[axsaxoman]

I had a machine shop mill mine to flatten it out. the bottom bar in this photo is flat. The manifold as you can see is warped in the middle

to be fair to DP or anyone making such a weird shaped +compact ex manifold ,they are always going to have issues with distortion
all those pipes of different lengths ,cold drawn bends +then welded --and not left in a jig and heated up to red heat to stress relieve them --its going to bend and nothing will stop it except for a much thicker flange on the head and then it will just move the bottom flange instead
I have to hand it them though they have managed to convince everyone that they need such a complicated manifold in the first place.
Its nonsense and just bling anyway .
same goes for the inlet manifold right over the top
a bmw 740 --4.5litre v8 runs a 60mm t/body up to 7000rpm and makes near 400 n/a ,runs like a sweety at all times --so why when you are cramming in twice the volume of air would you need a 70mm t/body on a 1587cc engine --you don,t
same goes for the primary pipe sizes on the ex manifold ,and to make it worse they all come together and the crossectional area at that point is very restricted
this cuases lots of heat to back up ,add to that the poor airflow around the manifold and you are going to get uneven expansion .

for those of you who want to prove /disprove what I am saying
take tapping from the ex manofld just before turbo --a bit of brake pipe extended out then rubber to a gauge --measure the back pressure required to get your boost required ,then watch it climbs as you go further up the rpm range -- all that wasted power and extra heat you don,t want to make .
I did this years ago with a saab 900 and by bleeding off the extra pressure I increased engine output with std turbo to get a 25% increase on what the turbo was rated at --
the turbo stopped glowing as did the ex manifold ,
this is why the VNT turbo came into being
which brings us nicely on to the choices of turbo,s --DP specifyied the gt28 ,as it was something old fashioned they could get cheaply and its why the lag is so bad and won,t spool up till more than half way up your rpm range
its really is not matched to the engine at all ,just happens to fit same flange as the r5/nissan silva turbo they started the project with .
its big its crude
yes it can make enough power --but its not a good choice due to its physical size .
a smaller turbo with a simpler more compact manifold will do all it needs to do and keep the heat problems to a minimum .
I am suprised some of the turboguru,s have not realised these simple things

If you must have a manifold of that shape then there should be slip joint on all the primaries to allow for this so it can expand and contract without moving everything about .
that will give leaks when its cold ,but on a race car of ultimate spec thats not a problem .
look at your favorite std turbo cars ,which with chipping etc can make more bhp than your saxo , + alot easier and you will find very simple cast manifolds and even some of those have a slip joint to stop cracking of manifold .
the bolt holes will be at least 2-3 mm larger than stud size for same reason to allow for expansion
the large primary sizes on the tubular manifolds will make more lag as the gas speed will be lower ,untill it hits the restrictive turbo flange,which will cause turbulance and not do what a std cast manifold does
most are split into 1+4 and 2+3 to give nice puses to the turbine
best example I can think of at present is the toyota mr2 turbo --which uses the twin entry ct28--boosts from 1500rpm --strongly and can give 2.0 bar below 3k--you can,t use that much boost anyway --,its just to show what a well matched manifold+turbo can achieve
If you study the manifolds of the cars your turbo came from you will probably find they are larger where they join your welded up flange but are much smaller in primary size--to keep gas speed up.
now common sense says if the turbo maker decided on that flange size ,then you do not need any larger sizes in your manifold ,anywhere
and if there were any great performance gains to be had with a fabricated manifold they would have used one --costs would be minimal for them when they are making thousands of them
If there are any small power losses due to the manifold --they just up the boost a bit .

now it is said that turbo,s sap less power than super chargers and I would agree to an extent --
with s/c we make the power then use some to drive the s/c unit ,but if the turbo installation is correct there should be less power loss than the s/c
studying lots of graphs and print outs for years from both s/c and turbo installs ,at boost levels the s/c can attain ,as its limted to around 1.2 bar for most installs ,
then the results do not back it up
the conclusion is simple
the expected gains from using a turbo do not manifest themselves because there are other losses --
the turbo has the advantage of being able to run higher boosts,so can hide the power losses very easily ,which show up by excess heat and fuel consumption .

It is well known I do not like turbo,s in small front wheel drive cars ,due to the non civilised way the power is deleivered with all that means to drivability and damage to transmission etc.
It is also why the std modern 200bhp+ turbo cars need all the tricky gadgets ,motorised throttle electronic boost ,ESP ,traction control etc to be acceptable
I am not against turbo,s at all and I am writing this ,as much to help with any new proects .not to slag off turbo power in general