V-twin Tech: Harley’s Hybrid Cam Kit
May 4, 2009
Filed under Uncategorized
Harley-Davidson introduced the Twin Cam TC88 engine almost 10 years ago. There have been hundreds of thousands of them made and they have an excellent reputation for performance and reliability. Its replacement, the TC96, is at least equally capable.
The Evolution engine, which the TC88 replaced, featured a series of fairly small but useful improvements over the earlier Shovelhead design, hence its name.
However, Twin Cam engine design changes were more like a generational leap than just overdue improvements. The TC88 motor was a much better powerplant than anything Harley had built before. It did, though, have a teething problem with its cam-support bearings.
While Shovelhead and Evolution Big Twin engines featured a single gear-driven camshaft, the TC88 has two cams driven by a pair of chains. The two cams allow a more compact engine, compact enough to allow for a crank-driven oil pump mounted between the cams and crankshaft. They also greatly improve the load path between the cam lobes and rocker arms.
Harley chose chain drive for at least two reasons: one, they run quietly and, two, they do not require the tight and expensive tolerances of gear drive. Most of the rattle of aftermarket Evo cams is due to loose cam gears, not cam-lobe design. I have a collection of about 20 Evo Big Twin cam-drive gears of various pitch diameters so I can get the whine out of a cam installation. At one time Harley-Davidson had to build a $400,000 cam-gear fitting room so they could build Sportsters quiet enough for Uncle Sam’s minions.
Unfortunately, some early TC88 engines suffered cam-bearing failures. News of the failures spread quickly among the many thousands of Harley enthusiasts. Harley quickly traced those failures first to manufacturing and then to assembly errors. Only a small percentage of engines carried these errors to failure. Still, the new engine’s reputation suffered for awhile; doubt had been cast. It was like politics; more noise than information. The failures measured in the hundreds, out of tens of thousands of trouble-free engines.
Harley quickly corrected the manufacturing-tolerance errors and made design changes that virtually eliminated the chance of installation damage to the bearings. Hundreds of thousands of these engines have been on the road for the better part of 10 years. Cam-bearing failure is not a common problem.
However, the spring-loaded cam-chain tensioner shoes do wear more quickly than other engine parts. Some owners have reported that the shoes need replacement at 25,000-mile intervals. A couple of Harley dealers I contacted gave less dire predictions.
Bartels’ Harley-Davidson recommends replacement at around 45,000 miles. They have had to replace shoes as early as 30,000 and as late as 60,000 miles. Tilley Harley-Davidson recommends changing the shoes between 45,000 and 50,000 miles and before 60,000. Ventura Harley-Davidson did not have a definite mileage estimate but said that shoe replacement is neither an issue with nor a common request from their customers.
Outright parked-on-the-side-of-the-road failure is rare. However, such failure can completely destroy every bearing in an engine. It is important that you keep track of shoe wear. Many and probably most aftermarket cams increase the wear rate.
The Motor Company has now addressed this small but important wear issue with a kit that essentially upgrades the TC88 cam-drive system to the TC96’s. The SE Hybrid Cam Plate and High Flow Oil Pump Kit (Part No. 25284-08) replaces almost everything inside the right engine-case cover except the cams themselves. The drive chains are now roller instead of link plate and the chain tensioners use oil instead of spring pressure. The kit’s larger oil pump is necessary to supply the additional flow requirements. (See page 60 for more information on this kit). This kit, however, is not the only option you might consider.
Almost as soon as the TC88 hit the street S&S Cycle, together with Andrews Products, had a gear-drive conversion for the TC88. No chains, tensioners, springs and so on. Just four gears; two for the cams, one for the crankshaft and one in between. Simple. Strong. Reliable. No plastic parts. They did what I like to think Harley would have done if Harley could have. Noise was, of course, the main reason they did not.
The S&S/Andrews Twin Cam gear-drive system does introduce some small amount of whine. A bagger rider behind a fairing, and with relatively quiet mufflers might notice it. Most of us would not, at least after the first couple of days. The ones I have heard are very much quieter than a set of Screamin’ Eagle mufflers.
As you may suspect, I prefer a geared cam drive over chains, especially when combined with aggressive cams. The S&S gear-drive system can be used with stock cams. However, few who install the gear drive are likely to stick with stock cams. This fact leads me to offer a caution.
The single most common mistake I have witnessed over the years is incorrect cam choice. No other element of engine performance is so poorly understood by so many. Almost all the aftermarket cams sold have too much total duration and, most important, they close the intake valves much too soon. The common result is an engine that must often exceed 3,500 rpm to run well.
Virtually all of us want and need useful power from about 2,500 rpm to an occasional 5,000. If you choose to install an aftermarket cam and want to be in this power range, pick one that closes the intake valves as near 30 degrees as you can find. For every additional 5 degrees of intake closing angle, you can add about 500 rpm to the start of good, responsive and useful power. Thirty degrees will get you 2,500; 35, 3,000; 40, 3,500 and so on.
The Andrews 21G grind is the one I’d use. The intake closing angle and success of this cam go back to the original Andrews EV13, the prototype of which I have in a box in my garage. There are some 100-horsepower-plus 95-inch Twin Cam engines out there that use this cam or one similar to it. Don’t let the short duration numbers mislead; it ain’t that simple.
I gathered total installation costs from two of the dealers mentioned above. The numbers are rough but reasonably close to what you might expect at a Harley dealer. They are of course offered for comparison purposes.
• Tensioner shoe replacement:
$400 to $500
• Hybrid Kit installation: $900
• Gear-drive kit: $900 or more
• Gear drive with cams: $1,200