First let us define the lobe centerline angle. In simplest terms it
is the angle between the intake and exhaust lobe peaks. Notably, it is
the only cam attribute described in camshaft degrees rather than crankshaft
degrees. Remember, the cam runs at half engine speed, and a cam producing
300 crank degrees of "off the seat" timing has a lobe which occupies 150
degrees of cam angle.
OVERLAP AND DELAY
The LCA dictates two important valve timing attributes: valve overlap around TDC, and how much intake or exhaust valve closure delay there is past the end of the relevant stroke.
When discussing LCA’s we talk in terms of "tight" or "wide." Tight LCA’s have the lobes closer together, making the angle between them smaller, wider LCA’s have wider angles. Generally speaking, the majority of cams fall between 98 and 120 degrees LCA.
Let's hold cam advance in the motor constant and look what happens to valve events with LCA changes. Tightening the LCA produces more valve overlap around TDC, while wider equates to less. At the other end of the induction stroke, a wide LCA produces a longer delay to valve closure after the piston has passed BDC. Tight LCA’s produce earlier intake closure after BDC.
Most of us are aware that extending cam duration moves the usable rpm
range up. If increased duration is the only change, then the longer cam
normally robs power from the bottom end of the rpm range and adds to the
top. When only cam duration changes there is usually little change in peak
torque. All the longer period does is move the point of peak torque up
the rpm range. Most of the increase in horsepower occurs in the upper 30-40%
of the rpm range. Changing LCA’s has a different but equally significant
effect on the power curve. Without a working understanding of this, you
cannot hope to effectively spec out your own cams, so here's what you need
Because of its significance we will deal first with that very important race engine event, the overlap period. By tightening the LCA, the amount of valve overlap for a given duration is increased. For the first and most important half of the induction stroke the intake valve is opened farther by a cam with a tight LCA than one with a wide LCA. This produces a greater flow area as the piston starts to pull in a fresh charge.
Increased valve flow area in the first half of the induction stroke has significant importance for many reasons. The principal one is that a typical production-based 2-valve race engine inevitably lacks adequate valve area in relation to its displacement. Starting the valve motion sooner means more velocity and lift before the beginning of the induction stroke. It is often argued that opening duration after BDC is more effective at producing power than opening before the induction stroke starts. In reality a cam for maximum output for a given duration must have a good balance of opening at both ends of the induction stroke.
If a valve is opened at a suitably early point, the intake port velocity tends, later in the induction stroke, to increase enough to offset any negative effects of a marginally earlier closing. This early opening can be vitally important, especially for an engine having effectively tuned intake and exhaust lengths. In addition, data from 'in cylinder' pressure measurements throw yet more light on the matter. For commonly used rod/stroke ratios, peak flow demand by the piston motion down the bore normally occurs between about 72-78 degrees. However at lower RPM the greatest pressure difference between cylinder and intake port may occur as little as 20-30 degrees after TDC. As RPM reaches peak power level so the point of greatest pressure difference moves back to 90-100 degrees ATDC. For a small-block Chevy, if that pressure point moves back much past about 115 degrees then no further power with increasing RPM will be seen. In other words the engine has, in no uncertain terms, hit its peak. By having the intake farther open during the first half of the induction stroke we can, to a certain extent, delay the retardation of the maximum port to cylinder pressure difference.
Looking at peak intake port demand, which is also peak velocity, we find it tends mostly to occur over a relatively narrow part of the induction stroke. It mostly takes place between peak piston velocity and peak valve lift that follows some 25-35 degrees later. This, and the effect of pressure wave tuning in the intake and exhaust, are important reasons why the initial opening point of the intake valve can be so critical.
Promoting good cylinder filling early on in the induction stroke allows a beneficially earlier closing of the intake. If practical, this increases the amount of charge trapped at valve closure and results in an increase in torque output. A late valve closure from a wide LCA decreases torque.
A cam ground on a wide LCA has less intake valve opening at TDC, so
reaches peak opening later in the induction stroke. This means as the piston
accelerates down the bore it creates a greater discrepancy between the
flow delivered by the valve and the flow required by the cylinder. Put
simply, this is because during the first half of the induction stroke the
valve is not as far open when a wide LCA is used as it is with a tight
POST BDC FILLING
When using a wide as opposed to tight LCA, the intake valve stays open longer after BDC. Because of this, it can be argued that if the cylinder wasn't filled by the time the piston reached BDC or thereabouts, there's time for it to go on filling. Here's some numbers to make the point. At peak power, the cylinder of a typical race engine receives as much as 20% of its charge after the piston has passed BDC. This technique to gain cylinder filling becomes self- limiting because of increasing piston velocity up the bore. Too much delay means a reversion process begins to expel some of the intake charge. This intake charge reversion (not to be confused with exhaust reversion) reduces torque and is most prevalent at 60-70% of peak power rpm.
Of the two techniques, earlier intake valve opening, as produced by
the tighter LCA, produces best results. High rpm cylinder pressure measurements
suggest that the port/valve combination needs to substantially satisfy
the cylinders demand in the first half of the stroke. If it doesn't then,
short of some very good shock wave tuning on the intake, it is unlikely
to make up for it in the second half.
WHICH WAY TO GO?
So far the case looks good for tight LCA's, and so it is, but there are tradeoffs. Increased overlap equates to reduced idle quality, vacuum, and harsher running prior to coming up on the cam. Probably the most significant factor to the engine tuner though is a tight LCA's intolerance of exhaust system back pressure. Remember, during the overlap period both valves are open. If there's any exhaust back pressure or if the exhaust port velocities are too low it will encourage exhaust reversion. The tighter LCA’s are, the more likely problematical exhaust reversion into the intake will occur. Put simply, we can say that a tight LCA cam produces a power curve that is, for want of a better description, more "punchy". At low rpm when off the cam, it runs rougher, and it comes on the cam with more of a "bang".
A cam on wide centerlines produces a wider power band. It will idle
smoother and produce better vacuum, but the price paid is a reduction in
output throughout the working rpm range.
THE STREET MARKET
Granted, this magazine is about racecars, but we have to tow them to the track, so this justifies a look at cams for street in general and trucks in particular. For a given type of engine the range of LCA’s offered by different cam companies is surprisingly wide. If you've had in mind that they can't all be right, score yourself 10 points.
Deciding LCA’s for a popular line of street cams is, apart from engineering requirements, a question of market perception. Corporate marketing policies dictate as much as anything what will be used. For instance, some companies tend to grind their performance street profiles on wide LCA’s typically ranging from 110-116 degrees. This produces what these companies feel to be the most marketable balance between idle quality, vacuum, economy and horsepower. Very often the choice of wide LCA’s is made knowing that some of the potential power increase will be sacrificed for idle quality and high vacuum for any accessories requiring it.
Wide LCA’s are not the only way to go. Not everyone wants the smoothest idle and the highest intake manifold vacuum possible. Many, building even the mildest tow vehicle engine, are more interested in maximizing torque. To satisfy this market, some companies will grind their popular short duration profiles on a tighter LCA. Such cams, though less civilized when longer street duration is used, tend to produce more torque. However, it is important to realize that a tighter LCA is totally acceptable if the overlap developed by the LCA and duration combination isn't excessive
We've already looked at the test results of extending duration and holding
the LCA constant, now let's try the reverse.
(continue) Cam Basics p. 2
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