HEADER DESIGN COMMENTS - A critical discussion of various header design concepts.
STEPPED HEADER DESIGNS
Stepped Header Designs seem to have caught the interest of many. Over the phone, by e-mail, and by regular mail, many ask us of our opinion of the design. Well, here it is - OUR opinion - as based on years of building, designing, and testing headers, observing the results of various other tests using stepped headers, DEALING WITH PEOPLE (this is VERY IMPORTANT!), etc. We'll warn you right now - we don't agree with what many others are saying about stepped headers PLUS we have a different point of view of them as well!A short discussion on the Stepped Header Design Concept.
Where the "conventional" header - at least what most see as being "conventional" or "normal" - consists of machine bent tubes of a single diameter that go from the header flange to the collector, the "stepped" header has header tubes that are made of at least two different diameters of tubing. Usually, the smaller tubing diameter emerges from the header flange port and, at some distance from the header flange, larger tubing is slipped over and welded to the smaller tubing. The location of change in tubing diameter is called a "step" (typically, a "step" UP in diameter). Most stepped headers consist of either two tube diameters (single step) or three tube diameters (double step). We have heard of people building headers with as many as four steps (five tube diameters used in the header tubes) but question if any gain came out of all that extra effort and expense (we'd bet money that they actually LOST power - by reducing the exhaust gas velocities excessively - by the addition of these extra steps).
Primarily, we (actually YOU) are told that the stepped header design creates a broader, flatter torque curve when compared to that produced by the conventional "straight" (or non-stepped) header design. Then we (YOU) are told that this produced broader/flatter torque curve will cause our (YOUR) vehicle(s) to perform better. These claims are often reinforced by theoretical scientific discussions. For example, we read in one magazine article an explanation where the action of combinations of resonating springs (that were attached to each other) were used to illustrate how various resonating gas columns (that were produced in the lengths of different header tube diameters in a stepped header) would actually reinforce one another to the degree that this reinforcement would create an improvement in exhaust gas scavenging and, from that, improve power output. All in all, a pretty heady theoretical discussion for most to absorb (especially for those who never took a course in Physics).
First, we'll NOT talk about bouncing springs!
Second, and far more important, we'll talk about we've seen as being important in header design and how that determines our opinion on stepped header designs.
Stepped headers have PROVEN to be inconsistent in gain - they work for a few and don't work for many others. When something is promoted as being a "step" forward in design (pun definitely intended), yet tests and other experiences don't prove it out - it basically becomes more of a marketing ploy than anything else.
We've seen NO consistency in results to make us think that a header - by simply being a stepped design - will outperform a conventional, "straight" design. Header tube sizing, collector tube sizing, tube length, collector length, collector transitional shaping, the accuracy of tubing lengths, how well the ports in the headers match the ports in the head are all design elements of a header that directly affect its ability to produce (or NOT produce!) a performance gain. Concentrating on a header having a stepped configuration and ignoring the importance of all of these other design variables can only guarantee a lessening of performance gain. Many times - in talking to others - we've been told of some stepped header outperforming a straight header yet, when we ask questions about the straight header being compared we can't find out anything about it!!!! We can't find out what tube size was used, what collector size was used, whether the straight header was equal length or not, etc. We've been amazed to find out that the person doesn't know anything about the other header (from a design point of view) yet, somehow, has become convinced that the stepped header was better by ignoring any BASIC design differences that may exist between the two headers!
The best way of illustrating the capability of stepped header designs is to relate a conversation that occurred awhile back with a local fellow that owns a shop that does dyno-testing. He called our shop one day and wanted to know "why do guys think that stepped headers are so good, when we've tested, maybe, twenty engines so far where the stepped headers did NOT prove to produce one horsepower gain over other headers used in the test". He went on to discuss his amazement as to how convinced these guys were as to the stepped headers being better yet, in test after test, on twenty-plus engines - all tested in a row - the headers did not increase power. Over twenty tests in a row - no gain! (Keep in mind that has to mean that some of the stepped headers had to cause a performance loss.) If you believe in statistical evidence, his testing PROVED that stepped headers don't work!!!! However, to be perfectly honest, we have encountered situations in which they do work but we also have seen that these situations are isolated and VERY dependent upon vehicle/engine design and usage.
Some fellows tell us about nationally known racers that use stepped headers and how fast their cars run and, guess what, we could care less. We don't know these people. We also don't know what other header designs they may have (or may NOT have) tried. The fact that they have this particular header design on their cars means absolutely nothing to us. Over the years we have seen many national record holding cars that, in our opinion, were using lousy headers (the cars ran good IN SPITE of the headers they had - NOT because of them). The cars had good engines, good drivers, good mechanics, well set up chassis, etc. - that is why the cars ran fast - because the headers had very little to do with the overall exceptional performance of these cars. Oftentimes, the headers were just there hanging on the side of the engine - maybe not even helping performance to any real degree but not getting in the way either. Looking at those cars and then saying that the headers on them must be good BECAUSE the cars are fast - where is there proof of that (or the logic) when you start to understand what MAY ACTUALLY be going on in these situations? Any statement about the headers being good in these particular situations is based purely on assumption!
We have seen no proof of stepped headers increasing the torque range of an engine. If you want to say that having a lower but flatter torque curve is of advantage, maybe they have something to offer BUT remember that the resultant torque curve was described as being LOWER as well as flatter.
1/5/01: I talked to a drag racer running a Hemi Barracuda (his latest race prepped engine cost him $40,000!). He spent $1,000 for a stepped header and lost 18 horsepower at peak - with power down everywhere else in the test rpm range. Not exactly a sterling endorsement for the stepped header design concept.
If one defines a stepped header as one with at least two different header tube diameters in it, then the second header Ed ever built was a "stepped" design! So this design concept is definitely NOT new since it has been around since 1962. (And who knows who else did it back then?)
Over the years we have run comparison tests using headers that were absolutely identical in all respects - identical except for the header tube sizes. These headers had the same tube length, same collector diameter, same collector length, same collector design, and the same TRUE Equal Length design. They were even built in the same header jigs so that the header tube layout or arrangement was identical. The ONLY design difference was the header tube size. Running comparisons as accurate as possible we have seen Elapsed Times at the drag strip change as much as .3 seconds just due to 1/8" changes in tube size.
REAL WORLD EXAMPLE: One of our past employees raced a 1969 Ram-Air IV Pontiac Judge. At one point when the car was capable of running consistent 13.00s @111 MPH with a 1 7/8" header, we built another header with 1 3/4" header tubes (remember that ALL other designs parameters were kept EXACTLY the same) because it seemed a logical size to try as we had already seen in other tests with faster cars that a switch from 1 7/8" to 2" headers had slowed them down so we thought that, if we were going to make another header, going smaller in this particular case simply made more sense. The following weekend with the new, smaller headers the same car ran two 12.66s and a 12.69 on its first three passes still running 111 MPH in the quarter mile. (Keep in mind that this test was run years ago and that tire technology has changed considerably since then so the same car - with today's tires - would be running much lower ETs.)
Knowing that something like this had happened - that the car dropped a solid .3 of a second on a tube size change - would you want to use a stepped header made out of 1 3/4" and 1 7/8" tubing? Build the first part of the header tube with the tube size that worked well and then build the rest of the header tube out of the tube size that didn't work well? Does that make any sense? Doesn't it seem likely that a stepped header design (assuming that half of it was made out of 1 3/4" tubing and the rest of it made out of 1 7/8" tubing) might only drop the car about half of the amount gained with the 1 3/4" (or .15 seconds)? OR, to put it another way, what if we had run a different test just comparing the aforementioned 1 3/4" and 1 7/8" stepped header to the 1 7/8" straight header, would we think the stepped design was better because it might have produced a .15 second quicker time than the 1 7/8" header? Of course, we would. (Yet, if we never ran the comparison to a straight 1 3/4" header we would never had known that it was capable of lowering the ET by twice as much as the stepped header design!) Can you understand why we're not too impressed with stepped headers? (By the way, by switching to the smaller 1 3/4" headers, the engine's mid-range power jumped considerably which showed up in the lower ETs of the car, yet, at the same time, because the MPH was NOT hurt at the dragstrip, it was obvious that the motor just didn't need a bigger header. On the street, the increased mid-range power simply made the car perform a lot better than it had done before. In this case, switching to a smaller header proved advantageous both at the drag strip as well as on the street.) This particular situation is just one of many, many we've seen where the BIGGER IS BETTER theory just didn't prove out when actually put to a test!
Another thing that bothers us about stepped header designs is KNOWING how a change of tube diameter can affect a car's performance and how a change of two tube diameters has PROVEN to be quite disastrous at times. While we can accept the use of a single step header in a few cases, we find it EXTREMELY difficult to accept the use of a two step header design as the cross-sectional area of the last step has to be anywhere from 25% to 50% greater than the cross-sectional area of the tube next to the cylinder head (and, by being much larger, reduces the exhaust gas velocities and attendant scavenging considerably). Unless the tube next to the head is deliberately selected smaller than what might be considered ideal, the last step diameter has got to be way too big.
One thing most fellows aren't aware of is the calamity of using a header with a "two tube size error" - in other words. where the header is two tube sizes larger or smaller than necessary. A mis-selection of design of this magnitude destroys the performance of a vehicle. By being MUCH too large, top end power can actually be reduced and mid-range and bottom end power can be absolutely "clobbered". If the error is the other way, being MUCH too small (which rarely ever happens), the top end power of the motor is drastically reduced although the mid-range and bottom end power isn't hurt as badly as one might assume and might actually be better at low engine rpms.
Over the years, when we have talked to fellows that were VERY unhappy about how some header turned out (as to how well the vehicle ran after it was installed), too many times we've seen them using headers that were two tube sizes too large. Very typical examples are fellows with Big Block Chev engines that only need 1 7/8" headers but, for some reason, are using 2 1/8" headers. They have no idea as to how much performance they have actually lost.
HEADER DESIGN COMMENTS