We have all seen mods on this forum for free flowing air filters, induction kits, hybrid turbo's, larger bore exhaust systems, decat kits and sports cats, all to improve air intake and exhaust flow. Having seen the exhaust and inlet manifolds off a car they do look puny and resistive to gas flow.

The questions are,

1. Do our mods listed above have as much impact on air & gas flow as much as we thought?

2. Are there any manifolds available to improve flow?

There must be someone who improved their manifolds, anyone?

Jason's had some manifold work done IIRC, or talk to Jabba

Steve

dont think its as inportant on a diesel as on a petrol.

Not as important, as to some degree the speed that diesel will burn is a limiting factor on how fast you need gas to flow, but at the end of the day the easier it is for gas to flow in and out the less power you lose. It's probably just that there's bigger gains to be had by upping boost or fuelling with the ECU, which is easier to do. I'd imagine that once you reach the limits of other things, a bit of manifold work becomes worth it, but at a guess a remap, FMIC, performance exhaust and bigger injectors would give you a better gain first. I'm sure Jason can reveal all.

Exhaust manifold is part of the turbo anyhow.

Exhaust manifold is part of the turbo anyhow.

Since when do you get a new larger flow exhaust manifold with a bigger turbo?

I have a ported out inlet manifold to match the ported head. Not sure what immediate effect that has had, but in theory should allow more air to come in than stock. Likewise my exhaust manifold is "custom" as part of my VT5 hybrid but I'm not sure if the custom refers to it flowing a lot more or simply custom to fit the big turbo on it?

Since when do you get a new larger flow exhaust manifold with a bigger turbo?

The hybrid turbos based on the VNT unit come complete with a manifold so changing it isnt an option was all I was pointing out.

The hybrid turbos based on the VNT unit come complete with a manifold so changing it isnt an option was all I was pointing out.

So do all the Turbo/Manifolds come ported/gas flowed as Jason says his has been done.

He has a seperate manifold and turbo as his is more than a hybrid.

Standard manifold is fine and has been ran up to 280bhp, we tried a better design with shorter runners and actually lost a few bhp so that idea was ditched.

And as Ross has already said, on the standard hybrid units the exhaust manifold is a part of the turbo anyway so cant really be changed for a "higher flow" item... You'll find the restriction is the VNT & exhaust turbine anyway as the general consensus is a small exhaust turbine with a larger compressor.

Seperate exhaust manifold and turbo, eg. GT25v etc, however is a different ballpark.

Kev

Thanks fellows, now that makes sense.

I'm old school petrol/normal aspirated in tuning knowledge, not much TDI experience.

From what you have said I deduce that too much free flow from the combustion chamber to the turbo impeller reduces the turbo's efficiency.

You're starting to get along the lines of compressor maps and VE, flow rates etc now

In short, i shouldn't worry about it, however a flowed head is a sound investment

Ok Kev. I'm not worried, just interested in the diesel technology.