Higher flow exhaust manifold will mean you can have more gas pass through at a lower velocity for the same pressure, what this translates to is more power (to a point) but slower spool, you are seeing this in the lower boost spike. with a PC16 file I dont know if you are benefiting from it or not, Max knows his shit though so if he thinks you are hes probably right.
There is a lot of miss information around on back pressure, when they are actually talking about scavenging. you do not need back pressure, in an ideal world then the manifold would be a vacuum and just suck all of the exhaust out of the cylinders, but its not.
First think of the humming into a pipe, the sound travels through the pipe in 'compressions' and 'extensions' of the air, kind of like this: |||| | | | | |||| close is 'compressed' air, far apart is 'extended' air. The same thing happens in an exhaust system, when a cylinder pushes out the exhaust then it creates a compression of exhaust gasses, when there is an intake stroke then there is an extension. Now from thermodynamics (well really from everyday experiences such as feeling the wind or using a hose) we know that pressure will always try to become even, eg wind flows from high pressure to low pressure to try and make everything the same pressure. So when designers make a manifold then they try to use this to create a slight vacuum in the cylinder, this 1) sucks the exhaust gasses out of the cylinder so that there is less work done by the piston to push them out (this uses power that now isnt going to the wheels) and 2) creates a small low pressure system in the cylinder compered to the intake so it sucks air/fuel into the cylinder (not important on a turbo/super engine as there is already a much lower pressure in the cylinder due to the intake being pressurized ie boost).
what happens when a manifold has too smaller runners for the amount of exhaust gasses that have to pass through it is it increases the pressure in the exhaust manifold and consequently in the cylinders so the pistons have to do more work to push the exhaust gasses out of the cylinder meaning less power to the wheels. The up side to this is that now the exhaust gasses have to go faster to get the same volume out, think of putting you finger on the end of a hose, same amount of water comes out but its higher pressure so the turbo spools faster.
to the opposite extreme if we use a manifold with far to bigger runners then we end up with the exhaust gas expands and cools down, this results in it not traveling as fast through the exhaust system, so slower spool but also, when the gas slows then it has to be forced down the pipes again as it is flowing slower, this time think if putting a hose into a bucket and expecting to get pressure out of the top of the bucket. (a side not is this is also why you should change to a test pipe not just bash out your cat) the cooler gas is now more dense and less energy it has so it will flow slower. somewhere in between is the perfect balance.
for an example of the first look for the threads on here such as my HTA2868 wont get over 220whp and my GT2871 wont get over 235whp (although there is argument that some of that may be due to being a restrictive FMIC while this is probably correct (cant get exhaust out if you cant get air in) then some of it is also going to be because you cant get air in if you cant get exhaust out, they have too smaller exhaust systems, and that is all those systems will slow with that engine turbo combination.) for the second then have a look at race cars, they never exit straight off the block they always have at lest pipes to get the exhaust out away from the cylinders into the atmosphere, sometimes these are just straight on drag cars such as rails but are usually headers.
The morel of the story is that this whole back pressure thing is a load of crap and that what you actually need is a system that will keep you exhaust gasses flowing with the pistons having to do the lest amount of work possible to push the exhaust gasses out of the combustion chambers. on the k04 then i doubt you will gain a noticeable amount from either stock or ATP high flow.
I am by no means an expert on exhaust design, this is just what I understand to be true from the research I have done, and am happy argue the point and to be proved wrong. After all doing this just gives everyone a deeper understanding of what is going on.
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Originally Posted by AforOuttie
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