Although my 400SS is one of the 20 officially imported into Australia in 1993, I didn't know anything about them until we had a second hand one come into Moto One a few years later, and we all stood and laughed at its slowness. I did however take the chance to dyno a 600SS once to compare the 2 into 1 header to the M600 40mm 2 into 2 header. All the 2V Monsters - 400 to 1000 - had 40mm headers std. When I did the M600/600SS comparison the whole "different bike" thing may have clouded the result somewhere, but comparatively the 2 into 1 600SS (blue) made a little more power under 4,500 rpm and less power above that. But bike and muffler differences may have clouded that.
So with the 400SS back together I took the opportunity to test the 2 into 1 versus the 2 into 2 that it wore in the Monster. As to what is the same, well - engine, cam timing, gearing, rear tyre (the same actual tyre), muffler (ever dependable Megacycle). Different - carbs, carb jetting, exhaust headers.
The 2 into 1 header came with the 600SS when it arrived in 1994. To help distance it from the 750SS price point wise (the 750SS also gained a second front disc for 1994), it had the 2 into 1 header set from the 350 / 400SS. The 350SS came with an aluminium wrap black steel muffler like a 750SS (not sure if it was the same part or not) and the 400SS had the aluminium sleeve muffler like the 900SS. The header itself was smaller in diameter than both the 750 (which was 35mm od) and the 900 (40mm od) and the primary tubes were quite a bit longer.
Photo shows 400SS 2 into 1 on left, M600 2 into 2 on right. The 2 into 2 features the stamped steel crossover which originally appeared on the 750 Paso and is truly an inspired design for an otherwise complicated part made simple for volume production. It works a treat. My 2 into 1 (hereafter known as "original header chopped up into kind of crappy 2 into 1") not so treat filled.
This being the original header chopped up into kind of crappy 2 into 1. Blue lines indicate the flow path from one header into the other, possibly part of the problem.
The result surprised me a little, given what I thought I might see based on the above graph. But it also backed up the few other tests I've done with 2 into 1 exhaust. That being, a 2 into 2 always works better. Never seen it go the other way.
As to what those other results are, I can think of two of my own. Moto Guzzi Sport 1100i, where I made a full exhaust for the bike with 2 into 1 and 2 into 2 cross overs. Otherwise same header and muffler. Green is 2 into 1. Had a noticeable top endy feel to it.
And Minne (M600 with a 750 engine) before the valves, cams and comp, fitted with an old Gio.Ca.Moto M600 2 into 1. 43mm od pipe into 48mm od collector, of similar design to the 400SS 2 into 1, but with lots of spring joints, etc. Red is 2 into 1.
I was also told many years ago that the improvement the Aprilia / Akrappovic accessory 2 into 2 full system brought to the first gen Mille (std 2 into 1 exhaust with the great big muffler) was more midrange. Which again is contrary to the 2 into 1 myth.
So that's my background to the 2 into 1 thing.
The graph below shows the 400 engine in the SS with the 2 into 1 in red, in the Monster with the original 40mm M600 2 into 2 headers in green and in the Monster with my original header chopped up into kind of crappy 2 into 1 in blue. Peak power at just over 80mph is 9,500 rpm. The 400SS top end air/fuel is an issue, and with more main jet it may make more top end power. The red curve certainly improved a couple with more air to offset its richness. The midrange air/fuel is similar to the Monster jetting, so comparatively at least should be similarly crappy.
While the original header chopped up into kind of crappy 2 into 1 did give a stronger bottom end (compared to the 2 into 1 of the green curve it has shorter, larger diameter primaries so it's not following the myth there either), it all went pear shaped over 7,500 rpm. Realistically, that's about as hard as I revved it anyway, so for me it wasn't a big issue. But the knowledge that I'd made it somewhat worse annoyed me. I think it's the way I modified the stamped sheet cross over into the 2 into 1 merge, and maybe the fact that it's all the same diameter isn't helping. But I'm not inclined to find out and that header set only fits a small block Monster with rear sets so it's a bit restricted in application to play with.
Comparison of the two 2 into 1's below, 400SS is blue. The funny thing is that the 400SS has a real pick up in performance you can feel around 7,000 rpm. Maybe it's as the fuelling goes from too rich to too lean, dunno. Definitely noticeable, whereas the Monster with the 2 into 2, which has a much less smooth dyno curve, wasn't.
The jetting also varies a bit - they're kind of wacky. The 350SS and 400SS std jetting variation is bizarre for what is essentially the same bike, produced at the same time. The jetting I'm using in the 400 is not quite std. They use a much softer slide spring than anything else - it's almost like a Factory Pro or Dynojet spring. I did have them in it when it first went together, but swapped the heavier "in everything else" springs into it to make it a bit richer. Maybe too rich now, not sure. Everything except the 600SS use larger holes in the slide with the heavier springs too. Maybe that transfers more vacuum to the diaphragm.
JD Hord made the comment on Facebook that "I did a bunch of messing around with springs on a Honda Hawk one time, and best I could tell, at steady state they had barely any effect, but throttle response did seem slightly better with heavier springs. I suppose that's because it would pull more vacuum and get fuel moving a bit quicker into the airstream."
It did seem to help the throttle response and starting, but I also richened the idle mixture a bit as well.
The 2001 M400 uses the same spec carb as all the M600, so I ran the original M600 carbs on this engine when it was in the Monster. The jetting is quite different in those two, but the midrange WOT fuelling is similar on the dyno. I've got a heap more mindless rambling to do on the jetting stuff, watch out for that blog post - it'll be coma inducing.