|01-20-2006 10:17 PM|
"Much ado about almost nothing."
Remember this: A longer rod "flops back and forth" less so there is less force trying to break it.
|01-19-2006 12:52 PM|
A couple of things to comment on.
1. I am not in the long rod or short rod camps, I am not talking theory or what works better, I'm presenting the difference in geometry of the setups.
2. If you're going to quote me, quote me correctly. You quoted me as saying,
"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.
I said specifically - "the rod-to-crank angle is closer to 90°".
There are two angles concerning the rod, the rod-to-bore angle and the rod-to-crank angle. The rod-to-bore angle in a 3.75" stroke motor does not exceed 19.7° with the 5.565" rod, and has a maximum of about 18.21° with the 6" rod. The rod-to-crank angle runs from 0° to 180° depending on the position of the crank. Neither one of these stays close to 90° for any appreciable time.
The rotational force transmitted to the crank is proportional to the sine of the included angle. If your crank is at TDC (included angle = 180°) and you apply 100 units of force, none of them are are transmitted as rotational force. The greatest percentage of force is transmitted when the crank and rod form a 90° angle.
Let's use 30° ATDC in our 3.75" stroke motor as an example:
With a 5.7" rod the piston is .3288" down in the bore, the rod-to-bore angle (RTB) is 9.47° and the rod-to-crank (RTC) angle is 140.53°.
With a 6" rod the piston is .3248" down in the bore, the rod-to-bore angle is 8.99° and the rod-to-crank angle is 141.01°.
Since the power transmitted to the is proportional to the sine of the included angle, the 5.7" rod will be pushing with about 1% more force on the crank. The wall loading will be about 5% more ( difference in sines ) but only about 25% of the force generated against the crank ( ratio of sines of the RTB and RTC ) is transmitted to the wall. I don't know what the coefficient of friction of oiled aluminum over cast iron is, but I would imagine it's pretty low, let's assume .02, a fairly high number for an oiled surface. The friction component would be less than 2% of the force acting directly on the wall, which is about 15% of the total force on the rod. .02 x .15 = .3% of the force is friction, the rest is actually loading the wall.
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?
Serious as a heart attack, and the numbers above represent the math of it. I never stated that 100% of the increased power goes to the crank, only that the rod pushes harder on the crank when the rod-to-crank angle is closer to 90°. Where does the increased friction come from? It comes from the rod pushing harder on the wall. You can't increase the friction on the wall without pushing harder on the rod. If you push harder on the rod with greater leverage on the crank, you make more torque.
Let's put up another example: You increase the cylinder pressure of the motor by some means, like more compression. The higher cylinder pressure will also increase the cylinder wall loading, there's no way around it. Would you theorize that the increase in cylinder pressure would cause a loss of flywheel power because the cylinder wall loading was increased?
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.
I didn't say there was any power being made past 150°, only that the crossover point of the piston position graphs for the rods in question is at that point. Let's talk about 90°, where you say the work stops. With a 5.7" rod, the piston is 2.1922" down in the bore, and the 6" rod has it 2.1755" down in the bore. That's roughly 20 thousandths more piston movement. Stroke is directly connected to the movement of the piston in the bore, and a piston that moves farther for a given crank angle would reasonably be assumed to have more effective stroke. One of the benefits of stroke is that a longer stroke motor does more work with the piston, since work is defined as force times distance. If you calculate the force at any point from TDC to 90° with the shorter rod, you will generate more force for any given cylinder pressure.
I am not including any theoretical arguments about cylinder pressure, wall distortion, vacuum, breathing, dwell, large eddies, small eddies, swirl, octane, cam timing, etc. I merely presented the geometry. Jere Stahl has an interesting treatise on the theory of long vs short rods. I'm sure Jere understands more than I do about the theory.
A couple of quotes:
C. Power Stroke -- Short rod exerts more force to the crank pin at any crank angle that counts ie.--20-o ATDC to 70-o ATDC. Also side loads cyl walls more than long rod. Will probably be more critical of piston design and cyl wall rigidity.
If other factors influenced by rotational speed such as the time distance between slug of intake air flow and valve opening rates relative to the acceleration of the air slugs were ignored, one should be able to predict the location (RPM) of the peak power as a result of a change in the (l/r) ratio. Note, that even though power is a funtion of air flow and air flow should be roughly constant for the same instantaneous piston speed (neglecting the afore mentioned factors), the power may not be the same because of the lever arm effect between the crank radius and the connecting rod. (As we noted earlier, the shorter rod should have the advantage in the lever arm effect.)
In reality, the analysis must be viewed by stroke (ie intake, compression, exhaust, power) the selection of exhaust valve opening time combined with the exhaust system backpressure and degree of turbulance the exhaust port experiences. If the exhaust port has good turbulance control then you may run a shorter rod which allows you to use more exhaust lobe which reduces pumping losses on the exh stroke.
|01-19-2006 11:12 AM|
In round tracks cars, short rods favor torque at lower race speeds. Long rods reduce the side loading a wall wear. Long rods lightens pistons. Long rods with narrow ends lighten rods. Piston guided rods lighten rods. Raised cams shorten pushrods and provide stroke clearance. Long rods help a restricted class engine fill the cylinders. A short stroke long engine rod will usually wear the bores less in more laps than a long rod short stroke engine.
I guess my real point here is you can't compare what a drag engine does with circle track, dirt or asphalt or otherwise. You can't compare drag with street for the rules are different. You gotta make sure you are comparing apples to apples or state the differences before hand.
Smokey was making round track engines when he made that qoute BTW. If your still reading books written more than ten years ago, Your wayyyy behind the game.
|01-19-2006 10:08 AM|
|predator carb guru||from what i've been told, the shorter deck heights on pro stock motors have absolutely nothing to do with rod lengths or rod/stroke ratios. they do it for improved valve train geometry. with the rpms those engines are running now (10,000 is pretty common now), they need as stable of a valve train they can get. shorter deck heights allow shorter, stiffer pushrods. lifter bores are relocated, etc. the new DRCE III GM blocks have seven cam bearing journals to help stop camshaft deflection. all done in an effort to reduce flex, improve stability, and allow for higher rpms. i've never personally been into a pro stock motor, but i know they spend alot more money on valve train stability, cylinder head port design, intake manifold tuning, and carb work than they've ever spent on rod lengths. my friend Jay Childs (Jay Childs race cars, anchorage alaska) got to talk to warren johnson and spend some time at his shop with him this last year while him and his dad were down in that area racing, and while warren wouldn't give up many of his "secrets" to him, never once was connecting rod length ever given a consideration when he's building his motors. he determines the bore size he wants, the piston crown layout and shape, and crankshaft stroke, then mearly connects the piston to the crank. so, again i go back to reher-morrison's quote. the purpose of the connecting rod is to mearly connect the piston to the crank. we can argue all day long about how long rods are better this, short rods are better that, but when it all comes down to it, it's a mute point. a rod connects the piston to the crank, nothing more. it only needs to be long enough to get the job done.|
|01-19-2006 07:48 AM|
Good points. The Pro Stock hemi block (NHRA) comes with a 9.28 deck compared with 10.72 on a regular 426.
What you've pointed out makes me wonder if the short decks are favored due to shortening and lightening the valve train (Push rods) for reliable 10,000 rpm + passes. Or maybe even intake/port considerations and velocities ABDC continuing to fill the cylinder due to higher piston velocities ??? I dunno.
|01-19-2006 07:21 AM|
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.
|01-18-2006 10:07 PM|
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.
|01-18-2006 04:02 PM|
|barnym17||the only reason for a 5.7 over a 5.565 for me is in the sock rods the the 5.7 seems to be stronger at the big end, more meat there. almost looks like gm made the shorter rod by removing metal from the big end.|
|01-18-2006 01:09 PM|
|01-18-2006 12:50 PM|
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...
|01-18-2006 12:32 PM|
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.
|01-18-2006 08:22 AM|
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.
|01-18-2006 07:36 AM|
No flames here
get your deck height, get your stroke, connect the pistons to the crankshaft, go
spend the extra $$ in the heads
|01-18-2006 06:40 AM|
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...
|01-18-2006 02:45 AM|
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.
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