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4.0L OHV & SOHC V6 Tech General discussion of 4.0L OHV and SOHC V6 Ford Ranger engines.

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Old 06-28-2014
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Cat removal and custom exhaust

Hi all,
I just bought a 98 ranger to take off-road . This vehicle will be use almost exclusively off road. I am adding on to it as I get money. My buddy owns a tuning shop and dyno. I have had him put a performance tune on. Also, I added a high flow intake (kn) . Now me and him are fabricating a custom exhaust. To get the maximum flow, he said I should remove the cats and he can tune the motor to run without them. We designed the exhaust to have enough back pressure so the motor will run right. What are you guy's thoughts on deleting the cats. Where I live I don't have emissions testing
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Old 06-28-2014
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??? back pressure..................

You don't want any back pressure, that is an old myth about exhaust systems.

Multi-cylinder exhaust can actually generate LOW pressure at the exhaust valves if done right, and in a low, mid or high RPM band, but only one, lol.

What you want are smaller pipes at the valves, this creates a "velocity" in that pipe when exhaust comes out, when that hits the Collector, larger pipe, its velocity and sudden expansion creates a lower pressure in the other 2 exhaust pipes on that bank, assume V6.
Like a siphon effect.

The correct pipe sizes are above my pay grade, lol, but it is calculated with cylinder volume I believe.

The myth of "back pressure" came from people putting on larger "header" pipes and losing performance, so the incorrect conclusion they jumped to was that the larger pipes have better flow so if performance went down engine must need "back pressure", oops, WRONG, lol.

What they did was to lower the "velocity" for that cylinder's displacement size, so they actually INCREASED the pressure at the valves with the larger pipe size.

Removing cat is a $2,500 fine, this is Federal not state, I believe full time off road vehicles that are trailered to sites, can get away with it but just guessing.

In any case cats and mufflers do not effect performance(assuming good condition), just the sound, lol.
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Old 06-28-2014
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Thank you for all of the info. You learn something new every day. I looked into the laws and you were correct that the vehicle has to be trailered, which it will be. Thank you
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