Exhausting

After removing most restrictions with the new heads, cam, intake system etc I have now turned my sights on the exhaust system.

I spend some time with a tape measure and clamp measuring the diameter of the various parts of the exhaust system.

Well its not too good. The cat back looks ok, 2.5". The headers are ok, ending up in try-y configuration with a 2.5" exit. The problem then is the 2" pipe into the cats with the O2 sensor in there too! Thats less than 3.14" sq area compared to the rest at 4.9" sq for the 2.5" section. It will even be less due to the O2 sensor in there. Now if I knew what I was doing with calculating exahust flow Id be able to verify that the exahust is a restriction. Though I have read another Vizard article which states dual 2.5" exahust will support up to 500bhp without restriction. So based on my hope to be at around that level at the flywheel the exhaust is going to be a restriction with its 2" bit, so out it goes.

My options are to replace the cat system with a bypass pipe for "off road use". This will allow me to replace it for "road use" and emissions testing. Hopefully I dont forget to replace it and just leave the offroad pipes on :)

As per usual good old Mr Vizard has an excellent article on exhaust selection and design here.

From what I understand the main summary is that exhaust tuning can produce up to 500% increase in cylinder scavanging over what the usual induction stroke of the 4 stroke cycle produces. Primary header length is not as important as collector length, and wise placement of open chamber or resonator boxes helps to terminate the effective collector length. A good rule of thumb is 2.2cfm of exhaust flow per engine bhp. So a 500bhp engine requires 1000cfm of exhaust. In addition a 1"sq pipe flows around 115cfm. So from this I can see my 2"sq pipe will only flow around 350cfm, or a total of 700cfm (2 pipes) a 50% increase in exahust flow is required to remove the current restriction! And worse because both 2" pipes have an O2 sensor blocking half of it off!

So anyway looking at options now, which include replacement headers as well. I think this is going to have to be the end of the purchasing mission. Finances have taken a hammering recently so hopefully there will be no mechanical restrictions, or at least only minor ones. (Like not having a Fast90/90 setup!) Still waiting for the thermostat and injectors. Im pretty sure the 30lb SVO's will be ok as Im not at 100% duty with the 28's yet. Unless the replacement exhaust makes this change a lot then I should be fine. RedHotSupra has recommended 42lb injectors as they will fit more nicely at the 80% level. In retrospect this is likely to be correct. However the 30lb ones should be ok, but will sit at a higher level.

Still aiming at 400rwhp