Correct Header Size

If you're contemplating a new header for your ride but you're not sure what size to get, you might want to pick up the latest copy of Hot VWs (Feb.'12). It's an enlightening study on the correct size of header for your engine, based on the expected performance you're after.
I realize, after reading the article, that I'm running too large of a system when I consider the type of driving I do.
Attached is the chart printed in the article.

I hope they pointed out that there's a little more to headers than just the OD of the zoomie tubes. Things like size of the exhaust valves, length of the zoomie tubes, bend radii, how many bends, size of the collector, placement of each tube in the collector, available back pressure, scavanging effects, little things like that. (No, I haven't read that Hot VW's mag. If it doesn't concern NASCAR or Bog Racing or turkey hunting, then I probably can't get it down here in Secessionville.)

i read the HOTVW article and it does cover that stuff Gordon.

It's a pretty decent summary. rule of thumb so to speak.

Stan, if you want I'll copy it and send it to you.
It is a relatively basic article that leaves the parameters a bit vague but it does give the reader solid information and will put them in the ballpark for choosing the appropriate system.
After I replaced the stock exhaust with an A1 1-5/8" system, I noticed less torque which seemed to increase pinging. Considering the piss they call gas in California, I find myself in a bit of a quandary. I've addressed timing, jets, additives, etc...
My 2110 has a mild 110 cam, 40 X 35.5 heads and 40 IDFs so it's a conservative build. After reading the article and talking with the builder, it looks like I would be better served if I went with an 1-1/2" system.
When I get a few extra bucks I'll have a system built and see how it works.
Hopefully, those who are thinking about a new system will give this article a glance before they fall for the hype that bigger is better.

Thanks for the link, Terry. I hadn't read the article, but would like to now. Perhaps I can find the magazine somewhere.

Gordon's right, there are many factors that relate to proper exhaust tuning. AG Bell wrote a book which was recommended to me called "Four-Stroke Performance Tuning". The book has a 25 page chapter on exhaust systems which gives a scientific (but clear) explanation of how a well designed system works, and formulas for designing a system to match the projected performance of an engine. This has been my get-away-from-business reading since about November. It's a wealth of information.

The Hot VWs chart is a simplification of the formulas and tables in Bell's book-- but it has a good deal of value, because the math in the formulas gets pretty involved, and there are a lot of other variables that effect things as well. Everything is a compromise.

The short explanation is that it's pretty close to impossible to build a perfect exhaust system within the limitations of a rear-mounted flat-4 anyhow. A "merged" system with a stinger comes pretty close, but I haven't met very many guys who can live with a collector and 30" long tube megaphone sticking out the back of their pride-and-joy. I certainly can't, so we're left with primaries that are generally too long, and collectors that are generally too small.

I'm deep enough into this that I'm having Tiger build a custom header for me with smaller (1-5/8") primaries for my 200 hp 2332. The key for what I'm after is that the primaries be kept as short as possible (under 36"). This runs completely counter to what almost everybody else recommends, but most builders insist on building for the dyno and racetrack, instead of for torque and "snap" for the street.

The key to all of this is the scavenging effect of exhaust inertia. Something I did not realize is that this "scavenging" is many times more efficient in emptying the cylinder, and drawing in a clean intake charge than is the pumping force of the piston pushing out the exhaust. That's why cam overlap works.

It's all pretty cool.

My combo seems to fit Hot VW's chart: 2276 with a 6500 redline. I'm running a custom merge 1 5/8" system. I like mine, but it's on the loud side. The big plus is I can adjust my valves without dropping my side mounted muffler.
Mine has plenty of torque-132 ft/lbs at 5000 rpm (rear wheel horsepower), and there is more to be found once I fully figure out my fuel injection system.

While Gordon is right (there is more to header selection than tube size and it's not just a collection of straight pipes), what I found interesting is that the chart lists the operating range for a tubing size with each displacement and that takes into account the level of tune needed to make power in that operating range. I've been around aircooled performance a long time (almost 38 years) and thought the chart was quite good.

Stan, I'm really interested in how your system turns out; I have a 2387 in the works and have been thinking along the same lines. Have you thought of stepping the primaries?

Terry- What's the compression ratio of your motor and how much porting is done? The reason I'm asking about the porting is that heads with big valves and seats but stock 1500/1600 ports exist.

If I have a 1915 motor what would be the best tube size be? I plan on having at least 3 feet of tubing after the collector.

It's a crying shame that more folks don't realize this stuff before they specify their engines. I feel luckier and luckier every time I read threads like this that I had an excellent balance between a hot-rod engine and a balanced, scavenging exhaust from the outset.

As much as it probably LOOKS like my setup was done by a rank amateur, it wasn't. The complexities of additional structures to work around were mathematically balanced against pipe lengths and diameters, a collector with the pipes entering it in firing order and a volumetric approach to through-put and resistance.

All for less than $200 spent, but with about 60 hours' labor. I probably should have had it coated.

(For those of you who've met Jimmy The Wrench, he's trying to make a comeback in Baltimore. Same old Jimmy -- but he's up and running again, somewhere off of Pulaski Highway. He's cutting and welding again.)

Cory: I honestly think that the key part of your exhaust system is that big, semi-baffled, ATV muffler, acting like an expansion chamber for ya. Pretty darn trick.

Matt:

That depends on what your cam specs are, the size of your exhaust valves, what the ID of the tubing after the collector is, the back pressure expected from the muffler(s) and so forth.

BUT, as a rule for a 1915, you can run 1-1/2" head tubes for most driving, and if you want it to rev past 6,500 in third run 1-5/8". THEN, make sure your tube after the collector has a small expansion area as part of the collector for scavanging tube-to-tube (there's a formula for that, but it's at my other house - sorry) and make the post collector tube ID between 2-1/4" and 2-3/4" through the muffler and out the other side.

Just my opinion.

Damn I wish I knew all those stats Brothers built it for me 110 cam and it was built for reliability not so much for racing. Why would I want to rev the engine higher, is there a benefit to that? Im torn between getting 15/5 or 11/2 but I was planning on 1 1/2.

Gordon-

The magic book says that the length of the pipe between the collector and the expansion chamber should be the primary tube length + 3", or any multiple of that length (2x, 3x, etc.). The volume of the chamber is supposed to >8x the volume of a single cylinder in the engine.

Got that, Cory?

Somehow I bet that Jim-the-wrench tugged at those figures here and there.

Then there is the theory of "choking" after the collector with a (relatively speaking) smaller pipe and then scavaging via the expansion chamber. That works, but I'm really not up on how or why it works. My late brother was into all that stuff, mostly on snowmobile racing sled engines (which he built quite a few of) but when he got into explaining it my eyes began to roll......All I know is his customers kept winning and coming back for more speed. Power truly IS addictive.

Over the years, I have come to realize that Stan's guesses are better than my facts.

Much like the advice you give me, Gordon. You guys have more technical information scribbled in notebooks you can't find than most manuals. It saves me a ton of reading when you post these little gems here.

(Now if I could just learn to TAKE that advice ...)

Two-stroke engines (snowmobiles) are doubly dependent on the design of their exhaust. Not only does it create low pressure to pull the exhaust gasses from the chamber and the fuel/air mix from the crankcase into the chamber, but it has to create a high pressure reflection to ram the fuel/air mix that has been sucked through into the exhaust back into the chamber. This creates a big increase in power, but only in a very narrow rpm range.

This page is a nice overview of 2-stroke tuned pipes:
http://www.southernskies.net/page_info/runningtwostrokeengine.html

4-stroke exhaust design isn't concerned with a positive pressure return pulse, hence the difference in layout. But the math is the same.

2-stroke exhaust tuning is for guys who can devote crazy amounts of time to it. The rest of us just buy expansion chamber kits from somebody else, and hope for the best.

4-stroke exhaust tuning is more of a balancing act than most of us understand. For example: the headers we run, for the way most of us use our cars should probably be built with shorter primaries, long collectors, and specific length (and diameter) pipes running into chamber mufflers. The practical problems can be summed up in a word: room. There is no room for all this, unless you are willing to hang the collector 6" out the back of the car, and pipe from there. If you turn the collector 90 deg (sidewinder) to get everything to fit, the primaries get longer automatically.

As a result, most of us end up with primaries longer than we'd like, and larger in diameter than would be good (to try to compensate for the long pipes). The collectors neck down too far from ideal so that it's not so important what comes after that.

I'm trying to get a custom sidewinder made (to clear a dry-sump pump) that has shorter, smaller diameter primaries (under 34", 1-5/8 for a big number 2332), with a larger collector (2-1/4" i/d). I'll build a tuned length pipe dumping into a large volume chamber (also built). I'll build the rest of the exhaust (with mufflers and all that) after that. That's a lot of bends and exhaust stuff from Summit.

Everything matters in determining what "should" work, from cam overlap to the size of the exhaust port in relation to the intake. What works for my cam may be nuts for yours. The science is close to absolute, but the application is a bunch of suppositions.

I'll let you know how it goes once I get the header.

Stan - is all this research and work just so you can hop in your Speedster and burn rubber just for the sake of burning rubber! 8-)

Jim- Pretty much.

I'm a gear-head-- always was, always will be. Stuff like this is "fun" in some twisted sort of way.