3.9 exhaust size

jeff · Jun 17, 2012

Hi all ,

I need to make a new exhaust for my hybrid , it has a 3.9 efi,

it has hedman tubilare headers , what size is best 3" or 2.5"

can do it in either , just not sure if 3" is too big?

thanks

dirtydiscov8 · Jun 18, 2012

I've got headman headers on a 3.9 and mine must be about 2.5 inch. 3" is for scoobaru impretza's isn't is?

jeff · Jun 18, 2012

dirtydiscov8 said:
I've got headman headers on a 3.9 and mine must be about 2.5 inch. 3" is for scoobaru impretza's isn't is?

yeah have 3" on my impreza (2.5" is fine upto 400bhp)

so i take it 2.5" will be fine for 3.9 v8

POAH · Jun 18, 2012

2.5 is more than enough for the 3.9

jeff · Jun 18, 2012

so no advantage for going 3" ?

, just i'm biulfding the exhasut myslef and its no more work to biuld it in 3"

discool · Jun 19, 2012

Mass of air that the engine breathes in + mass of fuel = mass of exhaust gases.
So unless you are increasing the fuel/air intake then the standard 2-1/2 inch size is ok.

Diesel Do · Jun 19, 2012

Go too big and you may lose too much back pressure and make it run badly. I'd go 2.5 to be safe

POAH · Jun 19, 2012

jeff. said:
so no advantage for going 3" ?

, just i'm biulfding the exhasut myslef and its no more work to biuld it in 3"

none what so ever

3.9 exhaust size

jeff

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dirtydiscov8

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