Thread: 4-2-1 sequential or non sequential

Hey, I'm going to be making a set of headers for my ve ( I have to its in a rwd datsun b110) but I've been thinking about the pairing a bit lately. I see chuck has made his with sequential pairing (1-3 and 2-4) but is that really the best configuration?
The reason I say it is with sequential 3 gets a bit of extra scavenging effect courtesy of cyl 1 and 2 gets a bit from cylinder 4 but cyl 1 and cyl 4 dont get any help.
Where as if it is paired no sequential every cyl will benefit from an even scavenging effect (albeit a weaker scavenging effect) happening at the last collector where as with sequential 3 and 2 will get a stronger scavenging effect than 1 and 4.
Does that make sense, I’m not sure if i’ve made my self entirely clear but let me know what you think? This isn't a stab at snickers, i'm just trying to get my head round the theory side before I start
Nick

This is pretty much beyond the scope of most people on the forum, I would give Burns a call.

Scavenging is very important and how the runners are paired is as well. IF the header is spec'd out right, a norma 1-4 2-3 pairing can aid in great power gains at specific tuned RPM.

On a sequential pairing (1-2, 1-3), (3-4, 2-4) you do gain from the preceeding cylinder firing because of the low pressure created at the end of the header primary at the first merge point, then the second cylinder fires. After that cylinder fires, there is a low pressure in that collector, as well as the second final 2-1 merge collector, now when the next cylinder fires, say cylinder 4, there is a low pressure zone in the last collector which does help in scavenging and then the process continues.

In the "traditional" 1-4, 2-3 pairing, there is 360 deg of crank rotation between exhaust pulses on the same collector. Because this is so long time wise, the low pressure zone created is gone by the time the following cylinder goes through its exhaust phase and thus you can not optimize on scavenging as much per say. If the header is tuned correctly and the runner diameter is not too big, the exhaust velocity will be very high and thus you will gain more from scavenging because of the higher gas velocity, but the top end will nose dive very fast in most cases.

I'm not sure if this makes sense, i'm pretty tired so I may have to re-word all of this again.

I had to read it a few times but I think I understand, so if I got it right what your saying is in reality, due to high velocity most of the scavenging effect is missed on a non sequential is in the first collector due to the time between exhaust pulses?

yeah. The main reason the factory OEM compaines pair the runners is for heat expansion, and for faster cat warm up. By alternating the exhaust pulses you keep the heat expansion at a constant rate as well as maintaining a more constant and even flow to the catalytic convertor.

I just had another read. So even though initially there is no scavenging effect on 1 and 4, once the scavenging effect has started and is under way and velocity increases 1 and 4 will benefit from some scavenging at the 2nd collector? would it still be less scavenging effect than cyl 2 and 3? Would this mean there would be any difference in a/f ratio per cyl?

So to summarize
non sequential = little or no scavenging due to the velocity but even pulses
sequential equals strong scavenging effect but uneven?