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Auto Repair Questions - What's Your Problem?

Wet Or Dry
Paul Serna, Delano, CA: In the Horsepower section of the Jan. '07 issue, you displayed a kick-butt dry-sump Pro Stock-style engine. My question is: What is it that makes a dry-sump application capable of such high rpm? I know through reading your magazine that the rule of thumb is 10 psi per 1,000 rpm unless you're running synthetic oil, in which case it could be less pressure. But what are the operating differences between dry-sump and wet-sump systems?

Jeff Smith: The engine you mention, Paul, is actually a 762ci Reher-Morrison nitrous Rat that unfortunately wouldn't be legal for NHRA Pro Stock racing's 500ci displacement limit. This engine is actually intended for unlimited heads-up racing. The reason dry-sumps have become popular is as much for reliability and endurance as for increased horsepower. Let's deal with the reliability factor first. A wet-sump engine stores oil in the oil pan. Most production V-8 engines use a typical 4-quart reservoir with an additional quart in the oil filter. In high-rpm drag race or corner-turning situations, the oil is subjected to high g-forces that can easily force it away from the oil pump pickup. This causes the oil pressure to take an immediate nosedive, generally at the same time that the engine is heavily loaded, which is a bad combination. Solutions to this problem include typical items such as deeper oil sumps with greater capacity oil pans, one-way swinging trap doors in the sump, windage trays, oil scrapers, and many other tools used to keep the oil pump pickup submerged in oil at all times.

Dry-sump oil systems are really not a new idea. The idea probably dates back to early '20s race engines. Our research turned up that Aviaid, started by the late Tom Davis, was the first aftermarket company to build dry-sump lubrication systems for V-8 production competition engines in 1968. The simplest dry-sump consists of a two-stage system with one suction or scavenge side and one pressure side with a remote reservoir to store the oil. There are several advantages to a dry-sump system. The most important is the remote storage tank, usually located near the engine. Moving the oil reservoir to a separate tank ensures that the inlet to the oil pump will always have oil, eliminating pressure fluctuations since the tank holds a large enough volume to feed the pressure side of the system at all times. Another advantage is that the reservoir is also an oil-air separator. A large percentage of the volume returned to the tank is actually air bubbles. The tank is designed to allow the oil to rest, which gives the air time to leave the liquid in much the same way that bubbles rise to the top of your favorite carbonated beverage. Wet-sump systems unfortunately subject the oil to far more turbulence, which does not give the oil time to displace the air, causing problems.

Creating an external storage location allows the engine builder to use a very shallow pan with no reservoir, since multiple suction locations remove oil from the engine as soon as it has done its job. Often you will see four to five separate suction lines as well as a high-pressure feed line into the engine. These multiple scavenge points ensure that the oil is evacuated out of the engine as quickly as possible. Additional suction points also create vacuum inside the oil pan, which reduces crankcase pressure and uncovers a little horsepower. Reducing the amount of oil bouncing around the crankshaft also improves horsepower, especially at race engine speeds. These are all good reasons why dry-sumps are so popular with race engine builders. The only real disadvantages are cost, at $2,000 and more for a professional-grade dry-sump system, and complexity, with all those hoses and extra plumbing.

As an interesting aside, the latest LS7 Gen IV 427ci engine used in the '07 ZO6 Corvette also employs a factory-built dry-sump system. As far as we know, that is the first production dry-sump lubrication system for a domestic car. While dry-sump systems are a bit of overkill for the street-as handling improves at well over a g lateral acceleration and pulling 1.5 g's or more on the starting line with the front bumper 4 feet in the air-wet-sump systems can be easily pushed beyond their limits. This is where an add-on accumulator tank can really be handy. An accumulator (such as the Moroso 3-quart unit, PN 23900, $185.95 at summitracing.com) is a cylinder connected via a small oil line to a main oil galley in the engine. Inside the accumulator is a piston. The accumulator is filled with several quarts of oil and uses engine pressure to pressurize the system. Should pressure drop when the oil pickup is uncovered, the pressure in the accumulator will automatically push oil into the engine to keep the bearings lubed until oil pressure is restored. Then oil pressure pushes the oil back into the accumulator.This is a very slick way of avoiding those zero-oil-pressure blues.

Also, your point about the "10 psi per 1,000 rpm" rule of thumb is valid, but the truth is that with a good lubrication system and synthetic or synthetic-blend oil, it's possible to run less oil pressure. We routinely run 45 to 50 psi in our 550hp, 420ci small-block at 6,000 rpm with absolutely no lubrication problems. This means we run a well-designed standard-pressure, standard-volume pump with tight clearances and have never had a problem.

Tuned Port InductionBrandon Munden, Sugar Land, TX: I have a standard 283-block with AFR heads. Will a newer TPI manifold bolt on?

Jeff Smith: You're in luck, Brandon. The Tuned Port Induction (TPI) system was first used in Corvettes and 305ci Camaros and Firebirds in 1985. The '86-and-later Corvette manifolds used in conjunction with aluminum heads employed the standard small-block Chevy intake manifold bolt pattern. This was true only for the TPI manifolds on Corvette aluminum heads, however. The Camaro and Firebird 305 and 350 TPI engines all employed iron heads that had a different bolt pattern for the two center bolts on each side. These center bolts used a different, more vertical angle to attach to the cylinder head. If you have one of these manifolds, it can be used on an earlier small-block, but you will have to modify the manifold to accept the older bolt angle.

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