manifold vs header

[BenG]

i've noticed a lot of people posting up problems with their exhaust manifolds lately...and the answer that they get is to switch to headers. This might be a stupid question, but what's the difference between a manifold and a header? Aren't they all just pipes to route the exhaust gas to the cat?

[5Sfte]

A header usually allows more airflow than stock and can dissapate heat better.

[bgrieger]

Manifold tends to refer to the log like object that collects the exhaust from each exhaust port on the head before moving it to the exhaust piping on NA vehicles. Turbo cars are always refered to as having manifolds, be they properly designed tubular ones or the good old log. In essence, they collect the multiple sources streams and combine into a limited number of outlets at the same location, or vice versa in the case of the intake manifold.

Headers do the same basic job, but properly designed can flow more, improve scavenging of gasses from the cylinders, and retain more heat in the exhaust gasses (hot exhaust is good...to a point). Headers are usually distinguished in how they combine exhaust flows, where the merging tubes are designed to help pull gasses from the other tubes as exhaust pulses travel down each one until all the streams have merged for that bank of cylinders. They are "tuned" by length of the runners. Manifolds are not.

Clear as I can make it this late at night...

[alltrac165]

Good explanation, Bob.
Quite a few cast manifolds today are far and away better than the old log manifolds of old found even today on domestic V8's. Some are good enough that it's hard to justify the expense of a header for the less than spectacular gains found at the top end of the power curve. If your I4 cast manifold looks like a nice tubular 4 into 2 into 1 set-up, for instance, you are probably better off keeping it over a header unless you have already modded the snot out of the rest of the engine. They tend to be more durable, naturally hold heat better for higher exhaust gas velocity, pose no issue for smog inspections, and you already have it. You could have it ported for a little extra flow and still keep your stealth mode.
If you have the log type, or a manifold with primary pipes that join up at sharp angles, or some other design that doesn't look like it would flow nice and smooth, then a header would be a more worthwhile consideration.
Headers have other benefits as well, such as the opportunity to use optimal tube size for the best velocity for a given application, and optimal length for wave tuning which, properly timed, can boost cylinder filling.

[grog]

altrac, is there a chance you might know what kind of gains are posible on say a stock 22r by upgrading? You say "less than spectacular gains found at the top end of the power curve" but this is a simple bolt on upgrade I am weighing right now.

check this post if you feel like helping

http://www.toyotanation.com/showthre...threadid=24040

[alltrac165]

As you can see by my sig, I have an 87 truck. The manifolds on these are pretty good. Unless you're doing some crazy high end build up, it's not likely you will find much power there. You can have it lightly ported to remove casting flash and roughness if you like. The rest of the exhaust is another story. Check out www.yotatech.com for more info on your truck specifically. I'll talk to a few friends of mine with built to the hilt 22Rs and see what they think. Essentially, right now it would be like putting a 2 inch valve on a garden hose.

[alltrac165]

Well, my friends unfortunately made all kinds of changes at the same time when they put on headers, so they had no info on just what the header alone did. I would venture to guess that if you do the exhaust and leave the manifold you would realize more bang for the buck. I would use 2 inch from the manifold to the cat and 2 1/4 from the cat back. The cat adds considerable heat to the gasses, which is why you can go bigger from the cat back.