Chevrolet Small Block Combinations - Small-Block Power

From Mild to Warm to Hot--Three Recipes for Small-Block Chevy Power

Jim McFarland Mar 1, 1998 0 Comment(s)

Step By Step

A typical street 350 small-block at 9:1 compression with stock heads, a dual-plane intake, a 650- to 750-cfm carburetor, 15/8-inch headers, and a good ignition is certainly capable of 275 to 325 hp. While it’s not killer power, it’s more than enough for miles of smiles in a daily driven street machine.

Increasing power is all about additional airflow. All the items in the photo contribute to increasing airflow. Cylinder heads demand a steep price, but they also deliver the most power per dollar. Combining this with a cam, rockers, and a more efficient exhaust system easily delivers more power. The latest generation of catalytic converters is far better at exhaust flow.

Carburetor sizing isn’t critical, but it does help. Many entry-level hot rodders tend to overcarburete their engines. A 750-cfm carburetor will feed up to 700 hp, so those killer 850-cfm carbs aren’t really necessary.

Increase the airflow going into the engine and you’ll need to retune. Improving exhaust flow with headers and/or low-restriction mufflers will require richer jetting to compensate for decreased levels of exhaust dilution in the chamber. Experimenting with jetting is in order whenever a significant change in airflow is performed.

Ignition timing can also be affected by engine-component changes. Even if you don’t make serious component changes, experimenting with timing can uncover impressive power improvements just by optimizing your pre-existing engine.

Conventional four-tube headers are fine for most applications, but don’t overlook the tri-Y-design header concept for mild street engines. This design tends to pump up the low- to midrange torque to enhance street fun.

For mild to medium-hot street small-blocks, a hydraulic flat-tappet cam and decent valvesprings and rocker arms can make a big difference in usable street power. The new line of Comp Cams Extreme Energy cams and Crane’s Max Velocity lineups offer lobe designs that can get the job done.

Performance mufflers vary in size, interior design, sound, and performance. Selections should be based upon anticipated power levels, engine size, and requisite noise abatement. Check the recommendations of your chosen parts manufacturer for the best options.

Intake manifolds, camshafts, and cylinder heads comprise the bulk of power-enhancing modifications for street-driven engines, although a good exhaust system can certainly complement a good engine package. This intake is designed to optimize runner length for gains in low- and mid-rpm torque boosts. The plenum divider is used (and cut to a specific height) to further improve upon throttle response and mid-rpm power.

The advent of multipoint fuel injection, lowered hood profiles, and a need for increased torque and driveability have brought new intake manifold designs to the domestic OEM and aftermarket. This small-block, LS1 intake manifold optimizes runner length for solid gains in the midrange of rpm. Don’t miss the fact that this particular piece is an aluminum casting. The production parts are injection-molded, glass-fiber reinforced nylon.

Much of a modified small-block’s power is derived in the cylinder heads; in particular, the combustion space. Here’s a look at a near-stock chamber, even though it contains larger valves, is cast of aluminum, and has a slight unshrouding on the intake side of the chamber wall. Such aftermarket heads are ideal for a mild version of the buildups described in the story.

This is a more "serious" chamber, showing a laid-back chamber wall on the intake side (for improved airflow) and a slight "hook" (or kidney) on the quench side of the chamber, between the exhaust and intake valves. This aids mixture motion more toward the exhaust valve and spark plug for improved burn and for quicker exit of exhaust gases (counterclockwise motion as seen in the photo).

Study this chamber for a moment. Note that there is a more pronounced hook (or kidney) between the exhaust and intake valve pockets for additional mixture direction back toward the exhaust valve and spark plug. Also note that the spark plug is pointing more directly toward the exhaust valve where it is desirable to have most of the burn take place. One other item: Distinguish the small ledge running from the plug hole to the exhaust-side wall. This aids mixture "tumble" immediately after the "swirl" that’s generated passing over the spark plug (clockwise motion as viewed in the photo).

For the "hot buildup" parts combination, here’s a good set of exhaust ports. D-shaped to improve mid- to high-lift port flow (where exhaust ports need to be efficient), this design allows the necessary mass flow while keeping flow velocity high. Small but efficient ports are enabled by the flat floor design.

This story is not just another "bolt it together" piece about Chevy engines. Rather, it's a collection of recipes designed to steer you through the myriad of parts and procedures heaped upon these engines over the years. To that end, you'll discover the subject is broken into three distinct, generic buildup categories: mild, warm, and hot. These are streetable engines, albeit the hot version may be considered marginal. Each can be fueled with pump gasoline and operated in a targeted range of rpm.

Be as realistic about engine rpm as possible. If anything, be conservative. For example, an engine built for 7,000 that doesn't see this rpm with frequency is built for the wrong speed range. So before you begin laying out plans for an engine build, first decide in what rpm range it will be required to make power...most of the time. It doesn't make much sense to have a package that produces 450 hp at 7,000 rpm when you're on your way to and from the workplace. So for street engines, build your combination to produce torque. Period. Just remember that horsepower is a dependent variable...dependent upon torque and the rpm at which it's produced. At the risk of oversimplification, that's one ingredient that you can use in any of the following recipes.

The Mild Side(Idle to 6,000 rpm)

This is bolt-on territory. Given the rpm range indicated, a flat-tappet, hydraulic camshaft in the range of 210 degrees of duration at 0.050 inch is a good starting place. If you elect to fudge this number another 5-10 degrees, then the use of bleed-down lifters will help maintain good idle quality and low-rpm torque. And unless you're concerned about reducing operating oil temperature, the use of roller rocker arms is probably an unnecessary expense.

For an intake manifold, the use of a two-plane design is generally the best choice for this level of power and performance. Plenum dividers can be used to shift torque (and power) slightly up and down, depending upon the amount of divider used.

Carburetor selection for a mild package can retain a stock Quadrajet or something from the aftermarket that approximates a Q-jet’s capacity (recognizing that there are several sizes of Q-jet carburetors). Generally, the tried-and-true method of matching carburetor size to piston displacement works well for this level of engine buildup. In fact, it works pretty well for just about any nonrace, carbureted, normally aspirated engine. To see the formula, click here:

Cylinder heads for this combination should be relatively stock. Fresh valvesprings, new valve guides, and a good valve job will generally suffice. In fact, at this level of projected power, you'll probably not even remove the heads.

For headers, examine the header selection chart provided. Low-restriction mufflers are good choices for this category of engine buildup. Mufflers generally don't make power, but they unlock lost power. Essentially, you'd like for them to reduce backpressure (combustion contamination)--not "overscavenge" the engine in the low- and midrpm range--and to produce acceptable sound/tone. Whatever muffler you select, make certain to use tailpipes.

In the ignition department, use of the stock distributor with a high-energy coil is usually sufficient. Use good ignition wires too. And for an appropriate spark curve, here's a short note on critical items.

When you change the volumetric efficiency of an engine, regardless of where this occurs in the rpm range, cylinder pressure is typically altered accordingly. In addition, chances are good that mixture density (and quality) will also change. Therefore, you can expect a change in ignition spark timing and rate of advance to be required for optimum power. Without the aid of equipment that enables pre-installation evaluation of distributor modifications, the best bet is to experiment...both with total advance and rate of advance. Available pump gasoline can also be a limiting factor to changes in spark curves. What you'd generally like to accomplish is a slow, somewhat limited advance rate and as much initial timing as possible short of pre-ignition. Even weather conditions can affect spark timing if you're on the ragged edge of optimum ignition specs.

Getting Warmer (1,500-6,500 rpm)

OK, we're inchin' up on something here. It's bottom-line cylinder pressure--limited by fuel quality, air/fuel mixture atomization, volumetric efficiency, and spark timing. So before we step up to the next level of bolt-ons, let's take a closer look at these variables, because each can have an effect upon a measurement of cylinder pressure called indicated mean effective pressure, or IMEP.

What's IMEP? At the risk of oversimplification, picture pressure applied to a piston as it approaches TDC on the power stroke versus pressure applied after TDC also on the power stroke. The arithmetic difference between these two values, which we will call negative and positive torque, becomes the net pressure, or IMEP. So maximizing IMEP, short of detonation, is a key element in making useable power. As we work through this next package of warm parts, we'll reference IMEP from time to time...now that you've become friends with the term. At this level of power, consider selecting a camshaft at 220 degrees duration and at 0.050-inch tappet lift. In order to minimize periodic valvetrain adjustments associated with solid lifters, you may want to again select a hydraulic lifter version. Advances in camshaft technology enable these designs to produce excellent power with the added benefit of low maintenance and performance repeatability. But if you're into the rigors of ongoing valve-lash adjustments or you just like to experiment with lash/valve timing changes, solid-lifter counterparts to the hydraulic versions are certainly available.

There's also a chance you should contemplate changes in rocker-arm ratio as well. Particularly on the exhaust side of the cylinders, an increase in rocker ratio (even with a change in camshaft) can help some of the flow deficit often associated with stock production cylinder heads. This assumes your choice of camshaft is of a single-pattern design. Should you elect to use a dual-pattern version that favors the exhaust side by 10-15 degrees of duration, then increasing exhaust rocker ratio may be an unnecessary expense. And don't forget about the options available from camshaft retard and advancement.

It's likely that you'll remove the cylinder heads for this combination. Whether you decide to cut the heads for a compression increase or to budget for an aftermarket version, it's wise to keep the ratio around 9:1 for iron heads and 9.5:1 for aluminum heads.

For an intake manifold, you're in an either/or situation. Take another look at the rpm range. Even though it's from 1,500-6,500 rpm, try to identify where in that spread you'd like the most power. If it's generally below 5,500, select a two-plane. But if it's from 4,000-7,000, you should probably bolt on a single-plane.

In either case, think about using some form of carburetor spacer. Think of spacers as a tuning tool to determine if you're using too much camshaft or as a means of addressing an air/fuel-mixture separation problem on the manifold's upper plane floor (two-plane design) or plenum floor (single-plane version).

Especially if you're using a small carburetor, mixture impingement on the bottom of either manifold design can be reduced by raising the carburetor...normally with an open-hole spacer.

While we're in the intake manifold department, we should mention that it's a good idea to match the manifold/head intake ports. In the worst-case scenario, check all port alignments to be sure there's no cylinder head material hanging into the intake port passages. If you can see head metal from the manifold side of the path, chances are good there'll be some air/fuel separation at the mismatch. If the alignment favors manifold material inside the head ports, there's usually less chance for a disruption in mixture quality. In fact, this has been known to capture a measure of reversion pressure, believe it or not.

With regard to ignition, carburetor, and muffler selection for this package, the rules don't change from what was discussed for the mild version.

However, if you plan to remove and disassemble the engine, a good balance job will do a number of good things to the package; in particular, it will add durability of parts in particular. It's also a good time to have all reciprocating and rotating parts checked for cracks. Crankshafts are prone to fail in the main bearing area and the connecting rods in or around the bolt/cap vicinity.

Hot Stuff (2,000-7,500 rpm)

What we'll now discuss involves some compromise, centered upon what you'd like to accomplish in performance for what you're willing to sacrifice in driveability and comfort.

For a camshaft, look for something in the range of 230 degrees duration at 0.050 tappet lift. Be mindful that the same considerations about single- and dual-pattern cams hold true for this higher level of performance. Cam advancing and retarding is still a tuning option too. For an engine destined for rpm, a roller cam is virtually a requirement.

Don't make the mistake of selecting a hydraulic-roller design with the thought that this'll eliminate your need for periodic valve-lash adjustment. When you've installed the valvesprings required for roller lobe profiles and increased rpm, chances are you'll collapse the hydraulic lifters...or at best render them ineffective as roller tappets. Among the camshaft manufacturers consulted on this story, the consensus is that if you decide to use a roller design, include mechanical roller tappets and the appropriate valvesprings. Otherwise, you're probably looking at about 6,000 rpm as a practical limit for a hydraulic-roller package (that's also costly), and you won't achieve the higher rpm powergains.

Cylinder heads: You'll need to choose from one of the current suppliers of true, high-performance parts. Race heads won't work, so stay away from intake ports larger than 220 cc's. But in any case, either perform or have performed a volume check of all combustion chambers. This will allow you to decide where to set the valve seats in order to balance the chambers. And unless you're equipped and experienced in this type of head blueprinting, it's a job to be farmed out.

For an intake manifold, a single-plane is about your only choice. Carburetor spacers will work, and a port match is a must. Carburetor sizing methods use the same recommendations shared earlier.

And once again, we refer you to the header selection chart. While a crossover pipe still serves the same function as before, you're now in a range of rpm where its effectiveness is diminished. If the engine will spend much time below 4,000 rpm, this modification is still a viable possibility.

As potpourri items, we offer the following. Check valvesprings for possible coil-bind. Ask your camshaft manufacturer what's acceptable. Depending upon specific lobe profiles, some cams will allow less piston-to-valve clearance and spring-height-to-bind specs than others. Obviously, for reasons of possible missed shifts, standard-shift transmission vehicles typically require more piston-to-valve clearance than those with automatics.

So there you have 'em...three recipes for power that outline many of the essentials required in the parts selection and engine assembly process for the power levels described. Use the header selection chart. To the best of our knowledge, it's never been published before now. Properly used, it will make a valid contribution to your performance personal recipe. CHP

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