I have a 91 Formula 5 speed 305 TPI, with a stage 2 chip and 1.6 roller rockers. I’m lookin to have a new exhaust done. I looking to leave the stock exhaust manifolds and go with with 3 inch mandrel bent pipes, cat delete and go with flowmaster 40 series transverse muffler. My question is if I were to put butterfly exhaust valves in the middle of the system will that screw with the computer settings when I run them wide open? Does anyone currently have the valves? Any complications I should be aware of?

Thanks in advance for your input….

 

Are you talking about exhaust cutouts to make it loud? Put it quite a ways downstream of the O2 sensor and you won't have a problem with how the engine runs. Putting it in the wrong location will let fresh air get to the O2 sensor and screw up the A/F ratio the engine is using to make decisions.

Personally, I would forego the exhaust cutouts and put money in headers to make more power. Loud exhaust just makes a car loud, while a powerful engine makes the car sound good.

 

Yeah they would be well past the o2 sensor. They would be welded in before the Y pipe. It's just to run open exhaust if I wanted to, that's all.
Thx for the info.

 

Owning a car that a dual mode exhaust a nice option. It gives one the option to have a mild vs wild exhaust note depending on your location. I plan on building a system like that for my TransAm. Mainly because I feel awful when I start the car up in the mornings I take it somewhere. An 11:1 cammed LT1 while being pleasing to my ears is definitely way too rowdy at 7AM for neighbors.

My Focus RS has a butterfly valve in the exhaust thats opens up depending on your drive mode. In Normal mode the valve is closed. In Sport, Track, and *gasp* Drift mode the valve is open. When open it routes the exhaust through a less restrictive portion of the muffler which adds a bit of noise.

Most manufacturers are doing this now due to drive by noise requirements. When open it offers a small gain in flow potential, but not much. Newer Corvettes, Camaros, and Mustangs all come with the dual mode exhausts either as standard or optional equipment.

I can tell the difference in the exhaust note when I start it up in the mornings. Ford determined they could light off the cat-converter faster if they opened the valve up. So when you start the car in the mornings, the valve opens up and stays open for probably 20 seconds or so. During those 20 seconds the idle is much louder, then suddenly it clams up and gets quiet. Inside the difference isn't really audible.

 

Thx for the input. I'm going to have to price it out and see what the cost would be. If I can't do it now maybe I'll add it later on.

 

If you haven't pulled the trigger on this yet, I would go with what QwkTrip said and go for headers first. I went with the QTP 3" cutout and loved it a lot, had a slight difference in power and the car sounded amazing. At the time it was my favorite investment, but if you aren't careful you can lose your cutout motor ( I had three passengers in my car, hit a dip too fast and ripped the motor right off). Now years later I am finally getting headers like I should have and if I get bored I'll toss on the cutout again, but start with the more beneficial investment and see what works for you.

 

I did hooker shorty headers then did a QTP...mustve ruined it 4 times between bumps/raised manhole covers/once i accidentally ran over a curb (i didnt see it)....It was fun, but it sounded like crap IMO. It was definitely loud, but it didnt have a good sound. I love my SLP Loudmouth (with Borla muffler), its loud as the QTP, but sounds good, plus i always lost power with the cutout. I will always have a spot in my heart for the cutout tho

 

I call BS on losing power with the cut-out. I have had them on 3 different vehicles now and all responded favorably to ditching the restriction of the muffler.

My M56S gains nearly 4 mph in the 1/8 with the cutouts open on my custom 2.5" X pipe setup. Exhaust runs into the factory mufflers.

My 350 in my Express van also has cutouts right in front of the muffler that bypass the muffler, enter a merge Ys before they hit the 4" tailpipe after the muffler. Basically the exhaust either enters a dual in, single out muffler or it run partially through the muffler and through the open cutouts into a merge Y that runs into another Y at the exit of the muffler. A bump of the switch virtually eliminates back pressure on the 350 allowing it to run cooler under load and make more power when climbing long grades with my travel trailer in tow. The difference is very noticeable. The MAF reading jumps up about 20 gm/sec on the Express with the cutouts open.

 

I still have yet to take my car to the track so i cant give you numbers, and i dont have anything to bring up any numbers from the onboard computer. all i have to speak on is the seat-of-my-pants gauge. When i have the cut out closed, my tires break loose easier, and i get better low end torque. When the cut out is open, i get a longer high end pull, but it holds longer when its just stagnant power. maybe its the placement of mine (where the cat used to be) but i'm happy yours work out well for you. I still have mine in, but i dont use it much anymore.