Inliners International Forums General Discussion Hi Performance Cam timing

I saw a thing that I have found pretty accurate for Harley Davidson engines that I would like to run by everyone here. All this info is based on .053 valve opening event.

After all, an engine is an an engine

Cam Basics:

Correct cam selection is the single most difficult choice facing the performance enthusiast. And, unfortunately, we get it wrong about as often as we get it right. An incorrect cam choice can result in decreased real-world performance if the selected cam's operating range is not matched to riding style.

The operating range of any engine is determined by a number of factors and the cam design is one of the most influential. If the cam design is not matched to the other components and is not matched to the desired engine power band (rpm range), then the engine's performance will be disappointing. No amount of carburetor tuning can compensate for having the wrong cam.

To get the "right" cam design, you need to do two things: First, decide upon the rpm range you want to improve. Secondly, use the list provided below and cam catalogues to select a cam design that begins to work at the lower rpm of the power range you have chosen. You'll probably be surprised to learn how few of the available cam designs fit your expectations. However, the list is accurate and if you follow its implied advice, you will get an engine that performs as you wish it to.

Cam timing: what matters

The most important cam timing event is the intake valve closing angle. The intake closing point determines the minimum rpm at which the engine begins to do its best work. The later the intake valves close, the higher the rpm must be before the engine gets "on the cam."

Long duration, late closing cam designs are necessary to drag the last bit of power out of an engine. Unfortunately, these same cams can perform poorly under more normal riding conditions. In the quest for maximum power output, many-too-many Harley owners choose a late closing, high-rpm cam for their engine. The problem with such choices is that the engine seldom spends time in the rpm range favored by such cams.

The majority of virtually any motor's life is spent in the mid-portion of its rpm limits, between 2000 and 4000 rpm. At open-road cruising speeds, that range is more like 2500 to 3500 rpm. With current.

Even the mildest of aftermarket cams do their best work above 3000 rpm. At 2000, the majority of these cams seldom perform significantly better than stock cam(s).

The rpm at which an engine gets "happy" can be predicted by the closing point (angle) of the intake valves. The angle is expressed as the number of degrees After Bottom Dead Center (ABDC) that the valves reach .053" from being fully seated.

The following list predicts the rpm at which the engine gets "on the cam" based on the closing angle of the intake valves. These relationships are approximate but should hold true to within 200 rpm or so. They also assume that all other tuning factors, exhaust, ignition, etc., are operating correctly.


30 degrees = 2400 rpm

35 degrees = 3000 rpm

40 degrees = 3600 rpm

45 degrees = 4000 rpm

50+ degrees = 4500 rpm



If you have one of the late closing cam designs installed, say one that closes the intake valves later than 40 degrees, then you cannot expect excellent performance at 2000 rpm. No carburetor adjustment, ignition adjustment or exhaust system can change this.

Has anyone seen any concrete informations that contradicts this?

The general remarks are pretty accurate, but then we sort of knew "water is wet".

To actually understand what's going on, you need another thousand pages.

The exact closing point vs. RPM data is way too specific; it's greatly affected by displacement and port size, somewhat by LSA, much less by rod angle. IMHO they're a bit too optimistic if applied to older engines - they show (example) that a 250° @ .053" cam (timed 30-40) has power at 3,600. This may be true, but it falls off the planet at 3,000 in a small engine.
Choose more conservative figures to be safe.

Greetings . . .

I agree with panic. The analysis seems very one dimensional - as if the cam is the only variable when building an engine.

In his book "How to HOP UP Chevrolet and GMC engines" Roger Huntington points out the various 'paths to power' - and shows how cam selection needs to be done in concert with other modifications.

He uses torque curves to show the difference between getting a 15% increase in HP from bore/stroke changes versus cam timing. When varying cam timing alone the torque curve performs below 'stock' for most of the RPM range (until the engine gets on cam) - whereas increased displacement performs above stock across the entire operating RPM range.

As panic points out, gaining HP from cam timing alone yields a steeper and steeper torque curve. Quoting Roger "In fact, we can carry this cam timing business to a point a where the peak torque will be above 3500 rpm - and it will barely idle at 1000. This would mean idling at about 22 mph in high gear."

Roger concludes that if one is going to go for a longer duration cam - one needs to do something to replace the lost HP on the lower end - bore, stroke, C/R or a combination - and this is most important if the displacement is small to start with.

regards,
stock49