ways to make the best of it. Fortunately, there is a ray of sunshine here. Knowing how best to deal with a short rod/stroke ratio is a substantial step forward and thus a distinct advantage over someone not so informed. In fact if what is said later in the book is absorbed you can actually shrink that short rod problem to near zero.
Delivering Results
So what is it worth to fix big-block shortcomings? To put it another way, What sort of power increase can you expect by parting company with the price of this book? Because I work on these engines almost exclusively, and most of the work I do is in the area of research and development, I can be relatively specific here. It is fair to say that probably better than 98 percent of the time someone who acts upon the information delivered here achieves a minimum of 50 extra hp, with an average of something around 75 to 100 hp.
Indeed, in many cases, target power levels are achieved at considerably less cost than would have otherwise been the case. How do I know this? Simple, I get plenty of calls for advice from pro engine shops asking if I have any moves that may show more power without a cost increase; as the shop down the road is beating their numbers for less money and getting their customers as a result.
Cam timing is critical. It’s entirely possible to lose 10 to 20 hp because you don’t know which factors you may have built into your engine. These can dictate a different cam advance than the cam manufacturer’s cam spec sheet.
Because of the short rod/stroke ratio, compression can be one of your closest allies. Be sure you have all the relevant facts because it can make a much bigger difference than with many other engines.
There are at least 20 hp locked up in simple block mods that cost only time. You need to ingrain it firmly in your mind that a max-performance big-block requires many cubic inches. That means nothing smaller than 427 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
