Racer Brown Chapter Fifteen

VALVE LASH SETTING

Now for the valve lash. There is a very good way of doing this and the first time takes the longest, but subsequent valve lash adjustments can be made quickly, easily and accurately. First, find maximum valve lift of number 1 intake valve with the dial indicator mounted on the valve spring retainer, then make a permanent mark on the crankshaft damper at the spot on the rim where the pointer is aimed, using a narrow stripe of nail polish or contrasting paint. Now rotate the crankshaft exactly one full revolution, right back to the same mark. This will place the rocker arm pad in contact with the centre of the heel of the cam lobe of number intake valve. Repeat the process for number 1 exhaust valve but use a different color of nail polish or paint. Then follow the firing order and do the same to the remaining valves. With L-16 and L-18 engines, you will have two stripes for intakes and three for exhausts because each mark will repeat itself at the crankshaft but no at the camshaft due to the 2-to-1 reduction between the crank and cam. For subsequent valve lash adjustments, start with number 1 cylinder and simply follow the marks on the crankshaft damper through the firing order until the valve lash has been adjusted on all valves. It isn't necessary to use the dial indicator again, but it is necessary initially to be certain the stripes on the damper are correctly located. Advancing or retarding the camshaft a few degrees will have no effect, so the original stripes on the damper can be used.
"Cold" valve lash, that is with the engine assembly at room temperature, should be set from 0.001 to 0.002-inch tighter than the specified "hot" valve lash. Hot lash adjustments must be made with the engine good and hot with the engine coolant and oil temperatures stabilised at or near their maximum levels. DON'T attempt a hot valve lash adjustment with the engine running unless the idea of drowning in hot engine oil has some masochistic appeal. Because of this, the hot valve lash adjustment must be made quickly, before the engine temperature level has had time to change significantly.
Any valve lash adjustment, hot or cold must be made carefully and accurately with feel gauge blades flat and smooth on both sides. Normally, lash adjustments are made between the cam lobe and the rocker arm pad; therefore any errors at this point will be multiplied as a function of rocker arm ratio at the valve end of the rocker arm. A more accurate method is to measure the valve lash between the rocker arm tip and the valve lash pad, but his means narrowing standard width feel gauge blades down to no more than 3/8-inch wide so the blades will fit in the lash pad slot. If this is done, the rocker arm ratio must be taken into account. If the specified valve lash is 0.012-inch (intake) and 0.014-inch (exhaust). This is the preferred method because it is consistently more accurate and also because any errors in adjustment are not subject to multiplication.

TUNING HINTS

Now for some minor tuning data. The L-series Datsun engines like to run with a sustained engine coolant temperature of about 200 degrees F.; fairly warm, like other liquid-cooled engines with aluminium cylinder heads. Engine lubricating oil temperature should be slightly less, say between 180 and 190 degrees F., otherwise an oil cooler, larger sump capacity, or both may be required. Assuming normal combustion condition, engine oil temperature is a better indication of power output than engine coolant temperature, but this is only true to a point, so don't get carried away and to a point, so don't get carried away and try to boil the oil. It isn't advisable to tune the engine around the coldest spark plug available because when the time comes that you really need a colder plug, you haven't any place to go. Besides, the engine will be sharper, crisper and more responsive with plugs that are one or two steps hotter, assuming the air/fuel mixture, total spark advance, etc., precludes any possibility of detonation and/or pre-ignition. A fairly cold plug is called for with an engine to be run on a dynamometer, where the same engine in a drag race vehicle would use a fairly warm plug, and a road race or circle track engine should use a plug somewhere between the other two.
About the only way and engine normally knows how to respond to a relatively rich air/fuel mixture condition is to demand lots of spark advance and a fairly hot spark plug. A lot of people in the world firmly believe that in order to get a power increase from an engine, it is necessary to really pour the fuel to it, and that it can't produce power without being dead rich. Their thought processes are confused. Obviously, if an engine does show a power increase, it will consume more fuel, but the specific fuel consumption will usually remain in the same range, if everything is clean and sanitary during engine operation. Brake specific fuel consumption, expressed in pounds of fuel per brake horsepower per hour, usually runs from about 0.50 to about 0.52 (pounds of fuel/brake horsepower/hour), based on gasoline as the fuel, with the best engines having the lower specific consumption because they make better use of the fuel. This roughly represents and air/fuel ratio of from about 12.5 to 1 to about 13.0 to 1. A not-so-obvious point is inherent in most carburetors. This is the fact that most carburetors are capable of handling the additional air and fuel required for moderate power increases, but at some point, the air/fuel mixture ratio will become progressively richer because the additional air flow demand by the engine is met with a more-than-optimum fuel supply from the carburetor. It is therefore entirely possible that a moderate power increase may be accompanied by the requirement for a leaner air/fuel mixture, particularly near the top end of the engine speed range. So why try to drown an engine with a surplus of fuel that it can't use and doesn't want? However, if an air scoop is part of the induction system with the intent of supplying relatively cool air to a carburetor air box or whatever, it may be necessary to make a compromise or two in the air/fuel mixture ratio toward a very slightly rich condition at lower vehicle speeds to take advantage of the additional air available to the engine as the vehicle approaches its maximum speed.
But most air scoops are junk, doing more harm than good. It takes some really intelligent and careful work to design and execute a very food air scoop/carburetor air box system, but when it's right, it is certainly worth the time and effort of making a few wrong ones. Depending upon rules, not all vehicles are permitted the luxury of an air scoop/air box system, but if it is legal and acceptable, do it. A good air scoop/air box system will do and/or not do several things: (1) It will supply all carburetor air, being sealed off from secondary air sources. (2) It will be removed as far as possible from any source of heated air (radiator, oil cooler, etc.). (3) It will equalise cylinder-to-cylinder air/fuel distribution so that all spark plugs and all piston crowns appear as close to identical as possible under all operating conditions. (4) It will be located at a safe point above ground level to preclude the possibility of inhaling rocker, sock, rags, beer cans or other trash. (5) It will provide easy access to all carburetors. (6) It will not direct high velocity air across the carburetor air horns causing fuel to be siphoned from the carburetors. (7) It will provide a significant power increase, particularly nearing maximum vehicle speeds. Sound simple? Try it. A good carburetor air supply system may indicate a camshaft with longer effective duration for even better top end power, if the minimum engine speed is kept well above the stagger-stumble-lurch range.

FUEL FLOW IS IMPORTANT

An accurate fuel pressure gauge, mounted where it is easily visible, can be a valuable instrument in determining if the fuel delivery system is adequate, particularly at the top end of the engine speed range in the higher gears when the load on the engine is heavier and the time the engine is under the heavier load is longer. If the fuel pressure diminishes toward zero, don't tempt fate; shut the engine down, junk the existing fuel pump and install fuel delivery system that is capable of handling more than the required volume of fuel when the engine is in the speed range of maximum power or beyond, and this means fuel lines of adequate diameter and a fuel pressure regulator capacity well in excess of the anticipated maximum fuel flow requirement. Hitachi (S.U.). Mikuno (Solex) and Weber are all pressure-sensitive carburetors, and this is not simply a game of fuel pressure. When the volume of fuel delivered to the carburetors is at least adequate, or more than adequate, which is the preferred state, any fluctuation of fuel pressure will be minimal. When the volume of fuel delivered to the carburetors is not adequate, then the fuel pressure will drop toward zero and the carburetors will eventually run out of fuel, possibly with catastrophic results. And the instrument that can tell you if things in this area are right or wrong is a good, accurate fuel pressure gauge.
The fuel pressure gauge should be hooked into the main fuel delivery line as close as possible to the carburetors and between the carburetors and the fuel pressure regulator. Some sanctioning groups (National Hot Road Association, for one) will not allow fuel lines in the cockpit. This makes sense, but it means that the fuel pressure gauge must be mounted on the firewall in the engine compartment, and the hood notched out for clearance. Other race sanctioning groups (NASCAR, for one) will not permit electric fuel pumps. This makes sense, too, because in the event of an accident, an unconscious or semi-conscious driver cannot be expected to fumble about for a fuel pump and/or ignition switch. So know the rules of your group and build a fuel delivery system with more than adequate flow capacity around them.
So what has all this got to do with camshafts and valve train pieces? LOTS! I wish I had a nickel for each time I've tried to tell some dumbhead that his fuel delivery system, such as it is, is very likely worst than a worn-out stocker because there is no way it can deliver the volume of fuel to the carburetors. Sure! Lotsa pressure! Enough to sink the floats of all carburetors to the bottoms of their float bowls forevermore, but not enough fuel flow to move the vehicle from point A to point B without running the carburetors dry. But the camshaft is really to blame. Not top end power. Sure.
With the fuel delivery system right, you should be able to literally drown the engine with a sloppy rich, dripping air/fuel mixture at the very top end by nothing more than two or three sizes larger main metering jets. This should tell you (1) that such a condition can be accomplished with nothing more than a change of jets and (2) that the fuel delivery system is probably adequate, if marginal.