Jebbysan

I really would not bother with the headers if you do not put some good exhaust behind it......the bendy, kinked up 2" system on these cars is dreadful.
Feel the whole benefit by purchasing an exhaust from Pypes or Wilcox........
The Magnaflow system looks nice....but necks down to 2" at the outlet of the mufflers......kinda dumb for $700.....

A nice system on these cars equals a big jump in torque and a moderate power gain......
Do the whole thing.

Jebby

ezobens

What was kinda dumb is that Chevy didn't use the 2.5" outlet ram horns from the fuelie cars and make the pipes 2.5" all the way instead of necking it down to 2" at the manifold and then going to 2.5"- What's the point in that?
That little detail robs more power than having the mufflers go from 2.5" to 2" way at the back. I doubt the difference between 2" and 2.5" tail pipe would be noticed or measurable by most engines running on the street IMO.

BLUE1972

If you are using a tuned pipe like on a motorcycle (2 stroke) or racing outboard ( racing - chain saws, etc) you need back pressure as the tuned pipe reflects the wave back into the cylinder. It scavenges the exhaust - pulling the unburnt mix out and then shoves the unburnt part back in to "super-charge" the cylinder. The length and diameter of the cones is critical for max power, and inefficient at all but the desired power band, usually near max RPM.

The car exhaust (4 stroke) helps pull the exhaust out of the motor, no return wave is involved, so back pressure will not help. Collector length is important , as is the header design for torque or RPM.

There is a little (a lot) more involved but that is close enough..

Back pressure will affect noise control ... not getting into that. Look up the NPP exhaust on the C6 -7.

Jebbysan

It does.....but necking down to 2" at the end is still a cork. Point is that even a stock 350 will run better with a dedicated exhaust......headers and all......Manifold kill torque for the very reason that another poster describes.....it will not help scavenge the cylinder....
Headers will in a stock engine help pull a charge out of the cylinder evn at low to mid RPM......it is a huge torque boost but only about 20-25 horsepower assuming the engine tune has been optimized.
If it is a question of budget.......sure...put the headers on now. It cannot hurt....but the real gain will be when a high flow setup is added.

I had a 69' 427/400 a few years ago that was all stock....right down to the factory 2" exhaust.......it never ran that great....even after tuning. I installed a 2 1/2" chambered setup from Wilcox and had the lower the idle about 300 RPM! It was corked to the max. It changed the whole attitude of the car. I never got time to put headers on it before I sold it....but wish I would have!

Jebby

ezobens

OP never mentioned the engine he has- BBC? SBC/350?
I agree that anything smaller than 2.5" on a BBC is restrictive but I question how much difference you would notice on your average, mildly modified 350 having a 2" tailpipe on the muffler with everything upstream being 2.5"?
The OP already stated he wanted the keep the stock exhaust tips without having to modify the body opening.

nix1981

350/300. Just a base coupe auto. Nothing fancy. The car is bone stock with the exception of 202s. For the most part I want to keep it stock but want the sound and for the engine to breathe the way it should with these headers installed. Carb has just been rebuilt with the heads and headers considered. I was thinking of just keeping the 2" stock system but was concerned about reducing it down from 3" to 2". I think the next thing I'd consider is to go with the stock 2.5" system. But as some have pointed out, the system is 2.5" all the way through then goes to 2" out the muffler. So, my inexperience leads me to think that it's still a 2" system
If just reducing it down to the current 2" is fine, and is not going to hurt anything, than Im good with that...or if I really should switch to the 2.5", thats fine too... iI just don't know enough about it and wasn't sure what to do.

ezobens

Being a 350/300 auto that is bone stock other than the headers, I don't think I'd spend the money on 2.5" exhaust at this time- You'd never notice the difference except for maybe the sound.
Now down the road, if you do additional engine upgrades, you can always go 2.5" at that point.

I think people get overly fixated with the 6 inches of 2" tailpipe on the mufflers.
No one thinks twice about the 3" to 2.5" reduction at the collectors and the exhaust gases are much cooler at the back of the car so why would a short 2" tailpipe matter on a mild build?
Elm

BLUE1972

THE REASON CHEVY WENT WITH 2" MANIFOLDS IS COST.

Low production parts cost more money. 2" was used on most cars.

The extra cost and breakage on 2.5" manifolds would require a surcharge on the car. Back in my engineering days the ratio was 5 or 6 to 1 for the cost transfer to the consumer. (in my industry, other industries were 8 to 10 to 1) Thus $1 equates to $5 to $6 for a customer.

That is why the engine options cost more between the SBC"s even though the block is the same. . That is also why a lot of Camaros and other "performance" cars came with single exhaust.

We have to remember the value of a $1 back then. $10 was a lot of money. When people purchased a new car - money , $ difference always played a role - or Chevy would have sold less Camaro's and more corvettes.

Also most engines in the day were rated at max horsepower, and the car's exhaust was not factored in.

Thus a large part of the major difference in HP rating between 71 and 72 cars with the same engine is due to the change in rating statistics / practices.

fazang

Nice discussion about exhaust, can't count the number of times i reference this little gem.
http://www.superchevy.com/how-to/exhaust/0505phr-exh/

to OP: once you understand how an entire exhaust system works you can build one to suit your needs. Good Luck.
-ALF out...