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Y connection into a slightly larger pipe, but enter an "exhaust box" (it's an old book, so the terminology is probably out-of-date) which has a fairly large volume, say 12-15 times larger than the displacement of one cylinder.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]

Maybe, maybe not. If Jake finds it adequate to use a single design header for everything from FP race engines to street 2056s to 2.5L monsters, all with substantially different cam timing, strokes, etc. then it's unlikely that same header wouldn't work decently on a Subaru engine. It may not be optimal, but it would likely be much better than the sucky Subaru stock systems. I've also always been led to believe that cam timing and esp. primary length need to be paired, but perhaps that's just not so.

Chris has mentioned that the primaries on the European header are the wrong length, and one would think that would depend heavily on cam timing and/or stroke and/or SOME other major parameter, but perhaps not.

I'm guessing this kind of thing really starts to make a difference when you're running so much valve overlap that exhaust tuning makes a really big difference. This kind of thing doesn't usually start to appear until you're way up the tuning scale, above 100hp/liter. An NA 2.5 Soob only makes 66hp/liter stock with it's stock exhaust, so it's quite a ways from being radically tuned (still way better than the 50hp/liter Type IV). Bike engines now are pretty heavily exhaust tuned, and they're all approaching 200hp/liter these days. Their torque curves are very lumpy by car standards, and exhaust primary length makes a noticable difference to the lumpiness and peak torque.

I have no idea what kind of power they're getting out of the FP Type IVs these days, but I'd guess it's still not in the 100hp/liter range. Those are certainly going to be the most radically tuned Type IVs in use right now, I'd guess.