Hey Mook...I love the effort. You probably already read this?
Hey all! I've been kinda lurking for a month or 2 now, getting ideas for 4bt conversion into a K5 Blazer that I hope to do someday (far into the future). Anyway, I noticed something about the 4bt manifold that I figured I'd bring to everyone’s attention.
Everybody knows that most turbos have a divided inlet to the exhaust housing. I can't remember the exact reason turbos are made this way, I think it has something to do with pulse tuning or something like that. Anyway, I do know for the divided exhaust housing to work the way it should, the pulses from the exhaust valves opening on different cylinders have to alternate between the two sides of the exhaust housing. So for example, on a 6bt, with firing order 1-5-3-6-2-4, the exhaust pulse from the number 1 cylinder goes to the front half of the turbo exhaust housing, the pulse from the number 5 goes to the back half, the number 3 pulse to the front half, alternating back and forth. This is why the 6bt exhaust manifold is designed so that the front 3 cylinders feed one half of the exhaust housing and the back 3 cylinders feed the other half.
However, with a 4bt things are a little different. The firing order for the 4bt is 1-3-4-2. Notice that instead of alternating between the front cylinders and the back cylinders like the 6bt, the 4bt alternates between the cylinders on the ends of the block and the ones in the middle. What this means is that the exhaust manifold must be designed differently to get the same pulse effect, and it is. If you look closely at the different factory 4bt exhaust manifolds, you'll notice that they're designed so that cylinders 1 and 4 feed one side of the turbo exhaust housing, while 2 and 3 feed the other, getting the desired pulse balancing into the turbo.
I bring this up because I notice many people are designing their own exhaust manifolds to fit the 4bt (often modifying a 6bt manifold) and coming up with manifolds that divide between the front and back cylinders. The effect of this is to send 2 pulses to one half of the turbo, then 2 to the other, instead of alternating pulses 1 for 1 between the 2 halves of the turbo. I don't know how much of a detrimental effect this has on turbo performance, but its something I thought I'd point out to everyone.
I see your header is configured 1-2/3-4 and you produced very good results (lower EGT's and greater boost) I wonder what the results would be if the header was plumbed 1-3/4-2? You may have hit on something like the Chevy "4-7" cam swap that changes the firing order on the SBC to gain better flow?
