Date:         Sun, 18 Nov 2001 11:46:13 -0500
Reply-To:     Gary Stearns <gstearns@OPTONLINE.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Gary Stearns <gstearns@OPTONLINE.NET>
Subject:      TiiCo Vibes...solution at last.
Content-type: multipart/alternative;

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

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