 Techline-Sept.08-Part V: Cam specs and choices
Part one of this story
Let’s finish discussing cam characteristics and then take a close look at Bob Wood’s Knight Prowler belt-driven cam setup.
Radical Cam Profiles
Over the years, I’ve been critical of radical cam profiles in regards to engine longevity, since they cause the rapid wear of other valvetrain components. I think it’s time to qualify my longstanding belief, which, quite frankly, evolved from my Shovelhead and Panhead days. The other principle I should qualify is that I am a proponent of reliable horsepower and torque gains, believing that many times engine efficiency and performance can go hand in hand.
I’ve watched a multitude of hemi-head engines — by that I mean hemispherical combustion chamber engines as found in Knucks, Pans, Shovels, and Ironhead Sportsters — demo or wear out right before my eyes. That statement is dramatic, and, of course, there is a sliding scale to its veracity directly proportional to the conservativeness or radical extremity of the performance options and parts used with these particular engines. Premature wear or parts and engine destruction made life difficult for me in the early years. I would say, “If we do that, your engine longevity will suffer, although the bike will be much faster.” The customer would say, “No problem, Don, make her real fast!” Foolishly, I would follow his instructions. “Hey!” The customer would loudly complain later. “Rebuild my engine for free; it wore out too fast.” Then the bad mouthing about my mechanical abilities would follow. I like to go into work every day. It is my hobby as much as my vocation. I began to shy away from anything that caused me grief.
So, why did premature wear happen? There are a few reasons. One is that the compression in the hemi heads, though low by today’s standards, was too high for the quality of the parts and technology of the day, resulting in detonation, the great engine destroyer. Extreme valve angles and pushrod angularity are other design features of those yesteryear engines that are intolerant of short-duration, high-lift cam profiles. These engines need longer duration cam lobes to ensure the lifters can follow them without flying off in a momentum-versus-inertia-driven frenzy that promotes valve float, another great engine destroyer.
Then along came the Evolution engine, which is much faster and more reliable. This is because, relative to what went before, the Evo head design can tolerate higher compression ratios while controlling detonation. Electronic ignitions, fed by multi-sensor information, began to modify ignition timing in a more exact and sympathetic manner than the weak, spring-loaded, mechanical points ignition systems used on earlier engine designs. These ignitions also allow more, but not enough, control over detonation. The biggie with the Evo was the reduction in valve and valve guide angles, which reduces side-loading dramatically. In itself, side-load reduction allows for higher lift cams, since it reduces valve guide wear and frictional loss of horsepower. Much-improved manufacturing techniques, newer technology, and better quality materials further contributed favourably to increased compression ratios and higher lift cams.
Moreover, rocker arm ratios increased to 1.625 in Evo and Twin Cam engines from the Shovelhead’s 1.420 ratio, which results in much higher valve lifts using the same cam lobe lift. For example, suppose we have a .290" lobe lift cam in both a Shovel and a Twin Cam. The valve lift in the Shovel will be 1.420 x .290" = .411" of valve lift, while the valve lift in the Twin Cam with the same .290" cam lobe lift will be 1.625 x .290" = .471" of valve lift. The rocker arm ratio increases the valve lift by a full 0.060". Stock Panhead lift at .390" evolved into a .412" lift for the Shovelhead via an increased rocker arm ratio. This then suddenly skyrocketed to an amazing, for the day, .490" lift in the Evo engine. A .490" lift cam in a Shovel was hairy, but normal in the Evo and Twin Cam. So, now we are getting somewhere.
However, the aggravating Evo and earlier engine pushrod angles from the lifters to the rocker arms remained a problem, particularly for the front exhaust valve and the rear exhaust. The center intake pushrods remain much straighter, so they are not as much an issue. Even the super-improved, automotive-style, strong Evo hydraulic lifters continued to fall victim to bleed-down because of pushrod angularity. With the advent of the Twin Cam, the valve angles and rocker arm ratios remained consistent with the Evolution engines. However, the location of the TC’s two cams allows for almost straight up and down pushrods. Gone are the problems of bleed-down with its resultant lifter ticking and the inherent inefficiencies. You can now see why radical cam lobe lifts and durations in a Shovelhead become very normal and safe in a Twin Cam.
The Twin Cam engine, because of the improved squish band combustion chamber head design, tolerates higher compression ratios than the Evo’s single-sided, D-shaped squish band. With the Twin Cam, we are starting to get an efficient, pushrod-operated engine, given the fact that we still have the restrictive inadequacies of a valvetrain and not valves actuated by overhead camshafts. Metallurgy, robot-precise manufacturing, and superior quality control are a vast improvement over the Evo and light-years ahead of the Shovels and similar engine types. In short, for an antiquated-but-beloved engine, we have huge changes and improvements that allow for radicalization of previous standards, as they now become normal. The current, stock Twin Cam engine management system attempts, better than anything before, to protect against detonation.
In fact, the ever-improving Delphi ESPFI (electronic sequential fuel port injection), instructed by the sensor-serviced ECU (electronic control unit), has a new feature called the idle temperature management system, which, in simple terms, offers a cooling feature for hot idle conditions, thus lowering cylinder head temperatures and the prospects of deadly engine-destroying detonation. This comes free from your H-D dealer; the engine calibration procedure is available for all FLHT touring models plus police-specific FLHTP, FLHP, and FLHPE motorcycles. The calibration offers limited rear cylinder cooling when cylinder temperatures reach a certain level. The EITMS (engine idle temperature management system) turns off the rear cylinder’s fuel injector at hot idle. The rear cylinder still functions without fuel. Air still sucks in on the intake stroke, compresses on the compression stroke, decompresses on the power stroke, and exhales on the exhaust stroke. This air acts to cool the engine, which is only running on the front cylinder. Sophistication is beginning its reign.
The aftermarket has been hot on the trail with automotive crossover technology and new innovative designs. Exhaust system O2 (oxygen) sensors feed information to the nonemissions control engine management computers that interpret and change input information to the ECM, thereby changing the fuel-correlated ignition tables or maps. Pulse width, which is the turned-on, squirt-time instructions to the fuel injectors, increases accordingly, as do the stock piggybacking, ignition-timing tables. I digress as usual, but an input fuel-ignition aftermarket module like Terminal Velocity is superior to an output module that only modifies fuel delivery, such as TFI, although, it does a good job doing that. The output Power Commander also has an ignition component, but any ignition changes require a sophisticated technician manning the helm. There are fewer competent people around than one might think.
In any case, improving technology, metallurgy, and computer sophistication is allowing, in my mind at least, former radical cam design to be viewed as an acceptable, safe, and necessary component of current go-fast modifications. I would never have dreamed of going above an Andrews B-grind .485" in my 1968 ex-cop Shovel back in the day. I did try an Andrews C-grind, which has a .525" lift, but I did not get any extra power because I, as most mechanics of the time, failed to correctly match other parts. We simply had not developed our knowledge levels sufficiently. The problem we had is that a wilder cam cost the same as the appropriate cam for whatever performance configuration we were after. We were still learning the two principles: there are no superstar parts (a coordinated, complementary configuration of components is better) and bigger is not necessarily better.
I relented a little in my many Evos, rarely going above a .525" lift and occasionally trying a daring .550" lift. However, I did not like the results provided by the higher lift cams, which are better suited to an overhead cam profile, because anticipated horsepower, and particularly torque, was not forthcoming with a powerband too far out of the range for normal street hot-rodding. The powerband was too high because the cam duration relative to lift was just excessively long!
I have changed my mind. I tried the heretical-to-some, .650"-lift/short duration Knight Prowler cams because I talked to Bob Wood and trusted his judgment. I’m having so much fun that I don’t care if the valvetrain/engine longevity of a new Twin Cam is reduced from, say, five or six years by a year or two. However, I still would not use these lifts in earlier engines, like some do. My pockets are not that deep. Of course, for those with deep pockets, it all reduces to more fun. For the knowledgeable, these are the best of times for the performance-
oriented H-D mechanic.
Story continues in next thread. Check back issue for pix and extra information. |
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