Hopping Up An 883: Part 3
Article And Photos By: S&S Cycles
If you have been following this series of articles about hopping up a Harley- Davidson® 883 Sportster® motorcycle, you’ll recall that we’ve already installed an S&S Stealth air cleaner and a set of S&S performance mufflers in the first article, an S&S 1200cc conversion kit in the second article, and a set of S&S 482 boltin cams in the third. We’ve taken this bike from a 45 hp weakling to a butt-kicking 71 hp wild hog. We’ll, we’re not done yet. As much fun as the bike is to ride now, we’ve got more. Too much is almost enough, right? In this article, we’re going to put together the biggest, baddest package that you can bolt together without doing any machining (you don’t have to machine anything anyway). We’re going to install an S&S 1250cc conversion kit to add even more displacement. We’re also going to CNC port the stock 883 cylinder heads, and get rid of those small stock valves. Let this thing breathe! To take full advantage of those high flowing new ports and bigger valves, we’re also going to install a set of S&S 600 cams.
At this point, we should mention that, normally, the 1250cc conversion kit would be chosen as an alternative to the 1200cc conversion kit we bolted on in the second step. While it is possible to bore 1200 cylinders for the 1250cc pistons, we figure this package is for the die-hard power junkie, and he or she will just go for the gusto and go big right away. It gets a little weird at this point because riders with stock 1200cc engines can also bolt this kit on. They can use their stock 1200 cylinder heads on the 1250cc kit, but stock 883 heads can’t be used …or shouldn’t due to the high compression ratio you’d end up with. Besides, what’s the point of bolting on the biggest cylinder kit you can get for stock crankcases and trying to suck air through those little 883 valves? So we make it simple and just say “Don’t use 883 heads unless you have them CNC ported”. Believe me, you’ll thank us. The 1250cc conversion kit has the same cylinder spigot diameter as a stock 883 or 1200 cylinder so they slip right into the stock crankcases of either engine. The stroke is the same in all Sportster® bikes. In fact, a replacement flywheel assembly for 883cc and 1200cc engines is the same part number in the stock parts book. That also answers the next question you were probably about to ask. No, you don’t need to rebalance your flywheels regardless if your engine was originally an 883 or a 1200.
The CNC porting done here at S&S will bring the chamber volume in 883 heads up to 60cc, which is the same chamber volume as a set of stock 1200 heads. That keeps the compression ratio to a manageable 10.3:1 with the flat topped piston or 11.2:1 with the pop-up dome piston. For this article, we went with the 10.3:1 compression ratio. Since we plan on using the S&S 600 cams, we opted for the S&S .650” lift valve springs as part of the CNC porting package. Assembling the 1250cc conversion kit was just like assembling the 1200cc conversion kit, which is to say just like a stock rebuild. It’s just that when we were done, we expected to be making close to 100 hp. It’s a good thing that we updated the clutch in the second article when we installed the 1200cc conversion kit.
As in the other three installations, we loaded a free EFI tune file from the S&S website with a DynoJet® Power Vision® tuner before we even fired the engine. It took under five minutes. When Ryan came back to Viola for a test ride, it was pretty plain that he figured it couldn’t get much better than the last ride, but he found out pretty quickly that it could, and it did! The dyno showed that this engine was now making over 95 horsepower. That’s 24 more than it did in the last test with the 1200cc conversion kit. Rest assured, a difference of 24 hp will register on the sphincter dyno (seat of the pants). Taking the big view, we have more than doubled the horsepower of the stock 883 engine. All we need now is a note pad to take names, because this bike is certainly kicking butts.
Here’s a comparison of the stock 883 piston that was originally in this engine and the piston in the S&S 1250cc conversion kit. Aside from being clean, the S&S piston is quite a bit larger. Stock 883 pistons are 3” in diameter, and the 1250 piston is 3-9/16”. Bigger is always better. Right?… Right!
Our technician decided to install the S&S 600 cams before installing the top end. It makes it easier to turn the crankshaft to line up the marks on the pinion gear and the #2 cam. The high lift cams are essential to take advantage of the increased flow of the CNC machined ports, and to make the big horsepower numbers.
Here’s the big 3-9/16” bore piston from the S&S 1250cc conversion kit being installed on the connecting rod. The pistons we used are the flat topped version for about 10.3:1 compression ratio. There’s also a 5.5cc pop-up dome version that will result in 11.2:1 for those who want a little extra squeeze.
The spigot diameter of the S&S 1250cc conversion kit cylinders is the same as stock 883cc and 1200cc cylinders, so the new cylinders slid right into the crankcases without any machining or clearancing. No rebalancing either! Truly a boltin kit. See Dyno Chart Top Left Is this the same bike? Yes, but with a lot more power. The DynoJet® 250i shows that the S&S 1250cc conversion kit, 600 cams, Stealth air cleaner, and slip-on mufflers added 50.3 horsepower to an engine that originally put out 45.4 horsepower. That’s over twice the power. It’s worth mentioning that if we hadn’t cranked up the rev limit to 6750 rpm, we’d have only made about 90 horsepower instead of 95. That’s still pretty respectable, but who couldn’t use 5 more horsepower?
