Racer Brown Chapter Six

VALVE LIFT CURVES, TIMING

With the camshaft lobe between the valve stem tip and rocker arm pivot point, as it is in the Datsun Design, it will be seen that obtaining a valve lift curve basically symmetrical on either side of the maximum valve lift point requires using an asymmetrical cam lobe profile. And so it is with the L-16/18/24 engines. A quick look at a stock camshaft lobe for any of these engines shows the lobe opening flank is almost a straight line. It isn't really, it is slightly convex. The closing flank has much more of a convex bulge to it. This is because the initial contact point of the opening flank with the rocker pad is as close as it gets to the valve stem tip and as far away from the rocker pivot, therefor the effective rocker arm ratio at this point is at its lowest. As the camshaft continues to rotate, the cam lobe/rocker pad contact point moves progressively further away from the valve stem tip and closer to the rocker pivot, increasing the effective rocker arm ratio until maximum valve lift is reached. The cam lobe/rocker pad contact point is as far away from the valve stem tip as it is going to get at maximum valve lift, so the effective rocker arm ratio is at its highest.
Beyond maximum valve lift on the closing flank of the cam lobe, the condition reverses itself because the contact point is moving progressively closer to the valve stem tip, calling for a somewhat "fatter" closing flank to accommodate the progressively-decreasing effective rocker arm ratio. Thus the asymmetrical cam lobe profile produces a valve lift curve essentially the same on both sides of the maximum valve lift point.
From the darkest and earliest days of the four-stroke cycle engine, right up until today - and maybe tomorrow - a symmetrical valve lift curve has seemed desirable.
So what happens to the valve lift curve when the cam lobe is basically symmetrical?
Right, the valve lift curve is asymmetrical. With a symmetrical cam lobe profile in the Datsun cammer engines, valves open more slowly than they close due to the system's physical layout. So who says the valve lift curve must be symmetrical? Not me! In fact, there are a couple of valid points to the contrary. First, the engines produce better torque at lower engine speeds, which contributes to the second; a broader engine operating speed range. This means the engines aren't as "peaky" because the torque curve and the power curve are both flatter. This is beneficial in nearly any application because the Datsun's, like any other relatively small piston displacement engines, need all the torque they can get, particularly at lower engine speeds
Datsun engines under discussion respond in an extremely encouraging manner to valve timing changes from ridiculously (perhaps deceptively) mild where exhaust emissions are a factor to some real giants in purely race engines. No one in their right mind would initially think that and engine with from 398.75 cubic centimetres per cylinder (24.32 cu. In.) to 500cc's per cylinder (30.5 cu. In.) - as represented by bored and stroked L-18's - could possibly use camshafts with effective duration well over 300 degrees with valve lifts from 0.600 to about 0.640-inch. Properly modified Datsun cammer engines can and do, and they love it. In fact, the intake port of the so-called FIA-cylinder head (not available on any vehicle imported into the US) continues to increase in airflow until nearly 0.750-inch valve lift. But enough of such exotica for the moment.

ROCKER ARM GEOMETRY

The biggest problem with a camshaft change in any Datsun cammer engine lies in restoring the rocker arm geometry as closely as possible to the stock condition. Because special semi-finished camshaft blanks are next to impossible to obtain for these engines, stock camshafts must be reground and processed. Any material removed from the base circle radius of the cam lobes (1/2 the base circle diameter) must be replaced at the valve stems as a function of rocker arm ratio. The stock cam base circle diameter is 1.300 inches 0 a base circle radius of 0.650-inch. Now assume that the base circle diameter of the new cam is 1.200 inches (0.600-inch base circle radius), 0.050 inches smaller on the radius than stock. The book gives the stock rocker arm ration as 1.5 to 1, but I have found that 1.48 to 1 is a more reliable number, although we may be quibbling. Multiply the 0.050-inch difference in base circle radius by the rocker arm ratio --say 1.5 to 1, and you come up with a number 0.075-inch. This is the amount the calve lash pad must be increased in thickness over and above the stock lash pad thickness for the rocker arm geometry to come out someplace close to stock, assuming nothing else has been changed.
It isn't all that difficult to make the Datsun cammer rocker geometry come out correctly, but it does take some time, some basic knowledge of rocker geometry theory and practice, plus some very close observation of the related pieces.
Theoretically the rocker arm tip radius should provide a purely rolling action across the valve lash pad face - with zero sliding action. This rolling action is desirable to minimise any frictional loss between the rocker arm tip and the valve lash pad, and also to minimise any side thrust that can be transmitted from the rocker arm tip to the lash pad to the spring retainer and finally to the valve stem. This all minimises any frictional loss between the valve stem and the valve guide bore, which can easily contribute to rapid valve guide bore wear.
In practice, some sliding action is inevitable but its effects can be minimised by following the rules and regulations of the game. With the Datsun cammer engines, everything you touch related to the valve seats in the cylinder head, valve faces, valve stem lengths and the camshaft affects rocker geometry - for better or worse!
Rocker arm geometry can be considered correct when the centre of the rocker arm tip radius contacts the valve lash pad on the valve stem centreline axis at exactly one-half total valve lift. Some say coincidence of the centre of the rocker arm tip radius and the valve stem centreline should occur at higher or lower percentages of valve lift. We have found that the one-half valve lift figure makes life easier on the entire valve train.
One highly important item may contribute some conflict of interest at this point: The actual area, and location of the area, of the rocker arm pad contacted by the cam lobe, hereafter referred to as "rocker pad contact patch." Some cam lobe profiles do not allow much if any latitude here. With these engines it is AB-SO-LUTE-LY ESSENTIAL that the cam lobe does NOT extend beyond either end of the rocker pad. For the most part, this occurs on strictly race engines, but if the cam lobe profile is sufficiently desirable, the rocker arm position must be adjusted at the pivot end so that the cam lobe does not override either end of the rocker pad. If, by mischance, miscalculation, or simply the wrong cam lobe profile, an override condition exists at either or both ends of the rocker pad, it spells instant death to the cam lobes and the rocker pads.
If the cam lobe is a tight squeeze on the rocker pad, then the first order of business is to make certain that the rocker pad contact patch is as correct as it can be, and rocker arm geometry must be forced to a secondary position.
Ideally, the rocker pad contact patch should be centred on the rocker pad. Assume that the opening flank of the cam lobe comes within 1/32-inch of one end of the pad and that the closing flank of the cam lobe comes within 3/32-inch of the other end of the pad. This is wrong. The rocker arm position must be raised at the pivot end of the rocker so that the contact patch is equalised at 1/16-inch from each end of the rocker pad. There is a good visual method of checking and measuring the rocker arm contact patch, and there is also a way to "cheat" a bit to gain a higher effective rocker arm ratio in some cases…but that comes later.

MEASUREMENTS: OPENING & CLOSING POINTS & LIFT

But back to more basic stuff. How are valve opening and closing points and valve lift measured? A great deal of confusion exists here and the Datsun service manuals are not too helpful. They do show valve opening and closing points are measured. By correlating their data with a chart of figures used to construct a valve lift curve, we concluded that they measure valve opening and closing points at one-half millimetre (0.020-inch for all practical purposes) valve lift with zero valve lash and with the rocker pad contact patch centred. Any other measurement method not only doesn't make sense but cannot be correlated with Datsun data.
So we have adopted the Datsun measurement method intact, but with one exception: We measure valve opening and closing points at a more Americanised 0.025-inch lift figure. Sure, it's arbitrary - just about any method is - but it gives four separate and distinct points of reference: Intake valve opening and closing points, and exhaust valve opening and closing points.
So why not measure these points at operating valve lash? Simple. There is too much room for error because a lot of crankshaft rotation results in only a very small amount of valve lift and the probability of error is too great at these particular points on the cam lobes. So we picked a point where the valve motion is not only quicker and can be measured much more accurately with repeatability - but on that also has much more significance to a functional engine. In other words, effective valve opening and closing points. Translated, this means that the valves are far enough off their seats so that gas flow into and out of the cylinders will have some influential and significant effects upon the engine operating characteristics.
I can hear it now, loud and clear. What about the split overlap and the so-called centreline methods of camshaft installation? They are both junk and should be buried and forgotten, but they can't be condemned without reasons and explanations. Both methods may tell you what you're looking for or want to hear, but they can't tell the engine what it must know to function properly. These are discussed in the sidebar titled Definitions.