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Anybody build a short-stroke Sportster?

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Anybody build a short-stroke Sportster?

Postby wjnielsen » Thu Sep 06, 2012 1:26 am

Now, I know there's probably a few guys getting ready to type some variation of "why on God's Green Earth would ya' wanna' do that!?"

Well, I was looking at the records for bikes with 650cc pushrod engines at Bonneville... was thinking a de-stroked Harley Sportster might be pretty competitive against those records.

I'm thinking there's a couple of ways of going about it: a stock (for 883) 3.00" bore with a set of flywheels for 2.79" stroke (basically an inch less than stock), or a set of 3.00" XR750 flywheels, and a set of barrels sleeved down to 2.90".

Of course, XR750 stuff doesn't grow on trees, and if you can find it, you'll need a bigger wheelbarrow of $20 bills than I've got.

Either way means having to buy long (and I mean looooooong) rods - the stock 883 rod is 6.926" - or short barrels. And if the barrels are shortened, how is a guy gonna' get an intake manifold between those heads? So, I'm thinking a set of the Ironhead rods (about a half inch longer at 7.44") would be the way to go.

So, anybody ever been involved in a small stroke XL? Have some experience where pitfalls may lay? Or have a crate of XR750 engines they'd like to donate to a guy in Seattle?

Thanks in advance!

-Bill
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Re: Anybody build a short-stroke Sportster?

Postby Belgian1979 » Thu Sep 06, 2012 3:51 am

The reason may lie in using a much longer rod that normal. Don't know anything about Harleys but there is much debate over the pros and cons.
They seem to be able to rev higher. If you can combine that with a shorter gearing it might be worth while if you can keep the engine in its power band and rev higher to attain the same top speed.
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Re: Anybody build a short-stroke Sportster?

Postby dezza » Thu Sep 06, 2012 6:18 am

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Re: Anybody build a short-stroke Sportster?

Postby wjnielsen » Thu Sep 06, 2012 9:13 am

Thanks for the link; an interesting read.

And it makes sense... when the guys with big budgets want to go fast, the winning combination generally seems to be bigger and bigger bores (unless there are rule constraints, such as bore spacing, etc.). Look at the bore/stroke of F1 engines.

But bringing all this down out of the stratosphere, to where I can get my grubby little hands around it... my main considerations are that I'm probably stuck with the deck height (give or take a bit), and what can be obtained for flywheels. Custom lengths in rods and pistons are available, but as the cylinder gets shorter, the heads end up being closer together. While this isn't a real consideration on a 90° V8 car engine, there isn't a lot of room on the 45° Sportster. All the cool stuff in the world doesn't count for a hill of beans if the intake faces of both heads are crammed tight together, and I can't get some sort of intake manifold squeezed between 'em!

Obviously, I understand that ports can be moved. But there is a lot of difference between 'fudging' something 1/8" by grinding and milling and a little epoxy, and 'moving' something a half inch or so. One of these is a lot harder for the guy who's working on the engine in his living room!

The 750 race bikes used different heads that didn't pair the cylinders into a common intake; don't ya' think I wish a set of that stuff would become available at a price I could afford. [-o< There's a set on eBay with a $5000 buy-it-now price. Somehow, I suspect I'll have to settle for Buell or Sportster castings. Which means I'll probably have a stock deck height, or close to it.

Now, the stock 3.500" bore of the 1200cc variant would mean I'd have a stroke of 2.06" or less to hit the 650cc limit. The aftermarket cylinders with 3.81" bores would push the stroke down to 1.739". Now, let's pretend that someone can make flywheels (there'd be almost no meat between the holes for the pins in the flywheels); we've gone from a 3.8XX stroke to a 1.7XX stroke - that connecting rod is going to probably grow at least an inch. Doesn't that get ridiculous at some point?

Anyway, I do appreciate the comments and ideas. If there's going to be some real issue with doing this, I'd rather I found it out on an internet thread, than by spending money on the hard parts and going, "gee, I see this can't EVER fit together".

-Bill
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Re: Anybody build a short-stroke Sportster?

Postby dezza » Thu Sep 06, 2012 4:26 pm

can the heads do a 180 ?
that would be different
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Re: Anybody build a short-stroke Sportster?

Postby dezza » Thu Sep 06, 2012 6:33 pm

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Re: Anybody build a short-stroke Sportster?

Postby Keith Morganstein » Thu Sep 06, 2012 7:24 pm

wjnielsen wrote: there isn't a lot of room on the 45° Sportster. All the cool stuff in the world doesn't count for a hill of beans if the intake faces of both heads are crammed tight together, and I can't get some sort of intake manifold squeezed between 'em!



Use two front cylinder heads.
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Re: Anybody build a short-stroke Sportster?

Postby DrillDawg » Thu Sep 06, 2012 8:15 pm

Make a single cyl 650 by removing the rear cyl/rod/ect and going big bore.
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Re: Anybody build a short-stroke Sportster?

Postby 63bobby » Thu Sep 06, 2012 8:24 pm

Xr750 bore is 3.125 x 2.7 stoke . To many problems for a 650 . Unless you did a single cylinder 650cc like the blast that 500cc just buy a S&S cylinder 3.625 bore and rebalnce .
Check CC on that combo I use Cubic inchs I dont know the stroke on the blast but the bore is 3.5 Something to think about . A record is a record !!!
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Re: Anybody build a short-stroke Sportster?

Postby panic » Thu Sep 06, 2012 10:10 pm

Based on how the factory dealt with this same thought (the 1970-71 waffle iron), remember to set aside an extra $100,000 and 2 years for R&D.
In the end, they started over with all new parts.

BTW: 2 fronts makes the front cylinder intake crash the rear cylinder exhaust. Using 2 fronts with the intakes rotated to the left is better.
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