10 Questions For Your Chevy Big-Block- Big-Block Checklist

8. Even if I'm turning lower street rpm, will the exhaust be critical?
Because of the limited valve size, the exhaust system becomes even more critical than for a 350 small-block. A good set of headers feeding into a low back-pressure exhaust system will considerably help a big-block realize its full potential. The key element is to aid combustion chamber scavenging as much as possible, so that the intake charge can start on its way into the cylinder as early as possible. In simple terms, the intake valve will be actively flowing mixture into the cylinder for a longer period during the opening phase. Like the intake ports, the header's primary diameter should not be too big, as this will allow reversion until high rpm is seen. This will defeat the header's ability to scavenge. If the engine is to be used on the street, make sure the mufflers have adequate flow. Target about 2.3 cfm per open-header horsepower.

9. On my street engine, do I really need upgraded valvetrain parts?
For the most part, the valves and valvetrain of a big-block Chevy can best be described as big and heavy. This means it borders on being a cam designer's nightmare. Anything that can be done to lighten the mass at the valve is a plus. The importance of valve lift was mentioned earlier. To be a real powerhouse, a big-block Chevy needs a lot of valve lift because that's where the big flow numbers happen. It also means that the valvetrain has to lift fast to get into that high-flow regime as quickly as possible. Lighter 11/32-inch stem valves (stock is ? inch) are an asset, but that is not the extent of all we need to consider. Valvesprings having a low mass for the poundage they deliver is also an asset.

Such springs are usually more costly because they are made from better steels that can be stressed to higher values without incurring a penalty to reliability. Better steel can lead to a reduction in spring diameter, which in turn can lead to a higher resonant frequency, and the use of a lighter spring retainer.

As for rockers, these are important to the extent that they have to deliver a high lift without undue flexure. Don't even think of using a stock-type, non-roller tip rocker unless you plan on using a wimpy cam. (And why would you do that?) Get a good brand of rocker, and if the budget is there, a Sportsman shaft setup from Jesel is a good idea. As for pushrods, remember that the valvetrain is heavy, and even a good spring will have a substantial over-the-nose poundage. Add to this the fact that even the shorter of the two pushrod lengths used is still a long pushrod. As such, the demands made in this quarter means that a premium pushrod is a must.

10. Do high-performance big-block heads need anything special for a cam?
The cam used with any high-flow aftermarket head is the make-or-break deal for the entire build. The high lift required (0.625-inch minimum) has several major implications. First, for anything but the longest duration, a flat-tappet cam cannot put sufficient lift into the valves. To get sufficient lift, a flat-tappet grind would have a duration of some 300 degrees; that's hardly a daily driver-type cam. Also, the heavy valvetrain mass plus the high rocker ratio brings about high surface loading. Couple this to the fact that many modern oils lack the ZDP (zinc additive) necessary for even a moderately loaded flat-tappet cam and lifters to survive, and you can see that a roller cam is looking like a better option.

For any given advertised duration, there is (for a typical 10:1 500 incher) more than a 20hp difference between a 280 advertised-duration roller and an equivalent flat-tappet. The reason is the extra lift, and a check on the numbers shows why. By way of example, a comparison between Comp's 280 Xtreme solid flat-tappet and a 280 Xtreme street roller shows the flat tappet lifts 0.580 inches, versus the roller's 0.646 inches. The result is no contest. But what about a hydraulic roller? How does this look in practice? The problem is that, once again, the heavy valvetrain and the high rocker ratios involved conspire to make things less than simple. These two factors, plus the need for a higher poundage spring (compared to a small-block) brings about a big increase in roller lifter side loading. This, in turn, causes the lifter to be more prone to collapsing. To minimize this, you need to make sure the heads are equipped with one effective spring (we are back to that resonant frequency thing again) that gets the job done with the least amount of spring poundage. Also, be aware that all hydraulic lifters are not created equal.

Get the best hydraulic rollers your favorite cam company has to offer. My advice is that unless you absolutely must have hydraulic lifters, go for a solid street-roller setup. Street rollers from Comp or Lunati will go more than 50,000 miles between adjustments. For the minor inconvenience of adjusting them about every five years, you will have the benefit of at least 15-30 more horsepower for a cam of the same (0.050) duration.

So are we done with cam comments? Hardly. Because the air demand of a big-block cylinder can so easily outpace the flow capability of the intake valve until high lift, we need to give it a greater head start than would normally be the case.

This puts the valve at a higher lift earlier in the intake event so that the valve reaches high lift (and consequently high flow) sooner. Because the cylinder can draw earlier, the port velocity in the first half of the opening event is higher. As a result, the ramming effect in the second half of the opening/closing event is increased. In simple terms, we find from port and cylinder pressure measurements that success in the first half of the induction dictates the success in the second half; if the first half is not optimal, there is nothing that can be done in the second half to rectify the problem. What this means is that the opening point of the intake valve for any given overall duration needs to be sooner than for the equivalent cam in a small-block. This translates into having cams with a tighter lobe centerline angle than we traditionally see.

So that you are not left wondering what you should use, here are three grinds that I can personally vouch for using Lunati's big-area-generating Voodoo street rollers: BBDV281-06SRH, BBDV286-06SRH, and BBDV292-06SRH. These are single-pattern cams using Lunati's VSR32, VSR34, and VSR36 profiles, respectively. These cams are optimal for a 489- to 511-inch big-block with typical aftermarket heads and a compression ratio between 10:1 and 11.5:1.

Note From The Editor:
This story is not intended as a complete comparison of all available cylinder heads, intake manifolds, or camshafts. Such a story would be impossible for a complete book, let alone a six-page magazine story. These technical illustrations are one expert's cumulative experience, which is understandably limited. Obviously, there are many other great big-block Chevy cylinder heads, intakes, carburetors, and camshafts that work superbly in a variety of situations, so if we forgot your favorite, please do not be insulted. Rather, it is the desire of the PHR editorial staff that you take away the overreaching technical concepts.