This step by step tech article was provided by Cmpfreak88.
This is a free mod intended to lower the RPM's at idle to give your stang's exhaust a slightly throatier idle sound while also increasing the engine braking effect. This is achieved by limiting the air going to the IAC (Idle Air Control) module, which in turn maintains a lower idle RPM. This mod works on both single and split port and on both auto and manual. The mod is more noticeable on a manual though as RPM's will drop faster in between shifts meaning you'll have to get used to the timing.
This was suggested by NeoTokyo and v6stang07 seemed to get the right hole sizes for out v6. All I did was follow what they said and take pics!
Felt gasket material ($5/roll at Autozone)
Socket wrench w/ 10mm Socket
Fine point pen
About 30 minutes time
Park your car open the hood and prop it up. If the car was just running you're probably going to need gloves because the IAC gets a little hot.
Take the socket wrench and loosen the two bolts on the IAC module, if you're looking at the car from the front it's on the left side of the upper intake and has one bolt on top and one on bottom. (Shown below) You may not be able to see the second bolt, but it's right underneath the middle of the IAC, fish around for it! And don't drop the bolts! I loosened both bolts with the wrench, then ended up taking off the wrench and just using the socket to take it off because it wouldn't rachet anymore!
Pull off the stock gasket and take out the bolts. Set the IAC module on top of the upper intake and take the gasket to the bench.
Note: Your gasket will have much bigger holes, mine already had my modified one on and you can see here that it ripped!
Open up your gasket material and cut out a small section about as big as the gasket, if you want, you can trace it out with the pen and then cut it or just lay it on top of the material and then cut around it, I did the latter. Since I was doing this for the second time for the writeup, I saw that my first gasket had ripped, so I recommend cutting out two!
Place the stock gasket on top of the new gasket material and trace out the bolt holes, then cut them out.
Measure a 5/8" hole on the center of the left side and a 3/16" hole on the center of the right side. Make sure your 5/8" hole is actually going to be all the way open by making sure it fits in the space of the stock gasket. I just drew lines from my center point and made it into a circle. Then cut these holes out. Do this for both gaskets! A razor would probably have been real nice, but I used regular scissors. My holes weren't perfect, but it worked!
I left out my stock gasket and then put both of the gaskets on for strength since the last one ripped. Simply place the gaskets on the IAC with the bigger hole on the left side, nearest the fender and the smaller hole nearer the engine. Put the bolts back in and tighten down snug.
Make sure all tools are out of the way and go ahead and start the car! Right away you should notice that the rpm's at idle have dropped and it almost sounds like the car is going to stall, but it's just the lower idle speed. Go ahead and rev it up and listen to the new tone! In a 5spd you really notice the difference and will have to shift faster or possibly double clutch it to keep the rpm's up. Nice and easy mod that worked pretty good! Makes my little 6er sound like it has a decent exhaust note! Once you decide you like it, go ahead and make the same thing out of sheet metal and make sure to put gaskets on both sides to keep the seal!
Idle RPM and Exhaust Note Video
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