Let's talk headers... Sound, frequency, and power - And eventually building a set.
 

Let's start a discussion about headers. Looking to compile some experience about engineering, designing and building a set. I know that many have close kept secrets on the details. But I think we can discuss concepts that lead us in the right direction.


Background. I have started this exercise because I want to build a new set of headers for my 72 3.0. Why? Because I want to change the exhaust note (tone) to a higher frequency. So... Higher frequency must be shorter primary pipes, right? Sorta...


First, You can calculate cross sectional areas and lengths for primary, secondary, merge, etc. as long as you know some cam timing events and a target peak HP/torque rpm.
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Now, I am going to through out some math. These are basic equations. I am using the simplified versions for this discussion. Yes, you can use the more difficult ones that use true exhaust volumetric flow rates, EGT corrections, etc. But real world comparisons and big picture discussions are what I am after right now. So we will be dealing with averages within a distribution and our tolerance will be something like +/- 1-2". (Give or take).

So let's get into some back of the napkin type calculations. Yes, I used my Google Fu to bring these all into once place.

The Primary cross sectional area is calculated using A = Cylinder Displacement * RPM [peak torque] / 88200. units are in inches.

So, My example 3.0 with 499CC cylinder and 5800 rpm peak torque target gets a 2.00" cross sectional area. What do you know? That matches a 1.6" internal diameter pipe. Yes, that is why 1 5/8" headers with 18g tubing are a good match.

Now, What about the collector? The collector is commonly calculated at 1.9X the primary. This gives a pipe dimension of 2.2". Again pretty close to the common tubing with 2.25" OD.

Now, I have some inexpensive headers. I measured the primary length at about 34". Its hard to measure the exact centerline distance of the tubing. So I measured the outer and inner radii and averaged. Its close enough.

Going back to my example, you calculate the header fundamental length for an open ended pipe (consider the exhaust valve closed).

L = ([850 - exhaust valve open angle]/RPM) - 3. In my example, I calculate 43. 3" primary length. I can tell you that this is different that what I measured? Why? Because a 43" primary exhaust header won't fit under the engine. So, What to do? Its "fundamentally" simple. Use Harmonics. So, The 43.3" length would be my fundamental L for a 1/4 wavelength on an open pipe. The harmonics would then go into odd numbers so L/3, L/5, etc. Our math leads us to the 5th harmonic which is... 34.6" length.

Now you know why SSIs and all the copy headers have this same build spec for tubing diameter, tubing length and collector diameter.

But, What if I want to change things up? Shifting the harmonic to get a higher resonance freqency would be like playing a key on the right side of the pipe organ. The pipe gets shorter and smaller to shift the frequency higher.

Well, That has also been done. There are shorty headers such as "georges" Euro headers and M&K that have what appear to be 7th harmonic headers. This accounts for their more unique (some say exotic) higher pitch sound at higher rpms.

In my example a 24.X" primary would give me a higher pitch and still be on resonance. Of course, I give up pulse strength on the scavenging but that's OK. More importantly, I can take my cheap, stainless headers, cut off 10" from the primary and put a new collector in place. And then, I can put that 10" collector pipe of the right size since that's what was calculated.

Now, I know that once I put a muffler on the end of these headers, it all changes. But we can still take advantage of some of the effects of properly tuning exhaust systems.

I will be building a set over the next month or so. I will document this within the thread.

Let's hear your thoughts and examples. Measurements are appreciated for the discussion.

Good Stuff, Jamie.

Looking forward to hearing the differences in your new setup.

I seriously love these discussions.

Do any of the engine simulation software packages allow you to model the exhaust parameters? I think it would be fascinating to model the end-to-end effect of cumulative changes — intake, heads, cams, exhaust, etc.

Yes, there are software modeling programs. I do not have any but that would be a great confirmation of my pad/paper which (as of last night) input into an Excel Spreadsheet.

Here are a few that I have found on internet search.

pipemax
ICE Engine works

And before anyone says... Well, negative things about design and calculations...

I do know that header design is around a single input rpm. Resonance is a concept that is focused on a near steady state location with respect to engine parameters. But, maybe that resonance can fill a hole in the torque curve, or soften torque/HP dropoff at the end of the curve. Or accentuate a corner exit rpm. Whatever the goals of a particular build, Let's keep these limitations in mind.

Again, My goal is sound quality. I am not a fan of booming, resonating, low tone exhaust notes. I prefer a smooth, high pitch exhaust not and that is the purpose of this learning exercise.

The organ pipe analogy leads you down a blind hole, There is a fundamental difference between an organ pipe w/ stops and an exhaust. The organ pipe receives its initial propagation or signal from the open end , the wave is then reflected from the stopped end. In an exhaust the wave is started at the closed end(exhaust port) and reflected from the open end, this doesn't count the multiple other reflections from the various transitions upstream of the open end or the effect of a muffler on the system which besides pressurizing the pipe also adds its own effect on the acoustics

headers are designed to enhance flow of exhaust gases of a certain configuration engine at a certain rpm, altering the length, width, curvature, merge and/or exit configuration in any way alters the flow profile and rpm range where they work best.

Ideally you would have a variable configuration exhaust as has been done w/ the intakes which are similarly designed for their function.

The various harmonics aka resonances are used to enhance that flow but they only have a positive effect over a relatively narrow but very specific rpm range. What is often ignored is that theses resonances also negatively affect flow over other rpm ranges, the most famous such resonance is the annoying ~2000 rpm drone many aftermarket exhaust impart to the acoustic signature of a 911, what is often unsaid is that this resonance is counter productive to flow.

The frequency of the exhaust is directly proportional to the rpm, w/o a muffler it goes from a more low frequency rumble at idle to a more high pitched wail at 7k, yes there are multiple harmonics in the mix. It is the mufflers job to absorb or transmit the various objectionable frequencies but even w/ a muffler the same frequency shift w/ rpm occurs, it's just the rough edges that are removed, different mufflers achieve this evening of the tonal quality in different ways and to different extents.

Cool project- subscribed. To add to the conversation, sound is also very engine-dependent. Cam and compression ratio hugely change tone.
I hated-hated the Magnaflows on my 525HP 10.5:1 compression small block Ford, but on my son’s mild 9:1 street cammed SBF they exhibited the nicest sounding musclecar tone I’ve ever heard. And yet the only difference was what they were trying to silence, so that confirmed for me that tone is a product of the entire package.

Stainless vs mild steel will also influence the sound

I have pipemax if you want me to run through some scenarios. There's also a free header calculator on the Wallace Racing website. I think for street drive Porsche's the header tubing is usually bigger than actually needed
If you're just after the exhaust sound, the other variable when compared to a pipe organ, is the temperature of the exhaust gas, and how it varies under engine load.

To add to JPNovak's post, it appears the pipemax website is gone, but iceengineworks.com is still alive. Here's some info on their header fabrication system:

https://www.google.com/search?source=hp&ei=HncTWu68A8HU0gKO5JbICA&q=iceen gineworks+exhaust&oq=iceengineworks+exhaust&gs_l=p sy-ab.3..0i13i30k1.2760.14731.0.14881.23.23.0.0.0.0.1 66.1861.15j4.20.0....0...1.1.64.psy-ab..3.20.1974.6..0j35i39k1j0i67k1j0i131k1j0i131i20 i264k1j0i20i263i264k1j0i20i264k1j0i10k1j0i10i30k1j 0i13k1.119.yyW8ip_FHcE

Videos:
https://www.youtube.com/watch?v=0wIgQeYYLHA
https://www.youtube.com/watch?v=lBmjU83kaow

Caveat. On their website, their kits assume common tube sizes used to fabricate headers. The 1.5" set is the closest tube size used on our cars.

Sherwood
(no affiliation)

Larry's pipemax site is still up.

MaxRaceSoftware

Can't argue with RPM

https://youtu.be/cyqJxXolico

I have rarlyl8 long tube equal length headers on my c2 turbo they do hit a nice higher pitch resonant frequency off boost starting around 3k RPM.

would like to hear equal length short tube.

never been impressed with the sound of B&B unequal length short tube headers.

Here’s another bimmer. This one, an '07 2.4 V8 in Kubica’s F1 car.

How about 19,000 rpm?

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I always thought that most of the sound variation you can get comes from what happens after the collectors (assuming the typical pair of 3 into 1 headers like most 911s use)?

This is good stuff. Seems like everybody assumes that sound quality comes from the header and the muffler. But so many other factors play into it. Subscribed!

Having just changed headers (and exhaust), I can offer anecdotal info...but no proper science or empirical evidence.

The exhaust pitch/note definitely went up 1-2 registers when I switched from 1.75" primaries to 1.625". (this is throughout the RPM range)

There has been a large increase in power, but I can't say if it's due to the headers, exhaust, or combination thereof. Everything else is unchanged. I even had to add fuel to the top 1/3 of the RPM range as the car was making more power and was going lean as it was moving more air.

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