Long Rod Conversion? I think so!

[Punkskalar]

Patches, this is the sort of info I'm chasing - Hard facts & good solid info about all the mods required to do such an engine mod including hi-res photos.
Lets face it - the crank is the easy part. The finer detail is required on the top end mods required & the pistons combos that work best with the least amount of work & costs involved. Looking forward to what you find out.

Try it for yourself...

Seriously, there will always be some details that get missed, and you'll only learn by trying things. I can do the most in depth writeups and details with good photos, and its still not the same as gettting your mitts dirty The crank is the easy part for now though, as I'm looking into piston mods to make this all work and there is no clear cut path to success just yet that won't require a strong amount of machining and modifying... Its not gonna be as simple as a bolt-in 750 kit thats for sure.

That said, I'm diving into a long rod engine ASAP If you talk real sweet to me, I'll document it for you folks

 

[hotdog]

I will build one...not just yet though, have just built my motor & its still on the bench unstarted LOL, have to get my arse into gear & get this bike finished....
Just always looking for new good ideas is all.
Might be an option if I end up blowing the mikes xs bearings to the shithouse.
I'm sure you'll keep us informed of your progress though Hugh - thanks.

 

[DirtyErnie]

The longer rod (higher rod/stroke ratio)... you lose some leverage on the crank (and side-load at the piston) by increasing the ratio, but that's only beneficial at low RPM. Longer rod reduces the 2nd harmonic component of piston motion, reducing piston time spent at the bottom part of the stroke and increasing piston time towards the top of the stroke. This gives you a little more piston 'dwell' time during the first phases of combustion as the piston isn't accelerating away from the combustion chamber so quickly, that helps with burn at high rpm. Also helps reduce rod load at the top of the exhaust stroke (more gradual piston deceleration), lets you spin a little faster before you snap the con rod and eject parts. :-D

 

[Jack]

The longer rod (higher rod/stroke ratio)... you lose some leverage on the crank (and side-load at the piston) by increasing the ratio, but that's only beneficial at low RPM. Longer rod reduces the 2nd harmonic component of piston motion, reducing piston time spent at the bottom part of the stroke and increasing piston time towards the top of the stroke. This gives you a little more piston 'dwell' time during the first phases of combustion as the piston isn't accelerating away from the combustion chamber so quickly, that helps with burn at high rpm. Also helps reduce rod load at the top of the exhaust stroke (more gradual piston deceleration), lets you spin a little faster before you snap the con rod and eject parts. :-D

You gain loading leverage(Dwell Time) and loose vacuum(from pumping loses) through the ports from TDC to Mid stroke and from that point, the ports are pulling like a freight train to BDC from BDC to mid stroke. They simply require a different approach when porting where velocity is the up most important factor over CFM to a certain degree, it's all a balancing act.

 

[Highside]

The longer rod (higher rod/stroke ratio)... you lose some leverage on the crank (and side-load at the piston) by increasing the ratio, but that's only beneficial at low RPM. Longer rod reduces the 2nd harmonic component of piston motion, reducing piston time spent at the bottom part of the stroke and increasing piston time towards the top of the stroke. This gives you a little more piston 'dwell' time during the first phases of combustion as the piston isn't accelerating away from the combustion chamber so quickly, that helps with burn at high rpm. Also helps reduce rod load at the top of the exhaust stroke (more gradual piston deceleration), lets you spin a little faster before you snap the con rod and eject parts. :-D

That's the theory at least. If you look at the actuall differences in piston position at a given crank angle (you can calculate this yourself using trigonometry), you will find that the changes in pison movement are often only a few thousandths. You need a MAJOR change in rod lenght to have a noticable difference on engine performance.

 

[Jack]

That's the theory at least. If you look at the actuall differences in piston position at a given crank angle (you can calculate this yourself using trigonometry), you will find that the changes in pison movement are often only a few thousandths. You need a MAJOR change in rod lenght to have a noticable difference on engine performance.

From a 477 rod length to Cr 500 rod is a major change,1/2 inch in rod lenght, that a lone is like adding more cam duration. All your theoretical calculations may seem small but in actual daily operations,the engines running characteristics are changed significantly.

 

[Ironriot]

Awesome thread thus far! Unfortunately I have absolutely nothing smart to add to the conversation...

That being said, Hugh, you are THE MAN if you do a write-up! Its good inspiration for us simple minded folk

 

[Highside]

From a 477 rod length to Cr 500 rod is a major change,1/2 inch in rod lenght, that a lone is like adding more cam duration. All your theoretical calculations may seem small but in actual daily operations,the engines running characteristics are changed significantly.

There's nothing theoretical about my "calculations" Jack. Trigonometry will tell you EXACTLY, the piston's position in the bore with any lenght rod, at any crank angle. The differences are small, and in most applications, barely noticable.

David Reher, Buddy Morrison, Bill "Grumpy" Jenkins and A. Grahm Bell have all done the dyno testing on short vs long rod engines, and all have reached the same conclusion. In all but a handfull of applications, there's NO noticable difference in power.

If you think I'm BSing, then lets see the dyno sheets that say I'm wrong.

 

[DirtyErnie]

good theoretical discussion, for sure. But is it worth it in the real world on a bottom end that can already handle getting into the 9kRPM range? Unless you're converting to a Scotch Yoke con rod... probabaly not a major issue.

 

[xjwmx]

then lets see the dyno sheets that say I'm wrong.

I'd like to see the same proof that any mod on these gives you significant power gains. Any mod that doesn't increase displacement, that is.

 

[Highside]

I'd like to see the same proof that any mod on these gives you significant power gains. Any mod that doesn't increase displacement, that is.

That's a good principle to stick by.

I spent over 10 years as a CNC Prototype machinist in the racing industry. I've seen more smoke and mirrors and BS horsepower claims than you can shake a stick at.

Untill I see dyno results, AND I can duplicate those results on another dyno, I call BS on anything that I haven't seen make more power in the real world.

 

[Highside]

good theoretical discussion, for sure. But is it worth it in the real world on a bottom end that can already handle getting into the 9kRPM range? Unless you're converting to a Scotch Yoke con rod... probabaly not a major issue.

Exactly.

In order to gain the theoretical advantage of a long rod, in the real world, you'd need connecting rods that were over 3 feet long in order to significantly alter the piston movement.

 

[hotdog]

Try it for yourself...

Seriously, there will always be some details that get missed, and you'll only learn by trying things. I can do the most in depth writeups and details with good photos, and its still not the same as gettting your mitts dirty The crank is the easy part for now though, as I'm looking into piston mods to make this all work and there is no clear cut path to success just yet that won't require a strong amount of machining and modifying... Its not gonna be as simple as a bolt-in 750 kit thats for sure.

That said, I'm diving into a long rod engine ASAP If you talk real sweet to me, I'll document it for you folks

Hugh - anything further on your proposed long-rod motor..??

 

[dwyatt]

I'm sticking to my story that it's not the size of the rod.........

 

[Tomterrific]

Given that the XS650 port size is considered large, there is a good reason the rod was made shorter in the later engines.

tom

 

[Jack]

Given that the XS650 port size is considered large, there is a good reason the rod was made shorter in the later engines.

tom

Tom.......... For someone like yourself who I consider highly educated in the engine field,I must say I'm quite surprised to see such a reply like this from you. Yamaha produced two versions of the XS head with different port volumes and shapes which also exhibited different velocities and CFM flow within the ports from each other and do you honestly think these two heads came about by accident? No,they where designed in direct correspondence to changes in piston speed from changes in the Rod/Ratio which inherently effect the draw within the intake track and when piston speeds change so does port volume flow percentages , now it's not to say that some ports aren't oversized some but that comes directly from poor casting. When you have a XS 650 knocking down 45 to 65 MPG be it stock or modified form pretty much blows that BS theory out of the water. When your intentions are to build a Hp motor and you actually take the time to flow several XS heads to see where they fall short for performance and educational purposes and only then to some extent that statement would be true for the track. It all boils down to the intentional usage and power band.

 

[Tomterrific]

Hi Jack,

My comment above is about a stock engine designed by engineers for the US market. I'm not disputing what longer rods can do for a higher rpm XS engine. The 750cc T140 Triumph also uses shorter rods than the earlier 650cc and this is coupled with the hottest intake cam (but the mildest exhaust cam so the transmission would not break). I can't test every combination but I'm basing why the Yam engineers used a shorter rod on theory (that we both agree with) and the belief that the engineers knew what they were doing. "It all boils down to intentional usage and power band."

Cheers,
Tom

 

[riksoto]

Michael Morse of http://www.650central.com is developing a long rod and piston with rings kit was told it should be done soon. The rod chosen is 1mm longer than a CR500 rod there are more and smaller diameter rollers for the main, the rod is also lighter than the stock xs650 rod. The piston I am told will be of a forged design with 1mm compression rings. I have already purchased a kit in advance

 

[KentMoney]

hugh, considering this mod for your new digger?


a question: you don't need to do any truing when welding the initial center pin? seems like truing would need to occur?

 

[jayel]

any thoughts on the brass bushing vs needle bearing small end of the rods?... didn't work for yamaha as they changed first year