header design with sequentially siamesed exhaust ports?

[renyo]

I was wondering about the impact about the odd heads on my engine on header design. I have a (Subaru) H4 - 16v (gasoline, spark ignition, naturally aspirated) engine. The exhaust ports on each head are siamesed together. The firing order however fires left front, back, then right front, back, resulting in the siamesed exhaust ports getting two "fires" sequentially. My question is how would this effect primary diameter and length calculations if it would?

Another question is I've seen primary lengths of around 38 inches to around 60 inches when tuning for around 3500 rpms. Any ideas as to which one is more correct and why?

Thanks

 

[marcusaurailius]

Renyo, not to familiar with the scooby doo engine,
but the old BMC 4 pot engines had siamesed ports, and the way they got round it ,was to have an exhaust called an L.C.B
long center branch,
the middle primary was way longer than the end ones.
the middle took 2+3 pots in one pipe, so was made longer to compensate,
the length of the pipe/s was made different for different rev bands/engine spec
might be of some help to you

regards Marcus

 

[renyo]

Wow, that is one odd looking header, interesting idea though. Doesn't help me too much unfortunately..

To get my problem across a little better...
my engine (drawn horribly in ASCII art) looks like:

front
____________
| 2 | O--||--O | 1 |
|----| || |----|
| 4 | O-||-O | 3 |
''''''''''''''''''''''''''''''''''''''''''''
back

(Clearly there's a reason they don't manufacture engines out of variable width font..)

The firing order is 1-3-2-4, however the exhaust ports in cylinders 1 & 3 are joined and cylinders 2 & 4 are joined.

 

[evelrod]

BMC siamesed port is not quite the same thing as the Subaru flat 4. The exhaust events on it are 360 degrees apart and offer little hindrance to flow (tuning? that's another story).

The Subaru fires 180 apart and unless it's supercharged, I see it as a problem in flow and tuning. I don't know anything about the flat four save I drove a WRX on the track at Vegas a few years ago and I really did not like it and it's crappy five speed gearbox. Personal opinion.

Rod

 

[IceStationZebra]

Are you sure the heads have a shared port? I tried to find a picture online and only found those showing separate ports.

http://www.headtune.co.uk/index.php...ional=images/Subaru/Impreza-EJ20-heads_02.jpg

If your heads do indeed have a shared port, how far back into the head is the port divider? Oldsmobile V8 engines had a shared port for the center two cylinders on each head. The hot rodder trick was to weld a piece of steel onto the header flange that protruded into the head and kept the ports separate. Granted the seal isn't air tight, but it doesn't need to be to get the tuning performance.

ISZ

 

[renyo]

I am sure they do have shared ports (I'm having trouble finding a picture myself). I'm unsure about how deep it goes in before it splits, however I'd prefer not go the weld route.

 

[izzmus]

Only some of them had shared ports. I know for sure the ones used in the later lesser Imprezas did. From the shape of the manifold, the gasket surface is just a round hole like the BMC, not like the Olds at all.

Is a head swap out of the question?

I was quite saddened to see Subaru revert to single exhaust ports per cylinder head like they'd had before 1990. At least they didn't revert to siamesed intake ports too!

 

[renyo]

Only some of them had shared ports. I know for sure the ones used in the later lesser Imprezas did. From the shape of the manifold, the gasket surface is just a round hole like the BMC, not like the Olds at all.
Cool, someone's heard of them.

Is a head swap out of the question?
I've considered this before, however the older model heads possibly flow worse.

I was quite saddened to see Subaru revert to single exhaust ports per cylinder head like they'd had before 1990. At least they didn't revert to siamesed intake ports too!
Yeah, the siamesed intake ports must have sucked. Good thing they only did the single port exhausts with the 96ish-01 2.2Ls.