Several people have emailed me and asked how I made the modification to my stock muffler 1 in 1 out, to a 1 in 2 out. So, I’ll post it here for everyone.
If you don’t want to spend a lot of money on SSI’s or M&K, but want better sound from your 911 and 4-5 more HP, you might want to consider this simple modification to your 1-in-1-out stock muffler.
And, it is very inexpensive! It's like going from one speaker on your sound system to 2 speakers with larger woofers. The sound will be lower and louder. My neighbor, Jim, did this last year to his 88 911, he gets all the credit/blame here!
If you don't want to cut a hole in your rear (the cars) you could add the second hole in the right side of the muffler and have the pipe installed and turned down. you will not see it, but will hear it.
I made a cardboard pattern of the hole in the valance on the driver's side. Then turned the pattern over and measured and transferred the pattern to the Passenger side valance. I marked it with a sharpie and cut it out with a round diamond blade/wheel on my Dremel tool. The blades are cheap at Harbor freight. It took about 1/2 hour, and two blades, as I went slow and was careful not to slip and damage my fender. I finished the cut out with a 1/2" grinding wheel on the Dremel and painted on a reddish primer with a sm. brush.
I took my 911 to the muffler shop. He took the muffler off as he did not want to cut and weld in such a tight spot and damage any oil lines etc.
He cut a hole in the end of the muffler the proper diameter I requested and then cut and bent a pipe to fit. He then tacked welded the tail pipe on and remounted the muffler to check the angles of the pipe in relation to the hole in the valance and if it looked the same as the driver's side pipe. He adjusted it and welded it on. Then he welded the SS tips on.
I got the SS polished tips on ebay for $9.99 each + $9 for shipping. Just type in... 3" Stainless Steel Chrome Exhaust Muffler Tip EX-1001, then have the muffler guy cut the 3 bolts off the SS tip and weld them on. They are rolled ends and angled.
Total charge was $67 for muffler work + $29 for the tips = $96
The new sound of the two pipes is incredible...very low and throaty... stereo like. Not loud and annoying, but really sporty, but not obnoxious. I have WebCam 20/21 High Performance Cams and that makes the sound a little lower and louder from the start. But, my neighbor has stock Cams and he did the same modification, 1 in 2 out, and his 88 911 3.2 sounds perfect!
Right side new pipe and tip.
Oh, I should mention that I put in a "Sport Muffler" Steve Wong Chip, but that does not change the sound, just enhances lower end RPM torque.
I am definitely no expert and I'm sure it is possible to find data that will support any theory regarding HP and mufflers.
But, in Bruce Anderson's book, EXCELLENCE Technical Notes, page 46, on Exhaust Systems, he states that his Dyno test data showed an increase in HP, with different engines and different muffler configurations, from 4.6% to as much as 10.5%. Some of these %'s were at peak power and some at 3500 RPM.
What really surprised me is that his data showed, that by simply adding a second pipe to a stock muffler on an 88 3.2 Carrera, an increase in HP at peak of 5.75%, and that's 11 HP!
Actually it is really more like 3Hp @ 5500RPM and 6 HP @ 6900RPM. Ok, I'm happy...I'll take better sound and +5HP for $100 !
...right foot all the way down... and Good luck. jhubs