I'd like to replace the original 4 speed in my 89 1200XLH with a 5 speed. Are there any original HD transmissions that will 'bolt on'?

 

No, it won't even fit, none.

 

Yeh ifn I could I'd make my '80 XLH a 5 speed but.....have to make do with 4.

 

If you could do this, what would be accomplished? Is there some performance advantage?

 

Early 5-speeds had the same overall gearing when in top gear, the 'extra' ratio gave a set of closer ratios between first and top. So there was an advantage while accelerating in having smaller drops in revs between each upward change.

 

All XL models use a 1:1 ratio in high gear. If you have a four speed bike that started life as an 883, it has a close ratio main drive setup that easily outperforms the five speed because when the five speed is coasting for 10-12 feet during that extra shift, the four speed is still accelerating hard. The percentage of gear drop is actually pretty friendly when you do your math and get your overall gearing setup correctly. The 1200 uses a wider ratio main drive setup that does cause some percentage of gear drop issues, but the close ratio 883 setup fits right in. The real issue with the 4 speed is that it has a lot of shaft flex that tends to cause breakage. I shot peened and back cut my OE gearing in my 1990 883 dragbike, then installed the Zipper's billet access door, the parts have been in service since 1990 with no issues. Same with the old four speed Big Twin trans, it's a cluncky old thing, but with proper setup you are holding the throttle wide open when the 5 and six speeds are stirring the gearbox. Harley's have a super flat torque curve and really don't need to have as many gears as an F1 car. Six speed trans are mostly a "keeping up with the Jones" thing. In fact, the 06-later big twin trans is a cheaply made POS, but it's been great for marketing.