Is a 1.75" exhaust too restrictive for a G40 turbo?

[Patttt]

Hi all, I recently purchased various exhaust manifold parts etc and with it came a full stainless powerflow exhaust. It reduces from the turbo downpipe 2.5" to the 1.75" exhaust. Is this going to be too restrictive for my g40 turbo build? I'm going with the k03 turbo route if that helps. Thanks

[cheys03]

I run a standard GT midbox and standard G40 back box which is about 47mm at it's narrowest point without problems at 1bar, 9.5:1 engine compression, K03. However I wouldn't like to go much narrower. 1.75" is approx 44mm.
I suppose it depends on the quality of the bends in the pipe, being aftermarket replica parts, some of the bends in the pipe on my setup do restrict slightly.

[Patttt]

Hmm so narrower than standard even! Put me off it slightly now due to the fact everyone seems to use 2.5" -3" 'generally'. It's a full power flow exhaust so I'd imagine it's strong enough. Just don't want it to be overly restrictive. What bhp are you running? Thanks

[prankstar2003]

Bigger isnt always better as the increased size can actually slow gasses moving through the pipe. However, too small and you have too much back pressure slowing the turbo.

[dub-disaster]

isnt less back pressure always bettter in the case of a turbo due to the fact the turbo itself is a restriction ? i thought this is why you can get away with over sized exhuasts on turbo cars such as imprezas and evos without them sounding completley rediculas

[prankstar2003]

But if you open up the exhaust bore too much, the exhaust gasses will be too slow exiting the exhaust. But yes, you are correct in saying that you can have a larger exhaust than on an NA engine.

Personally 1.75" would be too small for me, but it depends on how much power the engine is making and its size. If we estimate 150bhp from a standard 1.3, you should be looking towards a 2-2.5" exhaust for best results I would have said

[dub-disaster]

there is a post on here somwhere one exhuast size with turbos that goes along the line of a 2.5" will spool the turbo a couple of hundred rpm quicker than the 2.25" ive gone for 2.5" twin silenced on my turbo and i personaly think it sounds just fine nice burble at idle reasonably refined when driving

[Patttt]

Thanks for all the useful replies guys and personal experience etc, I've ditched the 1.75" exhaust (now on my mates loon) and I think I'm going to go for the phil J 2.5" system? As I don't want any restriction. And yeah I was hoping 2.5" wouldn't be stupidly loud, thanks

[Etches]

Not with a turbo it wont dude, similar problem ive got going turbo but got a stainless stardard system on !

[confusionhunter]

On a turbo, after the turbo, back pressure is bad and plays no aiding performance role.
On an NA different ball game. Anything after the turbo should be least restrictive as possible.

I run a 2.5 (61mm ish) system single large back box only and its quieter than most 2 inch Supergarded G40s with 2 boxes.

[Patttt]

Thats good then didn't want a stupidly loud system. Am i right in thinking this will reduce turbo chatter if there's less backpressure and it's more free flowing?