Chevy Big Blocks. David Vizard
inches. Starting with anything smaller sets you up for a power per dollar failure.
As far as blocks are concerned, many power production techniques involve the cylinder bores in some way. The fact that the combustion chamber overhangs the block and thus adds to intake valve shrouding means anything that helps unshroud it is beneficial. The difference in breathing capability of a small-bore (4.310 inches) 500-inch engine with a 650-hp output versus that of a lesser, shrouded, big-bore (4.5 inches) engine with the same heads is about 20 to 25 hp on peak and about 30 hp at about 600 to 700 rpm past peak. I estimate about 4 to 6 hp of that difference is due to the reduced ring/piston friction a shorter stroke engine has, but the rest is due solely to the increased breathing capability. Even a big-bore engine still has some shrouding in the vicinity of the intake valve where it most closely approaches the cylinder wall. For a 24-degree Chevy big-block head, minimizing this shrouding effect is more important than it may at first seem because a less-than-obvious factor concerning the intake flow pattern is developed in a typical 24-degree head’s intake port.
If you consider a typical pushrod V-8 port style, the dominant flow path into the cylinder takes place through the part of the intake valve circumference that is open to the center of the cylinder. However, a typical 24-degree Chevy big-block head’s ports have something of a flow anomaly for both ports. But the flow anomaly is more apparent for the bad port. (See Chapter 4, Cylinder Heads, for more information.) This anomaly brings about a potential high-flow area well toward the cylinder wall side of the valve, and that area is most shrouded by the chamber wall and the cylinder bore. Failure to appreciate its existence can cancel out this potential high-flow area, and as a result, you can lose a measurable chunk of power.
This is valuable knowledge that less than a handful of big-block engine builders probably know. I estimate that knowing what to do here to allow the motion of this flow anomaly through and past the intake valve is probably a 20-hp advantage.
Fig. 1.1. Other than typical reconditioning procedures, many moves can be done to a stock block to improve engine output.
Fig. 1.2. Here you can see how much the combustion chamber overhangs (red line) the cylinder bore (yellow line). This is a 4.290-inch bore and you can see from the valveseat (transparent blue) that a 2.3-inch intake valve only clears the bore due to its canted angle. Chamfering the top of the block drastically reduces the negative effect the sharp edge of the bore has on flow.
Just so you are primed, taking advantage of this flow pattern also involves piston reshaping when a big-dome piston is used. (See Chapter 2, Pistons, Rods and Cranks.) Small-bore engines are the worst bore-shrouding offenders but I am making a big deal of this point as they are the most common blocks with which to start a build. The first move is the block chamfer operation. It is important enough for me to cover it here in detail.
Let me say up front that regardless of bore size none of the block/head combinations I discuss here are free of shrouding. However, big-bore blocks, that is, from a 4.466-inch diameter (stock 502) on up, are very much better in this respect.
Fig. 1.3. Here is what bore chamfers (or deshrouding) look like. The intake side is a very effective power enhancer, but the exhaust side, even though it helps, makes only a relatively small difference.
Fig. 1.4. This test shows the difference in output without valve deshrouding block chamfers versus a block with deshrouding chamfers. Tests such as this are not as straightforward as it may at first seem.
Cutting block chamfers is easy enough. First check the fire ring form on a head gasket against that of the chamber. With aftermarket heads, in most instances, the combustion chamber perimeter closely matches the head gasket. If this is the case you can use the head gasket as a template to outline the block deck to establish just how far to go with a die grinder. As to how far down the bore to go this should be limited to about 1/16 inch shy of the position of the top ring at TDC. Just in case you are wondering if it is really worth it, check out the dyno tests showing before and after results in Figure 1.4.
Before 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