Technical Articles

Chain-Drive Cam Conversion for Twin Cam Harleys—Part II

THE STORY thus far: Serco in Capalaba fitted a Power Commander to our stock (apart from the Screamin’ Eagle air filter and mufflers) Twin Cam which made a hell of a difference, especially mid-range (see Issue #295).

Cyclecraft in Bondi then fitted a Screamin’ Eagle big bore kit which raised the compression from 8.8:1 to 9.4:1.

The bike was now taken the Motorcycle Weaponry in Mona Vale to have the Power Commander re-mapped. We were expecting a boost in performance but—shock, horror—the opposite occurred—the engine was pinging so badly we had to retard the ignition and we found that a low-compression motor with full advance runs far better than a high-compression motor that’s been retarded. Damn! However, although the power had dropped from 78 to 73 hp, the torque was up from 75.5 to 82 ft/lb.
The problem lay in the mismatch between the compression ratio and the original cams. In general, higher compression ratios need longer duration cams.

Richard Nicholls at Redgrave Motorcycles in Hornsby was enlisted to change the cams (see Issue #310). He suggested that since we were pulling the cams out, we might as well replace the original chain-drive set-up with the more reliable S&S gear-drive cam kit. H-D introduced the chain-drive in the Twin Cam to reduce engine noise (all previous Harleys used gear-driven cams) so we asked ourselves, how much engine noise would the new set-up introduce?

In deciding which cams to use, we relied on the chart in the S&S catalogue. The chart cross-references compression ratios with cam durations and pointed us to the 510G.

Because we didn’t want to pull the heads off again, the pushrods were removed with a set of bolt cutters and replaced with a set of Andrews EZ-install pushrods.

Within a couple of hours the Harley was running again and our first question: How noisy was the gear-drive cam kit? Apparently, as the lifter in an engine passes through maximum cam lift, the forces on the cam tooth change direction will results in an audible ‘click’ as the backlash moves from one side of the tooth to the other. Yes, the new cams do rattle at idle in our Harley, but it goes away as the revs increase, and it’s a healthy sound and no worse than an Evo. We can live with that.

The Harley was now off to Motorcycle Weaponry to be re-mapped.

Motorcycle Weaponry have a Dynojet dyno which can be loaded up to hold the bike at any revs at any throttle setting. This way the Dynojet computer can adjust the mixture at all possible combinations without going on the road. Roger, the man in charge of tuning, rolled the Harley onto the dyno, shoved an exhaust gas analyzer up its exhaust, the Power Commander and exhaust gas analyzer were connected to the Dynojet computer, and he started producing the custom maps that would ensure the Harley was producing its maximum power.

Within an hour the results were in: We had gained an extra 7 hp at peak revs. Mind you, that’s only 2 hp above our original figure and you’d have to ask yourself why bother. But the real gains were in the torque which had gone from 75.5 to 93.6 ft/lb—that’s a massive 18 ft/lb increase mid-range where you need it most.

Back on the road, the Harley was exciting to ride. It was producing its peak torque between 3000 and 4000 rpm which is where I like to ride it. Even down to 2000, she pulls away cleanly without bother. But it’s when you open the throttle that you feel the big difference—this pony hauls arse!

Next issue we’ll be back at Motorcycle Weaponry to compare the stock Harley exhaust with a true dual system. Stay tuned…

story & photos by Skol

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