Header design is EVERYTHING and exhaust flow is fairly irrelevant in the sense that stock manifolds, shorties and long tubes will all flow plenty of exhaust through them. They are by no means a restriction. But with exhaust you are not trying to “flow” exhaust. There is no exhaust flow in the literal sense. There are exhaust pulses (which are little columns of “flow” if you want to get technical) which if timed correctly can be used to increase momentum (scavenging) of the exhaust gases. A simple way of thinking about it is if an exhaust pulse from one tube reaches the collector right as another tubes exhaust valve opens it will create a kind of vacuum in the collector which will help the just created exhaust pulse move faster. Consequently if the timing of the pulse is off a reverse pressure wave can be created that will bounce back against an exhaust pulse and therefore slow down the exhaust gases.
Log style manifolds (stock) have no “timing” in them and they are effectively very short tubes dumping into a big plenum. And they don’t even have a “collector” to speak of. Stock manifolds, if they create any sort of scavenging effect is purely by accident. Stock manifolds are so simple that they can only be defined by their “flow”. It’s a good thing they have an abundance of it. The stock exhaust manifolds have proven to be a very small restriction to supercharged motors which don’t rely on scavenging like a naturally aspirated motor does. You can make a lot of power with a supercharged motor while still retaining the stock manifolds.
Unequal length shorty headers probably don’t have a flow advantage over a log style stock manifold and if they do have some scavenging effect it is by accident again and minimal. What they do have is a collector. I could go on forever about collector design and why some designs work with some combos better than others but the basics are that a collector should help the individual tubes to merge the gases together in an orderly, smooth and in some cases timed fashion to increase exhaust velocity. The collector is about the only measurable advantage that an unequal length header will have over a stock manifold. I have yet to see concrete dyno results that show unequal length shorty headers offer an hp advantage over the stock manifolds. I have seen a few dyno tests but the testing procedures were sloppy at best and the gains claimed within the standard un-repeatability of chassis dyno’s. Any result from a chassis dyno under 15hp can be easily attributed to inconsistent testing conditions. Simply getting the differential and transmission fluids up to temperature will show as much as a 5hp gain. And that is only one example of many possible variables. The lack of articles about shorty headers and their “gains” is also a big clue that they don’t offer much of a gain if any. If there was a set of shorty headers out there that made even as little as 10hp and 10tq the manufacturer would most certainly be going WAY out of their way to organize a test for one of the magazines to report on. A PROVEN 10hp gain from a set of shorty headers would assure that manufacturer a truckload of sales. And to add insult to injury shorty headers are only about 1.5lb lighter than the OEM manifolds so there isn’t a big weight savings either. In my opinion unequal length shorty headers are a complete waste of money unless you think the money spent is worth how much better they look than the OEM manifolds. I want to want some. They are relatively inexpensive, easy to put on, and may sound a bit better. But the lack of PROVEN hp gains makes them a dumb buy.
An equal length shorty again will “flow” the same amount of exhaust gas as an unequal length shorty and not much more than a stock manifold but by equalizing the tubing lengths there may be a slight advantage in scavenging effect because of exhaust pulse timing. But the primary tubes are way too short to be able to take advantage of exhaust pulse timing. F1 Motors use very short headers, but not as short as a set of “shorty” headers and the F1 motors rev to 18,000+rpm’s! They of course also have a collector which will help a little with exhaust gas velocity. The collector design has everything to do with how well that works though and the only nice equal length shorty with a good collector design I have seen are the JBA ones. Just like the unequal length shorty headers the equal length ones have not been proven to make power over the stock manifolds and for all the same reasons. Technically the equal length shorty header should out perform the unequal length shorty headers but if the unequal length headers are only worth 2hp, which is entirely probable, the equal length ones could be 50% better, which would be a lot and still only be worth 3hp. The equal length shorty headers are a colossal waste of money just like the unequal length ones are. There are literally hundreds of ways to spend your money smarter.
Now long tube equal length “tuned” headers are the cats a$$. Now the primary tubes are long enough to start timing exhaust pulses correctly (or most efficiently) and since they are so long the collector design can be very good because a long smooth merging collector is the most efficient. I’m pretty sure the optimum length for a long tube header on a 4.6L is 32” or there a bouts, that’s what I mean by tuned length. The length dictates the timing of the exhaust pulses and when they get to the collector. If everything is perfect every exhaust pulse helps the one behind it and it will actually start to “suck” the following exhaust pulse (scavenging) behind it. Tuned length and the timing of the exhaust pulses is the key to making a header work. There is a lot of Voodoo in the design of a great header. Take a look at some NASCAR headers sometime, they are incredible.
Other factors in exhaust performance are the heat inside the tubing. The hotter the gases inside the tube the faster they will flow. That’s why ceramic headers or heat wrapped headers are an advantage, they keep the heat in. On some motors, and I don’t know if the 4.6L is included will see a massive exhaust gas pulse timing advantage by taking one primary from each side of the motor and running them over to the other sides collector. This is a product of the firing order of the motor. Windsor motors are like this. This style of header is a “180 Degree” header. You don’t see them much outside of pure race cars because obviously packaging is a nightmare. Strangely enough the X-pipe design came from someone (Dr. Gas) who was trying to mimic the 180 deg style of header. X-pipes help with exhaust timing and balance. It can almost be looked at as a third collector. H-pipes help with balance only.
One last factor to consider in exhaust performance is if you have FI. The scavenging effect or the desire for it isn’t nearly as critical as a NA motor. Why you ask? Because the exhaust pulses are being pushed out with a lot more force because the next piston that is getting its intake charge is being force fed which actually turns the intake stroke into a sort of mini power stroke in the sense it is helping move gases somewhere else in the motor. Now with a FI car the percentage of actual true exhaust flow goes up in comparison to exhaust pulse timing quite a bit because the size of the tubing can now become a restriction and since the exhaust pulses have something behind them pushing scavenging is not nearly as important. A good example of this is that a 325rwhp-ish NA car can pick up 20-25rwhp with some long tube headers and a solid tune. That is a 7-8%hp increase and it will be all the way across the rev range. Putting the same set of headers on a 450rwhp FI car will pick up about 15-18rwhp. That’s only a 2% hp increase. Clearly FI motors are not nearly as dependant on exhaust gas pulse timing, or scavenging as NA motors are. I imagine that if you sat down to design a FI specific full length header you would find that the tubing size, primary tube length and the collector design would need to be a lot different than the NA long tubes to be 100% efficient.
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