Mailbag: What Does an Oil Restrictor Do?

(Image/MGA Guru)

Q: What is an oil restrictor?

A: An oil restrictor installs in the oil passages of the engine block. It limits the flow of oil to certain parts of the engine.

Why would you want that?

Engine oil lubricates and cools vital engine components. In most cases, more oil usually means the engine will run cooler and last longer.

However, in some specific race applications, more oil can actually reduce the performance of the engine.

When roller rockers and solid lifters are installed, the amount of oil required by the valvetrain is reduced greatly.

Installing an oil restrictor kit will limit the oil flow, reducing the windage created from excess oil draining past the rotating assembly. This keeps the oil flowing where it is needed most — to the rod and main bearings.

Important Note: Oil restrictors are NOT recommended for any street vehicles.

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  1. Thanks for the “Important Note” or I will run into issues.

  2. Can be a problem even on the street if parts not installed correctly. Way back when, I installed a solid lifter cam in a ’67 Pontiac GTO 400 CI engine. Shop sold me the cam and kit and they did a 3 angle valve job on the heads with new springs. The solid lifters were a problem, likely from a BB Chevy and flowed WAY too much oil to the heads. Much more than with hydraulics. It drove okay, ran pretty strong and I never lost oil pressure through the 1/4 mile, but it was not correct. (I sold the car eventually without ever pulling and tearing down the engine to install a restrictor.)

  3. Daniel Wilson says:

    Many thanks to the OAC Staff for providing the essential facts pertaining to screw-in oil restrictors. I get asked about oil restrictors quite often because of a particular engine that I prefer to build. Ford’s 351 Cleveland engine has gotten a lot of negative publicity because of its less than optimum oiling system design. Thanks to so many cyberspace experts talking trash online, the problem got blown way out of proportion. The basic problem is how the oil is distributed in the engine block.

    From the pump the oil goes up and feeds the right (US passenger) side lifter galley first. Then it is distributed to the main bearings and left side lifter bank. For stock and mild performance builds that don’t need to rev beyond 6000 rpm’s, the stock system is adequate.

    Taking into account the 4-V cylinder heads capacity to make huge amounts of horsepower at high rpm’s and the blocks thick bulkheads and stout main bearings, Ford obviously had big plans for the Cleveland in competition until the plug got pulled on ALL high performance engines coming out of Detroit. So why didn’t the engineers design a main bearing priority oiling system for the Cleveland like so many of the other performance engines of the day ? They apparently did but the bean counters in accounting got the last word and decided that they could save a few dollars on each block by machining only two primary oil feed galleys instead of the customary three. Not one of Ford’s better ideas as hardcore racers soon found out.

    In the early seventies when the Pro Stock pioneers started taking advantage of the heads superior flow capacity, that meant twisting the engines beyond 8000 rpm’s. Oil starvation at the main bearings caused eminent disaster. The fix was to limit the oil flow to the lifters especially since the stock hydraulic cam and lifters were being replaced with solid roller valve train components.

    By drilling and tapping the key oil feeds to the lifters at specific locations at the main bearing saddles in the block and installing screw in restrictors with 0.040” orifices, oil flow to the main bearings is greatly increased. The restrictors are inexpensive and the labor to machine for them is minimal but the gains in engine longevity are priceless.

    For optimum oil control in the Cleveland, there’s another modification in addition to the restrictors that I use and highly recommended for any level of performance build. The oil galleys that run the length of the block and intercept the lifter bores are another source of excessive oil flow where it’s not necessary. The easy fix is to enlarge the lifter bores and install brass bushings that have small oil holes for precise oil metering. If the additional cost is an issue then only install bushings in the right side lifter bank. That’s where the oil is delivered first from the pump on a stock block and the pressure loss is greatest. Oil feeds from there across to the left side lifter bank and terminates at that point.

    The oiling system modifications apparently worked extremely well. The early Clevelands that dominated Pro Stock and the current day Modified Production classes that run Clevelands with manual transmissions routinely leave the line in the vicinity of 10,000 rpm’s and shift around 11,000 without oil related bearing failures.

    And the beat goes on….

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