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 iron. Conceptually, a plateau hone finish might be just the ticket, but achieving it may not be within the realm of practicality in a local machine shop. In any event, I take a more practical but still conservative approach, which I feel is about the middle ground.
Fig. 1.7. This is a top-quality bore finish that works just fine, but can be improved upon if you are willing to work at it.
Fig. 1.8. Here is the bore finish on one of my engines. Note that the hone pattern is visually indiscernible, and the overall finish is nearly mirror-like.
I suggest you use a machine shop that works on race engines as a major part of its business. That means they have experience in this area. If the shop you are using does race engines regularly, they may well have an effective standard procedure. A number of viable routes can lead to an effective bore finish.
If you don’t want to go with what your machine shop is offering, here is what I do: I call for a bore finish smoother than that used for a typical production engine. This usually entails taking out the last half thousandth of an inch slowly to avoid overheating the bore and overloading the stones. This final operation requires a set of 400-grit stones, rather than finishing with the 320-grit ones used for a production rebuild. After sizing is achieved, the bores should then be final finished with a brush hone to smooth out the micro scarring that is inherent with any abrasive metal removal process.
Fig. 1.9. The Total Seal dry powder ring break-in lube has shown good results on the dozen or so engines on which I have used it. It is inexpensive and I recommend it for any budget-oriented build.
Final Finish
The following is the final finish procedure that I use, but I should also tell you that it is frowned on by some and embraced by others. Some of the tech guys at Sunnen, the company that makes the honing machines, agree this is one viable route to go.
