Honda explains the large step header here and collector design:ptuomov wrote: ↑Tue Sep 07, 2021 8:11 pm “Take a look at some of the exhausts of the V8 era cars, and you’ll see some of them have collectors the size, or very close to that, of their final primary diameter.”
I did take a look at those, and it’s a good a good idea to do so as the 2.4L flat plane V8 exhausts are just like two separate 1.2L four bangers.
The characteristics of those exhausts are massive steps in the primary, very steep merge angles in the 4-1 collector, and collector outlets not much larger than the primary ends.
I know of one company that replicated something like that for a regular four cylinder car engine and results seem to be blowing the commercially available competing products out of the water:
https://www.boefab.com/blogs/tech/84476 ... r-shootout
We have a winner!
Everything mentioned above about the construction can be said about these older style ForcedFed spec headers. They are a work of art! A bit different than the YYY variant, the step on these 4-1 headers is more pronounced and jumps to a full two inches following the 1.75” port matched primary. Immediately, we see cary over technology from the F1 header above, e.g. a large step in the primaries and a very steep merge. These headers also contained a choke after the merge to reduce the merge volume, which should result in a more pronounced sonic reflection to augment that second low-pressure wave.
DMC tried many iterations of this header before finally settled on the surprisingly small 2 1/8” choke. Yes, 2 1/8 inch! That is as visually stunning as it is to write it. The choke appears that it must be restrictive. However, the dyno shows otherwise when compared to the others.
We did try two versions of this header. One retaining the 2 1/8” choke and the other with a 2 3/4” choke but located slightly farther up the merge, which turned out not to be a productive modification. This modification made the midrange slightly softer, which is likely due to a softer sonic reflection from the modified merge.
One very interesting bit of data about the unmodified DMC ForcedFed Spec 4-1 header is that it was the only one to be very sensitive to the cam change event at 6000rpm. All the other headers did not register much of a torque dip at the cam change. On the other hand this header (which seems to be making the most of the exhaust signal) had a significant torque dip at the cam change. While we’re quite certain that we can tune this dip out with the cam phasing, it is interesting to see how sensitive a header that is properly making use of both exhaust mass and sonic reflections to generate power can be to cam phasing when compared to the others.
The unmodified ForcedFed Spec DMC header made 355whp and 227wtq while the modified version averaged about 1whp and 1wtq more. Hardly worth the effort! More interesting is that at 7000rpm this header made 9whp and 7wtq more than the next place header. The dyno results below exhibit a much more advantageous torque curve than all the others tested! Needless to say, we are going to work hard to convince DMC to bring these back to the market for us to enjoy again!
http://www.f1-forecast.com/pdf/F1-Files ... P2_10e.pdf
They have two things going on 1) those engines rev'ed high enough that the tuned lengths are short, which is where I think the stepped header comes into play for them. Some shades of Calvin's NASCAR Stub exhaust test. They did make a 4-2-1 exhaust that worked, but wasn't worth it in terms of weight and reliability, same with the balance pipes (I’m assuming they’re worried about welds cracking). The bike guys understand torque shaping much better than cars, and I think are ahead of the car world on this. I also don't think the primaries are unreasonably large for the power they are producing, they just look it relative to the really compact engine.
For what I am interested in doing, I look at something like Akrapovic's Honda CBR1000RR-R exhaust (developed with the factory WSBK team, and the engine has a convetional I4 firing order):
Here is their Yamaha R6 exhaust. It has balance pipes in the primaries and secondaries. The Honda paper talks about them and how they work:
At the end of the day, it's not secret stuff. It's a balance between pumping losses and utilizing pressure dynamics to shape the torque curve.