2ZZ-GE Header Shootout
Posted on 10 January 2016
BOE Header Shootout
There’s a lot of work, math, and artistry that goes into a well-designed exhaust system. Everything from the header flange, primary tubes, collector (merge), to the U-bend (on a Lotus anyway) and muffler can have a significant impact on the performance of the exhaust system.
There’s both an art and a science to header design. While we, BOE, don’t design headers, we certainly install a lot of them and are asked about them frequently. We thought it might be helpful to examine some of the more popular headers on the market, some that are no longer offered (but are intriguing!), as well as the stock ones and then install them on a car and do some pulls on our dyno.
Before we get into results, let’s look at some the very basics behind headers so that we may possibly better understand (or at least theorize!) why some headers seem to do better than others when the rubber meets the rollers…
Especially well designed headers are primarily making power by using resonance tuning to create a second low pressured wave in the exhaust using acoustical energy during the valve overlap period. The overlap period is the time that both the exhaust and intake valve are open around TDC of the piston travel.
The first low pressure wave is the larger low pressure wave and is created in the wake of the exhaust mass passing through the primary tube. The large exhaust mass pulls suction behind it allowing for better cylinder scavenging as the piston approaches TDC and prior to the valve overlap period. To illustrate this, think of two racecars drafting each other where the front car “pulls” the trailing car along. The faster the front car is going, the stronger the negative pressure wake behind it will be and the better the pull on the secondary car. Header primary diameter, and port matching have the most significant impact on this first wave. Just like the racecars, the faster the exhaust mass travels, the stronger the suction will be following it. A smaller diameter primary will speed up the exhaust gas. Too small of a primary will be restrictive, so finding the perfect balance between being small enough to keep the speed of the exhaust up and not restrictive is critical for best performance. Some headers have steps in the primary diameters effectively creating two primary calculations for each cylinder in an effort maximize flow speed and minimize restriction.
The potential for a second wave of magic happens during the valve overlap period. This is the period when the exhaust stroke is coming to a conclusion, the piston is around TDC (Top Dead Center) and both the intake and exhaust valves are open simultaneously. It’s during this period that a well-tuned header can use sonic energy to create a second and very productive wave of low pressure to help further scavenge residual exhaust out of the cylinder and make more room for fresh fuel and air. The more exhaust gas we can remove from the cylinder, the more fresh fuel/air mix that can enter to make power!
Here’s a 30 thousand foot overview of how the second wave works:
As the high energy exhaust pulse enters the collector, a reflected pulse of sound energy (a sonic wave) travels back to the exhaust valve. This reverse reflection of energy does create a very slight increase in pressure but only until the sound is reflected again off the exhaust valve toward the open primary tube. This reflection toward the open primary tube carries a low-pressure wake behind it very similar to the initial exhaust wave when the valve first opened. If the timing of this reflection is correct, a second low-pressure wave will result during cam overlap!
See the “Crude Cartoon” below to help illustrate this.
This second sonic wave is where the length of the primary and the merge collector selection are very critical. Since this second pulse or wave travels at the speed of sound, math can be used to get a rough idea of how long to make the primaries so that the sonic wave makes it back to the exhaust valve at approximately the right time based on the anticipated running RPM of the motor. Intuitively, a faster revving motor like a Formula 1 V10 will require shorter primaries than a slower rotating Lotus 2ZZ for the wave to reflect back in time to catch the overlap period! In addition to the primary length, we know the merge angle and potentially adding a “choke” to the merge can have a significant impact on sonic wave reflection.
Modern header design with aggressive steps in primary diameter and steep merge angles have evolved from ALMS and F1. This technology has trickled down to the hands of astute header designers in some aftermarket circles for us mere mortals to enjoy! Below you can see a modern F1 header. Notice the relatively short primaries, a HUGE step in primary diameter, and a very steep merge angle in these 5 to 1 headers. In our test, we had a few headers that implemented some of these modern and more sophisticated strategies and some that didn’t.
As you’ll see, the results speak for themselves. It becomes apparent that there’s a lot more to designing and tuning a header than what simply *appears* to be better flowing. In other words, the old adage of “bigger is better” most certainly doesn’t apply to headers!
The Header Tests
Now that we know the additional power a finely tuned header makes is largely done during the valve overlap period, we should know that making some fine adjustments to the cam timing on motors with variable cam timing like ours (which has an impact on valve overlap) can be important to get the best out of a particular header. In addition, spark timing changes can be made to take advantage of more fuel/air mix in the combustion chamber with the better headers. In this test, we did NOT do any tuning – repeat we didn’t touch a thing. We simply did plug-n-play header installations.
We utilized a closed loop fuel system so that we’re sure any power gains or losses were not achieved by leaning or richening the fuel mixture. We realize many installers will not have access to a proper tune for their headers, so we wanted to see which headers “naturally” seemed to work best with all factors equal. We carefully controlled conditions to keep the engine temps and the intake temps equal among all pulls. We performed 5 pulls per header installation and the car never left the dyno between tests. All dyno results are SAE corrected using our in-house Dynocom dynamometer.
The test car was a ~350whp Elise with a built motor, stock cams, and a REV400 kit. Its last quarter mile run was in the low 11s at 123mph+. It’s a good running car to be sure!<br />
Again, this is just one test and the results that came from this test, in these conditions, with this car.
The Headers Tested
• Stock with Stock Cat
• Stock with BWR Decat/test pipe
• PPE SC Header (no cat)
• 2bular SC header (no cat)
• DMC 4:1 (Old ForcedFed Header), no cat
• DMC 4:1 with modified choke, with 3” U-bend, no cat
• DMC/DRS 4:2:1, Tri-Y, with 3” U-bend
Stock Headers with the Stock Cat--- Our Base Line
We grabbed an low mileage stock header out of storage. Not caring much about the appearance of the part, we bent the EGR tube out of the way for an easy installation. Installed with a stock OE cat and did a pull.
At 8200rpm the combo made 337whp and 216wtq. Our baseline.
Stock Header with NO CAT
Next we removed the catalyst tube and replaced it with a BWR decat pipe. Surprise #1. Deleting the cat and running a decat pipe in its place was worth precisely Zero power. Yes. Zero.
The car made the same 337whp and 216wtq and the curve was nearly the same as well! A conclusion could be drawn that the stock header is a larger point of “restriction” than the catalyst, as every aftermarket header made considerably better power than the OE header and removing the cat did nothing to help the power output or power curve. Note that the stock catalyst is a high quality ceramic core. A quick Google search looking for information on whether OEM catalysts are restrictive or power robbing on a dyno will show similar results. So you may be wondering if deleting the cat is worth it when using stock headers. In many cases, yes—particularly if the car is not used on public roads such as track use and if it’s super or turbo charged. In those cases, the EGTs may be high enough to fail the catalyst, which can actually cause flow restriction. In addition, the catalyst runs extremely hot, which can be a fire hazard on a track car that is already generating more heat than the OEM ever anticipated.
2bular 4 to 1 Supercharger Header and No Catalyst
This header was purchased used and came with both a catless U-bend and a “high-flow” catalyst with a stainless steel substrate. We tested the header with the catless U-bend in place. This header was nicely welded. It had the thinnest mounting flange and was slightly warped. The stock bolts were able to flex the flange adequately to seal against the head.
The primaries on this header were considerably larger than the exhaust port at approximately 1.85”, which lead us to believe that exhaust gas velocity would slow considerably when diffused into the large port immediately upon exit of the head. The primaries were also the shortest in the test lot and did not have a step. The merge collector was reasonably steep at 20 degrees or so. There was no choke with the both the merge outlet and U-bend at 2 1/2 inches. Output on this header was considerably better than stock but was the lowest of the aftermarket headers with an overall curve and power at 8200rpm of 348whp and 223wtq. This was the lightest header by a considerable margin.
2bular Header vs Stock Dyno
PPE 4 to 1 Supercharger Header, No Cat
This was the only header made of mild steel, where all others were 304 stainless. This is also the cheapest and probably the easiest header to source in the lot. It made the same power as the DMC/DRS Tri-Y power at 8200rpm of 353whp and 223wtq.
The primary size at the port was the largest diameter of the lot at nearly 1.9 inch OD with no step. Like the 2bular header, we suspect the “bigger is better” concept that these two headers use on the primaries is hurting more than helping. The merge on this header was a pretty nice fabricated part and very shallow at about 10 degrees. The shallow merge angle increases the volume of the merge and seems to be contrary to what we see of ALMS and F1 headers. This header had the largest merge volume of the 4:1 headers. It’s hard to know if the extra volume of the merge or the primary design or both were the reason that it didn’t perform better. There was no choke on this header as the merge and U-bend are 2.5” OD.
PPE vs Stock Dyno
DMC/DRS Tri-Y with 3 inch U-Bend, No Cat
We were expecting a lot from this Tri-Y header. The YYY (tri-Y) design has become popular within the import aftermarket. The design pairs cylinders 1/4 and 2/3 together in separate 2 to 1 merges and then brought together a final time in a large 2 to 1.
The internet tells us a header like this will result in better midrange torque at the consequence of power in the upper registers. In a plug and play test like this, it performed nearly identical to the PPE 4 to 1 header. This DMC (marketed by DRS) part was easily the most pleasing to the eye in the lot. The welds were mystically perfect. If I didn’t know that DMC hand welded this header, I would have wagered a robot did the welding. The primaries started at 1.75” and were the smallest aside from the stock headers. The primaries contained a “mild” step and were perfectly port matched to the OE exhaust gasket in the first step of the primary. The DMC headers all had a similar primary layout to the header flange that made installation a five out of five with extremely easy ratchet access. Connections downstream of the flange were proper spring slip joints and v-band clamps. Not have any experience with YYY headers, we have no idea how the power would look, but we did like to see the F1 and ALMS inspired step in the primaries as well as a primary that closely matched the exhaust port diameter.
It should be noted that this header was built slightly different than it was originally developed with an enlarged choke and 3” U-bend. In talking with DMC, they believe that retaining the 3” U-bend and dropping down to a smaller choke might bring out the mid-range punch these YYYs are known for.
YYY vs Stock
DMC ForcedFed Spec 4-1, no Cat and DMC ForcedFed Spec
FF Spec DMC header as New with Cat Option
We have a winner!
Everything mentioned above about the construction can be said about these older style ForcedFed spec headers. They are a work of art! A bit different than the YYY variant, the step on these 4-1 headers is more pronounced and jumps to a full two inches following the 1.75” port matched primary. Immediately, we see cary over technology from the F1 header above, e.g. a large step in the primaries and a very steep merge. These headers also contained a choke after the merge to reduce the merge volume, which should result in a more pronounced sonic reflection to augment that second low-pressure wave.
DMC tried many iterations of this header before finally settled on the surprisingly small 2 1/8” choke. Yes, 2 1/8 inch! That is as visually stunning as it is to write it. The choke appears that it must be restrictive. However, the dyno shows otherwise when compared to the others.
We did try two versions of this header. One retaining the 2 1/8” choke and the other with a 2 3/4” choke but located slightly farther up the merge, which turned out not to be a productive modification. This modification made the midrange slightly softer, which is likely due to a softer sonic reflection from the modified merge.
One very interesting bit of data about the unmodified DMC ForcedFed Spec 4-1 header is that it was the only one to be very sensitive to the cam change event at 6000rpm. All the other headers did not register much of a torque dip at the cam change. On the other hand this header (which seems to be making the most of the exhaust signal) had a significant torque dip at the cam change. While we’re quite certain that we can tune this dip out with the cam phasing, it is interesting to see how sensitive a header that is properly making use of both exhaust mass and sonic reflections to generate power can be to cam phasing when compared to the others.
The unmodified ForcedFed Spec DMC header made 355whp and 227wtq while the modified version averaged about 1whp and 1wtq more. Hardly worth the effort! More interesting is that at 7000rpm this header made 9whp and 7wtq more than the next place header. The dyno results below exhibit a much more advantageous torque curve than all the others tested! Needless to say, we are going to work hard to convince DMC to bring these back to the market for us to enjoy again!
DMC FF vs Stock