The first exhaust collector in 4-2-1

General engine tech -- Drag Racing to Circle Track

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hoffman900
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Re: The first exhaust collector in 4-2-1

Post by hoffman900 »

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!
Honda explains the large step header here and collector design:
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):
Image

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:
Image

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.
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Re: The first exhaust collector in 4-2-1

Post by ptuomov »

Outside the motorcycle racing series where the flat torque curve appears to be very important, do any racing engines actually end up using balance tubes instead of simpler 4-1 and 4-2-1 exhausts?
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Re: The first exhaust collector in 4-2-1

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ptuomov wrote: Thu Sep 09, 2021 4:54 pm Outside the motorcycle racing series where the flat torque curve appears to be very important, do any racing engines actually end up using balance tubes instead of simpler 4-1 and 4-2-1 exhausts?
Not that I have seen, and I think some of that is it doesn't matter as much in a car and the players involved just aren't chasing things to that extent. Obviously F1 tried it, but the drivers didn't notice it. Mclaren in the V10 days had valves in the primaries to shape the torque curve. Mika liked it, Coulthard couldn't tell a difference.

Though, you can look at H, X, and single exit exhausts from a dual bank engine as an extension of that idea, as that's what those are doing. I think the breaking up the exhaust system in separate components is where most go wrong. It starts in the chamber where the valve seat / cuts start and ends at atmosphere... everything between has to be designed as a system.
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Re: The first exhaust collector in 4-2-1

Post by modok »

If the total length of the y is fixed, changing where the primaries end and the secondaries begin does not significantly change the frequency of the y branch. Tapering the secondary one way or another also does not change it either if the average diameter is the same.
making the secondaries larger dia (Or the open end of it in general larger) does raise the frequency, and makng the open end smaller lowers the frequency. This is something pipemax does not seem to take into account, but not a big deal. Apparently you can go very far just with length and frequency.
If the secondary was same csa as the two primaries, that would give the same frequency as one big one straight pipe, but from a flow perspective it would never need to be so large, as the flow out of the idle branch is much less than comes out of the primary. A step up of 30-50% is probably what is needed to keep the peak velocities about the same in the primary and secondary. Farther downstream the secondaries are, the larger they need to be... Makes sense from the "size at length" perspective.
Having the secondary the same diameter as the primary IMO could be a problem because it lowers the frequency, and/or there would be possible flow losses from the very high velocity at the small choke. But seems a lot more likely to work if the primaries are same length or shorter than the secondaries.
One possibility theory is.... is making the primaries and secondaries the same length would help to NOT excite the idle branch, and that would allow the secondary to be smaller.
While making the secondaries half the primary length WOULD excite the idle branch, thus requiring a larger diameter secondary.
It SHOULD work like that, BUT..... does not seem to be a big deal as long as the lengths are quite a lot smaller than "size" of the pulse coming out of the engine, which IMO will almost always be true in a 360 paired setup. That's a small factor, but mostly it's just....... the more volume is upstream, the larger the outflow path needs to be downstream.


Not needing to step up in size "from a flow perspective"
is only true if the idle branch is actually idle, and while at certain rpms it might be, more often it isn't. When the flow in the secondary is accelerating gas is driven up the idle primary, and when the flow in the secondary is decelerating will flow back out of idle branch. The idle branch will only be idle if how it is bouncing happens to match what the system is doing in a certain way.

I see no real advantage to making the primaries and secondaries the same length.
having the primaries 66% or even 75% of the length captures more energy in the longer primaries, and should produce a stronger performance boost if tuned right.
The longer the primary the more BOOST it has.....until you reach some limitation or another, and there are plenty of limitations.

AT LEAST from the perspective of the y-branch alone.

Which possibility is best at running with a small final collector size might be a different story. That could be very difficult to determine.
I feel the second harmonic is still a big limitation in either sense.
a small final choke can work great or maybe...best, IF the outflow becomes continuous at high rpms, and so you will have to tune the frequency of the Y branches to make that happen. If the y length is too long, the outflow periods of the y branchs become more than half the time, then the two Y's are trying to flow at the same time.... then the small choke is a big old roadblock and shuts it down.
going to 180 pairing allows a LOT higher rpm at the same lengths, or longer lengths to be used at the same RPMs
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