It all began in 2003 when I needed a header for the type 4 engine going into my '68 bug. Type 4s never came in bugs, just buses and 914 Porsche's. The U.S. VW after market has little to offer for type 4 engines. The German VW after market has a couple of nice headers available but they are pretty expensive, especially when you throw in shipping from Germany to California.
Many people that have done the type 4 engine installation into a bug used shortened VW 411/412 heater boxes and a modified bus header, not equal length and too restrictive for me. Fat Performance offers a header for the installation but I had heard that there were fitment problems with some of them and I really did not want the traditional bug header with the collector sticking out the back.
I found the Burns Stainless web site and read all their tech info, good stuff but not enough to tell me what primary pipe diameter, primary pipe length, collector diameter, and collector length I would need for my engine. I heard that Headers by Ed had some info on header design and construction so I ordered his info/catalog/cassette tape. Wow! Now there was some info I could use. Included in the package was a chart on primary tube size vs. HP level and collector size. This chart shows that for a 4 cylinder engine making about 130 HP I should use 1.5" primary tubes (this size should support up to 150HP or so). The chart showed I should run a 2.25", 2.5" or 2.75" collector dia., depending on where I wanted to focus the power band. I chose 2.25" because this is a street engine with a 6200rpm redline. I was off to a good start but still did not know how long the primaries and collector needed to be.
Burns Stainless recommended a book: Scientific Design of Exhaust and Intake Systems, by Philip H. Smith and John C. Morrison as deeper reading. They weren't kidding, the authors are scientists and write like it! It's pretty dry stuff and took me quite a while to get through it all and I'm still not sure I understand everything. I did get a formula for figuring primary tube length. Here it is: P=850xED divided by RPM - 3 Here's the explanation: P= primary tube length, ED= 180 + the number of degrees the exhaust valve opens before bottom dead center, RPM = the rpm you want to tune for. As you can see you need specific information on your cam to design an effective header. There are other things that come into play like displacement, max rpm, and other engine modifications that come into play and are more subjective in their impact until you get the engine on the dyno. I also spent over an hour on the phone with Ed talking header design, he is a very smart guy, a little opinionated but very willing to share his knowledge
After doing all the math I arrived at a primary tube length of 42" to focus my power band around 4000 rpm. Primary length includes the length of the exhaust port from the back of the valve and in my case the stub pipes that fit to the exhaust ports. I also found a formula for primary tube dia. but it came out the same as Ed's chart so why waste the effort-just use the chart.
There is a formula for collector dia. in the book also, after the calculations I arrived at a collector diameter just under 2", Ed's chart says a 2.25" collector will have good low and mid range torque so I went with the 2.25". Sometimes I second guess that decision, a 2" collector would probably work very well at my target rpm but may limit the HP before the 6200 rpm redline. How long should a collector/tail pipe be? Primary tube length + 3" is what it says in the book but modern header designers say you have to find the right length by trial and error.
Here's what my header looks like:
It took several tries to get the pipe layout figured out and tacked together with my little flux core mig welder. I then took the header to work where I have access to a Tig welder. I practiced for about 20 minutes on some scrap tubing and jumped right in on the header. Tig welding is very much like gas welding so it came quite easy to me.
Both Burns Stainless and Ed are very adamant that headers need to have equal length primary tubes, within .5" or less. I managed to get my primaries within that limit. I talked with a header designer and fabricator that builds race headers for very high RPM, very high HP engines, he has found that you will make more HP at high RPM's if the headers have more sweeping bends, even if it means the primary tubes are not equal length. By high RPM, we are talking more than 10,000 rpm!! With the gas velocities that go with that high of RPM and around 1,000 hp, I can see that the gentler the bends would be more important than equal length.
Another good web site is Stahl Headers , there are many tech pages on headers, cams, and dyno testing, good stuff.
I continued to read everything I could find on header and exhaust design. Circle Track & Racing Technology magazine had a couple of interesting articles on the subject. In an article about designing a quiet exhaust system that did not cause a loss of any HP I learned about using an expansion chamber at the end of the collector/extension. Basically it is just a chamber with a specific volume that tricks the exhaust gasses into thinking it has reached the atmosphere. Anything after the box does not effect the tuning of the header. .
I have learned that keeping heat in the header will increase exhaust gas velocity, increasing the scavenging effect of the header so I had the header coated inside and out with a ceramic/metallic coating. This coating will also reduce the transfer of heat to the engine where it passes close to the block or heads and make the header last much longer.
Here's the header coated:
I used a fabricated collector from Headers by Ed with the center of the primary tubes pinched together to eliminate the X plug most headers have filling in the center gap and causing unwanted turbulence. If I were to do it over again I would use a nice merged collector from S&S Headers or Burns Stainless like this and pick up several more HP:
Here's a recap of some of what I learned about header design and construciton:
1. All off the shelf headers are a compromise unless your engine is exactly like the one the header was developed for. A lot of headers on the market were never developed on a running engine, just designed/fabricated/sold with no testing at all!!
2. Most off the shelf production headers are poorly designed, not by intention to build a bad header but for economics. Next time you look at a header check the primary tube lengths to see if they are very close to equal, look in the collector to see if the primary tubes merge smoothly or if they have a plug welded in the center.
3. Primary tube diameter is based on HP output of the engine: it's all about velocity, you want to run the smallest dia. primary that does not cause a restriction.
4. Primary length is based on the RPM you are tuning for.
5. Collector outlet dia. and length is very important: for a street engine you want a 1.4 - 1.7 in/out relationship- that is the area of the primary tube outlets added up should be 1.4 to 1.7 times the collector outlet dia. Most people run collectors that are too short and too large of dia.
6. Step headers only make about 1% extra power/torque than fully developed single diameter headers. A "fully developed" header is one that is designed and fabricated using the latest technology and techniques then proven on a dyno. This often takes several prototypes to get the design optimized for a specific engine combination. That's expensive so few people actually do it. The consistent winners do it though....
7. 4-2-1 headers take dyno time and track testing to get correct and make about 2-3% more power than a fully developed 4-1 header.
8. By designing and fabricating your own header you can build a header better than most production headers.
9. Primary tubes need to be equal length. Most production headers aren't even close.
10. Most people think larger primary tubes and collectors are better but can really hurt performance.
11. There are many places to buy mandrel tubing bends with different radii and prices vary, so shop around.
12. Well designed headers are not cheap or quick to build.
13. It is possible to do it yourself and achieve great results.
14. Ceramic coating is not cheap, about $125 for a 4 cylinder header, but after spending that much time designing and fabricating a header you want it to last a long time so you don't have to do it again in the near future!
15. After all this effort to design my "perfect" header I have realized that all header designs are a bit of a compromise, but I was able to pick the compromises and minimize them.
What's next? I will design and build a header for my BMW 320is. I know I can do better than the Stahl and Pacesetter headers. I will start with a 4-1 header and then build a 4-2-1 header.
UPDATE: 20 Feb 2004
I started working on a 320i header this weekend. I have the #1 and #2 cyl. tubes laid out and tacked together, they are within .5" length of each other. This header is designed for a stock 1.8L and street use so it is pretty long to boost torque in the normal driving range. It should also help pull to the factory redline more easily. Here's a pic to see how it looks so far.
UPDATE: 11 March 2004
Well, just after I posted this picture I exchanged a couple more emails with the prospective buyer of this header and found out that the intended use is for endurance racing with a RPM band of 3500-6500 rpm. I assumed that a stock 1.8L engine would be used on the street... well you know what they say about "assume" (ass u me). So I did some more calculations and shortened the header by 8" to move the power band up where it needed to be for a racing 1.8L. Here's a pic of the final header tacked together before final welding. As you can see #3 and #4 cylinder tubes are pretty "snakey" to fit the total length in the limited space. I also found out this car does NOT have A/C so I could build a design using some of the space normally occupied by the A/C compressor and that would give more room to make #3 and #4 tubes with fewer bends and still achieve the desired overall length. Fewer bends = more power.
Here is a picture showing the location of the header collector. It is located fairly close to the floor so there will be adequate ground clearance.
