Which is best?

I can think of arguments for both but never read anything about it.

So 4 into 1


or 4 into 2 into 1

 

So nobody has any thoughts on this?

Bloody hell.

I'd have thought this would bring all sorts of opinions and contradictions out!?

 

With a forced induction engine you want the best flow you can get with minimal back pressure, but without sacrificing exhaust gas velocity.

So without any flow tests or dyno tests it's hard to say. Also the pics for the 421 don't show the merge collectors very well and neither give any idea to pipe diameter.

I would suspect that the 421 would out flow the 41 setup due to smoother merging of the primaries. As long as this hasn't compromised exhaust gas velocity they'd be the ones I'd go for. Depending on price too.

Pitfalls - The more they flow the less boost required to run the same HP. Or if you run the same boost then you'll see more HP. A win win situation.

Except, if the exhaust gas velocity is compromised, then you'll have less energy to power the turbocharger causing increased lag and possibly a higher boost threshold.

 

Well.
The 4-2-1's are cheaper. At just £140 delivered from the states.

THe 4-1 are £199 from a UK supplier.


I was thinking along the same lines as you.
But my thoughts were that the 4-1 would probably not flow so well as the 421 but would have an increased gas velocity..Possibly.
But would the increased flow be more significant than the loss of gas velocity?


The more I think about it, the more I confuse myself and come up with other arguments about it..

I've mailed them and asked for the pipe dimensions.

 

seems the majority tend to be 4-1.

i.e. GT Spec or similar

 

Hi Steve.

They do mostly seem to be 4-1 but that doesn't mean that thats the best way. It's just the easiest.

And I don't like to be a sheep and follow the heard blindly..

 

be careful of cheap copies though as you get what you pay for.

Mine are GT Spec, and suppliers like Zen, RCM etc all use similar design (probably from same factory)

 

true.

I am very tempted just to port mine.
Thats probably what I'll end up doing TBH as I've heard that once ported, standard are as good as most aftermarket ones.

 

ported ones are fine up to a certain level (about 400bhp i think) above that aftermarket ones would be required.

 

400's fine.

Thats the goal TBH anyway.

But I said that with 300 ..

Don't really want to get drawn into building an engine. I'd like to stick with the one i've got.

Will a VF35 go to 400?

 

VF35 will max out at about 350bhp.

 

bugger.
So all I'll get by swapping out the VF22 is earlier spool up.

****.

Back to the turbo choice drawing board.

 

the vf35 is great for spool up.

 

As I've stripped a stud on the VF22 it'll be the VF 35 going back in anyway so I'll just have to see how I feel about it.

Certainly earlier spool up will make it more driveable which may also equate to faster for the same power..

I'll have to suck it and see once I put her back together.

 

depends on what you want on the road, loved the quick spooling of the vf35, but wanted more power..........then even more lol

 

Another thing to consider is the manifold to up-pipe flange.
The 4-1 looks to have only a 2 bolt flange, these are aggro and you will be forever changing the gasket if not cracking the up-pipe itself.

Cannot see from the picture what the flange is on the 4-2-1 but it is well worth checking out 1st.

 

i wouldn't buy from somewhere that couldn't spell Impreza lol

 

the 4-1 on the gt spec have 3 bolt uppipe flanges

 

I'd be a bit worried about buying a set of headers with a flexi joint in them as per the 4-1 in your pic. My last set had this,and it blew apart in 4 months, ashave many others. The second set i had were Kakumei gt spec,with slip joints.These were amazing quality,and gave more power over the cheaper ones. And they only cost £295 with up pipe delivered from - Flat4online.co.uk Subaru Impreza Mitsubishi Evolution EVO Walbro Fuel Pumps Injectors Intercoolers