I posted this in the exhaust section too, but it has to do with the chip also.

I want to install duals (probably straight pipes) behind the cats. I've done this on a 5.0 Mustang, and it sounds terrific! Besides, a V-8 just isn't right without duals. I went to an exhaust shop to have them made, and was told that it couldn't be done, because the 02 sensors had to have backpressure to work correctly, and the check engine light would illuminate, causing problems that would require reprogramming to solve. Sounds hoaky to me. Any ideas?

I'm driving a stock 2001 5.4 4X4 SC, just bought it with 50000 miles

 

Hi Fordnatic,

That's one of the worst exhaust configurations you can do to your truck's torque production especially in the below 3000 rpm range - throwing straight pipes from the cats back will be louder than stock, and it may look "cool" - but it will also do a nice job of reducing torque below 3000 rpm (especially on part-throttle, where most driving time is spent) & causing fuel mileage to drop due to the loss of exhaust gas velocity.

Yes, you'll gain horsepower at higher rpm & at heavy throttle with straight pipes, but the part-throttle torque below about 3000 rpm will be less - not a good move in a heavy truck.

It will require custom fabrication to do a true-dual exhaust properly so that the OBD-II system still functions & you don't get any check engine lights - they weren't lying to you about that.

Now if you know what you're doing then this *can* be done - and if you can spend the money, you can even get a true dual setup that doesn't cause a loss of lower rpm torque - I run a true-dual system on my truck, as do some of our faster F-150 owning customers. But it's a very expensive setup (upwards of $2K) as it requires long tube headers, high flow cats, X-pipe, etc., & everything is in T304 stainless - so it's not a good bang for the buck value, it's only for those willing to spend whatever it takes to have the very best.

Exhaust system design both from the factory and in the aftermarket has changed radically in just the past 5-7 model years from what many of us grew up with - it's no longer appropriate to slap on a set of 2.5" or 3.0" true duals - that is *only* for forced induction engines in these late-model trucks & SUV's, or those with ported heads & aftermarket cams and other mods, etc. - trucks that are making much bigger power numbers, at least a true 60-80 HP over stock. For everything else, it's a 3" single or a 3" single in & 2.5" duel Out setup that does a better job - *if* it has been properly flow-engineered, and therein lies the problem - almost nobody actually does that for cat-back exhaust systems specifically for these trucks. The exhaust manufacturers spend what little actual flow R&D they do on much lighter & smaller vehicles - like 3400 lb. Mustangs, 2600 lb. imports, etc., where torque just doesn't matter anywhere near what it does in trucks & SUV's. So it pays to wait and "do it right" (even though a Magnaflow cat-back costs significantly more than just running straight pipes from the cats), as you get a better result especially in torque, what these trucks need most.

Yes, we can also actually turn off the rear O2 sensors by altering the PCM's programming in a custom tune, and eliminate that problem - but we won't do that when we know someone wants us to do that just so they can defeat the OBD-II system - that is highly illegal for any street-driven OBD-II vehicle, as are so-called "MIL eliminators." But the fact is, it's also just not necessary when a true-dual system is done right!

And that is done by simply custom bending new pipes welded on from just behind each of the secondary cats on each side of the motor, and then welding in O2 sensor "bungs" and reinstalled the downstream O2 sensors back into the new custom-bent exhaust tubing at roughly the same distance after the cats as they were before on each side of the exhaust. Then everything works properly - though you still have the loss of lower-rpm torque to deal with in your straight pipes scenario.

In short - the exhaust shop you went to probably just doesn't know how to do this correctly - it's so simple, just reinstall the downstream O2's the same distance after the cats on each side in the new custom-bent exhaust tubing as they were after the cats before with the original unaltered Y-pipe.

So with that information, now you can go back there (or better yet, go somewhere else where they actually *know* what to do) and have them do this for you and have the OBD-II system work correctly if the work is done correctly - but I still don't advise it. Instead, go with a properly flow-engineered single in dual out setup from Magnaflow, even if you have to wait longer to save up more money to do that - that is what will get you the best actual HP gains & torque results, over any other system - with one exception, our custom true-dual exhaust setup I was talking about before, that includes long tube headers, high flow cats, X-pipe, etc., etc. - that is one true dual configuration that will NOT cause a loss of low-end torque.

If all you want is sound and looks and doing that on the cheap, now you have the information you need so you can go have someone do this. I hope you'll decide to wait instead, & do this right by going with the Magnaflow SIDO setup so you actually get the best torque & HP gain results - but that is more expensive, and this is *your* truck - so the only person that needs to be happy with what you do is you.

Best of luck whatever you decide,

 

Mike,

Thank you for such a well written and detailed response. I wish there was someone like you available for tuning here (Alaska). One more question, if I may: How much torque difference are we talking about, comparing dual straight pipes to your recommended Magnaflow set up? I am sure it must be significant, and that you have seen it on the dyno. Is it something I could feel?

Thanks again for your time and sharing of your knowledge

 

What is the actual flow mechanism that causes the drop in torque with less backflow restriction? It seems counterintuitive.

Grim

PS--the dealer put dual exhaust on my '04 Ford 5.4L Supercrew. Dual flowmaster 40 series mufflers w/ 2.5" inlets and outlets. We had already set the price and the dealer threw in the mufflers through an odd chain of events (which I won't go into here). It sounds great but I have no idea what effect it has on performance!

Edit: Found a link here:
http://web.tampabay.rr.com/redroby/3liter/exhaust.html