First let me say that I'm waiting to scrape together funds for the Flowmaster American thunder system and the reason I did headers before cat was because my motor blew and while I ripped it apart and put it back together, I might as well do headers while I was in there.

So with all that said, will there be any gains by just swapping out the manifolds for the headers? Just curious. I understand that in order for the exhaust to get out faster, the catback would have to be enlarged. I just want to know if I have even 3 more hp/tq right now. So instead of 187whp and 270wtq, I now have an even 190 and odd 273 + whatever my Home Depot ABS intake pipe and ghetto filter give me.

 

You don't have 187whp if you have the 5.0TPI your info states- that's at the flywheel. Either way, the stock exhaust is horribly restrictive- small pipes and a poor flowing muffler. Full exhaust will reaaally wake that thing up.

 

No that's what it dynoed last year on the Dynojet. Everyone actually dynoed low that cool April day. It's a Speed Density Lb9 so it has the L98 cam. The peanut cammed auto Lb9s are the 190 flywheel cars. I'm not saying it's the norm, just what mine dynoed at a reputable dyno here in Chandler AZ.

 

You will see an improvement. Just not what you will have when you have the whole exhaust system upgraded.

 

Yes there will be gains. According to David Vizard, a legendary SBC builder. He dynoed an 81 t/a, with stock exhaust. Then dynoed the car with just the headers and there was an increase of 16 hp and 22 ft lbs of torque.

Then he replaced the cat with a high flow version, and brought the hp to 27 hp and 31 ft lbs of torque. Still with a stock exhaust system.

 

One of the worst things about the stock exhaust system is where the pipes "y" together. There are power gains to be had by changing out the y pipe.

 

How much whp can I expect to make from a catback? I used to run the catback unbolted before the headers and the power difference was noticeable. I loved the power, but the loudness got to me. Don't get me wrong, I love the V8 roar, but just normal daily driving and with my GF in there with me, it got old and the interior buzzed.

 

I have seen gains from a complete exhaust system up to 50 hp.. but that is entirely up to your setup... in your case perhaps 30 hp to the wheels. For Headers, Y pipe, and catback.

 

I know of a BONE STOCK L69/auto Z28 out here that put down 190 to the wheels, with almost 100,000 miles on it.

 

i just swaped out my 21/4 for a 3 inch with a high flow cat and an 80 series muffler. i nnoticed alot of gains. i did not dyno mine but the take off, throttle response and sound is sooo much better. i am very happy with itnow.

 

you could feel 10 hp to the wheels....

the only problem is when people feel the newly aquired 10 hp to the wheels they think it is like 50.

 

how much flywheel horsepower can be gained with headers, y-pipe and 3" exhaust if you say 10hp at the wheelsl wouldint that mean 25+ hp at the flywheel?

 

no more like 15 fly hp. i must agree with nelapse you dont NEED the cat back to see hp increases. i have hooker comps with a CRAPPY custom Y, no cat and thats it. now i know a cat-back would be nice, even just with that. i can feel a big gane over stock.

 

personally, I believe a full 3" exhaust like the one I have 3" collectors, 3" y-pipe, 3" rear exhaust... is over kill for a stock motor. throw some induction mods... like intake, cam, and heads... and your exhaust would be used to it's full extent.

It is like this really...
Let's say you have a 200 hp car.
You add a catback. You gain 15 hp <-(hypothetical)
Then you add headers. You gain... 10 more horse power.

Let's start over.. You have a 200 Hp car...
You add headers... you gain 15 hp...
then you add a catback... you gain 10 more horse power.

WTF?? Each individual component affects another one which affects another which some how is affected by another component.

We shall take the first mod example I gave you into consideration.
You have a catback... well your exhaust is a bottle... and the neck is the smallest part... so you have small exhaust manifolds but, since you added the catback you have a little less restriction. Your gain... 15 hp.
Now you add in headers, you open the neck in your bottle even more, since air is flowing pretty darn good right now the gains are not as great as the catback but since you are making a less restrictive exhaust ....even less restrictive... you are rewarded... 10 hp..

Now lets say you have the same car.... and you put on some AFR heads...
now you have a stock exhaust... we shall say you have 225 HP
You add on a catback.... now you gained 20 HP
Then you add on headers... You gained 15 HP
whoa... wtf? The heads control the airflow... You other modifications are directing influencing the efficiency of the exhaust.

Long story short.... You could gain 50 hp to the wheels from headers, hi-flow cat, and cat back... but not by itself.

You must have other components that will influence the mods. Things like Cam, Heads, intake... all influnce your exhaust gains. They are directly proportionate to your gains.

 

with that in mind i am trying to do to full 3" exhaust (from 2 and half inch from y pipe to tailpipe). I dont know if going from 2 and half to 3 inch all the way will free up that much more power though i do have AFR heads and comp cam....we shall see

 

Just keep the Bottle neck idea in your mind. The smallest part is your biggest restriction...
eventually, something else will become the bottle neck. Such as an intake plenum or t/b or even a carb.

 

if anyone cares... i have been playing around with my car for a wile now.. and i think i got a way to creat a true dual 2.5in exhaust.. all the way abck to the bumper... il show you next year wheni get it done.. i was told my tpi guys here that i will gain like like 20-40 hp at the wheels with no cat.. and headers.. so we will see eh... il dyno it before and after i get it done.... you will for sure see me next year if i can get it done!