Racers have long identified a critical airflow discrepancy native to all single 4-barrel intake manifolds designed for V8 engines. The problem — whether those manifolds are single- or dual-plane design — is unequal airflow to the front and rear outboard cylinders, due to uneven runner lengths.
This discrepancy alters the volumetric efficiency (VE) curve in the affected cylinders, reducing their power contributions when compared to the inboard cylinders. A few resourceful tricks have been employed to address this problem, including higher rocker-arm ratios on the outboard cylinders, air dams in the manifold plenum and adjusting the ignition timing in different cylinders.
One of the more aggressive and successful solutions is designing camshafts with unique timing specs for the inboard and outboard cylinders. NASCAR engine builders apply this technology with finite accuracy, based on years of dyno testing, sophisticated engine-cycle analysis and hundreds of hours with computer simulation programs. But not everyone has access to unlimited dyno time and proprietary software.
EngineLabs has learned that Comp Cams is bringing this NASCAR-inspired cam-design strategy to the street and grassroots racing and will introduce its revolutionary new 4-Pattern hydraulic roller camshaft at the SEMA Show.
“These are no-punches-held street cams with the most race DNA ever packed into a high-endurance hydraulic roller package,” says Comp engineer Billy Godbold, adding that the cams represent the first use of racing camshaft technology in street engines specifically to balance cylinder-to-cylinder air/fuel distribution.
These new grinds incorporate four different lobe sets specifically designed to equalize airflow between all the runners on contemporary 4-barrel intake manifolds. Matched against a comparable hydraulic roller, the Comp 4-Pattern cams can offer gains of five to 20 horsepower with the bonus of up to 1,000 more useable rpm–a worthy achievement for cams using relatively heavy hydraulic roller lifters.
These 4-Pattern hydraulic rollers are specifically designed for small- and big-block Chevys and small-block Fords utilizing 4-barrel carburetors or center-mounted EFI throttle bodies. The cams are CNC precision ground on steel-billet cores with pressed-on cast-iron distributor gears. Every cam comes with a full Adcole certification report, detailing the lift, duration and lobe separation specs and measurements of all 16 cam lobes and is accurate to 0.00001-inch.
The initial release of cams apply across the performance spectrum (moderate to high performance) for both Chevy’s and Fords, including retrofit versions for existing engines equipped with factory roller cams. Small-block Chevy cams are also offered for .900-inch base-circle requirements in both retrofit and standard hydraulic roller versions to clear the rods in long-stroke applications. All 4-pattern cams have lift values in excess of .600-inch to support the improved high-lift airflow characteristics of modern performance cylinder heads. Catalog listings will offer two distinct timing specs for these cams; one for the inboard cylinders and one for the outboard cylinders. Also, the lobe separation angles are tighter than traditional cams, with the outboard and inboard lobes one degree different to maintain equal overlap.
Unequal-length runner dynamics
The four center runners of any single 4-barrel intake are shorter in length than the four outboard runners. The difference averages about ten percent — enough to create measurable airflow variations between the inboard and outboard cylinders. This leads to disproportionate power production from the airflow deprived cylinders.
By equalizing airflow at all engine speeds, volumetric efficiency is improved with equal performance gains across the full operating range of the engine. Note inflation of the entire torque curve and peak-to-peak gains of eight horsepower in this ZZ4 350ci small-block matching a 4-Pattern cam against a comparable X’treme Energy cam. The test engine had a 9.5:1 compression ratio, single-plane intake manifold with 850cfm Demon carburetor and RHS Pro Elite 228cc CNC-ported heads fitted with Comp springs and rocker arms.
Individual cylinder tuning is required to correct this discrepancy, and it represents a significant path to power. Timing and fuel curves can be individually tailored to balance and increase power on EFI engines, but electronic controls can’t entirely compensate for unequal airflow. In its simplest form, the V8 engine is really just a collection of eight 1-cylinder engines, all racing for the finish line in close formation. Some of the cylinders perform well, but others run out of breath and need help. Those cylinders pulling hard at low rpm will likely drag at higher rpm, due to airflow disparities.
“Engines with sheet-metal, equal-length manifolds, especially those with multiple throttle bodies will not benefit from the 4-pattern technology,” warns Godbold. “Nor would EFI combinations with a large, common plenum and equal-length runners.”
Comp’s 4-Pattern cams can improve specific cylinder-to-cylinder airflow and tailor each cylinder’s individual torque curve (or VE map) to more closely match its companion cylinders, thereby raising the engine’s overall volumetric efficiency (VE) and power.
Single-plane, small-block Chevy runners average about 10 inches long on the outboard runners and nine inches on the inboard runners. Dual-plane runners are slightly longer at 11 and 10 inches, respectively; and the cross-sectional area of the runners are dimensionally fixed in commercially available manifolds. As a rule, cross-sectional area dictates the rpm for peak torque — higher in the power range with shorter runners and lower with longer runners.
Runner length controls airflow — and the eventual filling of the cylinders — largely on pressure-wave activity within the runners. Comp engineers reevaluated the discrepancy between runner lengths when testing distribution with both their EZ EFI system and carburetor applications. All the intakes they tested exhibited lean outboard cylinders at low engine speeds while trending toward lean inboard cylinders at higher speeds. And trying to trick the manifold with staggered jetting or some other band-aid solution won’t always correct the airflow dynamics.
“Jetting or port changes can’t alter the fact that the longer runners on the outboard cylinders will work harder at low rpm than the shorter runner inboard cylinders,” explains Godbold. “If you were to richen up the corner cylinders to help tune for critical low-rpm events, like launch and gear changes, then at high rpm — when the inboard cylinders are pulling more air — either the outboard will be so rich as to be low on power or the inboard will be dangerously lean. Basically, none of the standard tuning methods allow the engine builder or tuner to treat the cylinders differently, depending on rpm.”
NASCAR technology trickles down
To compensate, Comp applied NASCAR technology that employs dissimilar valve events to help equalize airflow to all cylinders. The new valve timing results in a significant increase in area under the curve with the added bonus of improved high-speed stability.
The Comp strategy gives the cylinders with longer runners (outboard 4 corners) more duration (time in degrees) and a later intake closing to move the same air volume as the center (inboard) runners. Shorter runners need less duration and earlier intake closing event to balance the air flow volume between cylinders.
The 4-Pattern cams incorporate four different lobe patterns with specific intake and exhaust timing for outboard and inboard cylinders. The four inboard cylinders utilize the same grind while the outboard intake opening point is delayed two degrees and the exhaust opening comes two degrees earlier. This initiates outboard cylinder blowdown earlier to lower cylinder pressure on the exhaust cycle, thus reducing high-rpm pumping loses and presenting less resistance and potential contamination to the incoming charge.
The later intake closing extends the flow event to accommodate the longer flow path of the outboard runners. The same overlap period is maintained by incorporating different lobe-separation angles on the inboard and outboard lobe sets. Most performance cams are ground 4.0 degrees advanced from the manufacturer, but Comp’s 4-Pattern cams are ground 5.5 degrees advanced to accommodate the lobe center and valve opening requirements to maintain the same overlap period in all cylinders.
Using pulses to sweep air
Pressure-wave activity in single- and dual-plane runners generally tunes to the 4th reflected wave. When the intake valve closes, a positive pressure pulse reflects back up the runner until it reaches atmospheric pressure above the carburetor. A weaker pulse is then reflected back down the runner toward the valve. When the valve opening event is timed correctly, it can take advantage of one of the returning pulses to help sweep more charge into the cylinder.
Second or third pulses (harmonics) generally produce the most torque, but the fourth harmonic is typically used for packaging induction systems under the hood — as found in single 4-barrel applications. The Comp 4-Pattern cam optimizes the valve timing in appropriate cylinders to equalize airflow to all cylinders and take advantage of pulse tuning, according to the known runner lengths.
These are no-punches-held street cams with the most race DNA ever packed into a high-endurance hydraulic roller package.
— Billy Godbold
Comp engineers evaluated individual air-fuel ratios based on oxygen-sensor readings taken from all cylinders on test engines. This data was vital to understanding airflow trends in different-length runners at all engine speeds. About half the gain on the outboard cylinders is obtained by making all eight cylinders move approximately the same amount of air at all engine speeds, thus realizing a brake specific (BSFC) improvement (higher efficiency). The rest comes from new lobe profiles which lead to VE gains and increased engine speed capability. The gains are split about evenly between the use of 4-pattern timing events and the new high-rpm lobe profiles. Since the same overlap period is maintained, they don’t specifically address potential effects of exhaust imbalance that may be caused by unequal length headers, but the VE gains generally outweigh any exhaust discrepancies — particularly with the earlier exhaust-opening event.
Regardless, these cams are specifically recommended for engine packages that incorporate premium cylinders heads and performance headers. Stock heads and/or stock exhaust manifolds will likely not see significant gains. Although the 4-Pattern cams are NASCAR inspired, their reach is well behind stock-car racing.
“There are huge categories of drag racing where center-mounted throttles are found on upwards of 90 percent of the engines,” says Godbold. “It may be closer to 99 percent in classes like Super Street, Super Gas or Super Comp. In these classes, consistency is the most important factor toward success. For that reason, proper tuning plays a huge part in winning or losing.”
To take full advantage of these cams, racers will need to incorporate the recommended spring and valvetrain package. With up to 1,000 rpm additional engine speed available, valvetrain compatibility remains important. Imagine approaching 8,000 rpm without resorting to titanium or hollow-stem valves with a durable, low-maintenance hydraulic roller cam? From a tuning perspective, 4-Pattern cams — which are more expensive than traditional hydraulic rollers, due to the steel cores and multiple grinds — will respond normally to necessary jetting and ignition-timing changes.