroke ratio. There's no magic - a rod's function is to connect the piston to the crankshaft. Period.
 
#16 ·
onovakind67 said:
This study has nothing to do with dyno data, it is a study in the actual geometry involved. The angles aren't somebody's theory, they are calculated using simple math.

I believe the original study was done in the 60s and it did involve a dyno and it also involved a mule motor that measured real flow. This is exactly why it is ashame that this guy didn't get the opportunity to get Smokey's data.

I can't see any difference between his definition of 'dwell' and yours. You use .050" as your dwell standard, he uses .250" as his.

My definition of dwell is more to do with the duration of time the piston hangs around TDC. Or, a given number of crank degrees for the piston to drop .050. A quarter inch away is a long way out to make a dwell comparison as you yourself point out in your own computations which btw are the right way to compute it.


If you calculate the piston displacement at 15° from TDC you will find:
A 3.48" stroke with a 6" rod has the piston 0.0762" down in the bore.
A 3.48" stroke with a 5.7" rod has the piston 0.0771" down in the bore.
Note that there is less than .001" difference in piston displacement.



A short rod has better leverage on the crank which helps it produce more torque at lower rpm's. The same effect that causes more cylinder wall loading, also causes more crankshaft loading. Note that a shorter rod has a mathematically longer effective stroke.
I believe a short rod has less leverage on the crank pin because it has more angle where the long rod has less angle and is delivering the force in a straighter line to the pin. In addition, it delivers this force with a straighter angle all the way to 90 degrees. The effect of wall loading is because of more rod angle. Any loading of anything (wall) except the pin will detract from available power. If I am scraping a wall with more loading, how do I increase force on the pin?

Please elaborate on the shorter rod having longer effective stroke. Thats an interesting concept.
 
#17 ·
camaroman7d said:
I just don't get the myths that are out there. My old 385ci 5.565" rod engine is still running to this day. It has been together for over 6 years and has been ran pretty hard. I used to shift it between 6300 and 6500RPM. My buddy bought it from me and now it's in his Nova, he has a bad habit of over reving the engine (stepped up from a 327 and still has not got the hang of how fast the 385 revs).

I bought into the hype and pulled that engine to install a 6" rod 383", guess what, same heads, headers, car, gears, etc.. ran the exact same ET. As far as 6" rods only lasting 15 passes, that is hog wash. I am still running the 6" rods (only because they were bought and paid for) in my current 388 with a blower on it. Lots of street miles and everything is still happy.

Moral of the story "Don't believe the hype". Use a good quality rod that will fit your engine and worry about things that really make a difference.

It would be awful easy to not believe the "hype" if it wasn't one of the fastest hotrodders in history that was peddling it. Everything that guy touched ran fast enough that everyone accused him of cheating.

Another interesting point is look at any motor that is a torque monster and is required to wring out every last drop of torque. Scrappers, Large Buldozers, Tug Boats and explain why those pieces of equipment all have compromises engineered into the equiment to provide for long rods. Why have a block height on an earthmover that accomedates a 4 foot long rod? Why not make the rod just long enough to clear the counter weights on the crank with the piston skirt? Why have a 20 foot tall motor on a tug when you only have a 4 foot counter weight on the crank?

Like I said before, I think the basis of all the confusion was that Smokey was referring to long rodding racing motors and I don't believe for a minute that a street motor like your 385 or 388 is ever going to benefit any from a 6 incher over the .565 rods.
 
#18 ·
rods

As some on here know i own a small automotive machine shop, I have been building engines for over 20 years. Anything about engines i love to study. I have spent countless hours on the net and on the phone about this subject. The one thing that is a fact.... nobody has any facts on this subject.Lots of theory's, sketches with long explanations, but when you get to the end there is never the chart showing the short rod to long rod power gain or loss????? I dare anyone to find me dyno sheets where the testing was done A-B-A and proved a long rod does anything all the "theory's" state they do...........car craft, hot rod, etc don't count... They get paid by the people that make the rods.... it's not fair testing...

I got into a knock-down drag-out fight on another site about this subject where the other guy threatened me with violence over this subject... He had zero proof, he was just mad at me because i asked him for it...

I have drawn up engines on cad, i have made working models, etc over this subject,,, still nothing. On that other site a guy sent me a PM and we got to talking about this subject. Turns out this guy ran a big block mopar in a rear engine dragster. He had alot of money and raced for over 15 years. He told me they tried every rod length combo ever invented in that car and nothing ever made the car race faster........If you sit down and look at the numbers and how they relate to how the engine really works you will soon see there is a bunch of hype over nothing..

If you had a unlimited budget and needed every last 1 or 2 horse power and you could calculate piston location in relation to tdc, throw in head flow at the lift the cam is at,,, so-on and so-on... there might,, and i say might with great hesitation be a tick of power there for you.... But the fact is if your asking on a site about rod length your not there!!!!!

I have parts for a 5.7 rod 406, if anybody wants to buy a 6.0" rod combo and pay for 1/2 the dyno time i'll donate the other half....

The smokey deal.....read what he said close, real close.. Plus we all need to remember smokey was a racer first,book salesman second,,, if he got his compitition off on a tangent looking at connecting rods that was less time they had to look at places that made power.... Plus that book was written, what 20 years ago. There are other things in that book that are no longer "the way" but back then it was the trick....

I was told this but cannot confirm it...Gm spent countless money or R&D about rod length in the nascar engines. They started with something like a 5.2" rod and went up to a 6.2" rod and found nothing more then 1 or 2 horsepower....

Of course this is all my opinion, yours may very...

Flame ,on

Keith
 
#20 ·
56Maynard said:
I believe a short rod has less leverage on the crank pin because it has more angle where the long rod has less angle and is delivering the force in a straighter line to the pin. In addition, it delivers this force with a straighter angle all the way to 90 degrees. The effect of wall loading is because of more rod angle. Any loading of anything (wall) except the pin will detract from available power. If I am scraping a wall with more loading, how do I increase force on the pin?

Please elaborate on the shorter rod having longer effective stroke. Thats an interesting concept.
The short rod has more leverage on the crank at the upper end of the cylinder because the rod-to-crank angle is closer to 90°. Any schoolkid knows this principle, when you want your bike to accelerate faster, you lean back on the bike and create more of an angle at the top of the crank stroke. Pushing straight down towards the ground gets you beat.

Next time you're torqueing a set of heads on, see which angle between your arm and the wrench allows you to make the wrench click with the least effort. It will happen when you are pulling at 90° to the wrench. Try pulling directly towards the headbolt as in your example and you will not be able make it click because you have no leverage.

Newton's third law lives everywhere, even inside your cylinders. If the piston is pushing harder on the wall, the wall is pushing harder on the piston. You can't have increased wall loading without increased rod loading, because it's the rod and crank that create the wall loading. The same effect or the short rod that increases wall loading also increases crank loading. Pushing harder on the crank increases the torque.

The effective stroke concept simply means that the short rod motor has the piston farther down in the bore at any point up to the crossover point, about 150° ATDC in the case of 5.7" and 6" rods in a 3.75" stroke. A piston that moves farther with more leverage has done more work, meaning more torque to the crank. Of course there's a point of diminishing returns as the piston begins to move away more rapidly than the flame front can follow.

One of the biggest reasons for using a longer rod is that the piston/rod assembly is lighter and takes less to accelerate. A lighter piston also puts less stress on the rod. It also allows the use of larger internal counterweights on the crank and the internal balancing of the assembly in the larger stroke motors.
 
#21 ·
k-star said:
As some on here know i own a small automotive machine shop, I have been building engines for over 20 years. Anything about engines i love to study. I have spent countless hours on the net and on the phone about this subject. The one thing that is a fact.... nobody has any facts on this subject.Lots of theory's, sketches with long explanations, but when you get to the end there is never the chart showing the short rod to long rod power gain or loss????? I dare anyone to find me dyno sheets where the testing was done A-B-A and proved a long rod does anything all the "theory's" state they do...........car craft, hot rod, etc don't count... They get paid by the people that make the rods.... it's not fair testing...

I got into a knock-down drag-out fight on another site about this subject where the other guy threatened me with violence over this subject... He had zero proof, he was just mad at me because i asked him for it...

I have drawn up engines on cad, i have made working models, etc over this subject,,, still nothing. On that other site a guy sent me a PM and we got to talking about this subject. Turns out this guy ran a big block mopar in a rear engine dragster. He had alot of money and raced for over 15 years. He told me they tried every rod length combo ever invented in that car and nothing ever made the car race faster........If you sit down and look at the numbers and how they relate to how the engine really works you will soon see there is a bunch of hype over nothing..

If you had a unlimited budget and needed every last 1 or 2 horse power and you could calculate piston location in relation to tdc, throw in head flow at the lift the cam is at,,, so-on and so-on... there might,, and i say might with great hesitation be a tick of power there for you.... But the fact is if your asking on a site about rod length your not there!!!!!

I have parts for a 5.7 rod 406, if anybody wants to buy a 6.0" rod combo and pay for 1/2 the dyno time i'll donate the other half....

The smokey deal.....read what he said close, real close.. Plus we all need to remember smokey was a racer first,book salesman second,,, if he got his compitition off on a tangent looking at connecting rods that was less time they had to look at places that made power.... Plus that book was written, what 20 years ago. There are other things in that book that are no longer "the way" but back then it was the trick....

I was told this but cannot confirm it...Gm spent countless money or R&D about rod length in the nascar engines. They started with something like a 5.2" rod and went up to a 6.2" rod and found nothing more then 1 or 2 horsepower....

Of course this is all my opinion, yours may very...

Flame ,on

Keith

My understanding was that Smokey was attempting to get Chev engineers to add a couple of inches to deck height but they wouldnt do it and he never even got to first base with what he was after. This fact suggests to me at least that the difference between 5.7 and 6.0 is negligible
at best. All we need is an 11.035 deck height small block to check this out on a dyno. I agree that even if it does benefit an all out race motor, it won't be the "holy grail" of horsepower by any means it will be like any other hp improvement in motorsports, 1, 2 or 5 hp and some torque. The paragraph where you start out " If you had an unlimited budget and needed every last 1 or 2 h.p", you have just perfectly described all forms of Pro racing. And I think you're absolutely right that valve timing and valve event would have to be revisited if you begin changing relative piston speed at either end of the stroke. For a person to simply change rods and thus, piston speed while disregarding valve event and expect an increase in power would demonstrate a total lack of understanding of whats going on in there.

No flames here dude, just discussing there is really nothin worth flaming for here anyway. What I dont understand is if a guy is building an engine anyway and he thinks that long rods may do the trick for him, and he has to buy rods and pistons anyway, why do people jump up and down advising him to spend his money on something else? He still needs rods and pistons lol. The rods cost the same whether they are 5.7 or 6.0 and I don't see any big deal money wise with pin height, why not just tell him "whatever floats your boat man but don't expect any gains".

I think Smokey's book may even be older then that because it seems as if I haven't seen a copy for 20 yrs but who knows. I also think Smokey was/got excited about rod length while building some 267 cu in. alky motor that made 1000 - 1200 hp for can am racing or something. The only thing I can't quite personally resolve with whole long rod issue is the increased weight and possible losses from flexing pushrods within a taller set up. I can see the increased leverage and dwell time but I can't quite get behind anything that increases the instability of the valve train.
 
#22 ·
rods

One thing on a street engine is that i would never buy a rod that put the oil ring into the pin hole unless it was needed...like to clear a crank throw...

I have built and will continue to build engines with 6.0" long rods and the same engines with 5.7" rods..like you said if the guy has 6.0" rods i am not going to make him swap them out, or if the guy is looking for rods and wants the 6" i am not going to say no, unless like above it's putting the oil ring into the pin on a street engine...

Much of this typ of thing is application driven, but it trickels down to the street engine sceen as " the end all hp adder" I blame alot of that on the parts sales promotions that hyp stuff like this up....

As a example,, last night a customer was talking to me about a cam i am going to put in to his engine. He said i would like one of those "voodoo" cams, Me.. i ask why??? I get the deer in the head light look. well he said there the new hot trick, says who ask me, same look. well... and he started into what he has been reading in the magazines.... I asked him if he thought it had more to do with marketing then performance???? he started laughing and said yea you pick the cam......

I would never tell someone not to buy something.. I don't think i did it here if i did i am sorry... but i try to inform them so they can see what some of the facts are...

I did not really mean that you would flame me 56 maynard it was just a jab....not at you just a in general jab...

Keith
 
#25 ·
O.K. I dont want to knock Smokey,but when he wrote that comment,he was building 350 inch engines that could barely make 1.6 hp per cube normally asperated,and even drag gurus in prostock were just tinkering with the 2 hp per cube mark.Today NHRA prostock motors are making about 3 hp per cube n/a,and if you looked at how they are building them,they are actually shortening the deck height of the blocks to run shorter rods in their engines.The current GM prostock blocks come machined with deck heights of over 9.500",but can be machined as short as 9,200",and none of the engines that I have seen are being run at maximum height,so they are techinically giving up some potential rod length for some reason.A stock bigblock chevy comes with a 9.800" deck height,and tall decks can be had at 10.200",yet for some reason prostock guys are running engines that are an inch shorter then they could potentially run.For some reason I dont think that a team with a 6 figure engine budget would pay an extra 5 grand for a special engine block with a shorter deck height if there were any advantage to running a cheaper block with a taller height.

My personal opinion on the matter is that if you can get a rod ratio near about 1.7:1,you will be within spitting distance of what you could build with any other rod ratio available,and if you can package that 1.7:1 rod ratio with the proper deck height to get the piston pin up high in the piston,but not comprimise the ring package,you can wash your hands and call it a job well done.
 
#26 ·
onovakind67 said:
The short rod has more leverage on the crank at the upper end of the cylinder because the rod-to-crank angle is closer to 90°. Any schoolkid knows this principle, when you want your bike to accelerate faster, you lean back on the bike and create more of an angle at the top of the crank stroke. Pushing straight down towards the ground gets you beat.



Um....as long as you are gonna talk down to me, I'll show you the same courtesy. I am gonna go ahead and guess that you and your school friends are taking the "Short Bus" to school. I won't respond to you after this because you have demonstrated that you don't posess any measurable understanding of the discussion regarding rods or bicycles. You lean back on your bike to go faster and this is why I should use short rods???? I will schedule a press release to let all of the bike racers know they've been leaning the wrong way for many years. After you get this bicycle business sorted out in your brain please install two pistons at the same height in two bores with two different rod lengths and connect these to a crank pin and marvel at how the long rod is at less of an angle then the short. You stated, "The short rod has more leverage on the crank at the upper end of the cylinder because it is closer to 90 degrees." It is well established for many years that the LONGER ROD remains closer to 90 degrees throughout the whole stroke. After you have figured this out, go let the schoolkids know that their "principle" is wrong.


Next time you're torqueing a set of heads on, see which angle between your arm and the wrench allows you to make the wrench click with the least effort. It will happen when you are pulling at 90° to the wrench. Try pulling directly towards the headbolt as in your example and you will not be able make it click because you have no leverage.


Your torque wrench example is as useless as the rest of your examples and theories when you're converting a vertical movement to a rotational movement. The wrench example is pure rotational and is correct until you, and this is where you loose your understanding, convert a vertical movement to do the work with the torque wrench.


Newton's third law lives everywhere, even inside your cylinders. If the piston is pushing harder on the wall, the wall is pushing harder on the piston. You can't have increased wall loading without increased rod loading, because it's the rod and crank that create the wall loading. The same effect or the short rod that increases wall loading also increases crank loading. Pushing harder on the crank increases the torque.


Newtons 3rd law does not in anyway tell you that for every increase in friction, parasitic or otherwise, there is an opposite and equal reaction (increase in work done). You dont get an equal and opposite reaction from increased drag, except that you get less work performed. Loading the piston against the wall only serves to unload the rod from the crank pin of at least some of the available power. Are you really serious?


The effective stroke concept simply means that the short rod motor has the piston farther down in the bore at any point up to the crossover point, about 150° ATDC in the case of 5.7" and 6" rods in a 3.75" stroke. A piston that moves farther with more leverage has done more work, meaning more torque to the crank. Of course there's a point of diminishing returns as the piston begins to move away more rapidly than the flame front can follow.


Aside from proving to me at least that you don't understand cylinder pressure and how it developes, Are you really under the impression that you are still making any measurable power at 150 past TDC? Thats rich man. Shortly after 90 degrees dude, its all baggage. Please, spare me man. Everthing else in that paragraph is nothing more then you making the long rod argument but you're confusing it in your mind with the short rod.


One of the biggest reasons for using a longer rod is that the piston/rod assembly is lighter and takes less to accelerate. A lighter piston also puts less stress on the rod. It also allows the use of larger internal counterweights on the crank and the internal balancing of the assembly in the larger stroke motors.

Like I said before, I don't believe that you'll ever see a difference on a street motor and there are a couple of concerns to overcome even on a racing motor before I think you'll see anything and thats what I have to say about that.
 
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