Ive decided to try to build my own log manifold for my car just to see if i can do it and because it will be much cheaper than a custom tubular one, and ill upgrade it later on.

My question is what diameter steampipe should i use?
It comes in 32mm or 40mm sizes and because of the oval shape of the exhaust ports im not sure which size to use.

I figured out the circumference of the port is nearly exactly the same as a 40mm diameter circle so im thinking i should use that size but just wanted to make sure before i buy some bits.

Also its going to coost me $36 for 2 90* bends and 2 tee peices. this seems good to me but i have no idea about prices for this stuff?

Thanks
Luke