GU 4.2 turbo airbox modification donaldson cobra head conversion

[_Laurie]

Hi all,
Just thought I'd add a small right up on how I did my airbox upgrade on my factory turbo 4.2 GU

I apologise for the lack of measurements etc as I was pressed for time

Pic 1 - The original airbox

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Pic 2 - I'm committed, basically I've just used a concrete cutting disc in a 5" grinder. Safety note, dont use gloves on rotating equipment and do not remove the guarding

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Pic 3a & 3b - Here is the custom through fitting I'm using Its just a piece of stainless 3" pipe tigged into some stainless plate. The other blank plate is the keeper plate for the top of the lid. The studs are tapped and threaded in then tig welded in place. The whole face is then sanded on a flapper wheel (bench grinder) The other pic is of the donaldson cobrahead with the reducer in the end of it to mate it up to the existing nissan intake

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Pic 4 - The installed through fitting. You have to grind out the inside of the airbox as there is some protruding plastic pipe. The top of the lid also has to be ground flush to accommodate the keeper plate. Once all the prep work is done, I used sika to seal all of it up, make sure you have fresh rags on hand to clean up the spillage as you tighten the nuts down.

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Pic 5 - The inside of the airbox with fitting installed. The diameter of these are so that it fits snugly inside the airfilter so it will still seal against the actual lid

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Pic 6 - The completed fit

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Pic 7 - The completed install in-situ

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[mudski]

Very neat job there mate....

[_Laurie]

Cheers, I looked at a bunch of ways others had done it but I didnt really like those so this is what I came up with in the end.
I'm fairly pleased with how it came out.
I might also mention I used 6mm stainless bolts for the flange and also trimmed the donaldson head down 5-8mm so it sat a little more snuggly on the filter and a bit better bonnet clearance.

[my third 256]

did you change the lid seal it looks as if it has a few cracks in it
you can get them on ebay for approx $20

[_Laurie]

Hi my third 256,
Yeah it's something I did notice. I did a temporary replacement with some spare rubber gasket img I had, it's not perfect and will be fine but I'll still order a new one because If I'm going to do it, it may as well be right hey

[Cuppa]

Ok, I’ve been waiting for someone else to ask, but no one has.

What is the purpose of this ‘cobra head’ thingo? Is it just a means of improving airflow by virtue of a bigger hole in the top of the airbox or does the cobra head itself actually do something?

[_Laurie]

Essentially it takes out the biggest restriction point of your intake pre-turbo.
Honestly 99.9% of people would not notice one iota of difference. I've noticed the faintest of signs of a slightly better pre-boost response and the transition to on song. Sounds good but like I said it's barely ever perceptible.

If you look at your intake it's mostly 3" or bigger except for the top of the air box where you're lucky to have maybe 1 1/2".
The aim of my mods are based around reliability and fuel usage and I notice with my truck it's still a bit rich pre-boost so with lesser restriction through the intake it should help very marginally to get a better a/f ratio at the very low rpm before the turbo takes care of it. It is a little trial and error for me as I'm still learning so we'll see what happens. I just wish I dynoed the car before I started with the modifications to track the differences.

[_Laurie]

Sorry I should add to better answer your question that the actual hole in the top of the airbox stays the same (3") but the cobra head design means it keeps the relative 3" volume capability through the 90 degree bend and doing so in a compact way otherwise it would never clear the bonnet