Racer Brown Chapter Seven

DEFINITIONS

The split overlap method of camshaft installation is defined as having both intake and exhaust valves of one cylinder off their respective seats exactly the same amount with the piston at exact top centre of the exhaust stroke; that is, during the valve overlap period when the intake valve is opening and the exhaust valve is closing. This method is valid ONLY if the following conditions are met: (1) The valve lift curve is exactly symmetrical and exactly the same for both intake and exhaust valves. (2) If the engine likes a given cam lobe profile in a statically determined split overlap position. It was earlier stated that some good things happen to the Datsun cammers when a basically symmetrical cam lobe profile is used to make the valves open more slowly than they close. If the split overlap method is used in such a case, the camshaft position in relation to piston position will be retarded - a position that nearly all-small engines and some big ones as well dislike intensely. So what happens if circumstances such as a lousy cylinder head job, or a lousy exhaust system. Or both, demand that the exhaust valve duration be longer than the intake valve duration? Same thing: The camshaft is retarded. Unless the two above conditions are met, which is very highly improbable, the split overlap method of camshaft installation can only lead to false indications and therefore false conclusions.
The so-called centreline method reflects its inherent stupidity in the worst possible choice of words. The centreline of what? The term actually refers to maximum valve lift points in relation to the TC; piston position. Maximum valve lift of the exhaust valve occurs at some point before top centre; maximum valve lift of the intake valve occurs at some point after top centre. As a camshaft installation method it leaves a lot to be desired because it tells you nothing about the camshaft or its characteristics. However, if it is used in conjunction with the opening and closing points of both valves of one cylinder, then there are six points of reference instead of four: Intake valve opening point, maximum lift point, intake valve closing point. And the same for the exhaust valve. But if the so-called centreline method of camshaft installation is used by itself, forget it. Effective valve opening and closing points are left to fall where they may, and his is not at all in the best interests of engine performance. In addition and to prove something to myself, I have intentionally moved the point of maximum valve lift by as much as five crank degrees in each direction, without changing effective valve opening and closing points. Results: The engine couldn't car less where maximum valve lift occurs, at least within plus-or-minus five crankshaft degrees. But if the camshaft is advanced or retarded a similar amount, there is an immediate and very measurable change in operating characteristics. So the engine does recognise effective valve opening and closing points, but the maximum valve lift points are relatively meaningless. Inasmuch as there is some physical effort involved in a camshaft change, do it right, do it once, and leave the guesswork to amateurs. It does take additional time to do it right, but it has to be considered as time well spent.

TOOLS YOU'LL NEED

Tools required for a camshaft installation - aside from the normal hand tools and the valve train hardware - are a fully-degreed crankshaft damper or a degree wheel - the larger in diameter, the better - to bolt to the crankshaft nose, or a fully-degreed flywheel that is easily accessible and visible; a rigid pointer located for easy reading of the degree wheel, damper or flywheel; a good one-inch stroke dial indicator with a large dial graduated in thousandths of an inch; a rigid magnetic base with rigid attachments for holding the dial indicator (flexible "snake" type attachments are not reliable for repeatability); a flat steel plate large enough to accept the magnetic base and one or two holes in on end so the cylinder head hold-down bolts can locate the plate on the cylinder head. Surface grind the plate on one side to provide a stable flat surface for the magnetic base. Two mechanical fuel pump springs are required to replace the valve springs for one cylinder, and an assortment of valve lash pads of different thicknesses. Time, patience, good temperament and cheer are also likely to be quite essential before it's all over.

STREET

Let's begin with a street application basically a stock engine, where idle characteristics, throttle response, general drivability and exhaust emission levels are all contributing factors. Based on measuring effective valve open duration at 0.025-inch valve lift with zero lash, duration should be in the low to mid-240 degree range with no more than about 25 Degree overlap. Datsun cammer engines respond very nicely to valve lift; it actually helps low and mid-range torque, as well as maximum power. But for this application, lift should be in the 0.430 to 0.450-inch range. A couple of years ago we proved two points: (1) performance level could be improved and (2) emission levels reduced with nothing more than a mild camshaft. Average road performance level in the 2,800 - 6500 RPM range was improved by 7+% with a maximum of 10+% at the higher engine speeds. Average exhaust emissions of unburned hydrocarbons, carbon monoxide and oxides of nitrogen were reduced by approximately similar percentages. The vehicle was an otherwise bone-stock 1971 Z-car with about 25,000 miles on the clock. A few carburetion modifications were indicated to help emission reductions even more but the primary objective was to learn what the camshaft-only change would accomplish. An incidental advantage was that fuel economy was increased by about 4%, all of which showed that the thermal efficiency was better than stock. The camshaft had 250-degree effective duration with 0.440-inch lift.
With the lower compression ratios of the later Datsun engines, camshafts for strictly street applications must be very mild indeed. The drop in compression ratio means a loss in cylinder pressure, which is contrary to improved performance. The gams plan here is to capture as much cylinder pressure as possible, yet retain normal combustion with pump-type fuels having very small amounts of tetraethyl lead, or none at all. This suggests very short duration camshafts in the mid-230 degree range with from 14 - 18 degree overlap. In 1972 and later engines, a camshaft like this by itself will usually wake up an engine to match the performance level of earlier stock engines with higher compression ratios. In most cases, such a camshaft can use all stock Datsun pieces, with the exception of valve lash pads, so long as maximum engine speed is kept within the 6,000 - 6,400 RPM range.
The next step up for the fours is for someone who is willing to bolt an extra stock two-throat progressive Hitachi carburettor onto a good aftermarket intake manifold, and still keep within existing exhaust emission limits for the year of vehicle concerned. Unfortunately, no such manifolds existed as this book went to press. In this case, a higher performance level can be expected, as well as higher average engine speeds. A camshaft for such and application should have an effective duration in the high-240 degree to low 250-degree range and from 34 degree - 38-degree overlap. For the privileges of higher power output in conjunction with considerably improved performance, and the realisation that this guy will usually have his foot buried a bit deeper in the carburettors when the occasion permits, a higher price must be paid, but not necessarily all in dollars or yen. A rougher and perhaps a faster idle and not much muscle below about 2,8000 - 3,000 RPM can be expected. The rougher idle brings with it less manifold vacuum at idle and the lower engine speeds, which can adversely affect the power braking system. So if you lean on the throttle harder, you can lean on the brake pedal harder. A camshaft assembly of this type will usually include special valve springs, spring retainers and lash pads that will permit a 7,000-plus RPM maximum safe engine speed.
It should be pointed out that the standard Datsun four-speed gearbox is not equipped with ideal (whatever that is) intermediate ratios. The first-to-second spread is OK, as is the third-to-fourth. But the large second-to-third gap is a factor that must influence camshaft selection because after the two-three shift the engine must have enough torque to pull itself out of the hole caused by the two-three gearbox ratio spread.
This is best translated into five simple words: DO NOT OVERCAM YOUR ENGINE! If questions arise about the suitability of two or more camshaft profiles, ask these questions of someone qualified to give the beast answers related to your particular application. If there is still some indecision, pick the milder camshaft, accept its limitations, and be glad you made the right choice.