 Techline-August08-PtIV Cam Specs and Choices (cont.)
August '08 story continued
Ratios & Heat Issues
The power-to-weight ratio is a biggie when it comes to performance. All things being equal, the lighter rider on a lighter bike with the same torque and horsepower will outride the heavier rider on the heavier bike. As obvious as this may be, the reason is that the lighter weight bike has a higher power-to-weight ratio. Is it time for a diet, anyone?
A bike’s power-to-weight ratio also has a big effect on cam choice. The heavier ride will need a higher-torque, higher-lift cam with shorter duration, which will move the engine’s powerband lower on the rpm range in order to get things moving easier. Valve overlap, LCAs, and LSAs will also need consideration. If the heavier rider with the heavier bike does not care about low-end power because he rides at highway speeds most of the time, then a longer-duration cam will assist at these constant speeds and for passing at speed. Lighter riders and bikes will move off the line easier, so their cam of choice might move the powerband into the midrange or a higher rpm depending on wants and needs.
Compression ratio choice is another big consideration. The greater the compression an engine has just this side of detonating the fuel/air mixture, the greater its power output will be. Detonation is the great engine destroyer. Detonation makes itself known through sounds like ball bearings bouncing around in the cylinders, hence the descriptive colloquialisms pinging, spark knock, or knocking. The cause is a spontaneous ignition of the fuel/air mixture occurring elsewhere in the combustion chamber independent of the correctly timed spark plug firing. Each ignition of the mixture forms a flame front and generates a pressure wave that travels across the combustion chamber. If the pressure and temperature of the as-yet unburned mixture exceeds its ability to resist combustion, the remaining unburned mixture all goes off at once, detonating.
There are many causes for detonation. Some obvious ones are poor quality fuel, overadvanced ignition timing, high-compression ratio, valve timing, excessive piston speeds due to a high connecting rod-to-stroke ratio, and the current lugging problem for some riders with the higher gear ratios found on 2006 Dynas and 2007 and later TC 96s. Too much heat causes the pinging. Heat and pressure are synonymous terms when it describes gases in a sealed container. The burning mixture creates heat, which expands the gases, thus creating pressure. The increased heat and pressure can make the fuel/air mixture spontaneously ignite.
Twin Cams generally make 35 psi of oil pressure at 230 degrees F at 2000 rpm. As Bob Wood says, “It is necessary to keep cylinder head temperatures under 210 to 220 degrees to combat detonation.” He recommends running the best oil cooler, which we agree is the Jagg. In fact, Bob says to run two 10-core Jagg oil coolers in tandem to control cylinder head temperatures on a performanced Twin Cam. I’ve been a proponent of the use of oil coolers on Harley-Davidsons since my early Shovelhead days. To me, the logic of using an oil cooler on an air-cooled engine is inescapable. I tried these dual 10-core oil coolers on my 10.5:1 compression Street Glide, and, as usual, Bob’s advice was excellent.
The major factor in raising cylinder head temperatures, aside from out-of-whack timing, is increased compression. However, we need more compression for power, so we must compromise. Cylinder pressure increases with an increase in compression ratio, which we can achieve in a variety of ways such as milling the cylinder heads, using higher dome pistons, thinner head gaskets, descending in altitude, or decreasing cam duration. Cylinder pressure decreases with a decrease in compression ratio via thicker gaskets, a lower dome piston, increasing cam duration, or ascending in altitude. An engine with long-duration cams needs a compression ratio increase to maintain sufficient cylinder pressure to maintain or increase power. Gasoline quality and octane ratings have a huge limiting effect on the maximum compression ratio used for a street bike.
However, cylinder head design coupled with sophisticated engine management systems are allowing for higher compression ratios over time. Hemi-head combustion chamber Panheads and Shovels used compression ratios in the neighbourhood of 7.0:1 to 7.5:1. These ratios were necessary even though the engines utilized lead as an excellent gas octane. However, the ignition was a relatively inefficient mechanical one, using mechanical points and a condenser. Later Shovels, from circa 1978 on, used an electronic ignition, which didn’t get good until the advent of the V-Fire III in 1983. Evos, which have a flattop piston design and a D-shaped combustion chamber, safely allow for a stock ratio of 8.5:1 with the V-Fire III, the beginning of an ever-increasing engine management system. Twin Cams began at a 9:1 compression ratio in 1999, using a flattop piston with a squish band extending right around the cylinder bore and an evolving, sophisticated, management system. Performanced Twin Cams using refined, electronic ignitions that are not hamstrung by emissions controls (like the download map Power Commander; the O2-sensor fed, self-tuning Terminal Velocity; and the IST S&S-proprietary engine management systems) allow for street compression ratios of up to 10.5:1 and sometimes beyond.
Other Factors
Other considerations when making cam choices are riding style, such as touring or hot-rodding (to use two extreme examples); bike weight, also taking into account trailer towing or side hacks (sidecars); operational rpm range relative to gear ratios; and the number of transmission gears: three-, four-, five-, or six-speed. Yes, early Harleys sometimes only had three forward gears. Engine displacement and compression ratios also have a huge impact on cam choice. In fact, any modifications like high-flowing air cleaners, air filters, carburetor size and jetting, EFI year, as well as throttle body size, ignition or engine management system, compression, cylinder head modifications, valve sizing, performance valve springs, and exhaust options all have small to great effect on cam choice. When entering the world of super-aspirated NO2 (nitrous oxide) and forced-induction supercharging and turbo charging, cam profile design makes some diametrically opposed changes to normally aspirated, natural-breathing motors. I will delve into these last subjects in the second book in my series Donny’s Unauthorized Technical Guide to Harley-Davidson, 1936-2008.
Rule II: coordinate performance components. All the above factors hopefully will give, rather than take away from, the cam profile design. Coordinated and complementary parts are the goal. There is no place in this equation for superstars, as performance can be severely compromised without compatible mating components. Parts choice must be coordinated so each component complements the other as a team that will perform better than the biggest superstar. Cam choice must be logical and harmonizing in this scenario rather than an attempt to bring radically opposing components into a difficult, contrarian fold.
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