Cam and Valvetrain
A carefully designed lobe profile is needed to crutch the inline cylinder heads, which can only pass a finite amount of air. On the cutting edge of modern camshaft technology, Sherman's custom Comp Cams profile is way beyond any "shelf" grind. Comp designer Billy Godbold really had to crunch the numbers to configure a profile that would both provide the needed lift and duration numbers while remaining stable past 9,000 rpm.
In theory, Godbold says, "This engine is almost like a restrictor-plate engine, but it's restricted at the port instead of at the top of the intake plenum. We're making a lot of power per cfm, so we need to be more miserly with the air that does come in." In other words, the smaller the heads are in terms of flow and port volume compared with the engine's displacement, the more cam you need. But after a certain point, simply adding overlap doesn't necessarily improve airflow into the engine, especially at the extreme high rpm this engine must turn to generate the big power numbers. Instead, Comp spread the lobe separation, which allows more duration without correspondingly increasing the overlap.
Cam development for this combo is still ongoing. So far Sherman has tried two different custom Comp Cams mechanical rollers. Both had more than 0.500 inch of lobe lift with less lift and more duration on the exhaust side. The difference was accentuated with radically divergent rocker-arm ratios-1.8:1 on the intake, 1.6:1 on the exhaust. In Sherman's experience, "More lift on the exhaust side rarely is needed, assuming the cam is close to what the engine likes. The exhaust side likes duration; it acts like a fifth combustion cycle, helping to pull air through the intake like a big siphon. Backing off on the exhaust lift makes more torque down low with no loss of top-end power." Since it was a custom grind anyway, why not just grind the difference on the lobe? "It's hard to manufacture-in such a big lift difference at the lobe; you'd need radically different base-circle sizes. Different length pushrods would also be needed."
After trying the first cam, "The engine peaking speed was only 8,400 rpm-not as high as I thought it would be," Sherman says. "It fell off significantly by 9,000." So on the second go-round, Comp added about 5 degrees more 0.050 duration and spread the lobe separation another 2 degrees (from 113 to 115). This kicked up top-end power at the expense of some midrange losses. The ultimate cam is probably somewhere between the two grinds.
As you might expect, the cam's small base circle, huge lobe lift, radical 1.8:1 intake/1.6:1 exhaust rockers, and tall valves play havoc with valvetrain geometry as well as require humongous valvesprings with titanium retainers to keep everything stable. Jesel's shaft-mounted rockers are longer than normal to help correct the geometry, while tall Ferrea valves provide the necessary clearance to install full-race Comp Professional Series valvesprings (PN 26082) at their proper 2.100-inch installed height. Thick Comp 31/48-inch Hi-Tech pushrods are used to resist flex; Jesel provides a tool to help determine the right length.
INDUCTION, IGNITION, EXHAUST
Rules permitting, sheetmetal tunnel-ram intakes have become almost standard-issue for competition engines. They can be specifically tailored to enhance the rest of the combination. Sherman went with a TRE intake, which features a unique plenum volume and runner profile, length, and shape, all derived using proprietary formulas that take into account the engine's operating rpm band and displacement. Like a funnel, the equal-length runners actually start out larger at the plenum and taper down linearly approximately 35 percent to match the intake-port entrance dimensions. This achieves the optimum air speed needed to cram additional volume into the limited cylinder-head ports without exceeding the critical Mach number that promotes air/fuel separation.
On the Dyno
An engine at this level is very sensitive to minor changes in the tune-up. Playing with the carburetor calibration and ignition lead can really move the power curve around. Run on 115 octane VP-115 racing gas with the initial Comp FW1924F/1871B cam and slightly rich No. 91 jets in the 825-cfm Demon carbs, the engine produced more low-end torque, low-end being a relative term here, as the effective operating range is approximately 6,000-9,000 rpm-but had only 774 hp at 8,400 rpm (Graph, Test 1).
Leaning out the jets to No. 88s and retarding the cam 3 degrees shifted the powerband up with the new 800hp peak now occurring at 8.900 (Graph, Test 2). This represented about a 26hp gain but came at the cost of 22 lb-ft on the bottom. Sherman says it's not unusual for a slightly rich combo to produce stronger torque numbers, but in a properly geared race car, the power number is usually more significant.
Installing Comp's larger FW1744F/1874F grind shifted the power curve even higher (Graph, Test 3). With this cam, leaner high-speed air bleeds in the carburetors, and 34 degrees of total ignition lead, the engine became stronger over 7,600 rpm, reaching a peak of 816 hp at 9,300 rpm, a gain of 16 hp over the previous test.
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