Compromise divided / twin scroll headers

[koukimatt]

I am considering a VH45 twin turbo swap for my street S14 and I have been thinking about exhaust manifold design for divided / twin-scroll turbine housings.

From what I have read, 180 degree headers are optimal for NA and most turbo setups; in NA engines, 180 headers provide the best scavenging effect, and for turbo setups, they provide a steady flow of exhaust to the turbine. However there is the obvious downside of plumbing -- 180 degree headers take up a lot more space than other header designs, and there just isn't enough room for this under the hood of an S14 (without extreme modification).

The alternatives are to use a log-style manifold (unequal length) or 4-to-1 equal length runners, both of which are less than optimal (in relation to the ideal 180 degree header design). However, I think I have found a good compromise between size and performance.

Using the VH45 firing order (1-8-7-3-6-5-4-2), I made a piano roll style chart which shows when each cylinder's exhaust valves are open (I forgot the chart at home, I'll post it later). On the left and right cylinder banks, I identified the cylinders whose exhaust strokes do not overlap and paired them together:

(viewed from the front)Left: 2+6, 4+8Right: 1+3, 5+7

Each of these pairs could be merged into a collector for a twin-scroll turbine housing. The timing is less than ideal (180 headers really is the only way to do it right -- or use a flat plane crankshaft) but this design spreads out the exhaust pulses more, enough so that I think a twin scroll setup would work pretty good.

I don't know much about runner length tuning -- I think unequal length runners have to be tuned for a certain RPM. But the runners for each scroll could probably be tuned a bit to spread out the exhaust pulses better.

Since I don't have a VH45 (yet) and I don't have any fabrication experience (yet) I can't test this theory. But I would appreciate it if anyone who does have experience designing manifolds could validate this.

[Carl H]

a 4-2-1 setup that you're proposing is much better than a 4-1 style header but due to space constraints a proper 4-2-1 isnt possible in an s chassis...there just isnt enough room, there is barely enough room for a 4-1 setup

[gs14racer]

I agree, while possible, i dont think its practical, your gonna have all kinds of issues getting that to work. Not to mention doing a 4-2-1 "the right way" is gonna be next to impossible due to packaging constriaints, especially on the passenger side for imo not a lot of gain over a well designed 4-1 with a good collector and even that is hard to do.

After i built the last set of headers for an schassis with the vh, i would not incur the extra labor personally but i would be curious as to the gain that could be had, assuming you could get them out of the car with out pulling the engine to do a back to back test.

[CrazyTrance]

I'll also have to agree with this. There simply isn't enough room in an S-chassis. Although i really wanted to do 180 degree headers, i was barely able to squeeze the headers i did make past the steering column.

[gs14racer]

Heres some pics from mine so you can get an idea of clearance. These where equal length with a merge collector