Racer Brown Chapter Sixteen

Minor Changes at the Track

So now we're at the racecourse and (almost) ready to race, but the indications are that the engine doesn't have enough muscle at lower engine speeds. What can be done to fix it quickly? Several things. Assuming the venturi diameter of the carburetors is not too large, and that the air/fuel mixture is "lean and clean" through the engine speed range (black smoke and the "blubbers" are out), and you're stuck with existing intermediate and final drive ratios, the most direct approach is to advance the camshaft a few degrees. The L-series Datsun engines are sensitive to this treatment and respond well to it. This improves low and mid-range torque by the simple expedient of closing the intake valve earlier, although the entire camshaft must be advanced to capture the beneficial effect upon torque output at lower engine speeds caused by the earlier closing of the intake valve. A "few" degrees means to advance the camshaft enough so the engine can recognise that a change has been made, but not so much as to hill the performance level at higher engine speeds. A good starting point is to advance the cam from 3 to 5 crankshaft degrees (1-1/2 to 2-1/2 camshaft degrees). Presumably, the earlier advice of lots of piston-to-valve clearance has been well taken, so that after this change is made, piston-to-intake valve clearance will not be dangerously diminished. Just make certain the cam is advanced and not retarded. This means that when viewed from the front of the engine, the camshaft must be moved in a clockwise direction in relation to its present position and everything else stays put.
Another change that can be made to improve low-end torque output is to increase the intake valve lash by from 0.004 to 0.006-inch, perhaps a bit more, but this approach is not as direct as advancing the camshaft. Most cam lobe profiles have a clearance ramp long enough to accommodate a reasonable change of valve lash before the ramp joins the cam lobe flank. There are a couple of practical limitations here. The length and shape of the clearance ramp has a direct influence upon the maximum safe engine speed of a given cam lobe profile, so the valve lash cannot be increased to the point where the clearance ramp is by-passed because this can make life very difficult indeed on the cam lobe/rocker pad interface stress conditions. Another factor: If the valve lash is increased too much, the valve lash will improve low-end torque by effectively shortening intake valve duration slightly, and also by a slight reduction in effective valve overlap. Again, make enough of a change so the engine will know a change has occurred, but not enough to ruin the performance level higher up, or to cause unnecessary valve train damage.
If this captures about half the missing torque, it is usually possible to gain some on the other missing half by applying the same trick to the exhaust valve lash. Don't expect to make and improvement of the same magnitude here, though, because the engine is not as sensitive to slight changes in effective exhaust duration as it is to similar changes in effective intake, and by observing the same limitations and precautions.
There are very few races in the world, and still fewer racecourses, where total, maximum; last-gasp top-end horsepower is an absolute requirement for the fastest possible vehicle speed or lap time. Some of these may include the Super tracks like Daytona (excluding the road race section), Talledga, Bonneville (the Great White Dyno), flying kilometer time trials for boats and maybe, if the gearing is just right, Ontario, Indianapolis and LeMans. The vast majority of race courses, ashore or afloat, have one point in common: all other things being equal, the races become a series of short, medium or long drag races as far as engine performance level is concerned. This means that both the engine and the vehicle must have the ability to accelerate - a word that strongly and correctly implies that the engine must work at its very best through an engine speed range. There are no high-performance engines - none - that are run at a constant, unvarying engine speed. If this were the case, an engine could be tuned to produce ultimate power output at a fixed engine speed to the utter exclusion of every other factor, and it would be very much easier to do so. Instead, all engines must operate through an engine speed range. Sometimes the range must be very broad, a condition that demands flexibility, even if it means sacrificing a few horsepower on the top end to gain the required degree of flexibility elsewhere within the working range. And flexibility, even if it means sacrificing a few horsepower on the top end to gain the required degree of flexibility elsewhere within the working range. And flexibility spells combination much more loudly and clearly than it spells maximum horsepower effort.
About the least likely place on earth to qualify, as a drag race is Bonneville, but it's true; it is indeed a drag race. The vehicle must start from zero miles per hour and reach its maximum speed within a fixed distance. If the engine doesn't have the ability to accelerate (that word again) because it lacks some essential factor for the right combination (that word, too), it's a waste of time because it isn't even an acceptable place for a vacation. The fire-time Bonneville competitor invariably arrives with his equipment overgeared, overcammed, underfueled, wrongly-tired, over-carbureted, under-jetted, wrong attitude of vehicle at speed, a totally inadequate air induction system and un-knowledged of the peculiarities and perversities of the place. Besides, he will have forgotten his metric toolbox. A week later, he leaves the wretched place with a truckload of fragments, a junk ex-race car and he has emergency hospital cases of sunburn, dehydration, malnutrition, hypertension, exhaustion, shock and a hangover. But it's a fun way to race, it affords a weeklong opportunity to find the right combination, even if it was left at home, and it does give some insight into the mysteries of tuning for flat-out maximum power. But it is STILL a drag race.
In such a case, as rare as it may be, if better maximum power output is required to satisfy a given condition, you have to rob Peter to pay Paul. In other words, it's a trade-off of low and mid-range torque for better output at the top end of the engine speed range. As on might expect, the correct approach is the exact opposite of that for improving low and mid-range torque; that is, the camshaft should be retarded a few degrees and the valve lash should be decreased, or a satisfactory combination of both. In retarding the camshaft, be careful. The maximum amount advisable initially is 3 crankshaft degrees (1-1/2 camshaft degrees); as previously mentioned, the centrifugal action on the timing chain at higher engine speeds will account for an additional 1 to 2 crankshaft degrees, perhaps more, depending upon the condition of the chain. Of course, this assumes adequate piston-to-exhaust valve clearance after the camshaft has been retarded. Again, make certain the camshaft is moved in the correct direction. To retard the camshaft, the cam must be moved the desired amount in a counterclockwise direction when viewing the engine from the front and all other associated pieces stay put. After the camshaft has been retarded, it is highly advisable that a valve timing check and a piston-to-exhaust valve clearance check be made to be dead certain there has been no slip-up. A relatively small error in this direction and you've got a handful of bent exhaust valves at best, and they don't even make decent paperweights.
Decreasing the valve lash will help top end power slightly but in this case too, retarding the camshaft is the most direct method and the results will be more positive. Moderation is required in squeezing the valve lash closer; initially by no more than from 0.002 to 0.004-inch, then work the engine good and hard, shut it off and very quickly measure the valve lash with the engine temperature as hot as it is likely to get. This is a necessary precaution because when the valve lash is decreased, the effective duration is increased slightly and as a function of this, the valves are seated for less time, therefore there is less time available to transmit the heat of the valves to the valve seats. As a result, valve operating temperatures are increased somewhat with a consequent increase in the thermal expansion of the valves, which further decreases the valve lash. Obviously, there must be some valve lash, even if the entire engine is at the melting point. Titanium and stainless steel valves are affected more by changes in valve operating temperatures than more conventional valve steel alloys. In any case, take it easy in this area and approach the minimum practicable valve lash condition gradually, if for no other reason than the increased thermal expansion of the valves could easily be more than anticipated, in addition to which, there is a point of diminishing returns.

When to Shift?

This data leads up to the questions of shift points and how fast to run the engine in top gear. Best point-to-point acceleration is usually obtained when the engine is permitted to run from 6 to 8% higher than the speed at which max power occurs. If max power occurs at 8,000 RPM, Shift points in the intermediate ratios should be at about 8,500 to 8,650 with decent intermediate gear ratios. If there is a gigantic hole in the 2-3 intermediate ratios, the shift-point in 2nd will probably have to be increased to about 8,800, perhaps 9,000. If max power occurs at 7,600 RPM, shift points should be in the 8,100-8,200 range. The purpose of over-speeding the engine beyond the point of max power is to be certain that after a shift has been made. Engine speed will drop back to a point between max-torque and max power so the engine won't have to pull itself up by its bootstraps in the lower engine speed ranges. Max-sustained engine speeds in top gear should be about 8,200 to 8,300 RPM with L-16's about 8,000 with L-24's and about 7,800 to 7,900 with L-18's. It doesn't do much good to run an L-18 faster than about 8,500 in any case because the engine has inherently better torque output at lower engine speeds than either the L-16 or L-24. These RPM limits are meant as suggestions for starting points only. Each engine-vehicle combination could very likely require on or more modifications to these limits for best overall performance and the only way to find out for sure is to try different RPM levels in the intermediate gears and in top gear.
While Datsun 510-610-Z-cars are very popular for anything from transportation hacks to flat-out race cars, Datsun engines are deservedly filling in some niches very successfully that have been dominated by other makes. A few of these would include midget race cars, small drag race vehicles, dune buggies, sand drag racers, Baja-type vehicles, marine installations, etc. And why not? The engines are very strong structurally as well as in power output, they are hard to break, are fairly light and they have the ability to absorb more punishment and abuse than they deserve. In addition, they have the capability of actually producing better power than other designs that may look better but can't produce.

AUTOMATIC TRANSMISSIONS

The foregoing is more-or-less related to Datsuns equipped with one of the several combinations of standard 4-speed or optional 5-speed gearboxes. A special note regarding camshaft selection for any of the L-series Datsuns equipped with an automatic transmission is necessary. The convenience of an automatic gearbox, particularly in bumper-to-bumper traffic, cannot be ignored. But from the standpoint of performance, the automatics used in L-series Datsuns take quite a bit from the engines because the torque converters sop up power like a blotter and the intermediate ratios are awful, the worst being from 2nd to top gear. However, a few tricks can be applied to the 3-speed automatic-equipped Datsuns to help close the performance gap, as compared to similar cars equipped with 4 or 5-speed gearboxes. If better acceleration is desired, and this would almost have to be the first consideration with the automatic, a higher numerical final drive ratio is in order. If the final drive ratio is say, a 3.5, then a 3.9 ratio, which is about a 10% differential, would automatically (no pun) improve acceleration without having the engine excessively buzzy at normal highway cruising speeds. Judicious use of the throttle foot won't destroy fuel economy; in fact, it may improve. An L-18 engine in an L-16 vehicle will give and 11% increase in piston displacement and an increase in torque of about 12 to 14%.
Any other internal-external engine mods must be very conservative indeed. The engine must idle, in gear, in a civilised manner and at an acceptable engine speed because the torque converter places a load on the engine, which makes it doubly sensitive at idle and just off-idle, and this is where any performance gain (or loss) will be most noticeable. The message here is to improve torque output in the lowest engine speed ranges. Presumably, an automatic-equipped Datsun is strictly a transportation chug suitable for driving almost anywhere by your wife, girlfriend or mother (or all three), but there is no valid reason for it to be dull. In this case, a compromise is obvious, and this must be in the engine speed range of max power, but with your wife, girlfriend or mother (or all three) aboard, you'll never get a chance to use it. However, acceleration can be pretty zippy (comparatively) if the camshaft is mild, MILD. M-I-L-D! Effective duration should be in the low to mid-220 degree range with from 4-8 degree overlap (no misprint) with lift in the 0.450 to 0.460-inch range so it won't die completely at 6,000 RPM or so. This isn't a budget-busting installation either; the only special items really required are the camshaft and a set of valve lash pads of the correct thickness. New stock Datsun rockers are a must, and a new set of late-type L-series valve springs is highly advisable. There are a couple of other indirect economical and ecological advantages too: Fuel economy will very likely be measurably improved along with significant reductions of exhaust emissions. The only real penalty with a camshaft of very short effective duration and reasonable valve lift is that the valve motion is fairly swift so the engine may be somewhat speed-limited to a safe 6,500 RPM.