Subie Conversion : What about exhaust ? Options

[jsteele22]

A couple of weeks ago, someone started a thread about DIY headers. Since I'm susceptible to that sort of thing, I got a book about exhaust design, and I've been trying (successfully so far) not to order Covell's "Working With Tubing" video, that Mueller suggested. The conclusion of that other thread seemed to be : don't DIY unless you really, really have to, or really, really want to. So for this thread, I'd like to find out if there's a need for custom headers for an EJ25/914 conversion, or if there is any reasonably affordable stock solution that works. And also to see what folks who are already driving conversions are using : stock manifolds, custom headers, ????


So far, my impression from looking at pictures is that the available Subie headers make too gentle a bend as they come down from the exhaust flange, ending up about as low as the stock oil pan. Not a problem in a Subie, but for us that's getting awfully close to pavement. Although sharp bends are usually undesireable, right near the heads they seem to be less of a problem. Also, I have no idea how they relate to various engine mount bars that people are using.

Another issue is primary length. For a 914, I think it would be more fun to emphasize torque at the mid RPM band, as opposed to the top end, so longer, smaller-diameter primaries would be the way to go.

Finally, the Smith book ("The scientific design of exhaust and intake systems", Philip Hubert Smith) talks about two categories of exhaust : "Interference" where reflections from one cylinder makes a significant contribution to scavenging from other cylinders, and "Independence", where the scavenging for each cylinder is almost entirely due to reflections from that cylinder. Most headers I see in piuctures look like the Interference type. Given that header design is so complicated and trial-and-error in nature anyway, I would think that the Independence approach would be less prone to failure. The key to the Independence approach is that the primaries do not merge in the standard Y connection into a slightly larger pipe, but enter an "exhaust box" (it's an old book, so the terminology is probably out-of-date) which has a fairly large volume, say 12-15 times larger than the displacement of one cylinder. Each pipe enters the box directly with no merge. So when a pulse comes down one pipe, it only increases the pressure in the box a small amount, resulting in very weak reflections going up the other pipes; but the original pipe sees a strong (negative pressure) reflection from the exhaust box. So there's really only two parameters to tune : primary length and diameter. Shortly after the exhaust box comes the "silencer", which is pretty much a standard glass-pack muffler.

Anyway, this last part was about how a custom design might look. But lets hear it : what are all you Converts out there (thinking of) using ?

[lapuwali]

The Smith book, as you say, is an old book (I also have a copy). From what I've been able to see, the "independent" scheme has pretty much fallen out of favor in all engines that are extremely exhaust tuned (look at motorcycles and F1 cars for perfect examples). It was the very thing to do in the 1960s in F1 engine design, but it hasn't been seen much in the last 20 years. The independent design doesn't even rely on a single collector, but can also be used in open "stack pipe" systems. Drag racers still use these today, but they're almost alone. F1 cars don't, partially for space constraints, but what little you can see of the designs they produce hint at an "interference" design.

Bikes have flopped back and forth over the years, and there have been some "mostly independent" designs made even recently, but this is often for styling reasons, not power curve reasons.

It's a fascinating subject, and one that's not really gone over enough, in my view.