Header and firing order curiosity...
 

I've been thinking about this for a while now, and I think this might be the kind of place to bring this up.

Should firing order be considered during header design?

In the case of a 4-2-1 header, wouldn't you want out of synch cylinders on the same merge? I don't know if that makes sense or not, the way I'm explaining it, but basically, if the cylinders fire 1-2-3-4, wouldn't you want 1 and 3 to merge, and 2 and 4 to merge?

Now I don't know if that's more confusing than what I first said. Basically, the idea being that there would be the least amount of turbulence possible at the collection point.

Just curious if this comes into consideration during header design.

The firing order is 1-3-2-4 with 1 being the passenger front, 3 being the rear, 2 being the driver front, and 4 being the driver rear.

Most design them where the two closest merge so 1-3 and 2-4.

I'm no header expert just relating what I have seen with other Subaru headers.

Good to know, thank you :)

This being the case, the stock header does exactly what I was trying to explain. Since 1 and 3 are on the same merge, by the time #3 is in the exhaust phase, the #1 pulse should be out of the way, creating less turbulence in the exhaust...right?

Now, if only I knew whether or not this was optimal or not XD

Hmmm? No expert, but I think for 4 cylinder engines you want the 2 cylinders firing 360* apart to be paired first, because if you pair adjacently firing cylinders then as the exhaust stroke is finishing on the cylinder that fired first, the cylinder firing next will have its exhaust ports opening right around that time, and shove a good amount of fresh exhaust into the previous cylinder. This primary pairing should be more important because these acoustic effects diminish after more merges and steps.

If it's 1-3-2-4, when #1 is in the exhaust phase, #2 (It's paired cylinder in the header) is in intake, so its exhaust valve is closed just like you said :)

Right, but dwx said the opposite, which doesn't sound right to me. Or at least that's how I'm understanding his post...

With the way it is setup, with cylinder 1 and 3 merging together, the exhaust pulse from cylinder 1 actually helps scavenge/pull the exhaust out of cylinder 3 as the exhaust pulse goes through the merge collector. This is because cylinder 3's exhaust ports open right after cylinder 1.

However, I could have it backwards... Maybe it's better to have 180º of separation (i.e 1-2 and 3-4 merge)?

Whatever it is, I think 4-1 is the best setup.

You are correct 1-3 merge, they are the same head. Of course the other issue is the exhaust ports on opposite sides sit pretty far apart it would be near impossible to build something different. :)

Phil

http://blog.perrinperformance.com/wp...05/header1.jpg
?

Oops forget I said that, generally they do tie together opposite sides, looking at some different header pictures. I thought I remembered otherwise...

Haha yup. If UEL headers tied together cylinders on the same head, they would be a pretty big performance disaster.