Читать книгу Chevy Big Blocks - David Vizard - Страница 7

INTRODUCTION

What makes one racer/engine builder a winner as opposed to another? In my experience, which is now spanning some half a century, I’d say it is attention to details. Not just any details but relevant details. That is what much of this book focuses on. It could be said these details come under the heading of “speed secrets” but within the top echelon of the performance industry’s big-block specialists pretty much all of what I talk about is well understood. But don’t assume every big-block specialist if they already knows what is revealed within these pages. There are maybe a couple of hundred at best, and if that sounds like plenty, consider how hard it is to find one of these specialists. They are spread out over the United States in about the same manner as the populace at large. Even if you found one, what are the chances they would part with any of their so-called speed secrets?

Often the reasons some speed moves are categorized as “secrets” is that their details are far from obvious. Indeed, they are very likely to be things you never gave a second thought to, assuming that it was a given and thus could be taken for granted. Well, you are probably going to be surprised at what you may learn within these pages.

Most of my engine building is for commercial clients who want the effectiveness of their products demonstrated by being used in a well-spec’d engine. Almost all of them comment on the amount of torque and horsepower I achieve because it is almost always 40 to 50 more than they had expected or hoped for. On one occasion a nationally known and well-respected Pro Stock cylinder head designer/porter was overwhelmed by the fact that Terry Walters, Jack Sain, and I made 105 hp more on his heads than he had expected. Was he pleased? Sure. But it was no accident. It was attention to detail, and that is a primary focus within these pages. Understanding which details are important and which are not means understanding one simple aspect of engine development. Here’s a phrase I’ve used before: Understand the nature of the beast. Let me expand on that statement. If you understand what characteristics the engine’s geometry and proportions are likely to give it, you can count on being one step ahead of your competitors right from square one.

I had a minor hand in helping out on this 565 street-build project. Built at Throttle’s Performance in Argusville, North Dakota, the builders wanted to see what was possible in terms of output with a set of off-the-shelf Brodix 24-degree heads. The result was more than 880 hp, and in this book, you will learn what it takes to make such an output (and more) for a lot less money than you might expect.

Proportional Consequences

I have covered the Chevy big-block’s size, proportions, and consequences before, but it won’t hurt to have a refresher because losing sight of it considerably erodes your big-block building prowess to the tune of at least 50 and possibly to a not-so-unlikely 125 hp!

The number-one factor limiting your big-block efforts in terms of power per cube is a valve size far too small for the displacement. Even when displacement is at the smaller end of the scale at, say, 454 ci, the valves commonly used in available heads are still far too small to effectively feed the engine. The consequences of this shortcoming are that the cylinder heads’ flow capability becomes of paramount importance. And when big-block heads are the focus of attention, the required port volumes to best get the job done are also of primary importance. Unfortunately a lot of what passes for accepted knowledge leads you down the wrong road. This is just another area where your build could lose 40 to 50 ft-lbs at low RPM for no gains at the top.

The plan for this build could be said to be only one rung from the bottom as far as available budget. But knowing what simple block mods could be done resulted not only in the saving of several hundred dollars but also some zero-cost power moves worth close to 20 hp.

The importance of having good cylinder heads is closely followed by the necessity to have a dynamically well-spec’d valvetrain. To be truly effective, the valvetrain must make the most of the intake valve’s flow capability. This, in turn, means you must diligently seek to maximize both intake valve acceleration and total valve lift. Now if this sounds easy, let me make this clear right away: That will not be the case unless you really work at it. The big problem here is the very substantial mass of the valvetrain itself, and this magnifies any negative issues a valvetrain can have. When you get to the discussions on heads and valvetrain, you begin to appreciate the value of this book as you learn about speed moves that you certainly won’t find in any other book on performance big-block Chevys.

Other Shortcomings

I have often heard that one of the faults of the Chevy big-block is that it can have more cubes than available cylinder heads can support. That situation may be so, but to look at it as a fault is indeed faulty logic in itself. Let me explain. Assuming the goal is to get power from the engine by any means (as opposed to power per cube), then at a fundamental level, the power achieved from an engine is only a function of the air it consumes. In simple terms, you can have either a 600-ci engine running 5,000 rpm or a 300-inch engine running 10,000 rpm.

When aircraft used piston engines, it was very clear early on in the development of such engines that the best power for a given overall size and weight of engine was to be had from the biggest displacement possible not the most RPM possible. In other words, bigger inches at lower RPM resulted in the best weight-to-power ratio. That also suited the engine’s application because propeller speed needed to be limited so that the blade tips did not go supersonic. So the big inches of the big-blocks are to your advantage. That is why I talk about heads and valvetrain in relation to moves, many less than obvious, that you can make to best utilize the cubes your big-block has to offer.

The valvetrain starts within the block. Knowing what can be done here can save you more than $100 if you are planning a short-cammed build. Not only that, but the torque output can increase along with that cash saving.

Choosing the best port size for the job is a critical part of a successful build. Adopting common wisdom can often severely impact low-speed torque while delivering no additional output at the top end.

Which is better, a roller cam or flat tappet? The answer you most often get is “a roller every time if you can afford it.” Unfortunately that is not possible when the budget is tight.

A Chevy big-block’s short deck height limits rod length, and as a result, it makes for a short rod/stroke ratio. You must constantly keep this in mind when attempting to spec out the best parts combination.

The short rod/stroke ratio of the big-block is another area of concern that is frequently discussed. The rod/stroke ratio is the center-to-center rod dimension divided by the crank stroke. It is common practice to use a stroker crank in a big-block, but even a stock-stroke 454 with a stock rod does not fair too well in terms of what it has versus what the engine may like. The stock rod/stroke ratio for a 454 works out to 1.576:1 (6.185 ÷ 4). This is on the short side for sure. To see by how much, let’s consider, say, the rod stroke ratio of a successful high-output Chevy small-block. Where power per cube, due to displacement rules is called for, the most favored rod/stroke ratio is in the range of 1.7:1 to 2:1. In other words, a much longer rod in relation to the stroke.

So what are the disadvantages of a short rod? The answer here is that if you take friction out of the picture: virtually none. Unfortunately, friction is ever present and carries with it a considerable negative impact. Studying the geometry, the shorter the rod the greater the rod’s angularity, causing the gas pressure to push the piston into the cylinder wall with greater force. My thoughts here are that this is something we are stuck with, so there is no point in fixating on it other than to look for 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.