Exhaust Back Pressure

[Jackson Ellis]

After reading a post on here about losing the bottom end from a CAT removal, I wondered if my exhaust upgrade has caused that problem for me. I have the Gen II V8, and had a Flowmaster 40 series installed in January 2014. The resonator and stock exhaust were removed, and then single into dual exhaust was run to the back bumper. The CAT was left in place.



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I also got the SCT 3 from Torrie at the same time (so my tunes would account for my exhaust upgrades), and I run the 87 performance tune. Everything else is stock.



Here are my symptoms, which are from a non-expert feeling my way through the RPM. From take off, the Trac pulls slower than expected (think minivan) until about 3500 RPM, at which point power comes in a rush. During WOT takeoffs, the RPM stays in the power band, and the Trac runs like a scalded dog. During normal driving (which my wife calls too fast), the gear changes seem to happen right about that RPM, causing the needle to fall back down into the dreaded minivan range. So the Trac feels underpowered to me all the time. I want the bottom end torque from the beginning, like a truck should be. If any of you are bikers, it's the difference between an inline 4 and big V twin.



Is getting some more exhaust back pressure the answer? If so, how?



If not, how can I get the power at the bottom that I want? I'm really not interested in dropping gears, or underpowered pulleys, etc. I'll consider intake if I need to, but would like to not spend a lot of money, and I figure I might have to reverse to stock when I eventually sell it.



Thanks in advance.

 

[Yardsale]

So, the exhaust was replaced from the cat back (a "cat back" system)? Are the diameters of the OEM and new exhaust pipes the same? If so, any backpressure change is due to the muffler (including the one-in and two-out design), and that is where you'll need to address your desires. If not, then maybe there is an exhaust pulse velocity issue.



With regard to regearing, I would do that if the current tire diameter is larger or smaller than the OEM by greater than 5%. Some of the acceleration and shifting can be compensated for with a tune, but the mechanical leverage demands of larger diameter tires cannot be - U-joints, bands, bearings have added stress.

 

[EddieS'04]

Did you have Torrie to take the ramp in, to the TB out. Ford's electronic TB has accelerator ramping from the ECU. It can have various effects at different speeds. As you know the ECU remembers driving habits. I would think, unless the ramp-in is removed. When you need the grunt you may not get it...

 

[Jackson Ellis]

Thanks for the replies. I've done some more reading, and my overall assessment of back pressure now is that it's probably OK because I still have the CAT, the Flowmaster design doesn't kill back pressure, and I didn't increase the diameter of my pipes.



Will be doing some more reading on the throttle ramping. I see a lot of posts on Ford 4.6L throttle lag - I mean A LOT of posts from the 1990s till 2014! Was scrolling through dozens when I saw Ford 4.6L throttle lag fixed!!! It was from 2008 lol!

 

[Tom Schindler]

Throttle ramp is from the TBW system. It is used to improve gas mileage by limiting jack rabbit starts.





Tom

 

[Lloyd Chaudoin]

I guarantee your loss is not in the exhaust. It is in the tune. I have my exhaust ( V8 ) opened up at the cat outlets to 2 1/2" (stock is 2 1/4"). Into a single 3" tube and a Magnaflow 3" muffler and 3" out the back. The area from the stock exhaust is up by 24% out of the cats and 20% in the single section and no resonator either. I not only picked up a bit of throttle response but gained about 1/2 MPG better out on the freeway average... Not to mention a much cooler floor board on extended drives or towing. I found the best change in my mileage and pulling power came from my electric fan conversion. My tune which allows me a bit of adjustment in timing control came from Brenspeed and works great. Since owning the truck I have done several mods, all done by myself. The most beneficial were the tune, cams, electric fan, Zabtek throttlebody and Steeda underdrive pulleys. I also have a K/N FIPK installed. Not much of a benefit compared to the others though... Even with the cams I have picked up an average of 2 mpg out on the highway, compared to stock on a trip I take several times a year, even using our crappy California fuel.