Exhaust theory.....

As some of you may know I changed my exhaust recently from a Longlife stainles one with 4 boxes, which went from anywhere between 50mm and 65mm in diameter, to an all singing all dancing new 2.5" 3 box stainless one from JP Exhausts. Now, it's an excellent looking piece of work, obviously higher quality than my old one, and I am 1000% certain it must flow better. Odd thing is, when I went to check out my system on the dyno, the power was down on my old one, as was the torque, pretty much across the board. Also while I can't feel that it is much if any worse on the road, it definately didn't feel any quicker/torquier.

Now the question is - given the system was mapped with my old exhaust (which was much more restrictive to flow) would changing to a larger bore more free flowing exhaust be a problem? The reason I'm asking, is that I was thinking about this.... The larger bore means the gasses will expand and cool quicker, therefore move slower, and so get less of the effect of pulling the gas out of the cylinder (scavenging from memory). Now my old system will have been set up to not produce a bang too hot, before the gas can be removed, and with the poorer flow of the old system that will be very different to the new setup. Presumably if there is now a big difference in how the system breathes then it will need a proper remap to get the best out of it. This presumably is where changing the timing will come in to play?

Ok ..
I think that a new exhuast will only be more free flowing to the head if the old one was a restriction in the first place.And jusy because the old one was narrower it doesn't mean it caused a restriction.
As you have intimated in your question,a norrower bore could infact help a particular engine in getting waste gasses from the cylinder head.
My non expert take on it.
Sometimes less is more.

As for if a remap could get more from an exhaust...
I'd like to hear from anyone who has had sucess with that.
If your willing to take a bullet for the team i'd be interested to know how you got on.

Oh it will be remapped at some point, but as per usual, the problem will be that other things will be changed at the same time, so it wont be a single variable experiment...

Change of exhaust becomes more noticeable when the boost is cranked up where it offers less flow restriction. It is more noticeable on a turbo over a charged car as it allows the turbo to spool more freely as opposed to a carger that is dictated to by the drive from the pulley.

Other things to keep in mind is ambient temperatures the warmer the weather gets the less power you make due to air density charge etc...

As for exhaust temperatures droping slowing air down this is not significant by the time you get to the exhaust side of things more significant at the manifold side of things where it has an immediate effect on the combustion cycle.

Lastly the gains from an exhaust on a charged motor will always come from the fact you can run more advance timing and increased fueling due to the improved scavenging of the system. As you have better scavenging you have more fresh air to burn so more fuel can be burned with greater advance timing as less hot combustion gases are oing to be going through the cycle again. Without mapping to suit you can potentially loose power from not enough fuel.

hth JP

Thanks JP, yes, I think that was where my mind was going with you comment about "more advance can be dialled in because of less heat build up".

I think it'll be time for another remap before too long - I now have the exhaust (on), 440cc injectors (not on), and an ISV reroute (not on yet, but going on to see what it does to the pressure at WOT). Just need the 65mm pulley now