Turbo manifold runner size
 

What would you consider an ideal diameter size for the manifold runners?

We use 1.25".

^^^ I'm assuming thats nominal size. In practice thats a pipe with the ID of 1.442" for anyone curious.

The 2 manifold I have build so far have have 1.5" tubing.

I'd imagine you would want to reduce restriction without reducing velocity of the gasses or flow.

There has to be a formula to calculate ideal I.d. For a given length runner on our cars...

http://www.pipeflowcalculations.com/...w-in-pipes.php

This will be fun

Sub'd

It's actually 1.35" with the thickness we use. :)

The smaller the diameter the higher the flow but less volume. The trick is the balance between the two. You need to have a length in mind. So for example for a UEL header the shortest length should be the largest diameter and the longest length should be the narrowest by theory. In practical it is the average length that you should use to calculate the diameter to get a good overall idea of where to start. Lots of headers for turbos are generally more narrow then NA headers.

This is exactly what i'm referring to. The issue is which diameter will be ideal for spooling a specific turbo

depends on purpose... 1.5 is great for regular folk who wont tell the difference.

Its not about the turbo in any way other then the entrance surface area of the turbo. You need to calculate the length to the turbo.

Rough idea is that you match the header outlet surface area which in our case is circular so its easy to use that diameter. If you want to increase the velocity of your exhaust to potentially quicken your turbo spool speed without changing it you simply step the header down to a smaller diameter for a short distance before the turbo. This will however increase back pressure and you may not get the results you are looking for if you are not careful and go to small.

Cannot find the thread that someone tested a few headers on his WRX. He originally bought a 2 inch runner diameter header and hated the spool time. I think it was around 4500 rpm. He bought new a new header with a 1.36 inch diameter and his spool time was reduced by almost 2000 rpm.

What turbo are you considering?

now i dont have numbers but...my fullblown base manifold with a gtx28 makes more torque substantially sooner than a few of my friends cars with similar kits and setups. It would be interesting to see the size compared to jdl, as a friend with their kit produced the closest results to mine vs some others. (albeit other factors nonwithstanding) I would ask lj what their size is, as it seems pretty ideal by comparision.
Just a start.

What size are the exhaust ports and what size is the stock runners?

ports are 1.5" not sure about stock runners