hippymill said:
if you have a downstream o2 sensor your idea won't work, besides backpressure is your friend with the 4.0. get a magnaflow cat (about $100) and a good muffler. also, if you have emissions tests and they have an exhaust sniffer, you won't pass.
Correct on the downstream HEGO, but backpressure is
NEVER your friend! Backpressure creates pumping losses (read: parasitic drag) and inhibits overall airflow - move less air, make less power.
If you want to "open up" your cat, do what I did - get a "universal" catalytic converter for BBChevvy, cut the bolt flange off of the old one and weld a pipe stub to it (makes it SO much easier to work on the exhaust!) and weld a bung into the pipe for afterwards if you have a downstream HEGO. You'll get improved flow through the unit (since it's designed to flow MUCH more air!) you'll have the unit last rather longer (same reason - it will take longer to coat up and clog up,) and your emissions will improve overall anyhow (I've got reports that say so - I just need to dig the up.)
I say again - backpressure is
never your friend, and I with people would stop passing that myth along. A well-tuned exhaust is going to contribute beneficially to exhaust gas scavenging to reduce pumping losses, but backpressure will actually cause reversion of exhaust gasses (meaning less air and fuel going
in,) and will cause you to make less power. That's why race cars run open headers, and why you can get "header mufflers" - that are just glasspacks that bolt onto header collectors. If backpressure is so useful, why aren't they into the idea? You'd think they'd have hit on something... Granted, the tuning is different, but the basic theories are the same.
BTW - the only reason that racers don't go all the way and run open heads is simple - it's a great way to warp valves. This likewise has nothing at all to do with backpressure - you just need some pipe (about two feet usually does it) to keep cold air off of the backside of that 1000* metal!
"Exhaust Tuning" is simple - you just need to figure an optimax ID for your pipe that will allow maximum airflow with minimum thermal turbulence - the exhaust pulses should decline in temperature at a constant and predictable rate, and should do so wholly, not from the outside in. Go too small, and it won't flow enough. Go too large, and you'll actually LOSE flow due to uneven cooling of the exhaust gas pulses. This has nothing to do with header primary length - but the two are usually settled at the same time, for simplicity (header length has to do with pressure wave timing - using the low-pressure area behind one pressure wave to help "pull" out the next. This sort of tuning takes place over a narrower range than optimax pipe ID tuning, tho...)
5-90
