'Bisimoto 4-to-1 Header:

This brand new header combines a number of cutting-edge design elements including: the equal length primaries with wide radius bends designed to maintain maximum velocity; equal length (13'') stepped primaries to prevent reversion; a true merge collector with a 2 1/4" venturi to a 2 1/2" exit pipe; and excellent ground clearance since only the downpipe travels under the oil pan. The design also appears to provide enough clearance to allow for the use of a full-width (ie. Integra) radiator. The fairly large primaries and downpipe should allow for increased volume and should therefore be very well suited for use on moderate to heavily modified engines.

This header is a very high-end design, similarly featured custom headers costing well over $1k U.S. At 'Bisimoto's price point, it seems like an incredible value for anyone looking for top flight performance out of a $500 header. And given 'Bisimoto's proven performance at the drag strip as the world's fastest SOHC Honda, it seems reasonable to conclude that this header will produce excellent power gains.

Well done 'Bisimoto!

Greddy 4-2-1 two-piece header:

Similar in design to the other 4-2-1 headers discussed, the Greddy header will be a bit heavier given the two-piece design. Also note the relatively short primaries for a 4-2-1 header, meaning maximum power output is pushed up the rpm range. The merges to the secondary runners and the collector appear a bit abrupt. This could create some unwanted back pressure and slowing of velocity. Overall, the Greddy header should produce good midrange power but the design does not appear to flow as nicely as many of the one-piece headers.

Hedman Chikara 4-2-1 two-piece with PowerPlus Technology:

Hats off to Hedman for designing a D-series header with a lot of interesting features. Not only does this design include stepped primaries, for improved velocity due to their anti-reversion inducing effect, it also includes an equalizer tube design to eliminate spent gases more efficiently. As a two-piece header it will be heavier than a one-piece (but easier to install) and it does appear to suffer from short primary and secondary merges, which look to be potential design limitations in terms of flow and velocity. Despite the merges, this header has a lot of potential to produce nice gains over a broad powerband and it's a very good value given the price and features like stepped primaries and an equalizer tube.

HP Racing 4-2-1 two-piece header:

This image is probably not for a D-series engine, given the resonator on the downpipe and the strange O2 sensor location. Nevertheless, it may give us some clues to the D-series header design from this company. In particularly, it would appear they use fairly long primaries for a two-piece 4-2-1 design, which will give better midrange output. However, the bend towards the collector on the secondaries seems rather sharp and thus potentially restrictive. The very long collector will limit peak rpm output but should enhance low and midrange performance. Based on this image, it would appear the HP Racing header is designed for good ground clearance and solid street performance in the low to midrange powerband. The fairly large primaries (1 5/8''), secondaries (1 3/4'') and collector (2'' I.D.) should provide good volumetric capacity increase and thus provide good gains for modified engines.

JG Edelbrock 4-2-1 two-piece header:

Unlike the Hedman two-piece header, this header has very nice long and smooth merges, meaning less restriction to flow and velocity. The very short primaries will have the effect of pushing the sweet spot on the powerband up the rpm range, and the long secondaries indicate strong midrange output. The 1 3/4'' primaries are larger than most of the other D-series headers, a feature that modified engines with greater power potential will benefit from. Overall the JG header has very few obvious restrictions in its design, suggesting it should be a solid performer across a wide powerband.

Kamikaze Endyn Modified 4-1 header:

A unique D-series design, with its 4-1 two-piece layout, this header is the popular choice among supercharged Hondas because of the long and large diameter (2 1/2'') downpipe and collector. The short primaries will push peak power up the rpm range. The fairly large primary tubing diameter (1 3/4'') and the large downpipe are clearly designed to move greater volume than most D-series headers. This great volumetric capacity is something a highly modified naturally aspirated or supercharged engine will benefit the most from. A stock engine would likely lose power with this header due to lost velocity caused by too much volume.

Landspeed Racing Powercore 4-2-1:

Another classic 4-2-1 one-piece design, very similar in dimensions to the DC Sports and Comptech headers. Fairly equal length primaries and secondaries indicate a design philosophy aiming for balanced performance and good midrange output. Definitely a solid choice for a street header on a relatively stock engine and an excellent price value compared to the other 4-2-1 one-piece stainless steel headers in the group.

Pacesetter 4-2-1 two-piece header:

This two-piece header features relatively large diameter primaries (1 5/8'') and collector (2 1/4'') and as such has greater volumetric capacity than most of the other headers listed here. A heavily modified engine will be able to take advantage of this extra volume without losing velocity, but a stock engine may not be able to and could suffer some power loss at low rpm as a result. The longer than usual collector will aid midrange power, though the collector merge appears to be a bit abrupt. The primary merge appears to be longer and smoother than the collector merge. Overall this header should provide good midrange and high rpm power output, though low rpm performance may be weak when mated to a relatively stock engine.

Ractive (Toucan) 4-1 one-piece header:

Sorry about the crappy picture quality, it's the best I could find online. Despite the blurry image, you can still make out that the collector is a Tri-Y design, which gives better ground clearance than the more common box style collector (as seen on the Stillen 4-1). Given the fairly large diameter primaries (1 3/4'') and reasonable collector diameter (2'' I.D.), this header should provide good peak power gains and high rpm performance, even on a fairly modified engine. It should also be noted that with 4-1 designs like this there is a possibility of running into ground clearance problems with the primaries that hang quite low and can easily be flattened on a lowered car. Beware of manhole covers and high curbs if you're using a 4-1 header and your car is lowered more than an 1 1/2'' or so!

SMSP 4-1 custom header:

This custom-built header by SMS Products uses extra long primaries and a long collector to maximize velocity and volume. Headers like this have been dyno proven to make more power than mass-produced off-the-shelf units, especially for highly tuned and modified engines. Because of the extra length of a custom header like this you will need to relocate the catalytic converter (by shortening the exhaust b-pipe or using a shorter length test-pipe). Being a true custom-built header, be prepared to pay more, but if you've got the budget for it and you want the best SMSP is a good place to start shopping.

Spoon Sports 4-2 header:

As you can see, this header is the top portion only of a 4-2-1 design. It is apparently compatable with a stock 2-1 downpipe (which can be cheaply and easily modified by any reputable muffler shop to feature any collector size you want). The primary merges are long and smooth for a 4-2 design, suggesting very minimal restriction to flow and velocity. But at $677 U.S., is it really that much better than the other 4-2-1 design headers in this group? To a JDM addict, maybe.

Stillen 4-1 one-piece header:

A classic 4-1 design, the Stillen header uses a tradition box style collector, which stacks the primaries in a 2x2 pattern, a design element that is thought to aid peak power output but decreases ground clearance. Ground clearance is also an issue where the primaries bend back towards the collector. However, 4-1 designed headers do generally offer superior peak power compared to 4-2-1 designed headers, though they sacrifice low and midrange performance in order to do so. As such, 4-1 headers like the Stillen version pictured here are ideal for racing applications but less street friendly given the ground clearance and low rpm performance issues.

T1R 4-2-1 one-piece header:

This header uses a JDM style 4-2-1 design, with its long primaries which make the bend towards the collector before merging and large diameter primary (1 3/4'') and collector (2 3/8'') tubing. The long primaries do create a ground clearance problem, though their length provides excellent midrange output. The large diameter tubing and collector provide higher volumetric capacity and higher potential peak horsepower, provided the engine is modified enough to take advantage of these design elements. Overall, this header should provide excellent all around performance and should produce strong peak power gains on modified engines. The price tag is JDM too.

D16Y8 '00 4-2-1 two-piece header:

This is the factory original header off a 2000 Acura EL 1.6. It uses an unsequential pairing of the primaries like many of the aftermarket headers seen above and the merges look to be longer and smoother than most factory D-series headers. The primaries are longer and have a larger diameter than the cast 4-2 portion on a D16Z6 header, which should add midrange output. The secondaries are also larger diameter than on the D16Z6. This header would appear to offer a good OE low cost upgrade over earlier stock D-series headers and with a little modification could potentially perform just as well as many of the more expensive aftermarket 4-2-1 two-piece headers listed above. Food for thought!

D16Z6 4-2-1 two-piece header:

This is a 92-95 Civic Si header. Compared to the D16Y8 header above, the primaries are considerably shorter, though they are paired in the same unsequentially 4-1 2-3 manner. Though it's difficult to tell from the picture, the collector merges are shorter and more severe than the Y8's. The loss of tubing diameter on the bends toward the collector on the secondaries also appear more severe on the Z6 compared to the Y8. It's also worth noting that the inner diameter (I.D.) of the collector is only 1 1/2'' due to the double walled tubing. This is considerably smaller than the aftermarket headers that use a single wall tubing and lose a small amount of I.D. as a result. Cutting off the stock collector and having a 2.25'' (or larger) I.D. collector tube welded on is a cheap mod that will yield some peak horsepower gains and improve velocity. Overall the Z6 header looks to have quite a few restrictions in its design and will prevent optimal peak performance, particularly on modified engines.

Modified D16Z6 4-2-1 header:

The pictures on the left are comparing a stock Z6 header and one with a modified collector. As you can from the top picture, the inner diameter (I.D.) of the collector on the modified header (on the right) is significantly larger (3/4'' to be precise) than that of the stock header (on the left). The modified collector is also slightly longer than the stock one since the point at which the original collector is cut off is positioned slightly farther back on the secondary merge so that the outlet diameter of the merge will match the diameter of the larger collector tube. We will be testing this mod on the dyno very soon, but we're confident this simple modification to the stock header will produce some gains in peak output and quite possibly strengthen the midrange a bit too.

For a more detailed explanation on how to modify a stock D-series header, check out Moose's guide