I have a bone stock z-28, I just recently installed catback 3" flowmaster system and gutted the cat (which had pieces coming out). The problem is now the Midas special exhaust that was on the car preformed like it had 50 more hp than it now does. I'm wondering if because it is now so loud the knock sensor is picking that up and killing timing? Or is this to be expected going with 3" pipes (If so I would love to trade it for a stock catback system again!). Help!

 

Help?

 

really cant help you. no clue. more details?

 

Maybe you didn't get all of the honeycomb out and now it's wedged in sideways blocking the exhaust flow.

 

That's a strange one. Make sure you don't have the muffler installed backwards, they only flow in one direction.

 

Its definitly not the knock sensor pulling timing since a catback wouldnt do that. Make sure you have no obstructions from the gutted car in the pipes of the catback. Also, is this car a 96+? If so you would need O2 sims for the rear O2 sensors, you did plug the o2 sensors in right?

 

Its a 94 Z and i made sure all of the honeycomb was knocked out. It runs okay, but before the car ripped for what it was. Only thing i did was the cat back, and as soon as i installed it the power loss was very apparanet. Did i kill the car with over sized 3"?

 

I would say get some real numbers and don't always go by what the butt-o-meter is telling you. A lot of times you will do a performance mod and it will feel like it is going slower but realy you just might have changed the rpm range that the added power is being apllied in. Unless something more serious is wrong and you are not describing what you are feeling to us clearly. Oh I don't think that you overkilled your exhaust by going with a 3" catback. The stock size pipe is like 2.75" in diameter to begin with.

 

Yeah the seat of the pants dyno is what im judging by. Before the car would break the tires loose pretty easily down low and now its like it struggles to scratch a patch.. I was major dissapointed! Anyway I have a set of gears going in next and hope it feels like i moving in the forward direction on the next mod. I hope!

 

Let's see, where do I start with this.........

Make a signature with what car you have and what you have done to it. That way, in the future, you won't have to describe your car every time you post a question.
Next, which Flowmaster exhaust do you have for your car? Flowmaster makes more than one catback exhaust for your car.
Also, where are you located?.....another one of those things to put in your profile/signature. Has the weather warmed up quite a bit since you put the exhaust on? Every 10 degrees increase air temperature = 1% loss of horsepower.
What I'm getting at is in EQUAL conditions, your Flowmaster catback should not have lost you power, you should've gained a MEASUREABLE amount of power. That means, you might not feel the 10hp you gained. Like 94blackz was saying, your butt-o-meter is not a dynamometer.
In other words, I'd say check something else. When you gutted the cat, did you take your y-pipe off? If so, I'd recheck the exhaust manifold to y-pipe bolts. An exhaust leak there would definitely kill some power.

 

I put the car up on a lift today and everything is intact no exhaust leaks? I put on 3" catback system with dual 3" outlets.

 

3" exhaust does not kill power, that's not the issue, sorry.

either your sotp meter is wrong, or they screwed up in the gutting process.

 

Just cause it wont break the tires loose as easy doesnt mean you lost power. On a cold day ( 30's 40's) if I take my car out I can burn out through first and second. But if it even warms up to say 60-70* out, the car just hooks and really takes off. THe hotter the road is, the better you will hook up. And the less power you will make.

Also, your not loosing power through the cat back. I have a American Thunder Flowmaster cat back on my car. It had a very noticable increase in throttle response and maybe some mroe top end over the stock set up.

Which flowmaster system are you running? There are a few different ones.

 

Ding-ding-ding-ding......we have a winner!

Also, there's a right way and a wrong way to gut a cat. If you did it the wrong way and just knocked all the crap out of it, your car probably sounds like @$$ right now too.
The RIGHT way to gut a cat is after knocking all the guts out of the cat.....take a 3" exhaust pipe about 18" long, ram in into the inlet of the cat until it can't go in any farther, and weld the case of the cat around the pipe. That will leave some pipe out on the inlet side to either just weld it to your y-pipe, or to weld a flange onto the "cat pipe" and y-pipe. Going the flanged route means you can have a functioning cat to bolt in when you need it - obviously you'd need to get another cat for that but you get the idea.

 

You could always take the cat back off and drive it to rule it out. Any codes being thrown? Hook up the data master and see what the computer is thinking.