Stock Exhaust Back Pressure???

07Softail · March 21, 2016

Yeah gotta love it Smart ASS! And this was taken from PipeMax if you even know what that program is used for.

Edited March 21, 2016 by 07Softail

Jon A · March 22, 2016

Yes, I'm quite familiar with Larry's work.

1) Do you see the words "Rough Estimate" at the top of the chart? Racecars tend to not have all that many complex bends, flat spots or restrictive mufflers in the system.

2) What makes you think running a system at it's listed "Maximum" somehow equates to "Optimum?" Do you really think at 467 HP 3.5" is "optimum" but if you make 469 you then need 4"? Do you not think when you're at the listed limit the larger one may make a couple HP more than the one that is "maxed out?"

3) I'm well over 468 with my 6.2 with simple bolt-ons. Your own chart shows a 3.5" system provides no room to grow on a 6.2 as mods add up. A full bolt-on 6.2 is well over 500 HP at the flywheel.

07Softail · March 22, 2016

Like I said go ahead and put a larger than life exhaust system on your truck.

RT

Jon A · March 22, 2016

Hyperbole works poorly in technical arguments. I have a 4" system on my truck. According to the chart you posted, that's exactly the size I need (as well as other 6.2 owners with more than a couple bolt-ons). While admittedly at this power level there isn't going to be a huge difference between the two, leading people to believe they're somehow going to make more power with the smaller exhaust is simply wrong.

auto_god · February 18, 2019

I realize this is an older post but thought I would share my results of doing pretty much the same test with my 5.3L a while back investigating whether a catback would be of any benefit to me.

Peak pressure of 6.6 psi occurred at just shy of 5500 rpm.

My cruising data was very similar to the original poster's, although with any sort of load at all above around 2000'ish rpm there would a consistent 1 psi or higher.

I'll update later when I've upgraded the exhaust system and have new data from that!

kennerz · February 18, 2019

There is some solid theory in this thread. In the "real world" the 6.2 has a necked down 3.5" system with various choke points.TO me a an exhaust is only as large as it's choke points. And that flapper is in now way a highflow item. There is also no way a 3" is adequate at this HP level. Ok but there is some HP being left on the table for sure.

When it comes time I"m going to do a custom true 3.5" cat back with just a 3.5" magnaflow muffler and attempt to remove all the restrictions and run no flapper or OEM resonator and see where it gets me. That said the bolt on magnaflow catback for the 6.2 with 4" tubing should tell us something. an industry leader in exhaust flow tech sized their system at 4" to optimize performance. Granted they know more about this topic than we all do (combined industry experience, hire or have in house engineers, and real world testing).

Most shops can't even bend 3.5 inch tubing, let alone test for different configurations of optimal set ups. Every time I went larger on exhaust and sized properly I picked up power everywhere, more frontside torque and better top end. now putting it all in perspective, being bolt ons we are not talking massive gains, just "better" and noticeable.

Only reason/argument I could see for keeping backpressure is if the factory cats need heat in the systems to work correctly/efficiently for emissions purposes. Everything on these new trucks is designed to run hot by design intension so as a system they can create a operating environment for the lowest possible emissions. It seems as if a lot of people forget that. Definitely not set up for best performance.

00pooterSS · February 19, 2019

On 2/18/2019 at 7:21 AM, auto_god said:

I realize this is an older post but thought I would share my results of doing pretty much the same test with my 5.3L a while back investigating whether a catback would be of any benefit to me.

Peak pressure of 6.6 psi occurred at just shy of 5500 rpm.

My cruising data was very similar to the original poster's, although with any sort of load at all above around 2000'ish rpm there would a consistent 1 psi or higher.

I'll update later when I've upgraded the exhaust system and have new data from that!

Interested in the results. This thread has already confirmed what I saw when I cut out my flapper. On my otherwise stock exhaust, when I had the stock muffler replaced with a magnaflow and the flapper cut out, I gained a couple of MPH and MPG. Now I have the stock 6.2 catback and my MPH and MPG keeps going up with each change I make to the exhaust.

damnthetorpedoes · February 23, 2019

I would love to find a 3.5” piece to replace the resonator & tailpipe section with. I can’t get over the fact that it chokes down to 3” pipe at that point. So far no exhaust shop seems willing to make 3.5 a reality for me.

SILVER SLED · February 24, 2019

22 hours ago, damnthetorpedoes said:

I would love to find a 3.5” piece to replace the resonator & tailpipe section with. I can’t get over the fact that it chokes down to 3” pipe at that point. So far no exhaust shop seems willing to make 3.5 a reality for me.

Check truck exhaust shops, like big trucks and diesels. I came across a 3 3/4 inside diameter in and out that had a bend that will replace a stock muffler.

auto_god · February 24, 2019

On 2/19/2019 at 6:31 PM, 00pooterSS said:

Interested in the results. This thread has already confirmed what I saw when I cut out my flapper. On my otherwise stock exhaust, when I had the stock muffler replaced with a magnaflow and the flapper cut out, I gained a couple of MPH and MPG. Now I have the stock 6.2 catback and my MPH and MPG keeps going up with each change I make to the exhaust.

So here is a question for those that have seen the valve inside (I have not......yet!).

Does it appear that the shaft that passes through the flap is a separate component from the flap itself?

Here is where my mind is at being the data kind of guy I am. What I would like to do is to remove the flap entirely from the system in a non destructive way and then redo my backpressure test to see what impact the flap really has on the measured backpressure. What I was pondering is if I were to cut off the end of the shaft without the spring (see attached image), could the shaft be pulled out of the flap allowing me to recover the flap out the intermediate pipe where it separates at the flex. I could then plug the holes where the shaft was & perform my backpressure test again.

I have 2 reasons behind my interest in doing this. 1. As I stated above, I'm very interested to see the flapper valves role in the measured backpressure. 2. I really don't mind how my truck sounds (I'm at the age where quiet is just fine with me), so if this modification provided a reduction of some or most of the backpressure without the expense or increased noise levels typical of aftermarket exhaust systems, that would be a complete win in my opinion.

00pooterSS · February 25, 2019

I can't be sure on that myself. I never looked. I just wanted it gone, see no good reason to have it.

auto_god · August 15, 2022

On 2/18/2019 at 8:21 AM, auto_god said:

I realize this is an older post but thought I would share my results of doing pretty much the same test with my 5.3L a while back investigating whether a catback would be of any benefit to me.

Peak pressure of 6.6 psi occurred at just shy of 5500 rpm.

My cruising data was very similar to the original poster's, although with any sort of load at all above around 2000'ish rpm there would a consistent 1 psi or higher.

I'll update later when I've upgraded the exhaust system and have new data from that!

So I realize this data is several years old now (got busy playing with my new Camaro SS 1LE!), but I recently finally got fed up with my flapper valve sticking on me and removed it and the stock muffler and replaced it with a OE replacement style muffler.

Being the curious person I am, I once again installed by backpressure gauge into the same location on my truck and logged another full throttle run.................and the verdict is.............No difference! Same 6.5 psi.

Now having said that, there is now no backpressure at all under normal driving. You have to be at full throttle now to observe any pressure.

So once I got those results my thoughts then turned to how much of that 6.5 psi was attributed to the secondary converter (I had been taking my backpressure readings from the downstream 02 sensor bung on the drivers side). I decided to install an o2 sensor bung in front of the muffler (and obviously downstream of the secondary converter) and ran the test again. This time I observed 5 psi of backpressure, leading me to the conclusion that 1.5 psi of my backpressure is attributed to the secondary converter.

After seeing the data, I've ordered a different muffler to see if that 5 psi can be banished (I'm confident it can). I'll report back when that happens!

Miguel Olea · March 23, 2023

Bump on the thread? So much info. I have AFM turned off on my 2018 6.2. Will I loose some torque off the line if I remove this flapper valve completely? I plan on keeping the stock muffler and resonator. People who have removed the valve on a stock truck - do you notice any off the line performance loss?