Originally Posted by kitw
That wasn't my point, I was actually curious as to whether or not the stock rear section becomes a bottleneck as power goes up.
I don't doubt that primary decats add power. What I am curious is that for cars with the turner test pipes, that retain the stock secondary cats but remove the primary cats, do you think any adjustment would be necessary, vs a fully catless tune? (I'm asking selfishly because I run the Turner test pipes with stock secondary cats too)
I've found that nothing behind the primary cats makes a lot of power, if the primary cats are in place. What I'm curious is that if the primary cats are removed, do the secondary cats and/or the stock rear section become a bottleneck?
Is a good question which has been investigated by many I am sure.
The rear exhaust does not really become a bottleneck in our tests which are not for the purposes of marketing material.
If we concentrate on gains and forget the final figures it all falls into place nicely. If we try and compare different cars on different days the results are not so consistent.
What we find is that we get roughly the same gain with an Akra/Evolve/Supersprint/ AA X Pipe with both stock rear silencer and any rear exhaust from the above mentioned companies.
We have fitted rear exhausts to cars on their own and the results are very inconsistent. Sometimes there is a gain, other times not. More recently where we have really become strict and data log every test we seem to gain very little power at all with a rear exhaust. That's when we compare runs where the ECU is being allowed to hit ignition targets before and after the rear exhaust is fitted.
Typically most tuning companies will take the lowest and highest power figures and then compare the results for marketing! Typically run 1 or 2 is compared to run 7-20!! The ECU would never hit it's ignition targets or the temperatures would be high enough such that a negative ignition offset is applied on the first few runs.
As for just removing the front CAT's - it should be incredibly close to an X Pipe because when we have seen secondary CAT's removed there is hardly any gain at all.
The difference between a tune for Turner Pipes and X Pipe is not really going to be that different. Not different enough that you will feel a difference and given the ECU is mostly target the changes made would need to be very large to actually take effect.