Would I see much loss in using 5.7 I beam rods vs. I beam rods in a 350?
I know of stock rod 400's that were raced for years with no unusual problems caused by the rod length.ap72 said:There is a increase in durability with a 5.7" rod- and for that reason I think they are best for street engines taht are ment to last more than 15 passes.
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?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.
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.
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.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.
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
Agreed! :thumbup:k-star said: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...
Keith
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.