Exhaust drone fix for 4th gen 4runner
After much review and many attempts to solve my drone issue I stumbled upon the scientific approach of designing and tubing a Quarterwave resonator. Sounds fancy but it is very simple. All it is is a J-tube of a specific length that targets a specific drone frequency. A single J tube of similar diameter to the collector pipe (eg 2.25 or 2.5in) ahead of the muffler and installed a few inches a head of the muffler. The tube requires a capped end to bounce back a sound wave with a phase offset to counter the wavelength of the resonating sound wave. Do not put an actual resonator or muffler on this branch as it will dampen the amplitude of the bounced back wave and be far less effective.
This quarterwave principle works when an exhaust produces drone only over a narrow frequency band. It seems that right around 2000RPM is the typical engine speed where many 4th gen 4runners with modified exhaust exhibit drone. My drone was limited to about 1800-2200RPM. In the cab the sound frequency was about 130-140Hz and max drone volume was 82-85dB. This frequency is directly related to my V8 engines RPM.
Using quarterwave resonator cals, the predicted J tube length at this frequency is around 30in. After trying different lengths my optimum J tube length to reduce drone is in the range of 28-29in. Any shorter than 27in or longer than 30 in produces noticeably more drone. This J tube drops the inside cab drone level to 69-71dB, which is very tolerable.
NB my exhaust system is totally custom with Doug Thorley headers and my own built 2.25in Y-pipe (all welded using stainless steel straights and elbows) with 2.5in collector feeding a 22in long 3in diameter Magnoflo muffler.
