Exhaust - Cylinder Pairing

[exFSAE]

Any takers on this? VFR? I'm curious... if you have a 4cyl with a 1-2-4-3 firing order what is the preferred pairing for a 4-2-1 collector arrangment.. or twin 2-1's?

I seem to recall always pairing 1+4, 2+3, but don't recall ever getting an explanation on what specifically it was doing.

[exFSAE]

Any takers on this? VFR? I'm curious... if you have a 4cyl with a 1-2-4-3 firing order what is the preferred pairing for a 4-2-1 collector arrangment.. or twin 2-1's?

I seem to recall always pairing 1+4, 2+3, but don't recall ever getting an explanation on what specifically it was doing.

[Dan Deussen @ Weber Motor]

This topic has been discussed on this forum in the past so the search function will let you find a lot of detail on this.

The basic idea though is to get even pressure pulses at the merge point of the primary collector. With the firing order of 1-2-4-3 you have 180?-180?-180?-180? between the pulses of the entire system. If you were to pair 1-2 and 3-4 into the primary collectors as the motorcyle stock exhaust does it you get uneven 180?-540? pulses at the primary collectors. If you pair 1-4 and 2-3 you get even 360?-360? pulses at the primary collector. This layout provides typically a broad torque band.

[exFSAE]

A search of "cylinder pairing", "cylinder matching", and "exhaust matching" yielded nothing of value prior to posting. I had checked.

Why do you want even pressure pulses at the merge point? What about that magically provides broader torque.

[Wesley]

You can tune either to just about equal efficiency.

The non-engineering explanation is this - if you have even firing, you have a nice orderly chain of exhaust gas flowing down the pipe. If you have them staggered as such (with even pipe lengths), you'll get two pulses in 360 degrees, then nothing for a rotation, trying to shove double the gas down the pipe in half the time. You can always double the length of one tube to make it even.

The engineering explanation I don't feel like thinking about - having to do with the fact that with unequally spaced exhaust events messes with your pressure wave reflections, and you have to be more sure that you aren't being destructive with your exhaust pressure waves, when with an even-fire, the return is more easily predictable, and the same across all cylinders.

[VFR750R]

since i got called out... i was under the impression that pairing opposite cylinders is optimum. In this senario each cylinder sees the same pressure waves, and like Dan said there should be the least losses in the pipes. From what i've seen, pairing this way gives the highest tuning peaks, with every cylinder tuning up at the same rpm. With a 4-2-1, you end up with multiple tuning peaks of lower amplitude then a 4-1, but each cylinder peaks at the same time.

When adjacent cylinders are paired together the summation of those pressure waves mean the cylinders still tune up, but at different rpm and with varying amplitude. Similar to having unequal length primaries.

Another reason i can think to pair the 'correct' cylinders together is the fuel will have to be trimmed accordingly when the different cylinders 'tune-up'. Not a problem with sequential injection, but one more thing to work on and next to impossible to fix with a single carb feeding all the cylindershttp://fsae.com/groupee_common/emoticons/icon_wink.gif

[BenB]

Kind of on this topic...Does anybody know if their is an optimal pairing for a 4-1 system? What I mean is should the 1 and 4 be located on oposite sides (accross from) of the collector and the same with 2-3?

It might not matter at all, but I was thinking that the pipe on the opposite side from another is "felt" less than one of the pipes that is located directly next to it.

[Wesley]

Well, in a 4-1, you'll always have two cylinders firing right next to each other.

I'd guess the effect is probably pretty negligible.

Also, don't forget the effect collection order has on noise output - header collection order and variation in lengths has a lot to do with frequency and order of the output sound. Not as important in FSAE, but loads of time is spent for both NVH and aesthetics in OEM cars, so it's worth looking at now, in my opinion.

This year is the first time we've done any acoustical analysis using WAVE, and it's fairly interesting.

[Marshall.Hagen]

Pairing also dictates collector size. When you have unequal firing on a 2-1 collector, you are moving more gasses through the collector at the point at which they overlap. With equal firing, you can run a smaller collector (weight...). Additionally, equal firing distributes the wave tuning more evenly across the torque band, so the torque curve is more linear/flat, with less spikes/waves.