Some major upgrade advice - Page 8 - Kawasaki KLR 650 Forum
2008+ KLR650 Wrenching & Mod Questions For repair, maintaining or modifying discussions related to the newly updated 2008 and beyond, Generation 2 KLR650 Motorcycle.

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post #71 of 147 Old 11-01-2019, 07:04 PM
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Originally Posted by DPelletier View Post
There was an interesting article in one of the car magazines years ago (Hot Rod?) where they went to great lengths on two identical big block chevy builds of the same displacement in an effort to "myth bust" the old tale that long stroke motors made more torque and short stroke motors made more hp......I wish I could find it again but the upshot was that it made virtually no difference. In searching the net of the old article, I did come across this reference from a race engine builder on another forum I used to frequent:


As long as the valve is unshrouded in both motors of the same displacement then the power bands will be near identicle. The reason the myth came about that a big bore makes more power is because they were still unshrouding the valves as the bore grew. Once you have reached the point of completly unshrouding the bore you will see no differeance in under square and over square engines. One of the magazines did a very good test of this a while back and accounted for pretty much everything includeing rod/stroke ratio and they ran steady state HP testing to cover the lighter/heavier rotateing assemblies and never saw more than a couple HP ANYWHERE in the power curves from top to bottom. I saw another test back in the 80s when I was a kid that came up with the same results. Another good example to look to is the chevy LS2 motors, they basically have a 318 bore size but make un holy amounts of power because they figured out how to move the valves and un-shroud them.

Mathmatically speaking no mater how you arive at the displacement the same PSI will result in the same TQ at the flywheel. Yes a big bore has more surface area to push on but the longer stroke engine has more leverage on the crank and they equal the same thing in the end.



...just food for thought

Dave
Yes, but the long stroke motor makes more torque at lower RPMs. Look at any statistics on long stroke motors, they make gobs of torque down low, usually much more than their over-square counterparts. The disadvantage is that the long stroke motors typically cannot produce as much h.p. as short strokers, without undo piston speed, which translates into excessive heat and wear. If you ride an old BSA or Norton or any long-stroke British bike you can feel it making gobs of torque seemingly right off idle. That's a feeling that I miss in the KLR and other shorter stroke motors, even though they make more h.p.. Longer stroke = more leverage = more torque = more energy that you can feel. It's simple physics and not a myth.

Jason

Last edited by Norton 850; 11-01-2019 at 07:10 PM.
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post #72 of 147 Old 11-01-2019, 07:56 PM Thread Starter
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Originally Posted by DPelletier View Post
.

Mathematically speaking no mater how you arive at the displacement the same PSI will result in the same TQ at the flywheel. Yes a big bore has more surface area to push on but the longer stroke engine has more leverage on the crank and they equal the same thing in the end.[/I]

So, really what you are saying is ... any engine of the same displacement will produce the same power at the same PSI - flow through the head I am assuming - regardless of the stroke. More or less.

So, a short stroke engine, a standard stroke engine, and long stroke engine all of the same displacement will produce the same torque at the flywheel - not wheel?

Just trying to clarify here. Assuming this is correct, then it definitely makes sense to strike an engine to a larger displacement regardless of a short or long stroke method.

The only real difference as I understand it is a short stroke creates its maximum power over a higher RPM range than a long stroke. A long stroke creates its maximum power in a lower RPM range. If this is the case I can see why you reference the flywheel rather than the rear wheel. The flywheel being the torque sink so to speak.

The flywheel weight would need to be increased to hold the torque to be released. While a log stroke could use a lighter flywheel. Both methods would then be able to maintain a similar acceleration curve.



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post #73 of 147 Old 11-01-2019, 08:47 PM
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"There's no replacement for displacement!"

Additional displacement, other things being equal, also results in power-creating higher compression ratio. (The 685 vendors provide thicker base gaskets, in case someone wants to maintain a lower compression ratio after big-bore piston installation). Stock KLR compression ratio remains rather low, in today's terms. Strangely (to me), few KLR hop-up aficionados, if any, have addressed raising compression ratio to get more power. Actually, a few have installed high-compression pistons. (Higher compression may require higher octane gasoline.)

The head can be shaved to raise compression ratio (a slight valve timing change will occur with the consequent change in cam sprocket center location and valve chain geometry (chain length between crankshaft sprocket and camshaft sprocket), but I doubt such valve timing change affects performance significantly (hey; a true-blue thoroughbred hop-up artist will degree his cams anyway (may even have adjustable-angle cam chain sprockets; thus, the valvetrain change from shaving the head will have zero valve timing effect).

Should anyone wish to calculate displacement increase from stroking, the forumla is: (0.78539816338) X (bore diameter squared) X (increase in stroke).

Tip in calculation: Use UNIFORM units, for coherent results. That is, use CENTIMETERS (not millimeters) for results in cubic centimeters, inches for cubic inches. Maybe someone wants answers in cubic millimeters; then--using millimeters consistently is OK!

ďYou better put down that gun. You got two ways to go, put it down or use it. Even if you tie me, youíre gonna be dead.Ē "John Russell" (Paul Newman), Hombre

Last edited by Damocles; 11-01-2019 at 08:51 PM.
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post #74 of 147 Old 11-03-2019, 02:11 AM Thread Starter
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Perhaps Iíve been thinking about this issue of a piston for a long striker modification the wrong way. Iíve been thinking I need to use a piston with the wrist pin located more toward the piston dome, but perhaps thatís not really necessary.

Perhaps, all I really need is to be sure the piston length is short enough that when the piston is at the bottom of the stroke it does not exit the cylinder skirt. Furthermore, it needs to be short enough to not intersect with the connecting rod during its travel.

Does anyone know the diameter and wall thickness of the stock piston and EMís 719cc kit? Along with the wrist pin diameter and location? Maybe I can calculate the rod angles sufficiently to be assured of a safe configuration.


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post #75 of 147 Old 11-03-2019, 09:58 AM
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Perhaps I’ve been thinking about this issue of a piston for a long striker modification the wrong way. I’ve been thinking I need to use a piston with the wrist pin located more toward the piston dome, but perhaps that’s not really necessary.

Perhaps, all I really need is to be sure the piston length is short enough that when the piston is at the bottom of the stroke it does not exit the cylinder skirt. Furthermore, it needs to be short enough to not intersect with the connecting rod during its travel.

Does anyone know the diameter and wall thickness of the stock piston and EM’s 719cc kit? Along with the wrist pin diameter and location? Maybe I can calculate the rod angles sufficiently to be assured of a safe configuration.

The geometry's obviously difficult to grasp, but . . . stock wrist-pin-to-piston-top distance is OK for re-ground stroked crankshafts (given shorter rods).

Stroking by installing a longer connecting rod only? EDIT: [Previous sentence nonsense, disinformation! My apologies. Crank throw length alone determines displacement; rod length irrelevant.] Then, the piston pin location relative to the piston top becomes critical, with stock-length rods, UNLESS . . . a raised DECK is installed to elevate the cylinder head (as in pwestman's Harley example previously posted). EDIT: [The raised deck would accommodate standard-length rods and standard-configuration pistons with a stroked crankshaft.] Oh, yes; Harley push-rods needed lengthening also when the head and its rockers were raised. With stock head elevation, and standard-length rods, the complementary piston's pin location relative to the top of the piston becomes an issue with a stroked crankshaft.

Maybe you mentioned earlier, ILove2; missed it if you did: What bike(s) do you ride today? Just curious!



-----------------

Thanks to DPelletier for his clarification and correction to this post.

ďYou better put down that gun. You got two ways to go, put it down or use it. Even if you tie me, youíre gonna be dead.Ē "John Russell" (Paul Newman), Hombre

Last edited by Damocles; 11-04-2019 at 11:18 AM.
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post #76 of 147 Old 11-03-2019, 03:32 PM Thread Starter
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[quote=Damocles;691303]The geometry's obviously difficult to grasp, but . . . stock wrist-pin-to-piston-top distance is OK for re-ground stroked crankshafts (given shorter rods). /QUOTE]

Yes, itís safe to use a shorter rod if it still remains in the cylinder skirt at its minimum rotation, but the compression would drop as the piston dome would not reach the top of the cylinder. Thatís not quite what I was thinking about.

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Stroking by installing a longer connecting rod only?
No. I still plan to relocate the crankshaft pivot pin. I found out last night that EM stared the stock connecting rod is 138mm from center to center. This is from postings on another forum. EMís was a direct measurement vs. a printed 139mm measurement. Iím not 100% sure itís correct, but it does give me some numbers to crunch.

So, depending on how far a custom rod could be relocated on the crankshaft will determine how long a rod I can install. EM used a modified stock rod which is pressed vertically. Iím going to use a custom rod by Crank Works Inc. that would use a more conventional bolt on lower cap. If I can. Got to talk to them. But, I was hoping I could relocate the crank pin 6mm. If that works it would give a 89mm stroke. So, a new rod should be 150mm from center to center to match the new stroke. Could go more in both measurements possibly. I donít know the limits yet as I donít have a crank to measure. So, itís all guess work at this point.

I asked about piston heights because. Want to keep the piston in the cylinder skirt. If the 719 piston is not as tall as the stock piston then it might fit. Generally, a long stroker moves the wrist pin location closer to the piston dome. The ring pack is condensed I think this relocation is more to increase torque - longer rod = longer leverage. Itís not a requirement though to just increase displacement. Basically, what Iím saying is if I can get it to all fit in the cylinder top and bottom it should work.

EMís kit is/was based on the 685cc kit I believe from references. A 102.5mm/103mm bore and 92mm stroke is 759.15/766.57 cc. If these numbers are correct and I could push the rod length to 156mm, pin relocated 9mm, and a stroke of 92mm then it would be a 796.63cc engine. Numbers are great, but real world is something else. I donít have the parts in front of me to measure. Ugh. Kind of drives me crazy just not knowing what would fit.

Quote:
Originally Posted by Damocles View Post
Maybe you mentioned earlier, ILove2; missed it if you What bike(s) do you ride today? Just curious!

Just the KLR right now. Iíve got room for two motorcycles after I sell the two scooters my wife & I bought for local transportation when we hit the road in our RV, but that didnít quite work out and sheís decided not to ever ride two wheels. So really I only have room for one motorcycle as the other space will go for a Can-Am Spyder - 3 wheels - if sheís ever going to ride with me.

Anyway, one motorcycle to do it all. So, thatís a big part of my motivation to maximize the KLRís potential.



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post #77 of 147 Old 11-03-2019, 05:54 PM
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Quote:
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Stroking by installing a longer connecting rod only?
Just for clarification, you do not change the stroke with a longer piston rod. A longer piston rod decreases the thrust angle.

Jason
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post #78 of 147 Old 11-03-2019, 07:38 PM
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[quote=ILove2Ride2Wheels;691309]
Quote:
Originally Posted by Damocles View Post
The geometry's obviously difficult to grasp, but . . . stock wrist-pin-to-piston-top distance is OK for re-ground stroked crankshafts (given shorter rods). /QUOTE]

Yes, it’s safe to use a shorter rod if it still remains in the cylinder skirt at its minimum rotation, but the compression would drop as the piston dome would not reach the top of the cylinder. That’s not quite what I was thinking about.
Not according to Boyle's Law, or the Wikipedia definition of compression ratio:

-------------------------------------------

In a piston engine, the static compression ratio (C R {\displaystyle CR} CR) is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.

-------------------------------------------

A longer stroke increases the swept volume of the piston. If the combustion chamber volume remains the same, and the piston ascends to the same height (as it will, with shortened rods on a stroked crank), the compression ratio (and maximum cylinder pressure) will be increased, seems to me.
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Last edited by Damocles; 11-03-2019 at 08:30 PM.
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post #79 of 147 Old 11-03-2019, 07:39 PM Thread Starter
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Jason,

My brain is going to mush in this. What I posted previously isnít right. I had a late night thinking on this and it carried over into this morning.

Iíve been focusing on this idea based on research and think Iíve gotten lost on the concept. The original plan - based on research - was a stroker engine uses a relocated pivot pin - outward on the crank - and longer connecting rod to increase displacement. However, I canít see how that could work without also relocating the wrist pin up toward the piston dome as well.

Anyway, long story short, I was thinking I could use a piston with a stock wrist pin location and just change the rod length and pivot pin offset. And then it hit me - this afternoon ... that would just push the piston through the valves. Moral of the story, never drink and think. I was having a cigar and bourbon when this epiphany hit me late last night and I posted earlier today.



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post #80 of 147 Old 11-03-2019, 07:58 PM Thread Starter
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Originally Posted by Damocles View Post

A longer stroke increases the swept volume of the piston. If the combustion chamber remains the same, and the piston ascends to the same height (as it will, with shortened rods on a stroked crank), the compression ratio (and maximum cylinder pressure) will be increased, seems to me.

Okay, you know my mind is going to mush over this concept, but let me see if I have this right because Iíve been thinking way to much about it.

1) Shorted the connecting rod.

2) Increase the pivot pin on the crankshaftís orbit the same amount as you decrease the connecting rod.

3) Stock piston - no change in wrist pin location.

So, at top of the stroke the piston is at the same deck height as a stock configuration. At the bottom of the stroke the piston is whatever change in length you make for the crankshaft pivot point/connecting rod change. So displacement change is Pi/4 x bore squared x (stock stroke + pivot point or rod length change)?

As an example the rod is shortened 6mm. The pivot point is moved outward 6mm. The stock stroke is 83. The stock bore Iím using is 105.

.785 * ((105) * (105)) * (83 + 6) = 770.65

That right?



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