Discussion in 'Harley Davidson Extreme Modification' started by 1Okie, Oct 5, 2012.
looking for any info or insight when building a 103 out of a 88
Gotta tell you...
An 88" TC and a 95" TC have the same Bore size of the 96" and the 103"
Shortened piston skirts and the wrist pin hole is moved up so These Special Pistons ar needed to use the same cylinders as the Old TC 88/95 do.
The change is the Stroke that makes the difference...:s
From 4" stroke on a 88" cu in motor to a 4.375" stroke = the 96" Newer TC of today.
from 4" stroke on the 95" cu in motor to a 4.375" stroke = the 103"
SO What are you asking here
Your response prompts a couple of questions:
How square is the factory 88" engine (bore vs. stroke)?
What is the benefit of increasing bore over stroke?
What is the benefit of increasing stroke over bore?
I seem to recall that the best possible combo is bore and stoke are the same. Please excuse my ignorance.
Aren't you an Engineer?
I think you should know the answers here.
The Sq of a motor is a good thing... HD came close in the first TC motors, shorting the stroke from 4.25" in the evo to 4" in the TC of 99 and increasing the bore closer to the 4" mark...
Thumpers with Long strokes had a good amount of Torque at slow turning RPM's + heavy flywheel, but the Sq. motors were well blanced at 4x4...SMooooooth at higher turning RPM's. I've been told. (hear-say)
Now that is about all I know and I May be Wrong.:small3d031:
You figure and post Your results. Be interesting to read. Thanks.
Square motors tend to have a longer life over strokers in most cases. Strokers tend to put more side loading on the piston skirts hence the longer stroke and more flexing and more low end torque. Strokers have been around a long time and if rode correctly wil pull good but not last as long as a square motor JMO
Found some Good reading to understand the differences::s
Stroke ratio | Ask.com Encyclopedia
This does explain the Sq. also under and over Sq.
I am indeed an engineer, but I write software and design electronic gizmos. I am relatively defenseless regarding engines. I did remember a hint of what you linked to, which was quite good. I thought some consideration in that direction might contribute something to the mix.
Stands to me that to simply increase one without the other (bore vs. stroke) you are changing the criteria determined to be important by the original twinkie designers. I think your link contributed nicely to the conversation. I don't have time to muscle up an engine, but think it would be kind of fun. I don't think I would do it to my primary scooter, since reliability and long life are nice goals there. It might be nice to see what could be done to a 2nd scoot, with respect to optimizing performance without building something that might beat itself to death.
You can bore to 98", and save from splitting the cases, to replace crank.
But IF going to a crank replacement, you can then, as well, bore your cylinders to the same dimension as the 98"s, and have a 107"er.
In tame form, you can easily see 110/120.
A great combination to upgrade an 88" is an "over square" 107" motor. A 4.125" bore x 4.00" stroke, with about 10.5 static, a decent set of cams with about .600" lift, good heads and the right pipe and you got yourself a torquey, dependable motor that is a hoot to ride. Of course, there are those that say once you start tearing into to lower unit, why stop at 107", why not go bigger but the over square configuration is strong and dependable and you really don't need more; it's a great setup for a touring model.
But if the OP has his heart set on a 4 3/8" arm, then at that point, a set of 4 1/8" cylinders on top of that will yield a 117"er.
That, my friends, is a HOOT to ride!!
Those in very tame form, will push off 130/130, and high-output versions, can bang on the 150 hp mark.
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