Racer Brown Chapter Nine

PISTONS AFFECT BREATHING, ETC.

With a lot of luck, many prayers, and maybe some intelligent letters to companies involved in the manufacture of such items, there may be a decent aftermarket piston available some day for the Datsun cammer. Venolia markets on forged type now and Interpart utilises a TRW piston forging. Both of these have terribly high domes. In any case, the design criteria for an acceptable piston, aside from a good stiffness-to-mass ratio, proper skirt design to prevent skirt collapse, good piston ring placement, minimum piston pin exposure between the pin bosses in the piston, etc., should be: (1) Acceptable compression ratio, particularly for modified L-16 and L-18 and "open" type L-24 combustion chamber cavities, but without a high piston crown resembling a misplaced Alp. (2) Adequate space around the spark plug to give a good strong point for combustion propagation. (3) More-than-adequate crown thickness in the area of the valve reliefs, so that the valve reliefs can be made not only deeper but with radii somewhat larger than the valves, and without weakening the hot-strength of the piston structure. (4) A piston designer with brains enough to recognise the importance of the fact that the air/fuel mixture and/or exhaust gases simply DO NOT and will not flow properly around sharp edges and corners. Perhaps as a very minor minority of one, I would like to see a piston in which the intake and exhaust valve reliefs are joined together to form a single "trough" type valve relief, similar to the original equipment Chevrolet 302 Z-28 piston.
Item 2, 3 and 4 are essential for proper cylinder breathing, particularly during the valve overlap period, and particularly at extremely high engine speeds, say above about 8,500 RPM on up. However, these items are contrary to Item 1, acceptable compression ratio. Nearly everyone has heard of some ridiculously high compression ratio numbers bandies about for these engines, but I can assure you that it is extremely difficult indeed to get and honest, genuine measured compression ratio of over 12 to 1, without shrouding the valves, the spark plug, or both, and still keep the power curve relatively flat above about 8,500 RPM. Again, this refers to the "open" L-24 type combustion chamber with valve unshrouding modifications, but it also applies to similarly modified optional L-16/L-18 large-valve heads. If the engine is going to run well and produce very good power in the 8,500-plus-RPM range, it must have breathing room to do so, even if it means a sacrifice in compression ratio.

PISTON-TO-VALVE CLEARANCE

Lots of piston-to-valve clearance does more than allow the engine to breathe well at extreme engine speeds. It also provides the engine with a "cushion" of space in the event of a missed shift or broken driveline component when the engine speed would tend to go completely out of sight, and thus minimises any engine overspeed damage. It also permits the camshaft to be advanced or retarded to suit conditions of the moment.
Given a food dynamically stable cam lobe profile, Datsun cammer engines have been known to run up to well over 10,000 RPM without damage of any kind, but no one in their right mind would do this intentionally. Peak power in a well and properly modified maximum-effort Datsun cammer engine usually falls in the 7,600 - 8,000 RPM range, so an overspeed condition to 8,800 or 9,000 RPM is acceptable. For once, this is a case where the valve train is not the speed-limiting factor of the engine. Happily, this is because the camshaft is well-supported in the cylinder head and is stiff by itself, the rocker arms are stiff, and the reciprocating mass of the rocker, valve, vale springs, spring retainer, lash pad, etc., is quite low.
We normally recommend a minimum piston-to-valve clearance of at least 0.090-inch between the piston and the intake valve at their closest point, and at least 0.100-inch and preferably more - between the piston and the exhaust valve when the piston-to-valve clearance is measured by rotating the engine by hand. In an operating engine at full blat, these numbers are usually reduced by about half because the crankshaft bends, the connecting rods stretch, the piston pins bend and the pistons stretch, and it all happens just before and after top centre when the pistons change direction, the least opportune period for maintaining adequate piston-to-valve clearance. The condition is made worse during a closed-throttle down-shift, or similar circumstance when the cylinder (under a pressure vessel) is changed into a vacuum vessel by the closed throttle, at which time the high vacuum in the cylinder tends to draw the pistons and valves together more closely than when the engine is under load.
The reason why additional piston-to-exhaust valve clearance is called for is because any camshaft drive tends to permit the camshaft to retard itself in relation to the crankshaft, thereby bringing the exhaust valve closer to the piston. This is particularly true with a chain-driven camshaft because as engine speed increases, centrifugal force acting upon the chain pushes the chain away from the sprockets and the chain rollers contact the sprockets increasingly closer to the outer edges of the sprocket teeth, and all the while the camshaft is resisting rotation due to friction, valve spring loading, etc. Chain stretch compounds the problem. It is a function of the load applied to the chain and of course the number of links in the chain. Datsun chains are quite long, about 42 inches in circumference, with 110 links, which means that keeping the valve timing exactly right at all times, is nearly impossible. It is therefore better to start out with a slightly advance camshaft - say by 3 or 4 crankshaft degrees - because there is no way in the world that it can be kept from retarding itself as the engine is run, particularly at high engine speeds. This is also the reason for the three different timing marks and three different dowel pin holes in the Datsun camshaft sprocket; they make provision for advancing the camshaft in 4-crankshaft degree increments, but they make no provision for retarding the camshaft. It does that by itself with no outside help required!
A high dome, high compression ratio piston that shrouds the valves and the flame front isn't all bad; it really wakes up the low and mid-range torque, but don't expect the engine to have good breath control at 9,000 RPM. It won't!
Naturally, any increase in cylinder bore diameter and/or crankshaft stroke will raise the compression ratio with a given combustion chamber cavity volume. In fact, a couple of 2.5 TransAm Datsuns were L-18 engines bored and stroked to just under 2,000cc's.
Obviously the cylinder head can be milled to gain compression ratio, but with the Datsun engines, there are some not-so-obvious factors involved. Milling from 0.060 to 0.070-inch is considered a "safe minimum," and then there may be head gasket sealing problems. If there are sealing problems, the cylinder head should be O-ringed with 0.030 to 0.040-inch diameter soft copper armature wire or soft stainless steel wire, leaving about 0.010 to 0.012-inch of the wire exposed from the cylinder head face to give the required seal around the cylinder bores.
O-ringing the cylinder block is not recommended unless the final cylinder bore honing operation is done with a honing plate, cylinder head gasket and O-rings installed and torqued down. If this is not done with O-rings in the block, the O-rings will cause the tops of the cylinder bores to shrink about 0.002 to 0.003-inch on the bore diameter, and will affect cylinder bore diameter for about and inch down from the block face when the cylinder head is bolted to the block. We really don't need sticking pistons.
When a Datsun cylinder head is milled, on and possibly two other items must be put right. With the camshaft in the cylinder head and the head milled, the centre-to-centre distance between the crankshaft and the camshaft is shortened, which means there is extra "slop" in the camshaft drive chain. To correct this condition, the lower end of the left chain guide (viewing the engine from the front) must be moved to the right. Chain guide bolt holes are slotted for this purpose, but it may be necessary to slot the lower hole more to take up the additional slack. This also causes the chain tensioner piston to extend itself further from the chain tensioner housing. If the piston extends from the housing more than about ½-inch, it may become stuck in the bore, unable to move in either direction, and worse, may not be able to take up the chain slack, perhaps causing the chain to skip one tooth or more on either sprocket, at which point the entire engine is in deep trouble. A sure fix for this is to make a thin-wall steel sleeve 1-1/4 to 1-3/8 inches long to press over the existing piston, then bore out the tensioner housing so the sleeve slip fits. Sleeve wall thickness should not be more than 1/16-inch. Sounds like a lot of monkey-motion, but it may be essential if the cylinder block face is machined for minimum deck clearance and if the cylinder head is milled the maximum amount. One more point: If the cylinder block and cylinder head are milled, it is mandatory that all cylinder head bolts be checked in their respective tapped holes in the cylinder block to be certain they do not "bottom" in the holes rather than clamp the cylinder head.
If you have a 68-69 four-cylinder engine with the ratchet-type chain tensioner, throw it away. Replace it with the later type or the FIA type in the competition parts listing. Do not attempt to interchange L-24 tensioners with those for the L-16/L-18 or vice versa.