assuming the tests in Figure 1.4 are an absolute, let me make a couple of points clear: A test like this cannot be done as a simple “A versus B” comparison. Cutting away the block means a reduction in compression ratio (CR). Sure, it is not much and if nothing else changed it would, in our 10.5:1 CR test case (a 475-ci unit), have amounted to about 0.2 reduction in ratio. Being aware of this I used a thinner head gasket to partially compensate. The reason for only partially compensating is that a thinner head gasket also tightens the quench/squish clearance between the head’s face and the piston at TDC. This also increases power so I estimated from quench tests what it was likely to be and settled on a working compromise. This means that you need to use the test results of Figure 1.4 as a guide to the value of cutting away the bore, rather than as an absolute.
Another point to bear in mind here is that this test unit had a 0.100 overbore. That in itself relieves some of the shrouding of the intake so block chamfers were needed less on this test unit than would have been the case for smaller bores. The fact that the chamfers are effective is also borne out by the trend of engines without them seeming to make less power than those with them.
Intake Versus Exhaust
As far as effectiveness goes the shrouding reduction of the intake is far more influential than the exhaust reduction. The intake seems to account for about 85 to 90 percent of the possible power gain. This means that unshrouding the intake is far more important than unshrouding the exhaust, which means that moving the heads across the block to further unshroud the intake at the expense of the exhaust is worthwhile. By using head-locating dowel rings that are offset you can move the heads across the block up to about 0.020 inch. Although this unshrouds the intake at the expense of the exhaust you are still very much on the winning side.
All the foregoing leads to the possibility of some additional power if you are committed to a certain piston size that is not at the bore limit or you have class rules limiting displacement. However, let me make it clear that bore size for increased displacement is always the number-one priority. With that in mind here let’s investigate bore offsetting and see how it plays into the production of a performance block.
In my previous Chevy big-block book, I discussed how to maximize bore size with a casting where the cores had shifted. This involved offsetting the bores to maximize the amount of overbore that could be accommodated. This involved shifting the bores up to about 0.025 inch in the direction of the thicker wall. Offsetting the bores can have a power advantage if the offsets are thoughtfully done.
Fig. 1.5. If the block casting is sufficiently thick, there is room to make favorable moves on the bores. Another way to further deshroud the intake valve is to move the bores in the direction of the red arrows. In a similar manner you can also get the effect of an offset wrist pin by moving the bores in the direction 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
