Compression Ratios III
TAKING AIM, by Chris Maida, Editor
Moderately boosting the compression, like going from 9:1 to 10:1 or 10.25:1, shouldn’t run you afoul of engine knock in a big way. However, a high-compression engine can be unforgiving and difficult to tune, especially a Twin Cam, if you go past 10.5:1. Even stock H-D engines have bouts with knock due to the very lean fuel/air mixture needed to meet EPA regulations. Thankfully, there are ways to prevent engine knock and an easy one is to not let the engine overheat. In case you haven’t noticed, a lean running engine will rattle and ping more often on a hot day, especially in heavy traffic. The combination of a very lean mixture and high engine operating temperatures aggravates the situation and gives engine knock an open invitation to trash your engine.
The method The Motor Company uses in its 2000 and later Softails, and 2002 and later rubber-mount models, is to mess with the ignition timing. All Delphi EFI control modules have a ping sensor, which detects whenever the Big K makes an appearance via a process called ion sensing. If the control module detects engine knock, it retards the ignition timing (moves it to a less aggressive setting) until the knocking stops. In fact, if the engine is not set up correctly for a high compression ratio, this system will retard the timing to the point of engine power loss, which, of course, defeats the purpose of having a high-compression engine in the first place. The fix for this is to have the module remapped for a high-compression engine.
Those with carburetor-equipped bikes can install a fully adjustable, single-fire ignition. This allows you to dial in the initial ignition timing and advance curve that’ll keep the combustion bogeyman away. The goal is to use the most aggressive advance curve possible, while still avoiding knock. Once correctly dialed-in, you’ll get the most power from your engine, while also protecting it from damage.
A word also needs to be said about riding style. Whacking open the throttle when the engine’s rpm is below its powerband will make even a properly tuned engine knock and ping. Down-shifting is the simple fix here.
Another way to eliminate knock is to use long-duration cams, which are camshafts with a lot of valve overlap. Valve overlap is when both the intake and exhaust valves are open briefly at the same time. This allows some of the engine’s compression to bleed off at low rpm, which is where engine knock always occurs. In fact, running the correct set of long-duration cams with a set of high-compression pistons will give you a nice gain in power. Be sure to talk with the cam manufacturer before buying to make sure you get the correct grind for your engine, bike, and riding style. Taking valve overlap too far, and ignoring other cam profile factors, can kill performance.
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