Tech Tips
1986 GM 'F' BODY LB9
by Vader (3rd Gen ID)

It has become common practice to remove protective screens from mass air flow sensors on these engines, in the hope of improving intake air flow. Analysis of the air intake system indicates the greatest areas of concern, none of which are this sensor unless other significant modifications have occurred first. The air ducts and air resonator box have minimum opening sizes of 11.05 in² and 10.42in² at their most restrictive points. The largest available throttle body is at least 3 in² smaller than either of those, so their flow rates are not a concern.


The following data was calculated from measurements taken with dial calipers, with the measurements calculated to indicate free area. Given are the resultant areas for comparison:



4.6648 in² OEM 48mm throttle body = 2.3323786 in² per bore, minus plate and shaft area
5.5600 in² 52mm throttle body = 2.7799564 in² per bore, minus plate and shaft area
7.0487 in² 58mm throttle body = 3.5243652 in² per bore,
minus plate and shaft area
6.8398 in² Air cleaner housing cover slot
5.7724 in² Air cleaner housing top filter opening
0.076699 in² 8mm drain hole on housing base
5.849099 in² Total air cleaner housing openings.

Since the incoming air enters the center of the filter and flows outward, any additional fresh air openings must be made in the ends of the housing, at the center of the filter. The photo shows the most restrictive part of the system - the air filter housing cover.

The outer slot in the cover has more than adequate free area, but the braces and center at the internal opening are restrictive. Since little can be done to improve the cover without making it substantially weaker, the lower end of the housing can be modified.

A = 5.7724 in² Total Area
B = 6.8398 in² Total Area

The Modification

The process is relatively easy and does not alter any of the body structure of the vehicle. Remove the air filter cover, filter element, and housing outer shell. Remove the two 10mm screws holding the evaporative emissions canister to the base, and tip the canister up and over toward the rear of the engine compartment. None of the hoses or connectors will have to be removed from the canister.

The base should be removed and cleaned. There is one 8mm drain hole in the base. The hole provides an additional 0.076699 in² of free area. The photo shows the base (upside down) removed from the vehicle, and the long stud removed from the base. 

Two additional holes can be drilled at 120° intervals, to form a relative triangle of holes. These three holes can be punched to 7/8" (0.875") without interfering with the stud mounting or the perimeter seal of the base. Any larger hole might break through the perimeter and weaken the base. The photo shows one hole drilled and punched, one drilled, and one spot-punched. 

This photo shows the three additional holes punched in the base. The holes can be punched cleanly with a ½" trade size conduit punch (shown).

The edges of the holes should be dabbed with paint to prevent the onset of rust on the bare metal. Notice the close proximity to the stud weld nut and the perimeter of the base which locates the filter element. This is the reason for the 7/8" maximum holes size. Four holes would leave very little material to support the stud and remainder of the filter. Three holes, 0.875" in diameter, provide enough additional air to equal the area of a 58mm throttle body.

While you have everything apart, it would be a good idea to replace the filter pad in the evaporative emissions canister. It rarely gets done, but is a recommended service procedure every 30,000 miles or 24 months. As you reassemble, a new air filter element may be a good idea also.

See Also:
MAF Article ] & [ Air Cleaner Housing Warning ]

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