Can we please put an end to the "long connecting rod" myth??

Highside said:
you don't have to take anyone's word for anything, you can test it yourself.

Science works BITCHES!!!!!
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" Straight From The Horses Ass Himself" finally we a agree to something Mr.Ed:bike: And one more thing Mr.Ed why don't you take a trip to Mooreville,NC to inform those NASCAR boys(Where automotive technology begins) that their usage of long rods for long tracks is pretty useless based on your Horses Ass Scientific Theories and one more thing Mr.Ed,when you start altering rod ratios or strokes,it changes pistons speed which in turn effects the vacuum draw through the ports and for someone who's claims to of been in charge of a cylinder head department should of know this from his elementary school training,after the all an engine is nothing more than big air pump. Changing strokes or Rod Ratios changes everything,IT"S THE FACTS MR.ED
 
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Highside said:
:laughing:

My reason is that some people waste a lot of money on something that they think will increase the power of their engine and it really doesn't. If you're doing the work yourself, it's one thing, but to pay someone else can get expensive, and it's downright fraudulent if the people selling the work are making HP claims they can't back up.

... and my John Deere tach is teh coolest!!! I take it off the bike and cuddle with it at night. :bike:
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Ignorance is bliss isn't it Highside ? If you were to read what the claims are, as far as long v.s. short rods, you will find that you misunderstood. Long and short rod motors make close to the same power. Within a few percent. I did not read any where in the long rod discussion where there is a superior horsepower advantage. As stated by a XS650 dirt track racer, the power on a full house 650(750) comes on like a light switch, breaking the rear tire loose. A long rod version all else being equal, has a different power curve. It does not spike like the short rod motor. Therefore no rippin the tire out of the corner. No more power, no less power. Different power curve. Applicable to 650 Yamahas and Pro Stock big blocks. The very small measurement that gets your panties in a knot, is not the the big deal. The effect on the airflow, piston speed in specific areas, piston weight and side load on the cylinder wall. That is what the discussion is about. It is better to keep your mouth shut and be thought of as a fool than to open it and remove all doubt.

Jim
 
mrriggs said:
Why is that too small to make any difference? In the world of blue printing motors, thirty-one thousandths is like a mile. If you changed your valve lash by 0.031" or decked your block 0.031" there would certainly be a difference.

I'm not saying you are wrong, just that it needs further explaination. Most of us don't have your vast experience and knowledge so it's not as obvious to us.
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Well, if we were talking about this at something close to TDC, then you would be right. But this .031" is occuring when the piston is already more than halfway down the bore, too far away to have an effect on either combustion or valve timing. Valve timing would be the closest, but you'd have to have an enormous cam, and even then, .031 is only a small fraction of cylinder volume at 90 deg.

Long rods COULD have an effect on engine performance, but in order to do so, you'd need stroke to rod ratios in the neighborhood of 6 to 1 with rods that are a couple of feet long.
 
JimR said:
Long and short rod motors make close to the same power. Within a few percent.
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No, they make the same power, within 0%.

I did not read any where in the long rod discussion where there is a superior horsepower advantage. As stated by a XS650 dirt track racer, the power on a full house 650(750) comes on like a light switch, breaking the rear tire loose. A long rod version all else being equal, has a different power curve. It does not spike like the short rod motor.
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No it doesn't. As I have already proven, the difference is too slight to be noticed. A maximum of .0012" @ 10 deg ATDC, when cylinder pressures at at their highest, is undetectable.

The effect on the airflow, piston speed in specific areas,
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There is no difference in piston speed, that's already been mathematically proven. When you graph the piston movement of both a long and short rod, what you end up with is an almost identical curve.

piston weight and side load on the cylinder wall. That is what the discussion is about.
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Pistons might be a bit lighter, depending on the application, the rod angles are almost the same with a 1.5 deg difference @ 90 deg. between long and short. Again, an insignificant difference.

It is better to keep your mouth shut and be thought of as a fool than to open it and remove all doubt.

Jim
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What I am doing is challenging an accepted "truth" with facts and evidence. The more you people talk about your long rod "theories" the more it sounds like religious belief. I'm starting to feel like Galileo facing the Spanish Inquisition.
 
Highside said:
Well, if we were talking about this at something close to TDC, then you would be right. But this .031" is occuring when the piston is already more than halfway down the bore, too far away to have an effect on either combustion or valve timing. Valve timing would be the closest, but you'd have to have an enormous cam, and even then, .031 is only a small fraction of cylinder volume at 90 deg.
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Why would it have to be close to TDC to effect valve timing? The intake closing event, even with a relatively mild cam, happens nearer to 90 BTDC than it does from TDC or BDC.
 
Highside said:
Pistons might be a bit lighter, depending on the application, the rod angles are almost the same with a 1.5 deg difference @ 90 deg. between long and short. Again, an insignificant difference.
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One and a half degrees sounds small but that is a 10% reduction. That basically means a 10% reduction in friction between the piston and cylinder.

You keep saying that these changes are "insignificant" but nowhere have you qualified what a significant change would be. For example, it would take a rod angle change of ___ degrees to change the horsepower by ___ %. (fill in the blanks).

With an established baseline, determined from real world testing, you can approximate results through calculations. Though, actual testing could prove that inaccurate if there were variables or curves that you had not originally considered.

We can speculate and calculate, point fingers and call names, but without any real data you can hardly call it scientifically proven. The fact that racers have built long rod motors, and been pleased with them enough to continue building them, tells me that there must be more to it than just numbers. Of course, that is in no way scientific proof but if they are winning races then who gives a shit.
 
Before the Yamaha factory approved the budget for the construction of the OU version of the XS engine several things had to happen. First of all, overall layout of the head was specified by CR Axtell and the factory engineers had to approve the layout to ensure the casting was doable by Yamaha's vendor. Secondly, Ax would receive the "blank" heads and he and Mike would hand port them on their flow bench to optimize the actual port contours. Third, the Yamaha race department had to review and approve the proposed OU specs and plan.

Importantly for this discussion was how the rotating group specs were derived. To keep costs under control, Ax used as many stock XS components as possible plus some off the shelf items. Therefore, stock cranks, beefed up cases, stock oil pumps, etc. were used. Suitable GM auto pistons (!) were sourced and remachined to work with Carillo rods which were then detail-machined by Warren Machine.

Ax - in concert with Yamaha's race department - determined that 144mm rods were the best choice to work with the proposed combustion chamber, piston, valve angle, etc.

Now, of course, they could all be delusional or just plain stupid. And, I suppose it's possible none of the engineers at Yamaha had ever heard of trig, actually ever worked on a real engine or graduated from college.

However, if these people did know what they were doing I believe it is telling that when they had the choice of any length to choose from, they chose a 144mm rod. I doubt they did so for any reason except that they believed it would enhance the performance of the XS engine.

My take: if long rods are good enough for Ax and the Yamaha race department, they're good enough for me. Full disclosure: I've never taken trig (in fact, I thought Trig was Sarah Palin's son's name)

But, like I said, they could all be wrong. Maybe they could have saved a lot of money by talking with Highside and just done a run of uprated 256 rods. I guess we'll never know.

Craig
 
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Craig,
Glad to see you drop by and speak your two cents worth of actual hands on knowledge with what actually took place back in that era and not plotting Trig on paper.
You and I and others all know to well that CR Axtell and the Yamaha racing engineers where no dummies just out of grade school and where faced with the agonizing question of how to beat the XR750 and used all possible resources to meet this goal,which like you said included the usage of the 144MM rod to reap it's benefits and Highside can preach all he wants to default the long rods actual benefits but at days end
there's the real world documented evidence of satisfied customers and racers staked up against him stating otherwise, period.
 
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Is this thread dead?
I wish I had been there to weigh-in.
There's a simple mathematical concept that explains why long rods produce more power...
 
Hi Hotdog, great!
Yeah, I'm always late to the party, keg's empty, everyone's gone.

Look at the long rod concept from the point of view of the block. The crank produces torque, but there's an equal counter-torque realized in the block (cylinder). This counter-torque comes from the sideload thrust of the piston, directly across from the wristpin.
The longer rod does indeed induce a shallower thrust angle on the piston/cylwall, which is good, less sidewall thrust = less friction.
However, with the longer rod, this sidewall thrust occurs over a much longer torque arm (rod + 1/2 stroke).
The reduced sidewall thrust does not cancel out the increased torque arm.
The result is net increased counter-torque realized by the block (cylinder).
Long drawn-out mathematics proves this out. Easier with an engine simulation program.

Smoky Yunick was right...
 
Stan Weiss's program ( http://users.erols.com/srweiss/ ) can calculate thrust side load due to changing rod lengths.

Smokey basically said in as much to stuff the longest rod you can into a motor. Doing the math (or letting a program do it for you), you can see where the difference lies (increase tdc dwell, lower piston speed, less side thrust). The biggest, by far, advantage of a longer rod is being able to run a much lighter piston.


That being said, I don't think anyone on here is working on the level where any of that matters. The difference is small enough that being slightly off on jetting or a change in the weather will negate any difference, and practically everyone isn't using tuned length exhausts and intakes. There is a lot more power to be had there. Besides, you need a cam specially ground for the above to take advantage of pretty much everything but from friction and lighter weight and even then it's debatable. Most seem content on building motors exactly as they were 35 years ago.

Dirt racers (car and bike) use long rods to shift piston speed around which effects the way it transmits power. It's more about that and less about making more hp.

Highside's posts agree with everything I've read by high level engine builders.

Here is an example I was playing around with:

Here is a graph I made comparing a SR500 crank/rod and 87mm bore and a SR400 crank/rod and 97mm bore. It shows piston cfm demand for a given crank rotation at 7300rpm and at 44hp for both motors. I adjusted VE % in PipeMax so they had the same exact hp. It wouldn't work out this way in real life, just trying to demonstrate the differences in where the peaks occur. Keep in mind, piston cfm demand is linked to velocity of the piston.

The other graph is the same set-up as above, but the y-axis is showing piston speed in ft/min for a given crank rotation. The values are for 7300rpm.

The engine specs are as follows:
SR500: 3.425" (87mm) bore, 3.307 (84mm) stroke, 5.709 (145mm) c-c length
SR400 crank/rod + 97mm bore (this is so both motors equal 500cc): 3.833" (97mm) bore, 2.640" (67mm) stroke , 6.030" (153mm) c-c length.
 

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Here is what Highside was trying say. It just doesn't matter. It really just comes down to potentially less piston weight.

Values taken at 7200rpm.
Stock 136mm rod vs 145mm long rod.

Peak piston speed (FPM):
136mm: 5685 FPM @ 76 degrees crank rotation.
145mm: 5661.2 FPM @ 76.5 degrees crank rotation

xs650_piston_zps1b5d7f25.jpg


and Piston CFM demand. Sure, the long rod pulls harder at the bottom part of the stroke, but the difference is 1cfm at best. That's not going to show up on the dyno.
xs650_cfm_zpsc8308816.jpg


Cheers,
Bob
 
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I have done the math and the long rod does noticeably reduce the piston side load.

LongRodChart1.jpg


So that's it then, less side load equals less friction, less friction equals more power.

Of course, that's only looking at the sideways force at the piston end of the rod. At the big end of the rod you have an opposite sideways force pushing on the crank.

Picture a crank throw at TDC. If you push straight down on it there is no torque. If you pushed sideways on it there would be torque. If you turn the crank 20° then only 34% of the downward force is converted to torque and 83% of the sideways force is converted to torque. Even though the sideways force is only a fraction of the downward force, it has more leverage near TDC. As shown above, a short rod has greater sideways force so it will produce more torque.

Which is greater, the increased friction or the increased torque? It would appear from the following chart (which calculates both) that the increased torque, from the shorter rod, is greater than the increased friction, on the power stoke.

LongRodChart2.jpg


There are however three more strokes before it gets back to the power stroke and the reduced friction from the long rod is an advantage there.

All said and done, the long rod only makes 0.6% more power, but does so with a 3% reduction of the instantaneous torque. I imagine that is what the dirt track guys are referring to when they say that the long rod "softens" the power delivery.

LongRodChart3.jpg


Rumor has it that Yamaha experimented with re-phasing back in the 70's but they raced 360° motors. It would stand to reason that if the long-rod's 3% reduction of instantaneous torque was advantageous, then a re-phase motor would have been a disaster.

Let me finish by saying that none of this is, in any way, conclusive. It's just one mathematical model with as many variables as I could figure out. I've studied a bit of math and have a real love-hate relationship with it. It can be a useful tool but there is no real truth to it. Math does not exist, it is a concept. We observe things that do exist then make up this thing called math to quantify those observations. The goal is to use math to try and predict what real things are going to do. The deeper you get into math the more you realize that it's all a bunch of hot air.
 
Mrrigs,

Thanks. That .6% is under a perfect scenario. There are so many other little things that could eat up .6% gain from friction reduction (the list is too long).

Agree with the math, but it's helps remove the 'witchcraft' associated with some things. We don't have everything defined, but we've uncovered a lot since the days these motors experienced real development.

It's this type of work and analysis that separates the fastest from the fast and helps pick through the bs.

As smart and as great as the old timers were, it's been 35 years. It's time to use what they did as a foundation, and if people are serious, apply 21st century racing know how to update where they left off.

Cheers,
Bob
 
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hoffman900 said:
Thanks. that .6% is under a perfect scenario. There are so many other little things that could eat up .6% gain (the list is too long).
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There are also "so many other little things" that could add up to more than 0.6%. The discussion wasn't about other little things, it was about rod length. The model shows that if all things [big and little] are equal, changing only the rod length, there is a difference in power output.

I think the friction part, for now, would be a mute point as I have yet to hear of anyone honing with a torque plate. A very slightly out of round cylinder will cause more friction than you gain from the decrease in side loading.
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Tell me the friction coefficient for a "very slightly out of round cylinder" and I will plug it into the equation.

Just for fun, I doubled the friction and the long rod made 2% less instantaneous torque than the short rod, and 2.5% more mean torque.

Cutting the friction in half, 3% less instantaneous torque and 0.1% less mean torque.

In every case you are reducing the instantaneous torque, and in all [but the most extreme] cases you are still making as much or slightly more power.

Does a 3% reduction in instantaneous torque really make that much of a difference? The guys with actual experience building and racing these engines say, "Yes."
 
It's impossible to put a value on it. I just know what I've seen with other examples. Not using a torque plate will pinch the bore a bit near the studs. Talk to any performance machinist and they'll tell you can feel the chuck on the boring machine 'shudder' a little for the first couple passes as it takes out the high spots in the bore with a torque plate installed.

I PM'ed Highside. He says he's a machinist for NHRA teams so he can certainly comment on that. Hopefully he comes back to the site.

As for friction, there is this:
http://www.ricardo.com/Documents/Do...ulationofFrictioninHighPerformanceEngines.pdf

There are a bunch of peer reviewed papers, but they're not accessible for free. I can view some of them, but I can't share them (still have student privileges for now).

A simple way to measure this is to assemble the engine and measure how much torque it takes to turn over. Granted, there are many things that contribute to this.

A top rider would notice it on dirt. On the street or road course, it doesn't manner. That's why Smokey Yunick, etc. all say just jam the longest rod you can in the motor, but it's not going to make a measured increase in horsepower. I think that's what Highside was arguing. Anyone who says otherwise is just a good pitch man. You're not going to feel .6% more horsepower from friction losses. Long rods aren't going to make a poor cylinder head magically flow better, as I said, the instantaneous change on draw in the lower parts of the stroke at 28" depression is 1cfm - the shorter rod has a 1cfm advantage higher up in the stroke.

Apparently you'll feel the change in instantaneous torque, and you'll certainly feel much lighter pistons. The former isn't from increased horsepower, it's just less punchy. Lighter pistons effect acceleration, however, and reduce the load on the crank and rods.

As I've shown, there isn't much of a change at all in piston speed and it occurs .5 degrees later than with the short rod.
 
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This is great. I'm glad it went from name calling to a thoughtfull process. I ran a long rod v8 on dirt 20 years ago and it was great. My engine builder specified that the lighter pistons, and lightened crank was more of an advantage than anything.

Smokey was right about getting long rods into those straight sixes and early v8's as the pin length to top of piston was ridiculous. The pistons looked like coffee cans and weighed quite a bit. Putting the piston pin extremly close to the combustion chamber is not a good idea for getting 50,000 miles out of a motor, on long rides and slow traffic, heat soaks that area and burns the oil into carbon deposits that score the cylinder.

With all that said, I hope everybody keeps expirimenting and building it the way you like. I've got more invested in my bike than I care to admit. I could have bought a used rocket that outperforms my xs in every way, but what fun would that be.:bike:
 

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