Chevy Big Blocks. David Vizard
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of the blue arrow. Combining the moves of the red and blue arrows results in the bores moving in the direction indicated by the green arrows.
For instance if the block can only be bored, say, plus 0.040, and the casting is thick enough, the bore centerlines should be moved toward the intake valve because this relieves the shrouding to a greater extent than if the bore stayed on its original centerline. Also there is the possibility of simulating the effect of an offset piston wrist pin.
The point here is that if the bore diameter is limited before that point, these centerline moves are a way to get back some of the possible deficit. With that in mind let’s consider the effects of offset pin-to-bore centerlines.
Piston Wrist Pin Offset Potential
This subject has generated a lot of controversy about whether offsetting the piston wrist pin creates power. I should tell you now that you will find plenty of theories and even some dyno evidence to the contrary. You should take my findings on this subject and put whatever value you feel is worthwhile on it.
Pin offset is the practice of relocating the piston’s wrist pin so that it is offset toward the major thrust face of the bore. Having the pin offset in the opposite direction of crank rotation means that TDC occurs slightly sooner than otherwise would be the case. At TDC of the power stroke, the pressure is still considerable although still somewhat short of peak. However, because the crank centerline, rod journal centerline, and wrist pin are in a straight line, they are “dead-locked,” so no torque is transmitted to the crank via the prevailing cylinder pressure.
Fig. 1.6. Here you can see that offsetting the wrist pin means that the rod transmits its downward force to the crank at a more favorable angle. This geometry advantage comes into play immediately after passing TDC.
Because of the offset, the rod-to-crank angularity comes on faster after TDC than it would without the offset. The application of the pistons’ downward force on the crank has a more favorable geometry. The initial result is the pressure in the cylinder is communicated sooner during the power stroke than would otherwise be the case.
Let me clearly state that something like 80 percent of the power that is generated in a high-performance engine occurs in the first 20 percent of the stroke. So if the pin offset allows more power to be extracted early on, it should be a move for the better. This effect is seen to a greater extent with the shorter rod/stroke ratios. Big-blocks with short rods are in theory at least, prime candidates for such a move.
Of course the dyno is the place to see if that is so. I ran such a test, not in a Chevy big-block, but in a 2-liter Cosworth YB engine in 1989 when I was racing these engines in the United Kingdom. These tests showed a 3½-hp advantage with a 1-mm offset in a 250-hp engine. With short rods and large displacement, this result bodes well for big-block builds, but it is not without certain issues that must be addressed.
Acquiring Pin Offset
You can acquire pin offset by two means: First and most efficiently, you can offset bore the cylinders. Second, you can use custom-made pistons with offset pins. The original reason for offsetting wrist pins is to reduce piston slap, especially on a cold start-up. If the piston is symmetrical and balanced about the pin centerline (so that one side of the piston is not heavier than the other side due to a dome or valve cutouts) as the piston comes to TDC, the loaded side reverses and the piston rattles in the bore.
By offsetting the pin the cylinder pressure loads one side of the piston more than the other thus tending to keep the piston in contact with one side of the bore more than the other. The fact that the piston has a tendency to become “cocked” in the bore means the skirt is being pressed into that bore with more force, and as a result, piston-to-cylinder wall friction increases. This is the price of a quieter running piston. If the offset goes toward the major thrust face, the piston assembly has the benefit of a better geometry, but too much offset means that the advantages of the geometry are countered by increased piston friction.
Bore offsetting, in contrast, does not carry any significant friction penalty.
Gains and Issues
If the piston assembly-to-bore friction were zero, pin offsetting would work every time, but it is not. That means that if a piston offset is used, great care must be taken over the preparation of the bore finish, and you must select a piston/ring that has minimal friction.
There is no simple answer as to how much pin offset can be used before geometry gains are erased by increased friction. However, as the compression ratio is increased the amount of useful pin offset becomes larger. The limits of offset, friction, and increased compression ratio would take a dyno test session beyond anything I can afford, so I tend to err on the conservative side. The most I have ever used has been a 0.020-inch offset of the bore along with a 0.040-inch (1 mm) offset of the pin in the piston. As near as I can tell that move is worth about 8 to 10 hp in an otherwise 650-hp 468-ci engine.
Modifications to the Bottom of the Bores
I received another block modification from one of my former students who has since become a premier engine builder in the United Kingdom. Much of his work is in the field of 20,000-rpm MotoGP motorcycles. The mod was applied to his championship-winning Mini Cooper engines, which won every race for the championship in 2011. This mod is totally contrary to common practice. Typically, the lower edge of the bore has nothing other than a small chamfer on it. The intent here is that it scrapes off excess oil from the piston.
Although this seems like a logical function, let’s consider what happens when the piston moves up the bore rapidly: It has to draw up the air and oil mist with it. When trying to optimize the flow of air into a hole such as on an induction system, we usually go to great lengths to make sure it flows as easily as possible into the system. Ram stacks are typically used because they all have a nice entry radius. When the piston goes up the bore, the air moves into the space beneath it in the same manner. It was discovered that when the bottoms of the bores had a generous radius applied the power increased.
I recognize that this big-block Chevy is not a 20,000-rpm MotoGP engine, but neither is a Mini Cooper engine, and this modification produced positive results for both. Even on this small 79-ci engine, a power increase of a couple of horsepower was seen, so on all my serious big-block builds I now radius off the bottoms of the bores. (See Chapter 3, Lubrication Systems, for photos.) And, for the record, there seems to be no ill effects on oil control.
You would think by now the subject of bores and what finishes they should or should not have would be wrapped up. If the trends I see in the new century are anything to go by this seems not to be the case. Some top pro engine builders are within my circle of close friends. The bore finishes in their championship-winning engines range from a significantly finer honed finish than normal to a mirror polished finish. You need to pay attention to this because the big stroke and large pistons of a big-block are prime candidates for a substantial loss in output due to piston assembly-to-bore friction. Sure, you will hear stories from many machine shops about customers’ engines in which the bores were too smooth and the rings never seated well. But the reasons for this happening could be due to factors other than too fine a bore finish.
You might also hear that plateau honing is apparently the answer when it comes to honed surface finishes for a high-performance engine. This may be so at the high end of the engine building scale, such as for the Cup Car guys, but it is not necessarily the same or the best approach for the serious enthusiast having a block prepped by a competent local machine shop. Your local professional machine shop may have all the equipment to apply and measure the finish for the particular material content of your block. But your block and a Cup Car block are not made of the same cast