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Proud Infidel
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What concerns me is the op's name is "SD CHOPPERS", which to me, sounds like the name of a shop.

I personally don't think I'd open a shop around building choppers before I learned how to build choppers... part of which is understanding different kinds of motors and builds.
Very few bike builders know engines. They know how to fab tanks/fenders and weld frames. Motors are bought in the crate and bolted in.

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Curmudgeon
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I'm having a stroke after reading all this! ..................................................wait, never mind it was just my left eye twitching from eye strain.
 

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In a maximum effort race engine, the difference of a short rod/long rod can be measured on a dyno, and/or 1/4 mile time slip....street engine?.....not worth it.
 

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Banned
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Here's my $0.02 worth: I believe Gman started to bring some sense to all this when he indicated that there was some confusion in the term 'stroke' as it related to either rod length or crank throw. In reality, the 'stroker' aspect of most any engine is simply a relative commentary about a change in 'stroke' from it's original design to a greater length as related to the piston travel. Someone in the early part of this thread mentioned two different engines, both with the same bore and stroke, but different displacements. I don't believe that's a physical possibility, since the formula for displacement is simply the 'area' of the bore diameter X the stroke. Simple geometry for the volume of any cylinder. The bottom line here is that you simply can't increase the length of a connecting rod to make a 'stroker' out of any engine without compensating in other areas. Consider the early HD TC 88 motor. In it's 'original' form at 88 cubic inches of displacement, you could go to a 95" motor by simply boring the cylinders to, IIRC, 3.875". The original bore was 3.75".....so, a change of 0.10" increased the overall displacement by 7 cubic inches without any change in stroke. HD later came out with the 96 cubic inch motor from the factory. In this motor, they increased the stroke to 4.375 (from 4.0 in the OE TC88). The piston diameter remained 3.75". HD later introduced the 103" motor using 3.875 pistons. BUT....if you tried to use the ORIGINAL TC88 big bore (3.875) pistons to make a 103 out of a 96....you'd be in big trouble!! Why? Because the pistons would bang on the top of the cylinder head due to the longer stroke of the motor. The answer was that HD used pistons that had a raised wrist pin location. This permitted the piston to be at TDC at the top of the increased throw (stroke), and still travel the increased range of the 'stroker' motor. They also had to shorten the skirts of the pistons as well. These pistons are clearly marked as 103" on their tops......so someone doesn't try to use them in a 'stock' 88 motor with a 4" stroke. If you do, they'll come to about 1/8 of the top of the jugs....giving you a compression ratio of about 1.5 to 1. Engine won't run well at that!!......lol

So, as you increase crank throw, you increase rod angle.....and thus need to increase case clearances, etc. Generally speaker, there are 'builders' out there that offer pre-engineered kits for most any motor.....and this is by far the safest way to go. Speaking for myself, I HIGHLY recommend Zipper's Performance Products for stuff like this, as the amount of R&D that this company goes through prior to introducing anything is really amazing. They have a really sophisticated shop, some really skilled people, and decades of experience and knowledge! If you call them, ask for Pete in sales, and tell him Tom told you to call!! Other than that, you're on your own!! Good luck! Tom D.
 

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Rod length and piston pin location have to be worked together in order to get correct compression and good piston strength for longevity. On a street motor, you generally don't want the piston pin up in the rings, so that will limit how long of a rod that you can use. A shorter rod would force the pin location lower in the piston skirt, increasing rod angularity and increasing piston rocking and shortening piston skirt and cylinder bore life.
 

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Asylum Inmate
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The overall governing specification is piston speed. That is it. It all has to be done to manage piston speed. All the other variables affect performance sure, but the constraint is always piston speed.

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Barney Fife - Poser
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The overall governing specification is piston speed. That is it. It all has to be done to manage piston speed. All the other variables affect performance sure, but the constraint is always piston speed.

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True, but piston speed can be limited by engine RPM. That's why those short-stroke rice burners can rev up to 15,000 without burning up.
 

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Asylum Inmate
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;7329167 said:
True, but piston speed can be limited by engine RPM. That's why those short-stroke rice burners can rev up to 15,000 without burning up.
Yep, because the stroke length is damn near Zip!

Keep it under 4000 FPM or replace the engine a lot!



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Very interesting to learn the nomenclature, thank you! I thought that a stroker could only increase on the down stroke, because of more limitations with the heads, but that can be remedied as well...?
 

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Very interesting to learn the nomenclature, thank you! I thought that a stroker could only increase on the down stroke, because of more limitations with the heads, but that can be remedied as well...?
Its a limitation of where the heads sit. Long stroke motors can overcome this limitation by using longer cylinders as one possible remedy, frame constraints not withstanding.
 

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Its a limitation of where the heads sit. Long stroke motors can overcome this limitation by using longer cylinders as one possible remedy, frame constraints not withstanding.
It's common to just raise the piston pin in the piston. A shorter rod would also work. It was common, so I was lead to believe, for sportsters to have a spacer installed under the cylinders.
 

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It's common to just raise the piston pin in the piston. A shorter rod would also work. It was common, so I was lead to believe, for sportsters to have a spacer installed under the cylinders.
Right, but you can only raise the wrist pin so far before it obstructs the rings. To go further you need either longer cylinders or shorter connecting rod.

You can only shorten connecting rod so much before piston skirt runs into flywheels.
 

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Right, but you can only raise the wrist pin so far before it obstructs the rings. To go further you need either longer cylinders or shorter connecting rod.

You can only shorten connecting rod so much before piston skirt runs into flywheels.
That's why you don't want to try this at home :devil
 

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Stroker plates are a thing of the very distant past.
S&S Ironhead stroker pistons, use an aluminum button which bisects the oil ring pack, as the C/L of the wrist pin location requires that.
 

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i feel alot of people have gotten confused over the rod length discussion

rod length has very little to do with the stroke of an engine, someone did touch on the fact that it does effect the angle of the rod when at its 90 and 270 degree locations

the longer rod ratios will keep the con rod straighter along with moving slower at tdc thus allowing ignition timing to be altered and providing more time for the force of combustion to act on the piston and crank, also longer rod ratios move faster at bdc, so cam timing and mainly overlap requirements are different compared to short rod ratio engines

in a short rod ratio engine the piston moves through tdc faster, more ignition advance is needed to get a complete burn/use of the combustion stroke, also overlap can be increased because there is a harder draw on the intake ports, shorter ratios also move slower at bdc

at the end of the day most do not have the know how or resources to change rod length or utilize its function, most are stuck with whatever manufacturers have set out for them

most common rod ratios fall between 1.6:1 and 1.8:1

2 identical motors say 1 is at the small end of the ratio and 1 being at the long end, will ideally like 2 different heads/ports and 2 different cams, and because of this will produce 2 different power curves
 
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