The pathways to more power are to make an engine bigger or make it more efficient…or both. The Evolution Big Twin was only ever an 80-inch (81.77-inch, actually) engine. The Motor Company would never make that mistake again, because more efficiency was (pun intended) an evolutionary cul de sac.

At the time, the lucrative power game was in the hands of the aftermarket. The factory simply didn’t have resources to do much themselves, but looked on closely while lamenting having no immediate way to play.

What the aftermarket of that era could whip out in weeks, the Motor Company had to slowly develop, test, refine and test some more to ensure that its answers to the power questions were the right ones in the long run.

Here it is, before the loopholes closed. An early Screamin’ Eagle catalog (a 20-page pamphlet really) from 1993. Mostly Sportster goodies for a one-make racing class at the time, it presaged what was to come when the floodgates of factory hot rod parts finally opened all the way. Up until then, “off road” (illegal) performance stuff was in the main accessory catalogs every year.

The Evo (and Sportster and ’Shovel before it) always had (ahem!) aftermarket ‘mufflers’ available to free up some stifled ponies. Almost without fail you could stick a stick right through ’em, and of course they were noisy. Even back then the Guv’ment frowned on that kinda thing, so the work-around to get manufacturers off the hook was to proclaim the things (tongue firmly in cheek) as being for ‘off-road competition use’ only. Ha!

Anyway, the ploy worked well enough and long enough to let both aftermarket and factory build big profit centers purveying pipes using the proven “off-road” label. Expanded to include practically any part that wasn’t noise and/or emissions legal, the list included items from air cleaners to cams to heads and back; anything around or in a V-Twin engine that made it more efficient was thusly coded that way. Meanwhile, the EPA (and CARB, or the California Air Resources Board) were busy sticking their noses up automobile pipes. Meaning, even the few rules on the books were lacking enforcement. The whole deal lasted nearly into the 21st century, just like the Evo Big Twin. 

Good thing, because…a couple years after the Evo Big Twin arrived, the Evo Sportster showed up in two displacements. The smaller one was part tradition (since Sportys had seemingly always been 900s) and part insurance-beater. Any swab, grunt or working class hero could handle the insurance rates for an “under 1000cc” motorcycle, whereas, often as not, the target demographic for an 1100/1200cc machine was shocked to hear that insurance could cost more than the payments on the bike itself. Shrewd types figured out real quick that you could buy an 883, bump it to 1200, not tell the insurance company and cover the costs of the conversion with a couple years worth of cheaper insurance rates. 

A quarter century later the “Screamin’ Eagle Performance Parts” catalog is a tome of five times that many pages. The M8 stage kits within are legal. We’ve come a long way in a big way!

News trickled back to the factory that lots of folks liked having choices like that, and soon enough the concept of selling power as added displacement rather than improved efficiency was off and running. As were all the goodies ancillary to that approach, things like beefed-up clutches, stronger belts, etc. 

Meanwhile, the EPA/CARB consortium was done with cars and was now focusing on…us! Wait, that’s not exactly right. They actually gave up on us, incorrigible as we are, and instead cracked down on OE manufacturers, because they are easier to control.  

This chart of a ‘legal’ stage IV kit for a 117-inch Milwaukee Eight engine indicates the same levels of efficiency, four valve heads and all. But, peak power means damn little on the street where off-the-bottom torque is king. There’s really no substitute for cubic inches! By going big, the M8 (and extra-large Twin Cams, for that matter) would simply crush an Evo in a roll-on. That is real ‘truth to power!’

Only Harley-Davidson could afford to get cams, heads, big-inch kits, exhausts and a whole lot more certified as legal under these ever-tighter rules. A precious few outfits like S&S and Vance & Hines could manage something close. Crucially, however, they couldn’t get dealers to install, finance and warranty their stuff in new machines. So Harley-Davidson cleverly turned the tables on the whole performance parts thing. 

In other words, after years of playing catch up to the aftermarket in the high-performance race, H-D brought forth in all its glory and with every Screamin’ Eagle option they could think of, the expandable Twin Cam 88. (Designed from scratch, as everyone knew from the outset, as a 120-incher.) Fewer of the hot rod parts the factory offered were of the ilk formerly known as ‘off road use only’ but lots more were perfectly legit. And that’s when Harley really began to win the horsepower wars, with the money, development time and strategy of having bigger and better performance available at the same time as the motorcycles. It was pretty much game over at that point. Come the introduction of the M8, even more so! After a slow start, suddenly it was game, set and match to The Motor Company.

Of note is the power-per-inch of the 80-inch Evo with its “off-road” stage IV kit (seen here). It still looks pretty good at over one pony per cubic inch. It was done by improving efficiency.

Still, one question remains. Since Harley-Davidson has certified legal and warranteed performance “stage” kits for M8 and Sportster models, why aren’t they delivered as “standard equipment” (or even factory optional) on anything? I’ll get back to you on that one. 


  1. The Big Twin Evolution engine is actually 81.686 cubic inches of displacement, or 1338.33 cubic centimeters, with its 3.498 bore and 4.25 stroke. But the real question is, why did the factory under advertise the cubic inches to 80 by nearly two inches, and over advertise the cubic centimeters by near two to 1340?


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