Stock Exhaust Back Pressure???

BuyAmericanIron · March 18, 2016

Agreed that even with the flapper bolted open, there is still a large piece of metal interrupting the exhaust flow. Another interesting point is the OE 6.2L exhaust. The pipe from Y pipe to muffler is 3 1/2 inch, pinched down to 3 inch right at the muffler inlet, 3 inch coming out of the muffler and through the flapper and then the tailpipe is 3 1/2 inch again to the resonator and then 3 inch from resonator to tip. Why the hell do they feed 3 1/2 inch pipe into a 3 inch muffler? It has to be for sound attenuation because in stock form it sounded like an electric car. Almost no exhaust sound at all.

Now with catted AR Headers it has quite a bark through the stock exhaust and it sounds aggressive when accelerating and quiet when cruising. Kind of sounds like a big block with flowmasters. Most of my friends like the sound. I like it but hate the back pressure. Not sure how to make it better and keep it quiet.

07Softail, to answer your question, I purchased the back pressure gauge about 15 years ago from Snap On. It is a 10 lb gauge with an O2 sensor bung and a piece of high temp stainless covered flexible hose. All goes together with 1/8 inch pipe threads. Don't know if you can rent from auto parts store but I doubt it.

Edited March 18, 2016 by BuyAmericanIron

TxTruckMan · March 18, 2016

You only data from one point in the whole exhaust system. Did you measure the any pressures on the far end of the exhaust past the flapper valve? Exhaust flow on exit will create negative pressure upstream and could easily negate most of the 5 psi you saw in front of the flapper.

Jon A · March 18, 2016

No, that's not how it works. The pressure will gradually decrease as you get toward the end (not in a perfectly linear fashion, but as s trend) and what matters is the pressure closest to the engine--that's what it sees. This represents the sum total of the entire exhaust.

And guys, 5 psi is a lot. I remember an article by David Vizard in testing mufflers and 2 psi from the muffler alone represented a huge power decrease on the test engine. He advocated rating mufflers in flow (CFM) the way we rate carbs and throttle bodies. He determined you need X number of CFM per HP to not cause a significant power reduction.

Which goes back to what I was saying before--the more power you have, the more flow you need. An exhaust may measure virtually no power loss on a low power engine and yet the very same system may cost a high HP engine a bunch of power. These 6.2's need to breath.

I just got back from the dyno--I measured a 47 RWHP increase from simple breathing-related bolt-ons. I sure as heck didn't gain all that from the headers alone.

Victory14red · March 18, 2016

The following is copied from http://www.dsmtuners.com/threads/exhaust-the-straight-scoop-on-backpressure.168578/

Granted this is a completely generalized answer not in any way specific to our trucks bit the science makes sense. The copied text follows:

There is a common misconception that engines need backpressure in order to run properly, generate low end torque, etc. That is simply untrue. Backpressure is a bad thing. Always. Take a look at a top fuel dragster...how much backpressure do you think those zoomie headers make? Very little, and those engines produce 6500 hp.

So, what is backpressure? Any fluid flowing through a pipe experiences drag on the walls of the pipe. This depends on a number of factors, including the diameter of the pipe, the smoothness of the inside of the pipe, the viscosity of the fluid, and the velocity of the fluid. This drag results in a pressure drop through the pipe. In order for the fluid to flow at all, the pressure on one end of the pipe must be higher than at the other. In an exhaust system, that pressure drop is what we refer to as backpressure. It's pretty obvious that the engine has to produce this pressure differential, so the less power it has to spend making pressure to push the exhaust out, the more power it can send to the wheels.

Given that exhaust pipes are pretty smooth, and that we can't change the viscosity (thickness) of the waste gas being forced through the pipes, we are left with basically 2 parameters we can have any control over: The pipe diameter and the gas velocity.

Unfortunately, the pipe diameter controls the gas velocity since the volume of gas is prescribed by the engine. So, we really only have one thing we can change. So, bigger pipes allow less pressure drop for a given volume of gas because the velocity is lower. The pressure drop (backpressure increase) is proportional the gas velocity squared, so if I double the gas velocity (by reducing the cross sectional area of the exhaust pipe by half) then I quadruple the pressure drop.

Well, there's an easy solution for that: Just make the exhaust pipe bigger. Bigger pipe, lower gas velocity, less pressure drop, so less backpressure. Wow, that was easy. After all, this is the way it's done for basically any type of commercial plumbing system. Need less pressure drop on a chilled water pipe or a natural gas line? Just make the pipe bigger.

But wait, there's a problem....Having a huge exhaust pipe has killed my low end torque!!! What's different? Oh, there's no backpressure!! Therefore backpressure makes torque!

Wrong.

An exhaust system is different than just about any other plumbing situation. How? Because the flow is pulsed, and this turns out to be a big deal. Every time a pulse of exhaust gas runs through the pipe, a strange thing happens: it as it passes, it has a little area of vacuum behind it. Just like a NASCAR stocker running around the track, the pulse generates a little bit of a vacuum behind it. In NASCAR, a driver can take advantage of another driver's vacuum by getting right behind him and driving in it. The wind resistance is drastically reduced. This is called drafting.

Well, how big the vacuum behind each pules is depends on the gas velocity. The higher the velocity, the bigger the vacuum the pulse has behind it.

Now, this means that I can "draft" the next pulse, just like in NASCAR. In NASCAR, it's called drafting, in an exhaust system, it's called scavenging. You've probably seen this term used when talking about headers, but the same concept applies in the pipe.

I get the maximum scavenging effect if the gas velocity is high, so the pipe needs to be small. By maximizing the scavenging effect, I help to pull pulses out of the combustion chamber, which means the engine doesn't have to work as hard to do that.

This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished.

So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart).

So, the loss of torque has nothing to do with backpressure, and everything to do with gas velocity. So you need exhaust components that are not restricive (manifolds/headers, mufflers) and that are sized correctly for your application.

To further dispel the "backpressure is necessary" theory, try this if you want. If you have access to a vehicle with open headers, make a block off plate that will bolt to the collector. This plate should have only a 1" hole in it for the exhaust to flow through. That will give you PLENTY of backpressure, and zero scavenging. Then you can report back on how much low end power it has.

The one exception to sizing an exhaust is for turbo cars. Since the turbo is in the exaust stream, the gas flow spinning the impeller tends to come out of the turbo with the pulses greatly diminished. In this case, you can get away with running a larger pipe than on an equivalent HP N/A engine because you can't take as much advantage of the scavenging effect.

BuyAmericanIron · March 20, 2016

Victory14red, that is a very interesting article and I have read it before. Great information and very relevant to this discussion. Here is something I thought about today to add.

I own a 2010 Corvette with bi-mode exhaust. For those not familiar, the bi-mode has four exhaust pipes out the back, two on each muffler. One of the pipes has what looks like a throttle plate inside. When it is open, you have virtually a straight pipe from the engine to the tip of the tailpipe. When it is closed, the exhaust gases are forced to pass through the baffles of the muffler and exit out the other pipe on the muffler. In that series of Corvette, GM rated the HP at 430 with standard mufflers and 436 with bi-mode mufflers. Now this is just a guess, but if the standard exhaust has a comparable (to this discussion) 5 psi of back pressure and the bi-mode has near zero psi back pressure, it is good for six HP on a stock engine.

I would have to again guess that if you install add on's like Jon A discussed above which improve volumetric efficiency and create additional air flow through the engine there would be a bigger effect.

Victory14red · March 20, 2016

Here's my question. Which I have been trying to figure out on my own to no avail.

On the 5.3 if you get the metal mulisha exhaust it's 4" however the piping that it connects to is 3" so because it's a thru flowing exhaust you're essentially "ending" the exhaust at the point where it becomes 4" correct? So you'll still have a 3" exhaust for the scavenging effect but it's "shortened" which should be good for low end torque? Is my logic sound there?

harley_man · March 20, 2016

I installed the metal mulisha exhaust on my 2014 and don't noticed anything different as far as performance. But also I only have a 100 miles since. Gibson says it takes 4-500 miles before the computer adjusts to it.

Victory14red · March 20, 2016

I installed the metal mulisha exhaust on my 2014 and don't noticed anything different as far as performance. But also I only have a 100 miles since. Gibson says it takes 4-500 miles before the computer adjusts to it.

I'm a professional mechanic. I don't know what it would adjust to (other than possible Leanness due to increased airflow)? Maybe it has to relearn stft and ltft. Not sure.

But just going based on the science above and logic it seems to me that shortening an exhaust would be better for low end torque......therefore adding a bigger pipe after a smaller one effectively shortens it. It makes sense in my head lol.

07Softail · March 21, 2016

Actually the larger the pipe the slower and cooler the exhaust gasses are unless your engine size and HP level support it. Optimum pipe size is your key not too small and not too big based on you HP level and RPM you drive in. Ideally you want Hot Tight exhaust pulses as far as you can get them down the pipe which aides in scavenging. 2 into 1 exhaust systems are the best for our trucks because each bank of the engine pulls gasses down the pipe like a merge lane on the interstate. Personally I would not go any larger than 3.5" on a daily driver unless you drive your truck Wide open all the time and I doubt that you do.

RT

Edited March 21, 2016 by 07Softail

Victory14red · March 21, 2016

Actually the larger the pipe the slower and cooler the exhaust gasses are unless your engine size and HP level support it. Optimum pipe size is your key not too small and not too big based on you HP level and RPM you drive in. Ideally you want Hot Tight exhaust pulses as far as you can get them down the pipe which aides in scavenging. 2 into 1 exhaust systems are the best for our trucks because each bank of the engine pulls gasses down the pipe like a merge lane on the interstate. Personally I would not go any larger than 3.5" on a daily driver unless you drive your truck Wide open all the time and I doubt that you do.

RT

Interesting info.

What about the fact that the y-pipe would still be 3" but after the second cat it increases in size? Still detrimental? I would think it would act as a restriction and cause the "hot and tight" situation you speak of up to that point.

I'm just trying to figure out gibsons logic using 4".

Jon A · March 21, 2016

The logic is they probably found it made more power. The Magniflow for the 6.2 is also 4".

Guys, worrying about scavenging of a catback on a big, powerful V8 is really chasing your tail. I have never, ever, ever seen a dyno of a smaller exhaust making more power on such an engine.

If you're worried about scavenging, read up and focus on header design when making your selection. Primary diameter (as well as stepped or not), primary length ("Longtubes" "mid length," shorties, etc), primary design (equal length, unequal length, tri-Y, etc) along with the collector design ("merge spike", "scavenging shape" etc) and length can and will have a huge influence on the torque curve.

Don't expect any such magic to be happening with the cat-back. After the headers, after the cats, after the Y-pipe....all that's really left is backpressure. A smaller, more restrictive cat-back is generally going to make less power across the board. Changing to a true dual system gives you a little control over the torque curve with differing H-pipe, X-pipe configurations, etc, but with a single cat-back system don't expect anything from a smaller system other than less weight and less noise (and likely less power).

07Softail · March 21, 2016

A 3" pipe will support 339 HP single pipe and the L-83 pops 355 flywheel HP

A 3 1/4 pipe will support 401 HP single pipe

a 3.5 pipe will support 468 HP single pipe

So you can see a 420 HP L-86 will work perfect with a 3.5 pipe (Optimum)

Also the HP is made at 6000 RPM do you drive at 6000 when going to work? No you do not. Build for torque not HP!

Torque is what moves you, HP is the side affect of how long the torque is carried. Folks Bigger exhaust pipe is not better.

RT

Jon A · March 21, 2016

1) "Will support" is a conditional term that means something different than "optimum." General rules of thumb like that do not account for bends, muffler performance, etc. He measured 5 psi with the stock 3.5" system, no engine builder in the world will tell you that's "optimum."

2) I've added more than 30 lb-ft of torque at just a mere 2000 RPM, mostly with the exhaust. I didn't do that by making the pipes smaller.

3) Just so you know, "Torque is what moves you, HP is the side affect of how long the torque is carried." is laughably incorrect. You're confusing torque with HP and HP with work.