Milltek vs stock

[P_G]

It is a cross link between the two exhausts that allows scavenging of exhaust gases and often will increase the pop you get on throttle lift off. Most manufacturers bar OEM have something like this but it depends on the diameter. Milltek is tradionally small (a .75 inch section IIRC wheras JHM or others wil use the same section pipe as the rest of the system 2.52 or 2.75"

On x it is the shape of an x on H you have the two straight pipes joined by a straight cross section. Tubi are straight through pipes IIRC as are Capristo and others. Does theoretically reduce loss of backflow pressure created by using bigger pipes which increase tone.

[RIV]

Interesting, thanks for the info

[Shadow6ix]

It's not thought.

There is an overlap in valve timing so that the exhaust gas exiting the cylinder helps draw in fresh air charge.
If the exhaust is 'free-er flowing' the more of the exhaust gas has already left the cylinder before the intake valve opens and so the scavenging effect is lower reducing the amount of air the cylinder draws in.
The ecu can't really adapt to this as it only sees the maf which is way up the intake system.

Well that's different again to what Dave was saying, ie less back pressure - more scavenging - more air drawn in, which equated to a weaker mixture. You think less scavenging, less air, richer mixture?

Either way, any air coming into the system must have been measured by the MAF, providing the appropriate amount of fuel, added to which is a reading from the lambda sensor which should also be able to detect mixture at the other end?

Still struggling to understand why less back pressure could result in less power

[RIV]

This is getting more confusing than the wife

[adsgreen]

Its all a matter of timing.
The problem is the whilst the ecu can see raw airflow from the maf it is basing many calculations on assumptions.
One of these would be exhaust back pressure exit speed an the timing. It would have a table that would say for example based on maf x, thottle y and air temp z we could say that this is the correct setting.
Back pressure isn't something it can measure so it would throw out the assumed values on the look up table.

[adsgreen]

With Less back pressure then there is the capability for more scavenging however the valve timing for the required overlap would be completely different. You want to time the valve overlap so the pulse wave of pressure exiting the cylinder is timed perfectly with the intake port. Too soon an you end up with exhaust gases going into the intake, to late and the pulse is gone and halfway to the back box.

Also consider if there was no exhaust manifold and open the air - virtually zero back pressure and no scavenging. There is definitely a 'sweet spot'

[RIV]

Sounds like you know your stuff regarding the rs, do you have a mechanical background?

[PetrolDave]

Well that's different again to what Dave was saying

It's what I was trying to say but in more detail.

[Shadow6ix]

My applogies then, I understand the concept of scavenging, but as far as I could see Dave, you were suggesting more air into the cylinder (leaner), and Ads was saying less (richer?), as a result of reduced back pressure, in these specific circumstances (ie slightly reduced as a result of Milltek).

[PetrolDave]

My applogies then, I understand the concept of scavenging, but as far as I could see Dave, you were suggesting more air into the cylinder (leaner), and Ads was saying less (richer?), as a result of reduced back pressure, in these specific circumstances (ie slightly reduced as a result of Milltek).

I was saying trying to say less burnt gasses got scavenged out of the cylinder and hence less fresh air was sucked into the cylinder, and less fresh air = less fuel (as the MAF senses a smaller mass airflow).