Backpressure and Removing Cats
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Senior Member
Joined: Mar 2009
Posts: 290
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From: Northwest Ohio
Backpressure and Removing Cats
I recently installed an ISSPRO backpressure gauge (tapped into the left manifold) for grins and have been noting my backpressure. My truck is a '05 F-150HD 4x4 (I call the payload package trucks "F-150HD"... bear with my affectation, huh). The truck just turned 20K miles, has all the factory cats but a Dynomax 3-inch catback, running a Gryphon tuner, generally on Level 2, and it has an AEM CAI. Driving around normally, the needle doesn't move off the "0" peg. When I give it the boot, it will reach a max of about 3.5 psi at about 4800, which is my set upper rpm limit. Up to about 3K, it stays at or near 0 psi (less than 1). From there, it starts to rise. Most likely, the majority of the restriction comes from the cats.Read my earlier post to see the performance specs for the truck.
Thing is... 3.5psi at 4800 is nuthin' and the difference in performance between 3.5psi and zero, or near-zero psi backpressure (which you may or may not attain by removing the cats), is also next to nuthin'. My point is to say that removing the cats is a waste unless you are replacing them with long-tube headers (exception would be for supercharged engines wich need a lot more flow in the exhaust).
If anyone has done before and after backpressure tests on our trucks with and without cats (rest of the exhaust system remaining the same), I'd like to hear about them, but I have seen the modern honeycomb-style cats flowbenched and they generate very little flow resistance... less than a muffler usually. The one (a stock GM unit) I saw tested flowed about 20 percent less than a piece of straight pipe the same diameter. That was a few years back.
Unfortunately, I didn't get the gauge when the truck was all stock, but since I have all the stock stuff, in theory, I could go back to see what stock looks like. Come warmer weather, maybe I will.
Thing is... 3.5psi at 4800 is nuthin' and the difference in performance between 3.5psi and zero, or near-zero psi backpressure (which you may or may not attain by removing the cats), is also next to nuthin'. My point is to say that removing the cats is a waste unless you are replacing them with long-tube headers (exception would be for supercharged engines wich need a lot more flow in the exhaust).
If anyone has done before and after backpressure tests on our trucks with and without cats (rest of the exhaust system remaining the same), I'd like to hear about them, but I have seen the modern honeycomb-style cats flowbenched and they generate very little flow resistance... less than a muffler usually. The one (a stock GM unit) I saw tested flowed about 20 percent less than a piece of straight pipe the same diameter. That was a few years back.
Unfortunately, I didn't get the gauge when the truck was all stock, but since I have all the stock stuff, in theory, I could go back to see what stock looks like. Come warmer weather, maybe I will.
Technical Article Contributor
Joined: Apr 2009
Posts: 1,080
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From: Paintsville Ky.
I recently installed an ISSPRO backpressure gauge (tapped into the left manifold) for grins and have been noting my backpressure. My truck is a '05 F-150HD 4x4 (I call the payload package trucks "F-150HD"... bear with my affectation, huh). The truck just turned 20K miles, has all the factory cats but a Dynomax 3-inch catback, running a Gryphon tuner, generally on Level 2, and it has an AEM CAI. Driving around normally, the needle doesn't move off the "0" peg. When I give it the boot, it will reach a max of about 3.5 psi at about 4800, which is my set upper rpm limit. Up to about 3K, it stays at or near 0 psi (less than 1). From there, it starts to rise. Most likely, the majority of the restriction comes from the cats.Read my earlier post to see the performance specs for the truck.
Thing is... 3.5psi at 4800 is nuthin' and the difference in performance between 3.5psi and zero, or near-zero psi backpressure (which you may or may not attain by removing the cats), is also next to nuthin'. My point is to say that removing the cats is a waste unless you are replacing them with long-tube headers (exception would be for supercharged engines wich need a lot more flow in the exhaust).
If anyone has done before and after backpressure tests on our trucks with and without cats (rest of the exhaust system remaining the same), I'd like to hear about them, but I have seen the modern honeycomb-style cats flowbenched and they generate very little flow resistance... less than a muffler usually. The one (a stock GM unit) I saw tested flowed about 20 percent less than a piece of straight pipe the same diameter. That was a few years back.
Unfortunately, I didn't get the gauge when the truck was all stock, but since I have all the stock stuff, in theory, I could go back to see what stock looks like. Come warmer weather, maybe I will.
Thing is... 3.5psi at 4800 is nuthin' and the difference in performance between 3.5psi and zero, or near-zero psi backpressure (which you may or may not attain by removing the cats), is also next to nuthin'. My point is to say that removing the cats is a waste unless you are replacing them with long-tube headers (exception would be for supercharged engines wich need a lot more flow in the exhaust).
If anyone has done before and after backpressure tests on our trucks with and without cats (rest of the exhaust system remaining the same), I'd like to hear about them, but I have seen the modern honeycomb-style cats flowbenched and they generate very little flow resistance... less than a muffler usually. The one (a stock GM unit) I saw tested flowed about 20 percent less than a piece of straight pipe the same diameter. That was a few years back.
Unfortunately, I didn't get the gauge when the truck was all stock, but since I have all the stock stuff, in theory, I could go back to see what stock looks like. Come warmer weather, maybe I will.
I my self had thought about removing the cats, but as I am not running F/I or lots of power, the only thing i would net, would be a sound change, if that.
Technical Article Contributor
Joined: Oct 2005
Posts: 25,641
Likes: 19
From: MI
Yea, it gets louder of course, but a HUGE depreciation in low end performance. Embarrassingly. Not enjoyable to drive w/a factory tune w/o converters. Been there done that, learned the hard way.
Last edited by jbrew; Dec 28, 2009 at 02:00 PM.
Senior Member
Joined: Dec 2008
Posts: 761
Likes: 1
From: Carson city Nevada
With stock compression there is no reason to gut the cats what so ever. Blowers and turbo which is also referred to as a blower in Webster’s = much higher compression ratio when in positive pressure.
Technical Article Contributor
Joined: Oct 2005
Posts: 25,641
Likes: 19
From: MI
I believe Mike Troyer markets a good system for basically stock trucks, but like with any mod over stock, I beleive you have to dial in it's potential,- to get any advantage to be had.
Good Posts!
Senior Member
Joined: Aug 2004
Posts: 2,824
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From: North Huntingdon,Pa.
tested my 03 5.4 with an in cab mounted magnahelic gauge mounted in 2nd o2 sensor bung, up a hill in 2nd gear to redline, with oem cats i was 1.5-1.75 in each bank at 5400 rpms-factory cats running properly are not restrictive to flow-sound yes,