Contour Enthusiasts Group Archives Forums Modifications Duratec Performance ? about back pressure how little is to little?

Backpressure is not a myth, maybe a poor term to describe what is going on, but not a myth.

In an exaggerated case, Open headers allow tremendous flow out of an exhaust valve at high engine rpm, where scavenging really isn't a necessity. On the other hand open headers don't allow the the airflow velocity necessary for maximum power at low-mid range rpm where scavenging is necessary for maximum torque which directly relates to hp.

Even the best exhaust system is a compromise, and is tuned for optimum performance at a given rpm on a given engine.
Obviously, airflow obstructions like catalytic converters and restrictive resonators are not the best for maximum flow, or keeping up the exhaust gas velocity, but when the air goes from a small pipe to a large open chamber (like a gutted cat), then back down to a smaller pipe, velocity is really slowed down, though maximum flow may increase. Now the question is, do you gain more from the increased flow than you lose from the decreased velocity? sometimes yes, sometimes no, sometimes its a wash, it is dependant on too many other things to give a definative answer.

I guess the bottom line is, if you want maximum peak power at high rpm, go as big as you want on your exhaust, but if you don't want to kill you low end, keep it a bit conservative (of course as smooth as possible too) and maintain your exhaust gas velocity.
Personally, I'll stick w/ a bit of a conservative approach, simply because my street driven SVT doesn't spend too much of its life at redline

I think its the main cat thats giving me the problem. the gutted precats show good results for everyone else on here and terry haines said he gutted precats and kept the main cats for a 10% increase. I'm simply losing to much flow as the turbulence builds in the main cat. or I've concluded

Merlin I have an obd1 car so I don't need the elimanators and I've seen your GREAT dyno results thats what made me get the offroad y-pipe now if I can just get it to fit.

Now I got another question can A spacer of some sort be put between the throttle body and the manifold for an increase in intake velocity.

If you have access to a plasma cutter (or some other cutting device) try this.

Cut open your gutted main cat, place a pipe straight thru it, then weld that to the flanges and bolt it back up. You retain stock look, lose any turbulance and have an off road pipe that looks like a cat. IIRC somebody had their Contour smog checked Catless in CA an still passed the sniffer test. Normally I don't agree with removal of polution control systems (indeed I would love to see race cars of all sorts forced to use some sort of polution control so they can get some really good technology for street cars....) But seeing that the Contour is a very clean running engine....