Chris B

First off, I like headers. I haven’t replaced my stock manifolds with them yet, and may never, but I have always believed headers to be the best ratio between price, performance, and ease of modification to be had, and will definitely use them on the non-stock '57 Chevy I would like to build someday. However all the input on the "Think Before Buying Headers" thread has me wondering:

- Is there such a thing as "properly tuned" stock manifolds? Is it possible that the exhaust pulse from each port can join in a flow in certain manifolds that is similar to the flow in properly tuned equal-length headers?

- Is it true, as Oger said in the “Think” topic that an engine that doesn't need more flow than the stock exhaust allows will not see any gain with headers? I mean, it does make sense that, if the flow is not impeded, then greater flow is not necessary. Unless backpressure exists regardless of flow capacity??? ...and that matters only if backpressure is always bad:

- A friend of mine (who runs stock manifolds on a T/A haha) even argued that in some cases a certain amount of backpressure is desired, something about preventing valve float at high RPMs. I think the idea was that zero-backpressure situations, or even a vacuum situation (if possible) caused by headers may “suck” on the valves increasing the chance of float already present because of high RPM.....??

- Do the dyno results alone of one or even several engines prove that headers are always better?

I realize there is a reason race engines use open straight pipes, not cast ram’s horns. But I don’t run wrinkle-wall tires, either.

What do you think? Have other thoughts? Weigh in!

Chris

Gordonm

I will say it again An engine is nothing more than an air pump. More air in more air out gives you more HP. Headers are about the best bang for the buck N2O aside. They will flow better than stock manifolds all the time.
Your friend about helping valve float is out to lunch. Valve float is is caused by the spring being able to keep the valve on the lobe of the cam or the heavy valve in contact with the rocker arm. It has nothing to do with back pressure. A small amount of back pressure is ok. There are reams of articles about this and custom tuned headers to eek out the most HP you can get. A set of small tube headers on a stock to mild motor will always help HP and MPG. I'm no expert on this but use some common sense. Manifolds were never meant to be high HP extractors. The first thing that was done was to put a set of headers on and see what the car is really doing.

gkull

Well just think about sucking on the back of the exhaust valve. It would be held closed not floating.

With proper primary pipes an exhaust port will flow more cfm. No C-3 cast iron manifold was ever tuned and all of them are a restriction. Just some are less than others.

I do not believe the old wives tale about needing back pressure. Because uncapped headers make the most power. That's why I have 3 inch exhaust pipes and mufflers


[Modified by gkull, 2:10 PM 1/4/2002]

427V8

As anyone who has been on this forum knows, headers are one of my favorite subjects...

I've done quite a bit of research, but little actual testing on my own, instead relying on testing by others ( I'd LOVE to spens a week with a dyno and a pile of headers but it's a little expensive )

Here are my thoughts...
Cast iron manifolds are designed to route the exhaust gases to the exhaust pipe which sends them to the back of the car.
They are cheap to design and to make.
They are designed to work reliably. They don't leak, they dont rust through.

Headers are designed ( the good ones ) to make use of the exhaust gasses from the previously exhausted clyinder to help pull the gasses out of the currently exhausting clyinder.
They are more than just a freeflowing hose, if that were the case race cars would use one big tube rather than 8 smaller ones. They uses the momentum of the gasses to help draw the gasses from the next clyinder.
They are more expensive to design and make.
They are made from thinner metal and compromise the accesability in the engine compartment.

The only engines that use straight pipes or 'zoomies' are cars with blowers, since the blower can push the gasses right out of the clyinder ( but some people argue that even they would benifit from tuned headers )

Tuning, BTW is the process of designing the headers for a particular application.
I.e. a drag race car would have large short primaries and big collectors. The headers would be tuned to work their best at maybe 5000 RPM for example. This would produce more horsepower in the higher RPPM range where the engine produces more power.

A street car would want smaller primaries and a longer smaller collector to get the torque at the lower RPM range where it spends more of it's time.

Equal length headers mean that all of the primaries are he same length +- 1 to 2 inches. This is important more from a tuning perspective than a performance perspective. The problem with unequal lengthness is it becomes necessary to tune the engine to a compromise. Some clyinders will be running lean, and some would be running rich, only the average would be just right!

If you had a fuel injection system that monitored each clyinder and gave each one the correct fuel mixture then unequal length headers would flatten the touque curve since different clyinders would perform differently at different RPM's ( the clyinders with long tubes would putout more than their share of power at low rpm, while the clyinders with short tubes would do more at high rpm's )

But now some clyinders will run rich, some lean.

Flareside

Headers will have a positive effect on performance every time. The problems come when you select a header of suspect quality, low ground clearance, or if you don't install it properly.

nunus79

The exhaust system that produces the greatest exhaust velocity with the less amount of back pressure in your targeted rpm range is optimal. For a low rpm range, stock manifolds may give you just that. For upper rpm range, headers are designed to boost exhaust velocity by scavenging.

Restrictive catalytic converters or lo-perf mufflers add back pressure with no benefit to exhaust velocity. You don’t want that kind of back pressure if you can help it.

AlwaysWave

I put headers on my basicly stock 350,72 tied into the stock exhaust pipes. The improvement felt was immediate in low end, top end performance and gas mileage. Then later I had 2 1/2" exhaust pipes and Dynomax SuperTurbo mufflers installed and that is even better yet.

Bottom line..you can't go wrong by swapping the pipes. :)

ddn

I think the backpressure myth comes from people with 2-stroke experience. Or no experience but have heard it from 2-stroke. With a 2-stroke, the exhaust is everything. Look at a motocross bike. The pipe is tuned to create perfect backpressure to make the most power.