Racer Brown Chapter Eight

OTHER ENGINE MODIFICATIONS

So far, no mention has been made of other engine modifications, and for a purpose. For vehicles that serve as basic point-to-point transportation on freeways and surface streets in areas of high vehicle population, as well as more rural parts of the countryside, efficient and enjoyable vehicle operation pivots about on work: Drivability. Along with providing such basic transport at a relatively nominal price, Datsun's are "fun" cars to drive. A mild camshaft, perhaps with a more efficient induction system, actually improves drivability. Modifications such as larger valves, large intake and exhaust ports, tricky competition-type exhaust header systems, etc., all have one point in common: Singly or in combinations, they seriously inhibit drivability by taking too much torque away from the engine in the most frequently-used engine speed ranges, and simply do not belong in engines caught at stop lights, bumper-to-bumper traffic or short-hopping. Again - either singly or in combination - these modifications will cause poor idle, poor throttle response, poor part-throttle operation, so who needs 'em in a street driven vehicle? You don't if your vehicle fits this category.
There is one other exception: Compression ratio. High compression ratios equate to higher thermal efficiency, higher torque and power outputs. However, with the quality of available pump gasoline steadily diminishing, the advisability of raising the compression ratio is questionable, except possibly at higher altitudes, or where one is still blessed with the availability of decent fuels. A higher ratio does help, but there is a very fine line indeed between balancing the highest useful compression ratio with valve timing and average fuel quality. This says nothing for the seemingly insignificant details that must combine to make a higher compression ratio function properly such as reasonably constant (and correct) air/fuel mixture ratio (no lean spots under full load), accurate and consistent total spark advance, correct spark plug heat range, adequate duel delivery system, proper spark advance curve, acceptable engine oil and coolant temperatures, and absolutely zero detonation and/or "dieseling" (when you have to beat the engine with a club to make it quit running after the switch is turned off). These are the trivia that will make relatively high compression ratio engine function as it should, or can break it into more tiny bits than you'd car to count for a strictly street engine, the small increase in compression ratio that could be tolerated probably isn't worth the effort.

Weekend Warriors

Enter the "Weekend Warrior." He has one vehicle and a strong desire to compete in motorsports events. These could be slaloms, gymkhanas, rallies, road races, drag races, hill climbs, etc. He knows a bone-stock engine hasn't a prayer of being competitive in any type of off-road event where modifications are permitted. He also knows that he must sacrifice some drivability and loss of operating economy on the street in order to be reasonably competitive in the off-road event of his choice. And he is willing to accept these sacrifices --up to a point. But where is that point?
A good question and a difficult one, and one compounded by increasingly stringent exhaust emission limits. He may be called upon at any time for a roadside emissions check by state or city authorities, a practice that is not only entirely legal but one which is increasing in frequency all over the US. It's a very safe bet that if his engine is modified to the point of being marginally streetable, any exhaust "sniffer" will turn thumbs-down on the exhaust emission levels. Then he had best be prepared to convert the engine to stock condition and submit his vehicle for a recheck, or face the consequences and penalties of the law.
The best advice is to keep all emission control devices hooked up and operative when the vehicle is driven on any public road. These devices do help reduce exhaust emissions and if nothing else, they give tangible evidence to any vehicle inspection officer that the guy's implied intent was conformity with the law, even if the exhaust emission levels are unacceptable. Guy: "I've been thrashin' it pretty hard lately. Guess it needs a sharp tun-up." Inspection Official: "Yeah. Sign your citation here, and do visit our humble inspection station again. Within the mandatory 30-day period, of course. And with the emission levels right." Sound ridiculous? Perhaps. But it's happening every day!
The problem is further complicated by trying to explain to some aspiring John Morton or Bob Sharp that he has to use his Datsun as a transportation hack for six days o week to compete in an off-road event on the seventh day. This very strongly suggests conservatism for any and all internal/external engine modifications. In the area of camshafts for such applications, effective duration should be in the low-to-mid-260 degree range with 44 degree - 48-degree overlap. A camshaft assembly must include special valve springs, spring retainers and lash pads of the correct thickness. Usually, this type of speed of around 7,500 RPM; but it may require notching the pistons for the required piston-to-valve clearance.
Before we depart for more exotic worlds, a few more words about exhaust emissions. It has been conclusively proved that a relatively mild camshaft can do two things in Datsun engines: It can improve road performance and reduce exhaust emissions at the same time. Similarly, a good aftermarket intake manifold could do good things in both areas by a vast improvement in cylinder-to-cylinder air/fuel mixture distribution and by maintaining relatively high mixture velocities throughout the entire induction system. A mildly but expertly modified cylinder head has proved beneficial in both areas but it does more for performance than it does for exhaust emissions. A good set of street-type steel-tube exhaust headers, with provision for air pump injection nozzles, will also help but the engine noise level is increased over the stock exhaust manifold. With these mods, a slight increase in compression ratio becomes feasible. Don't get carried away: A good, honest, genuine measured 9 to 9.5 to 1 should be considered adequate because the extra heat generated by higher compression ratios increases oxides of nitrogen (NOx) emissions. Then come the little things that count such as correct carburettor calibration, correct and stabilised ignition advance curve, perhaps a breakerless magnetic impulse ignition system, a good set of wire-type radio-shielded secondary spark plug and coil cables, and so on. Each item individually will improve performance and most of them will reduce exhaust emission levels, while the remaining few will at least not be detrimental to exhaust emissions.
So what have we got? With all of the preceding done with precision and moderation, we have a combination; an engine assembly capable of producing a very good level of performance, again in the most frequently-used engine speed ranges…without extremes in any direction. We also have a more efficient engine assembly, on which at least has the potential of reducing exhaust emissions significantly. Whether it does or not is up to the individual more than it is to judiciously-applied and moderate modifications.
Race Engines - Now let's go on to fully-modified Datsun cammer race engines. Generally, these engines are quite sensitive and respond in a most gratifying manner. This doesn't mean "biggest" or "most" is always best in any single component or combination. Again, the strictly competition engine that is most successful in a given application must necessarily be compromised in one or more areas. Assume that you're stuck with a four- speed gearbox. This means that the ultimate, last-gasp, maximum-effort engine will be out of place because the gearbox ratios won't let the engine run where it is happiest and can do its best job. Compromise: You need better torque at lower engine speeds to overcome the gearbox ratio penalty, which usually involves some sacrifice in maximum power output. Solution: Use a camshaft with slightly shorter effective duration, retain as much valve lift as possible, and perhaps advance the camshaft three or four crankshaft degrees, assuming no piston-to-intake-valve hang-up. In addition, a slightly smaller diameter exhaust header primary pipe from four to six inches longer will also help this condition, as will longer carburettor air horns. Secondary solution: If the bankbook and rulebook have no serious objections, purchase the optional Datsun intermediate-ratio five-speed gearbox with the necessary drive shaft, and the appropriate ring and pinion ratio, the right diameter tyres - and go race.
If mid-range and upper mid-range output is a major factor, effective duration should be kept in the high-280 degree to mid-290 degree range with from 70 degree - 75-degree overlap. Valve lift should be in the 0.580 to 0.610-inch range. If otherwise rightly equipped, such an engine will be strong from about 4,800 RPM on up to about 8,000.
If a maximum-effort engine seems to be the correct plan, and vehicle is properly geared so that minimum engine speed doesn't drop below about 6,000 RPM then the skies (almost) the limit. In this case, effective duration should be in the 310 degree - 320+ degree range with from 0.620 to 0.650-inch valve lift. With such a camshaft, the best effective engine speed range would normally be from about 5,800 to 6,000 RPM through at least 8,800. But a word of caution here: L-16/L-18 four-cylinder engines are better equipped to handle extreme engine speeds that L-24 sixes. The L-24's weak link is the 50% longer crankshaft, therefore torsional oscillations of the crankshaft are more severe. Consequently sustained maximum engine speeds in L-24 engines should be limited to 8,000 to 8,200 RPM, and even then the crankshaft damper, the crankshaft damper bolt, flywheel, clutch pressure plate bolts and camshaft retaining bolt must be constantly checked for torque and replaced, preferably before something breaks. Four-cylinder engines are not as bad in this respect, although there are some unbalanced secondary forces at work, so they are not completely immune from similar problems.

DISPLACEMENT ANGLE

This brings us to another definition, that of displacement angle. Displacement angle is defined as the angular relationship from a given point on the exhaust lobe opening and closing flanks to the same given point on the intake lobe opening and closing flanks of the same cylinder. Displacement angle is expressed in CAMSHAFT degrees (exactly one-half the amount of crankshaft degrees). Previously-shown camshaft examples have a displacement angle of around 108 degrees as an average figure. With the longer and taller competition camshafts, the Datsun cammer engines like to have the displacement angle squeezed up to about 102 degree to 105 degree. Example: Assume an effective duration of 280-degree with a displacement angle of 108 degree. The nominal valve timing event, in CRANKSHAFT degrees would be: Intake opens 32 degree BTC, closes 68 degree ABC. Exhaust opens 68 Degree BBC, closes 32 degree ATC. Add 32 degree to 68 degree plus 180 degree and the effective duration is 280 degree for both intake and exhaust. Now add the intake opening point (32) to the exhaust closing point (also 32) and we have 64 degree. Next subtract the degrees of overlap (64) from the effective duration (280) and we have the displacement angle of 216 CRANKSHAFT degrees. Finally, divide the 216-degree number by 2 to obtain the displacement angle of 108 in CAMSHAFT degrees.
Now advance the camshaft one camshaft degree (two crankshaft degrees). The valve timing then becomes: Intake opens 34-degree BTC, closes 66-degree ABC. Exhaust opens 70 degree BBC, closes 30 degree ATC. Three things remain exactly the same: Intake valve duration, exhaust valve duration, and overlap. All we have done is to change the valve opening and closing points. If the camshaft is retarded the same amount from the original figures the valve timing becomes: Intake opens 30-degree BTC, closes 70-degree ABC. Exhaust opens 66 degree BBC, closes 34 degree ATC. Intake and exhaust valve duration and valve overlap period still remain exactly the same; all we have done is to move the valve opening and closing points in the opposite direction.
Next assume a camshaft with an effective duration of 310 degree and a displacement angle of 105 degree. Nominal valve timing would be: Intake opens 50-degree BTC, closes 80-degree ABC. Exhaust opens 80 degree BBC, closes 50 degree ATC, these numbers again being expressed in crankshaft degrees. As before, add 50 degree to 80 degree plus 180 degree and the effective duration is 310 degree for both intake and exhaust. Again, add the 50-degree intake valve opening point to the 50-degree exhaust closing point for the overlap period of 100-degree. Subtract the 100-degree overlap period from the effective duration for the displacement angle of 210 crankshaft degrees, then divide by 2 for the displacement angle of 105 camshaft degrees. The advance-retard game can be played here as well and again, the valve opening and closing points are the only items that will be changed, and many times this is certainly enough.
The basic idea is that displacement angle is a fixed value. Once the camshaft is made, the displacement angle cannot be changed unless the camshaft is reground, and then only within fairly close limits.
Now let's take our same 310-degree effective duration camshaft and squeeze the displacement angle up from 105-degree to 102-degree. The nominal valve-timing event in crankshaft degrees will be: Intake opens 53-degree BTC, closes 77-degree ABC. Exhaust opens 77 degree BBC, closes 53 degree ATC. The intake and exhaust valve duration remain the same at 310-degree. However, the valve opening and closing points have been changed, and the valve overlap period has been increased from 100 degree to 106 degree. By applying the above equation (duration - overlap divided by 2) the displacement angle is 102 camshaft degrees. There are a couple or three messages here. The first seems pretty obvious: As effective duration increases, the displacement angle should be decreased, within reasonable limits, of course. Why? Primarily because the effective intake valve closing point can be kept at a sensible number, which will keep mid-range torque from dropping absolutely dead. Secondarily, assuming the engine can breathe well during the overlap period, the increased overlap will let it do jus that. The third factor that rears its ugly head as effective duration is increased, or displacement angle is decreased, or a combination of both, is maintaining adequate piston-to-valve clearance through the operating cycle.