[becauseican]

I am wondering what the ID of the tubes that are on your aftermarket 1jz manifolds?, or even what size the mk 4 manifolds are?. I am starting my twin GT28R turbo set up and I am not sure if I should go 1 1/2", 1 5/8" or 1 3/4" inside diameter. The exhaust ports are about 1 3/8", but I ported them out to 1 1/2" the size of the gasket.

I know that the smaller the runners the faster the exhaust will be moving to get the turbo's spooling, but I dont want it to be choked in the top end.

 

[Ckanderson]

Mine apear to be 1 1/2

 

[flubyux2]

i plan on using 1.75" cast steel weld els for my manifold... which have an ID of 1.61"... so 1 5/8" primaries may work out pretty well. possibly 1.75" ID for all out stuffs.

 

[becauseican]

I have some of those weldels in 316 stainless steel. They are to big, the outer diameter gets way to close to the bolt holes,also the inside diameter is way to big, the stock ports are like 1 1/4" I ported mine out and they are still under 1.5". The 1.61" bends are like 1/4" bigger than the gasket holes.

If the runners are to big you will lose exhaust velocity, which will cause slower spool up.

 

[flubyux2]

ahh, the only exhaust ports ive measured are the 7M and those were almost dead-on 1.5" diameter... so perhaps using a T.E.M. calculated for a 7M on a 1JZ would be too large.

if the stock 1j ports are about 1.25" like you mention, then i would just drop down a size and stick w/ the 1.5" ID runners for the same reasons you mentioned... not to mention, itd create an anti-reversion dam of sorts.

i didnt think 316 would be good enough for a turbo exhaust manifold... i know 316 is really good for aqueous corrosion resistance. i was going to use the low carbon steel and just have it ceramic coated inside and out to prevent any potential oxidation that tends to occur when metal heats up to extreme temps. it seems that the metal is more vulnerable to attack from oxygen and results in the porosity when its hot or near molten... like welding.