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1963 Buick Riviera Replacement Distributor - Ask Marlan

Or you need a good rev limiter! Seriously, the problem isn't in the vacuum-actuation portion of the valve (at full-throttle and max rpm manifold vacuum is essentially nil), but instead the culprit is high coolant pressure that blows the valve off its seat. The valve isn't needed if you're not running air conditioning, but if you are, take a leaf out of the '87-and-later 5.0L HO Mustang book: Plagued by high coolant pressures that blew apart their heater cores, the factory solved the problem by inserting a restrictor inside the intake manifold-to-heater core hose. Although the late molded hose won't fit early chassis, you can fabricate an equivalent restrictor from a round aluminum or steel disc: Drill a 1/4-inch hole through the disk's center, then trim its od so it fits snugly inside the 5/8-inch-id manifold-to-valve hose (the valve's "upstream" side).

Manifold Destiny
I am working on a sheetmetal EFI intake manifold. Could you please offer some speculation on how to size one for various applications? I have found that a rule of thumb for sizing a plenum box on normally-aspirated engines is to build one 50 percent the size of the volume it is feeding for engines producing under 1 hp/ci, and 75 percent for engines producing over 1 hp/ci. As for engines producing 2-plus hp/ci, I have no idea. If any sort of tech information is offered on this subject for naturally-aspirated as well as forced-induction designs, I would appreciate knowing about it.

Paul Tholey
Roslyn, PA

Your assumptions are pretty much in the ballpark. EFI specialist Ken Duttweiler likes to see plenum volume at about 75 percent of displacement for engines making 1 1/2-2 hp/ci, reducing the volume progressively as engine hp/ci output declines. Plenum volume for current Pro Stock engines at the 2 1/2 hp/ci level is typically 80-90 percent of engine displacement.

These generalizations serve only as a starting point. In the real world, you have to factor in the specific engine design, establishing the proper plenum volume and runner length to tune the system for a specific rpm as well as displacement in much the same way headers and exhaust systems are designed. A higher rpm engine sucks in more air in a given amount of time, so the manifold must be able to move the extra air. As rpm goes up, runners need to get shorter-which leaves more room for plenum volume. In "dry air" EFI manifolds, runner length, and design are critical, with their taper and overall volume more important than the specific plenum configuration. Duttweiler also says that on supercharged or turbocharged engines, plenum volume changes have relatively little effect on the engine's power output. Of course, whatever you come up with also has to fit in the engine bay and clear the hood!

Over the course of internal combustion engine development, dating back now over a century, there have been scores of studies and engineering papers written on optimum exhaust and intake system design. Most of them assume that engine cylinder volume remains constant (equal to total displacement). But in reality, the area above the piston is a continuously changing variable as the piston moves within the cylinder; this cylinder volume factors into the overall volume and length of the induction and exhaust systems.

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