Читать книгу Chevy Big Blocks - David Vizard - Страница 8
ОглавлениеThere 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 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 minimal friction.
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.
After the block is back from machining, spray Gunk engine cleaner down one bore at a time and use a green Scotch-Brite kitchen pad to rub the bore surface in an up-and-down motion. This process only needs to be done for a couple of minutes or so. To test the finish, I wipe the bore clean and see if a finger nail runs up and down the bore with a very smooth feel. You can easily tell the difference by checking a bore that is “as honed” compared with the one you are working on. It usually takes about an hour to prep 8 bores like this and maybe another 30 minutes to wash and dry them. Cleaning the bores in this manner processes the finish at a micro level in the direction of the piston/ring motion.
To make full use of this friction reducing process you also need to prep the rings in the manner I describe in Chapter 2, Pistons, Connecting Rods and Crankshafts.
After a good brush hone finish from the machine shop, you can apply the above final finishing step so you smooth off any micro raggedness at the tips of any pinnacles, which might still be sharper than you would want.
With the bores prepped, a long break-in, although still advisable, is not necessary. I cannot definitively say how much power this produces, but I can tell you that I have used this technique on engines that have finished first in every race in the process of winning a championship.
After “Scotch-Briting,” the bores should be repeatedly washed, first with detergent and hot water, then with lacquer thinner. Be sure you have a good air line because after the water wash the bores will rust in seconds. Also have some WD-40 ready.
The lacquer-thinner wash continues until a white paper towel wiped over the bore remains totally white.
As a last check, apply some Total Seal dry powder break-in lube to the bores. If it turns green when it’s rubbed on the bores’ surface, the bores are not clean enough, so start again and reclean them until they are absolutely clean.
Bore Finish and Break-In
Regardless of your bore finish specification be very aware that the break-in procedure is important to get the best output and life from your big-block. Over the years I have tried many break-in procedures and felt that they were less than optimal. This led me to develop my own break-in lube and procedure. I did this in conjunction with Oil Extreme. You should note what I have to say about this in Chapter 2, Pistons, Rods and Cranks.
Following my Oil Extreme–based break-in regimen is worth about 7 to 10 hp on a typical 750-horse engine and as much as 12 or more on one of those 1,000-hp-plus builds. This extra power comes with a longer bore/ring life.
Modifications to the Lubrication System
There are quite a few things you should know about the lubrication system in your big-block. You have probably heard of “mains priority oiling” and wondered exactly what it means. If you follow the oil routing from the oil pump, on stock two-piece-seal engine blocks, the camshaft bearings are fed first and from there, the oil is routed to the mains. On one-piece-seal Gen V/VI and aftermarket blocks, the preferable discharge route is where the oil pump feeds the mains first and then the cam bearings.
Although the lubrication system is pretty good on all big-blocks (and better on the later ones), you can perform modifications to improve the system as a whole. The importance of the lubrication system, both in terms of power and reliability, is covered in detail in Chapter 3, Lubrication Systems. Be sure you read it before doing anything to your own block!
Also in connection with lubrication read what is said about timing sets, thrust bearings, and cam buttons in Chapter 10, Valvetrain Optimization.
If the budget is so tight you cannot cover the cost of new ARP bolts, be sure you polish the threads of the ones you have with a wire brush in a drill gun. Only when the threads are very smooth do you achieve the full clamping loads, so don’t shortchange the build on this account.
Fig. 1.10. If valvespring clearance allows it, use low-cost ARP hex-head bolts instead of the 12-point items. For the record, they deliver just as much clamping load.
I am often asked whether or not the heads should be held down with studs or bolts. The easy answer is studs. But they cost a lot more than good hex bolts (from ARP, for example). A set of studs provides a marginally better clamping load than a good set of bolts, such as ARP’s. Also, you need to consider ease of service when the engine is installed. If the engine is equipped with studs, you need to have at least 8 inches of clearance above the studs in the direction that the heads are lifted off the block. If this is not the case, you cannot remove the heads without having to take out all the studs first. This is not the case with bolts. My advice? Unless it’s an “on-the-limits” build, use ARP bolts.
When explaining the use of a stroker crank in my previous Chevy big-block book, I spent most of the time covering what is needed with a 1/4-inch stroker and the Scat cast-steel crank, which is a budget-priced item with great power potential. Whether you choose a cast-steel or forged 1/4-inch-stroker crank for either a 454- or a 502-style block the installation is pretty simple as stroker builds go. For the most part, cutting the block for rod clearance is minimal assuming you are using the right rod for the job (covered in Chapter 2, Pistons, Connecting Rods and Crankshafts). In some cases, the block has enough clearance to allow a 1/4-inch stroker to drop right in but you should not count on it.
Fig. 1.11. This factory 572 is based on a tall-deck block with a 4.560-inch bore and a 4.375-inch stroke. This is a good combo from the point of view of RPM capability and rod-to-stroke ratio.
Let’s focus on strokes of 4.375, 4.500, and 4.75 inches; that is 3/8-, 1/2-, and 3/4-inch strokers. The good news is that almost all 454 and 502 blocks accept a stroke increase of up to 4.5 inches, which is a 1/2-inch increase. If you opt for a forged crank, you should go for the longer-stroke crankshaft because the price difference between a 1/4-inch stroker and a 3/8- or 1/2-inch stroker is virtually nil. That being the case, the minimal amount of extra work for additional block clearance is worth it in terms of results.
As of 2014, I have experience with four 454 1/2-inch-stroker test engines, bored 0.060 over, and each has delivered gratifyingly good results. In spite of having a smaller bore and thus a breathing penalty, the resulting 525-ci build does remarkably well, as long as the combination is right. The principal aspects to focus on are the heads and, most important, a cam spec that suits both the heads and the displacement.
Fig. 1.12. Dart’s short- and tall-deck blocks are almost certainly the most popular choice. The taller deck allows for a longer stroke that is typically worth about 40 ci over the short-deck variant.
Fig. 1.14. Here is a BMP aluminum tall-block. Going from cast iron to aluminum saves about 100 pounds. If big displacement is what you are looking for this particular block can accommodate 632 inches.
Putting a 1/2-inch stroker in a 9.8-inch short-deck block means the rod has to be 6.385 inches long and that’s 1/4 inch longer than stock rods. If the rods are any longer, there is insufficient room for the ring pack. With this length of rod the piston pin height is down to 1.165 and packing a regular set of rings into much less than that for a budget-conscious build is impractical. Another factor that looks a little bleak for a performance build is the rod/stroke ratio that, at 1.42:1, is really short. However, if you keep this in mind and work to minimize all the factors that can make a short rod/stroke ratio a liability, the final result can be more than acceptable.
With longer strokes in a short-deck block it becomes very important to minimize piston/bore friction, so bore prep and piston/ring selection become critical factors. I realize that sounds a little scary, but if these factors are taken care of, the end result is an engine with great performance potential, especially for the street. If you use a taller-deck block, the engine’s geometry becomes significantly more favorable.
Before deciding to go with a tall-deck block, you need to determine whether a tall-block engine will fit into your chassis. Sure, any chassis can be ultimately made to accept any engine, but the hassle and expense may be more than you can afford or want to deal with. With that caution in mind, let’s talk tall-blocks.
It appears that other than some Chevrolet Performance Parts Bow Tie blocks the biggest displacement production 10.2-inch tall-deck blocks that General Motors made were 427s. Fortunately, these had a 4.25-inch bore and on occasions could be rebored 0.100 over even though 0.060 was a more common limit. Off-the-shelf Scat 6.7-inch rods or Callies 6.8-inch rods can be installed in these blocks. With these units, the rod/stroke ratio increases to 1.60:1 for a 4.25 stroke or 1.51:1 for a 4.5 stroke.
If you find a tall-deck production block and you verify through sonic testing that it is useable, by all means use it. But if you can afford an aftermarket block, a whole new world of big inches opens up. How big? Try the topside of 710 ci while still utilizing production-style heads.
Fig. 1.15. This short-deck aluminum block was the basis of a build that used Brodix heads, intake, and block for a 565 fuel-injected build, which was intended for a Corvette for the 2013 SEMA show. Mark Dalquist of Throttle’s Performance built this engine. I helped with the dyno testing and can vouch for the output, which was ultimately just a few horses shy of 900.
Aftermarket Blocks for Big Inches
Some exotic blocks are available for big-inch builds. The spreading of the cylinder bore centers is the most influential dimension of these exotic blocks, and it directly affects the displacement potential. Stock bore centers are 4.840 inches but some manufacturers are spreading the bores to 5 inches or even as much as 5.3, thus allowing larger bores. This can make for displacements in the region of 900 ci.
I don’t want to get into repitched bore-spacing blocks in any great detail here as it is out of the scope of this book. Basically, four sources produce standard bore-spacing iron blocks. In alphabetical order, they are: Blueprint, Dart, GM Bow Tie, and World Products. If it’s an aluminum block you are after, Bill Mitchell Products (BMP), Brodix, or Dart are the available options. But be aware that they are about twice the price.
Fig. 1.16. This short-deck aluminum block was the basis of a build that used Brodix heads, intake, and block for a 565 fuel-injected build, which was intended for a Corvette for the 2013 SEMA show. Mark Dalquist of Throttle’s Performance built this engine. I helped with the dyno testing and can vouch for the output, which was ultimately just a few horses shy of 900.
By a margin of about 8 to 10 percent, Blueprint has the least expensive block, yet it’s all American made and is of American high quality. As I write this chapter, I am about to start building a tall-deck 652-inch. This block is available at 9.8 and 10.2 inches deck height and can be had with bores up to 4.6 inches.
Dart has the greatest range of blocks in terms of heights and bore-size capability; most of my aftermarket block experience is with their blocks. However, I do feel it is worth mentioning that Dart has a 4.9-inch bore spacing variant of the block that accepts heads closely patterning regular bore spacing heads.
Brodix and Pro-Filer offer heads to suit 4.9 bore spacing blocks. These heads differ from others because the chambers are slightly repositioned and some head bolts are repositioned. Other than that, they look much like heads for a regular 4.840-pitch block. These blocks are available with deck heights up to 11.1 inches, and they accept strokes up to 5.5 inches. With a maximum bore of 4.7 inches and a 5.5-inch stroke, a short-block assembly of 763 inches can be built without undue hassle.
With the right heads, an “all engine” build can produce right around the 1,400-hp mark with torque in excess of 1,180 ft-lbs. So far the biggest Dart block build I have been involved with was a 712-inch unit that made 1,098 ft-lbs and 1,346 hp on nonoxygenated 116-octane race gas. I built this engine with Terry Walters at TWPE. There is no doubt in my mind that oxygenated fuel would have added about 40 hp.
Maximizing Bore Size
The entry-level Sportsman blocks from Blueprint, Dart, and World Products represent a good return on investment in either 9.8 or 10.2 tall-deck configurations. The cylinder walls on these blocks are much thicker than stock. Often, the bore limit is 4.6 inches, but a sonic tester can often find a block that can go significantly bigger. I have gone to as much as 4.67 inches with a Dart block and have had no subsequent problems. A 4.5 stroke in a short-deck block produces 616 ci. If engine bay space accommodates the block, the tall-deck version of this block can, with a 4.75-inch stroke and a 6.7-inch rod, go to 652 ci, and that many inches has some serious torque and horsepower potential.
My last word on Dart blocks here is that you should visit their website and check out all the variants they offer. However, don’t let the name “Sportsman” in any way make you think you are buying an “also ran” member of the Dart performance line-up.
GM Performance Blocks
I have used GM’s one-piece-seal tall-deck block for a 572 (4.375-inch stroke) and a 588 (4.5-inch stroke). Both instances used a 4.56-inch bore; they were street/strip pump-gas builds and produced very satisfying results. In round numbers, these were 850 ft-lbs and 904 hp for the 588 and 828 ft-lbs and 883 hp for the 572. That was with some super-ported 24-degree heads and a 10.8:1 CR. You should be aware of these Bow Tie blocks. They are available in short- and tall-deck versions, and most can bore to 4.6 inches. Compared to stock blocks, the Bow Tie blocks have revised oil passages and much stronger upgraded mains caps.
World Products Blocks
World Products has gone through some big changes during 2011–2012. First, the production of blocks has been split. The “trade only” parts manufacturer/distributor PBM of Louisville, Kentucky, manufactures the iron Merlin blocks. BMP, formally World Products, exclusively produces the World aluminum blocks. What does this mean? Trade customers, such as pro engine and big speed shop outlets, buy from PBM. As a retail buyer, you can purchase your Merlin block either from an engine shop, a speed shop, or directly from BMP. Regardless, you should go to the BMP site to review the lineup of blocks, details, and prices.
The iron Merlin blocks have gone through a series of important updates. Without the need for any additional clearance, the short-deck block is good for 582 ci and the tall-deck, 632. With a little grinding for additional rod clearance some extra stroke increases these figures by about 12 ci. I have not used the latest block but I used the previous version for a street/strip nitrous build that made more than 1,500 hp. That was some years back and the block is still in one piece.
Most aluminum blocks cost a pretty penny but sometimes you can find one advertised on eBay, Racing Junk, or at a swap meet. Usually they go for comparatively little money. However, aluminum blocks are susceptible to far higher rates of corrosion than iron blocks, so you need to diligently inspect a prospective purchase.
Almost without exception aluminum blocks have a lesser bore capability than their iron counterparts. The smaller bore means less displacement, but they more than make up for this by weight reduction. A 9.8-inch-deck aluminum block weighs about 90 pounds less, and this difference is substantially more when considering taller-deck blocks.
If you can afford to pay a little more than twice the cost of an iron block, you should consider what Brodix, Dart, and BMP have to offer.
Although you need bearing housings, bores, and the like precisely sized, an often-overlooked dimension can cause a substantial power loss, and that is an incorrect crank-to-cam centerline distance. When a new set of main caps have been installed, the main caps and journals have been line bored or honed and sometimes the cam tunnel needs to be cleaned up because the centerline between the crank and cam closes up slightly. As a result, the timing chain is sloppier than would otherwise be the case. You can install a timing set with a slightly larger cam gear to fix this. Although it’s only by a couple of thousandths, it can take a considerable amount of slack out of the timing chain.
At the other end of the scale, a timing setup can be too tight, which can be worse than a loose timing chain. A simple check is to notice how much the timing chain can be moved back and forth at the midpoint between the two gears. I typically expect 1/8 inch or so of play in the chain. The problem here is that this test is a little on the subjective side.
The best simple check is to install just the crank and use light oil on the main bearings and the cam bearing. Install the cam first without the timing chain and verify that it rotates freely. Now install the timing chain and recheck. This procedure allows you to feel how freely, or not, the crank and cam rotate. With the timing chain installed, the amount of effort it takes to rotate the crankshaft, cam, and timing gear should be barely perceptible.
The reason I have gone into detail here is that I had a significantly tighter than normal timing chain on one engine due to an incorrectly packed timing chain set. The gears were slightly oversize to compensate for a crank align hone job that moved the centers closer. With the timing chain installed, the assembly took about 5 ft-lbs more to turn. I dyno’d a 468 build with this setup with the intention of swapping out the timing gear for one that gave the proper tension.
Fig. 1.17. Guarding against failure of flat followers is very important and that is why I am emphasizing the need to take steps against it. Using Comp Cams’ lifter grooving tool, a groove such as seen here can be cut. This allows a stream of oil to spray onto the cam face just before it contacts the lifter. This is a very effective move and only takes seconds per lifter bore.
I was expecting to see about 5 ft-lbs and about 6 hp difference. Surprisingly it was much more than that. The torque, with the correct timing chain tension, increased by an average of 9 ft-lbs, and power went up by 11 hp. I am also sure that cam bearing life also increased somewhat. When a well-used timing chain replaced the new one, there was quite a bit of slack in the chain. The power dropped only minimally, but at part throttle, the ignition timing danced around far more than before. This indicated that the cam was oscillating back and forth as much as 2 to 3 degrees.
Lifter Bores and Flat Followers
A big-block Chevy has the same lifter diameter as a small-block Chevy; this size is too small for a small-block and way too small for a big-block. (Refer to Chapter 9, Camshafts and Valvetrain Events, to see the effects of size and geometry on the opening envelop of a lifter, whether it is a roller or flat tappet.) For a flat-tappet lifter, the peak lifter velocity is dictated solely by how far off center the cam-to-lifter face line of contact is. As a result, diameter of the lifter totally dictates maximum velocity. A bigger-diameter lifter means that more velocity as well as more lift can be designed into the cam profile. As previously stated, one of the factors to take care of is building a valvetrain that has high-lift capability.
Just how large of a lifter diameter you can use depends on how accurately the lifter bores are located with respect to the camshaft. Also, big-block Chevys with flat-tappet cams have a reputation for eating cam lobes and followers. I have experienced that problem at least a couple of times. According to my friend Billy Godbold, Comp’s wiz kid cam profile designer, General Motors had a batch of several thousand big-blocks come off the production line in the early to mid-1970s that had miss-machined lifter bores. This made an already marginal situation worse, and these big-blocks had a propensity for destroying the cam lobes and followers.
Fig. 1.18. I tested this oil additive in the 1990s and was so impressed with how well it worked that I bought shares in the company. From that, I developed my own break-in lube. Use this in the oil and a steel-on-steel pin boss will last virtually forever. I have also proved to top pro engine builders on their dynos with their engines that my Oil Extreme break-in lube is the best there is—bar none.
Most cams are designed to utilize up to within 0.025 inch of the edge of the lifter. That being the case, a 0.842 lifter offers a diameter of 0.792, and delivers the performance it has to offer. This margin is a common standard within the aftermarket cam industry. However, if the lifter bores are both bored and positioned accurately and increased in diameter, the ability to utilize more aggressive flat-tappet cams is available. The most practical size to use here is the 0.904 Chrysler lifter diameter. By accurizing the lifter bore positioning, cam grinders (such as Comp Cams) push the diameter utilization envelope to within 0.010 of the edge of the follower. This means the working diameter has increased from 0.792 to 0.884 inch. That’s an increase of 12 percent in velocity capability, which translates into an 8-percent increase in opening area.
Lifter Bores and Roller Followers
Boring out cam follower bores to accept a larger size also is advantageous for a roller lifter but not for the same reasons as for a flat tappet.
When using a roller cam the bigger the base circle diameter and the larger the follower roller is, the better the dynamics and the more aggressive the opening event can be. The big problem with a roller cam (contrary to popular belief) is that the system is acceleration limited. In the initial off-the-seat acceleration phase, a flat tappet can easily beat a roller (typically used for a big-block). The roller lifter’s problem is side loading (see Chapter 9, Camshafts and Valvetrain Events, for an explanation). The bigger the roller diameter, the lower the side loading for a given acceleration rate. This means that a big diameter here can allow higher acceleration rates to be designed into the cam profile. This is exactly what you need for an under-valved engine that desperately needs high lift.
As you have probably guessed, not many machine shops are equipped to machine out lifter bores. The four shops I use are: The Checkered Flag in Desoto, Missouri; Terry Walters Precision Engines in Roanoke, Virginia; Blanks Machine in Clarksville, Virginia; and Jesel in Lakewood, New Jersey (which, of course, did much of the pioneer work in this area).
Complementing the bigger-diameter lifters equipped with larger rollers is the big cam journal move. Here, a larger-diameter cam allows for a larger base circle to be used and (like the larger lifter roller) it reduces the side loading and allows for greater lifter accelerations before once more reaching the limit. At the time of this writing, a typical cost for boring lifters and cam tunnels larger is about $900. Just how much bigger can you go on the cam and lifter bores? It is best to consult the block manufacturer. At the time of this writing, the typical Dart blocks accept a 55-mm cam core and 0.937-diameter lifters.
Before selecting a block and building an engine, be sure to review and understand the information in Chapter 3, Lubrication Systems; Chapter 8, Electronic Fuel Injection; Chapter 9, Camshafts and Valvetrain Events.
