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Small Block Chevy Angle Plug Combustion Chambers - Ask Marlan

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It is confusing, but big numbers sell product! Putting the best face on this issue, remember that the installer can vary the lash-setting on a mechanical cam, which of course affects lift; so, it could be said that most aftermarket companies publish theoretical valve lift figures and leave it to the tuner to figure out the "real" lift (theoretical valve lift minus the lash setting).

And even assuming zero lash, the official published valve-lift figure is somewhat arbitrary-invariably, there's some valvetrain deflection, and stock-type rocker arms may not really yield their advertised ratio. Hydraulic profiles generally don't achieve their theoretical published valve lift figure either; they lag a bit because the pushrods slightly compress the lifter.

As for duration-whether advertised, seat, or 0.050-it's always calculated at the tappet, not at the valve, so even with "zero" lash, the actual working duration is somewhat higher due to the rocker arm's multiplier effect. A gross rule of thumb is that measured working duration increases about 2 degrees for each tenth of a point increase in rocker arm ratio. But the actual increase in total area under the curve makes the cam act as if duration increases about 4 degrees for each tenth of a ratio increase. Professional racers often decrease duration (as measured at the tappet) as much as 10 degrees when moving up from 1.6:1 to 1.8:1 rocker arms; this maintains the same horsepower, but increases torque.

Nevertheless, due to the considerable lag induced by the necessary lash clearance setting, a mechanical cam needs to have about 8-10 degrees more duration than a comparable hydraulic cam to achieve an equivalent powerband when both cams are installed in the same engine.

The Bucks Stop Here
I'm looking to build a 400 or 383 small-block Chevy blower motor. I would like to make 400 hp (with an 8.0:1 or 8.5:1 compression ratio) on just the motor, no blower. With the base motor at this power level, I'm gonna bolt on an ATI ProCharger for some real fun! All the local machine shops say it can't be done without major dollars invested. I was wondering if you could help.
Dan Conklin
Hewitt, NJ

The key to keeping costs reasonable is staying under 10-psi boost and investing in a good set of heads-like Air Flow Research's $1,250, 195cc intake runner offerings. Cam selection won't be critical under 10 psi; you can get by with a single-pattern 292 Isky MegaCam or equivalent hydraulic flat-tappet grind at this level.

In contrast, achieving maximum performance potential under higher boost conditions requires a custom dual-pattern blower cam with wider lobe separation than the Isky cam's 108-degree LDA; such dedicated "blower" grinds generally aren't best for unblown running. High boost levels also cause head gasket sealing problems on a 400 due to the block's thin deck and thin siamesed cylinder walls. If you must have a 4 1/8-inch-bore engine and high boost, it's time for a pricey Bow Tie block with its beefy siamesed cylinder walls and extra-thick deck. High boost also mandates an aftermarket forged crank with a bigger big-block-style front snout-serious boosters have been known to rip the snout right off the crank.

An alternative is to forgo the 400 and build a 383. Boost builds torque, so a 383 will get the job done fine without any head gasket-sealing problems. KB Pistons offers hypereutectic 383 pistons (PN KB121) that produce about 8.8:1 compression with 64cc combustion chambers. With the AFR heads, above-recommended cam profile, reasonably competent machining, and prepped stock 5.7-inch rods, you can achieve 400 hp with the 383 in an unblown configuration with no sweat. When you bolt-on the supercharger install an 850-cfm Holley double-pumper, minimum.

If you shop around for good-guy parts deals, assemble the engine yourself, and only farm-out necessary machine work, you ought to be able to put an engine like this together for around $4,000.

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