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I decided I would start a new thread for the mods that I'm doing on the stock cast iron exhaust manifold for my 1st gen B6. Although I would prefer to have an actual long tube header, I decided to make the most of what I have.
IMO, the factory B6 manifold is a pretty good design in factory form. The runners are large, relatively long (compared to a B3 unit), smooth flowing and stay seperate all the way until the psudo-collector portion of the manifold. There are, however, a couple issues that I believe really limit the potential of the factory set-up.
The biggest issue I can see is the pathetic factory downpipe. Although it "looks" good on the outside, it's actually a double wall design that is actually only ~1.5" ID and is crush bent at the 90 degree turn. To make matters worse, the outlet opening on the manifold is ~1.75".......meaning there is an 1/8" wall or shoulder that the exhaust gasses run into immediately after merging. Pure speculation on my part, but it seems to me that this obstruction inhibits the scavaging potential the comparitively long runners could provide. Couple this with the rather small volume of the collector area and the tiny downpipe, and I think we're leaving some power on the table.
The second issue I see is the casting of the merge section. The runners appear to be paired up correctly (1&4, 2&3) and the outlets of each runner slightly off-set.....so it give the impression that some thought was given to the design with flow in mind. There is, however, a pretty nasty amount of casting flash and (on one side in particular), a pretty obstructive flat wall to inhibit a smooth merge of exhaust gasses (shown in orange circle).
The first task was to fabricate a new downpipe from 2-1/4" OD (2-1/8" ID) mandrel bent tubing. The larger 2-1/8" ID will provide more volume, allowing the exhaust gasses to merge and more easily manipulate the 90 degree turn. The mandrel bend retains a consistent ID through the turn as well. Based on some calculations I did, the optimum collector diameter for a B6 should be between 2" and 2.125". I chose the larger, specifically because of the 90 degree bend in the pipe and the ported head/FMS cam combo. If I was doing this to a stock B6, I would probably go with 2" ID tubing. Since my brother has all the equipment, skills and free time (he's divorced w/no kids), I gave him the manifold and stock pipe and let him do his thing. He cut the 3-bolt flange off the original 1.5" pipe, drilled the center hole out to 2.25" and welded the flange onto the new larger pipe. He used the manifold and factory downpipe support bracket to get everything in proper position. Here are some pics of the new downpipe, with a sliver of the old 1.5" ID pipe held up to it for comparison.
The next task will be grinding on the manifold. I plan on re-shaping the merge section (shown in blue in above pic), rounding off sharp corners, removing casting flash and seeing if I can construct somthing that more closely resembles a traditional "fire cone" to improve the merge of gasses in the collector. In the world of tube headers, the cheap ones have 4 individual tubes that just get welded into a clover-leaf shaped collector. The more expensive headers, that are actually engineered for flow, use some type of pointed cone that is installed at the center of the intersection of where the 4 tubes meet. I will also try to increase the opening size of the manifold outlet from 1.75" to around 2". This should help gas merging and increase flow volume as well, and the slight mis-match of a 2" dumping into a 2-1/8" pipe may provide some anti-reversion properties.........at least that's my theory. I'll post more as progress continues. :p
IMO, the factory B6 manifold is a pretty good design in factory form. The runners are large, relatively long (compared to a B3 unit), smooth flowing and stay seperate all the way until the psudo-collector portion of the manifold. There are, however, a couple issues that I believe really limit the potential of the factory set-up.
The biggest issue I can see is the pathetic factory downpipe. Although it "looks" good on the outside, it's actually a double wall design that is actually only ~1.5" ID and is crush bent at the 90 degree turn. To make matters worse, the outlet opening on the manifold is ~1.75".......meaning there is an 1/8" wall or shoulder that the exhaust gasses run into immediately after merging. Pure speculation on my part, but it seems to me that this obstruction inhibits the scavaging potential the comparitively long runners could provide. Couple this with the rather small volume of the collector area and the tiny downpipe, and I think we're leaving some power on the table.
The second issue I see is the casting of the merge section. The runners appear to be paired up correctly (1&4, 2&3) and the outlets of each runner slightly off-set.....so it give the impression that some thought was given to the design with flow in mind. There is, however, a pretty nasty amount of casting flash and (on one side in particular), a pretty obstructive flat wall to inhibit a smooth merge of exhaust gasses (shown in orange circle).
The first task was to fabricate a new downpipe from 2-1/4" OD (2-1/8" ID) mandrel bent tubing. The larger 2-1/8" ID will provide more volume, allowing the exhaust gasses to merge and more easily manipulate the 90 degree turn. The mandrel bend retains a consistent ID through the turn as well. Based on some calculations I did, the optimum collector diameter for a B6 should be between 2" and 2.125". I chose the larger, specifically because of the 90 degree bend in the pipe and the ported head/FMS cam combo. If I was doing this to a stock B6, I would probably go with 2" ID tubing. Since my brother has all the equipment, skills and free time (he's divorced w/no kids), I gave him the manifold and stock pipe and let him do his thing. He cut the 3-bolt flange off the original 1.5" pipe, drilled the center hole out to 2.25" and welded the flange onto the new larger pipe. He used the manifold and factory downpipe support bracket to get everything in proper position. Here are some pics of the new downpipe, with a sliver of the old 1.5" ID pipe held up to it for comparison.
The next task will be grinding on the manifold. I plan on re-shaping the merge section (shown in blue in above pic), rounding off sharp corners, removing casting flash and seeing if I can construct somthing that more closely resembles a traditional "fire cone" to improve the merge of gasses in the collector. In the world of tube headers, the cheap ones have 4 individual tubes that just get welded into a clover-leaf shaped collector. The more expensive headers, that are actually engineered for flow, use some type of pointed cone that is installed at the center of the intersection of where the 4 tubes meet. I will also try to increase the opening size of the manifold outlet from 1.75" to around 2". This should help gas merging and increase flow volume as well, and the slight mis-match of a 2" dumping into a 2-1/8" pipe may provide some anti-reversion properties.........at least that's my theory. I'll post more as progress continues. :p
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