This one's for TiiConauts plagued with the I-4
droning as I was... After spending 3 months trying everything, I finally
have success.
Apologies to the engineers who may cringe at my
misuse of terminology. First observation: It is the exhaust system, nothing
else.
I have noticed that some other TiiConauts don't
seem to have this problem (or maybe aren't bothered by it). It seems that
those with no complaints have stayed with the stock muffler etc. I had
changed to an "Ultra-Flo", basically a 6" diam. glass pack. An important
difference is that a stock muffler weighs about 15 lbs. more than the the
aftemarket one. This is significant.
I have tried every configuration of support, flex
joint, ball joint, bracing for strength etc. Some would have minor success for
awhile. Finally I had my son run the rpm's up while I watched the header
downpipes from the side. Right at 3800 rpm I could actually see the flex
in the header pipes causing them to vibrate in a blur. I remembered that
years ago I had put an aftermarket exhaust on a 16V Scirocco causing the 4 cyl
drone that wasn't there before. Pack rat that I am, I still have the
resonator that came out of that system. Looks alot like the old "Thrush"
tin cans that we used in high school, but it's welded heavy gauge
stainless. Look inside, almost straight through. But, it must weigh
18 lbs. Far more than you would expect. Gotta be a reason for
this.
Aha. increase the mass of the exhaust system
at a strategic point, and change the resonant frequency of the whole
system. Experiment time. Went to Home Depot and bought a 2.5 x 6" length
of cast iron threaded plumbing tubing with a cap for each end. Loaded it up with
scraps of steel bar stock, topped off with sand from my son's
sandbox. Attached my torpedo weighing about 10 lbs. to the top
side of the header collector (very securely, wouldn't want this thing dopping
off on the highway). Test drive time. Turn around in the driveway,
coupla heavy vibes as the rpm's dipped below 1000 when releasing the
clutch. Never did that before. Out on the road. 2000, 3000, 4000, 5000
rpm. NOTHING! Wooooweee! Doesn't even try to drone.
Feels like a damn Honda engine! Mass increased, resonant frequency
decreased.
So, go hang something on your header, and
enjoy smooth sailing!!
Gary