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This 81 SC is new to me as of March. The car is getting better and better. My major projects have included rebuilding the Control Pressure Regulator (WUR), reviving the O2 computer electrical system, installing a full Griffiths a/c kit, and most recently, dropping the engine to replace the air box and changing a handful of gaskets, rebuilding the cv axles, etc.
Engine is running excellent, much better than when I started.
Yesterday, I drove from Dallas to Fort Worth, enjoying my air conditioned cabin, but really driving for the first time with the windows all the way up. Around 3,200 rpms and not letting off until about 3,800 is a ear-ringing resonance from the exhaust. My system appears stock except there is not catalytic converter - just a pipe to the muffler.
I need to get rid of the resonance, it is not good. I really don't want to put back in a cat as I don't need it in Texas or should I purely for exhaust resonance? Or is there a better solution than a straight pipe, some kind of pre-muffler setup?
Check if it's a test pipe or a pre muffler ... a test pipe is straight through and in the case of a pre muffler your best option should
you need to install one is the MK , best one out there and outstanding quality.
Check if it's a test pipe or a pre muffler ... a test pipe is straight through and in the case of a pre muffler your best option should
you need to install one is the MK , best one out there and outstanding quality.
Yes, thanks. I've been thinking/studying/reading. I wonder if it is a straight through pipe or if there is some baffle installed? Thinking I might experiment with a baffle first if there isn't something in there already. I'll check MK's website about their pre-muffler. I spoke with a guy at Fabspeed and he said their pre-muffler wouldn't really help the resonance/drone problem.
When my 89 had a 3.2 in it i installed a MK premuffler to get rid of the cat and there was no resonance , even with a Fabspeed muffler.
Now that the car has a 95 3.6 with headers i had to install inserts in the exaust tips as the resonance was unbearable around 3000 rpm , that
can be an option for you ,lots of reading regarding exaust drone !
Should you decide to go with a pre muffler , the MK has the O2 sensor bung picking up both exaust banks , whereas the other brands only pick up one.
Nothing against Fabspeed as i have their full header/muffler combo in my car , but MK is really tops for pre mufflers ...
I've done some more reading and studying. I see some drivers have had drone/resonance reduction by the use of a pre-muffler and some using inserts. I ordered a 3-disk insert unit from Car Chemistry in Waxahachie, TX. About $70 with shipping. I specifically asked for an insert to replace the cat in my 2.5" straight pipe to reduce drone/resonance. This is what he recommended. I'll report back on the change. https://www.carchemistry.com/3-disk-...-1-2-diameter/ 3 Disk Insert from Car Chemistry
Yeah, that's what i have in my car, although i only used two discs per exaust tip... those inserts really kill the exaust note,
you might find three discs to be too much with a stock exaust.
I still suggest the MK premuffler , it's a real piece of art !
Yeah, that's what i have in my car, although i only used two discs per exaust tip... those inserts really kill the exaust note,
you might find three discs to be too much with a stock exaust.
I still suggest the MK premuffler , it's a real piece of art !
Is your 911 an SC? You say "two disc per tip" - What system did you have that made it too quiet?
I'm planning on inserting this in the straight pipe in lieu of a catalytic converter, thus still have stock muffler afterwards. I don't mind quieter exhaust. I used to have a 427 BB Corvette side pipe car so I've had loud.
If it is too quiet or notice too much restriction, I'll remove it and cut off one of the disk and try again.
I believe the instructions say to install the discs after the muffler , not sure installing them in the cat bypass is a good idea.
My car is an 89 911 with a 95 3.6 transplant and Fabspeed headers and muffler ... i have the inserts in the exaust tips on both sides of the muffler.
I believe the instructions say to install the discs after the muffler , not sure installing them in the cat bypass is a good idea.
My car is an 89 911 with a 95 3.6 transplant and Fabspeed headers and muffler ... i have the inserts in the exaust tips on both sides of the muffler.
His website does state it can go before or after a muffler. My theory of course is the inexpensive solution. I don't want a louder aftermarket exhaust, although the Fabspeed video of the pre-muffler and their Maxflo muffler combo sounds really cool to me.
Insert Installed - video of straight pipe and insert alone.
Yesterday afternoon I installed the 3 disk Car Chemistry insert into the straight 2.5" pipe where the cat used to be. The insert is installed about 5" "up" the pipe and I drilled two holes through the pipe through the trailing disk to steel pop rivet the insert in place.
I ran a quick video of the straight pipe with insert clamped in place near the end of the pipe without the muffler attached to see how much noise reduction the insert actually accomplished. I don't think the audio of my iPhone does justice to how loud the exhaust at this point actually is. I didn't think it made much of a change. I wanted to try this worried the insert would silence it too much thus creating too much back pressure when combined with the original muffler. After driving the car with the muffler attached, it helped the drone a little but it is still there.
First, I'd like to ride in another SC with stock exhaust including cat to see what it sounds like at 3200-3900 rpm.
Second, I wonder if I should remove the insert and try wrapping the pipe in between the inserts with steel wool to create a packing and more muffling. Edit - the insert came with two caps for the center pipe - one with a hole, one without a hole. Ken at Car Chemistry just told me to try the cap with a hole on the intake end of the center tube. He reiterated that cats are very restrictive and it might take one or both caps to replicate the muffling characteristics of the cat. Of course, I'd like a nice drone free exhaust noise without too much back pressure.
I wish I had measured the exhaust noise with the straight pipe and no insert. Of course, it is really loud. The video shows at 1,000 - 1,100 rpm the exhaust with insert is producing about 102-103 db's.
If you're looking for a bit of an upgrade while you're at it, get an SSI exhaust ...very well established over the years as a solid unit that offers better flow yielding a bit of a performance boost, even on a stock engine. My SSIs go into a Dansk 2-into-1 Sport muffler and it sounds sporty with a good tone, nowhere near annoyingly loud, and no resonance or "drone."
07-05-2019, 11:11 AM
81 SC Exhaust Resonance w/o Cat
