I have read this site for a while, and thought you may be interested in seeing some differences between the 6 and 4 stud exhaust, as well as just how bad the inner flange welding on these parts is.
I have a F VVC with plenty of modifications, and my exhaust needed replaced. I do not think that the majority of aftermarket parts are good value for money and I do not race so I decided to use stock components and modify them.
I chose a 6 stud TF manifold and downpipe, a replacement F cat and a F Bosal backbox, as I perceived it to have better quality construction and pipe bending than similar replacement parts.
The manifold was £18 delivered, £44 for the downpipe (with a lambda sensor in it!), £40 for the cat and £105 for the backbox.
I chose the 6 stud system as I wanted to see if it had longer primaries or a bigger pipe diameter. The conclusion I came to was that the primaries were around 6-9mm longer on the TF/6 stud manifold than the F/4stud. This was done by the fairly poor 'string and Blu tak' method, but did appear repeatable. The old 4 stud manifold had to be butchered to find this out (see photo album). The pipe inner diameters were the same. On this basis I went ahead.
The following are photos of the various stages.
This is the system, minus backbox
This is the size of the typical burr on the EX manifold inlet:
Here is the downpipe exit flange burr
This is the amount of burring at the EX manifold outlet (primaries are visible inside)
This is the EX manifold about to be matched to the gasket to the downpipe:
And here is the nasty bit - the sizes before....
That equates to a 15.9% increase in throat area. Obviously this does not mean the whole pipe is bigger, but the throat restriction is sizable.
I used a tungsten carbide cutter bit and an air driven cutter to do the work. The welds are quite hard.
Now on to the EX manifold inlet. These are the burrs:
That's a 18.9% increase at the throat area. Bearing in mind that this is a restriction right where the hot exhaust flow wants out from there must be a considerable pressure loss in the pipe right there.
The cutting made some parts too thin, so I beaded some weld around the outside, and then cut back the drips.
Now the downpipe. It's inlet needs to be matched to the gasket and the exit from the manifold.
This was the diameter as standard:
That's a 9.2% increase in area, and shows the mismatch between manifold exit and downpipe inlet caused by the burrs (I did fettle the two sections to match almost exactly after those photos BTW).
The downpipe exit to the cat (of which I lost the photos) had a burred diameter of 48.79mm and was opened out to 51.10mm. This joint was heavily radiused as the pattern catalytic converter had a significantly larger diameter, so it was a case of trying to blend as best as possible. The cat also has a nasty step where the tube is pressed in, and I did consider machining the flange so it was at least flush, or running a bead of weld around it, but as I did not want to hurt the matrix I decided to to live with it.
With this all done it was exhaust removal time. I chose a large angle grinder to help.
and decided to simply remove the old 4 stud manifold from the top with the remainder of the downpipe on it
Here are the manifold differences:
You would say the primaries on the 4 stud are longer but I found the 6 stud to be longer, as mentioned above. You can see inside the 4 stud here:
The whole thing was ready to be put on after that:
I will write separately about the backbox surgery, the short version is I removed the baffles and crosspipes to reduce the backpressure and make a sportier noise.
My conclusion from this;
The car seems slightly better in low range and will comfortably trundle in 5th gear at 1500rpm and pull cleanly, if not strongly from there. Beforehand it would slug if I tried to do that.
The noise is nicer and sportier, but neither too loud or tiring.
The power above 4000rpm seems to have improved. I tried timed before and after 30-50mph 4th gear runs on the car with an overnight cold soak and a 15 minute warm up on the same roads. Before was 6.38 seconds (averaged over 3 runs), after was 6.16 (av. 3 runs). The book figure is 6.5, but I already have other tweaks on my car. I was toying with the idea of getting someone else to drive it to try to eliminate bias, but I did not have time.
I had no intention of paying for before and after rolling road runs at £50 a time, so this was the best I could do.
As I needed a replacement anyway I felt all of the effort was worth it.