Back Pressure - Can anyone explain?

[Bruceynz]

Hi Guys,

Ok my car has the 360 mag in it with 209/217 degree roller cam and headers. It has a single 2.25" to 2.25" to 2.5" system on it. It has one muffler what they call a "turbo muffler", its a local built thing, oval and about 14" long.

The engine sounded quite raspy and well was noisy! It has annoyed me for quite some time and I finally got another smaller muffler added which is tucked in between the leaf spring and the tank.

The car now has a deeper quieter note to it, its really nice! I like it, but I dunno if it is my imagination but the car feels to me that its way more responsive down low, I mean noticeable so. Have I just got a vivid imagination, does it feel faster because its more quiet now or is it simply more grunty down low??

As far as I know I have added more restriction to the system which would mean harder for the exhaust gas to get out thus robbing power.

Anyone care to have a crack to offer an explanation?

Thanks
Bruce

 

[AJ/FormS]

This thread could go off the rails pretty quick.
That cam has almost no useful overlap so the Fifth Cycle will be pretty much insignificant. And that makes headers sortof optional... which leads to the exhaust system sizing being a lil more important, IMO. Really a 360 should have duals, but I understand on your car that is tougher to make happen.
Now, here's some more opinion;
People say the front pipes need to remain small to keep the velocity up. I call bs on that, especially on a log-manifold combo like yours.
So anyway . My take is this ; after the headers any exhaust at all is too much exhaust. But if you have to have an exhaust, then it should at least mimic no exhaust.
to be continued;
My daughter from Thompson showed up, so sorry man FamilyFirst.
Ok so girls have gone to bed; now where were we? Oh yeah;
When a gas is heated it expands, and in a given sized container, the pressure rises. If you pump more heated exhaust into said container, eventually the pump pressure will reach equilibrium with the pressure in the container, and no more gas will be able to enter the container. In fact, without a check-valve in the supply line, there will be times when the pressure would/could actually back up into the pump. Bad news.
But I get that your exhaust system is not a closed chamber, and that some exhaust will always escape, so making high pressure nearly impossible.
Ok so now, My belief is that since hot exhaust occupies a lotta space, I want to cool it off as soon as possible, so there is more room to jam more hot stuff in behind it.
To that end, I have dual 3 inch exhaust, not for the flow cuz dual 2.25/ 2.5 would be large enough for my engine; but for two other reasons;
1) the larger pipes have a larger radiating area,and
2) the larger pipes have a larger storage volume.

So, a pair of 2.25 pipes 6 ft long have a volume of 510cubic inches and 1018 sq inches of radiating area,
whereas a pair of 3s have 935 cubic inches volume, and 1357sq inches of radiating area; which being 83% more volume and 33% more radiating area.

So now, in
my belief, my header equipped 360 is pumping hot air into an adequate expansion area, which then cools the gases and preps them , to move thru the 3in/3out mufflers; Which suck even more heat outta the exhaust. After the mufflers; by this time the exhaust should be cool enough to run smaller exhaust pipes, but I wasn't having any of that either.
So why do I have this belief? Well because altho I haven't timed it, the car doesn't feel any different to the top of second or third gear with or without the exhaust system. So that takes care of the power situation.
Now lets talk about the low-rpm torque situation.
First; anytime the engine has back-pressure, then it has to work to push the exhaust out.That takes away from the power it coulda been using to propel the car. And that also costs you fuel economy. Ok but with dual 2.25s into a single 2.5, you shouldn't be having this problem. If you think you do then it is a simple matter of plumbing a pressure gauge into the pipe and monitoring it. They say up to 3psi in the exhaust is OK. But they don't say if that is at idle, or at WOT, so to me saying 3psi is ok ,I call bs. But if they say 3psi at 6000rpm , well that is useful information.
Now, if your car seems to have more down-low torque with increased back pressure, then I have to point the finger at the tune, or even the combo.
However;
It's hard to argue against opinion; so if you like the new personality and it doesn't slow your car down at WOT, or if you don't care if it does, then let it be.

done I think

 

[Bruceynz]

AJ I am hanging out for more...whats the rest??

 

[AJ/FormS]

My daughter from Thompson showed up, so sorry man FamilyFirst.

 

[Bruceynz]

AJ more moore mooore, you left the whole FJM body community hanging lol lol or maybe its just me, enjoy your time with your daughter I can wait

 

[Opticon77]

Look up videos on exhaust scavenging effects and hypothesize from there. The point at which your exhaust expands from 2.25 to 2.5 is likely big drop in velocity (at low RPM and part throttle) and thus the end of any scavenging advantage you MIGHT have retained back to the muffler. Installing a more restrictive muffler MIGHT be keeping the effect going for another 14" and MIGHT have been just enough to effect a particular driveable throttle/RPM range...

... But if you actually pulled that off without any math you should definitely be playing the lottery.

 

[Bruceynz]

No math was used All I wanted to do was shut the noise up, but seems to have a benefit of feeling like its got more grunt! Something I thought it would not do, I thought it may rob some top end hp but I don't drive around in 1st gear at 5500rpm all day so HP was not the biggest concern, ear comfort was

 

[MBDale]

Hey AJ/Form S !!

You still have that car you wanted to show me??

 

[MiradaMegacab]

Bruce nice car!!!!!!

Aj/FormS..... I enjoy your posts, here and on FABO. Always full of great technical tech and positive vibes!

 

[Duke5A]

You have a single 2.25" on that thing? That motor has a potato up its ass. Dual 2.25" would be perfect.

Anyways, yes, you can go too big on the exhaust and end up adversely affecting low end torque. The total volume the exhaust is capable of flowing is important, but so is the velocity.

 

[Bruceynz]

2.25" from header into a Y then 2.5" single, I have had big exhaust systems before on other cars. J body has gas tank offset leaving no room for exhaust pipe on left side, no ribbing on gas tank if you move over to right. What's the stock size exhaust on a 360 magnum truck?

You have a single 2.25" on that thing? That motor has a potato up its ass. Dual 2.25" would be perfect.

Anyways, yes, you can go too big on the exhaust and end up adversely affecting low end torque. The total volume the exhaust is capable of flowing is important, but so is the velocity.

 

[SixBanger]

Found a small paper on the web about the mathematical formulas about mufflers for ICE.

Maybe this could add to think about. Like calculation of optimum tail-pipe length, muffler volume.

But at the end it is theoretical and, of course there is something to say about available space and sound.

Design Principles for an Automotive Muffler

Nothing to do in the evening. So set it up on excel.

 

[Camtron]

general rule is basically 1” exhaust diameter for every 100hp.
Here’s a nice little write up from Hot Rod network.
What Size Exhaust Pipes Work Best?
Your magnum would ideally have a 2.5”-3” exhaust off your headers all the way back to the tail pipe for your single pipe set up

 

[kkritsilas]

it is entirely possible to have pipes that are too big for an engine. When that is the case, there is a drop in low end torque. Increasing the back pressure, in that case, can increase low end torque. For example, putting 3" exhaust headers and pipes on a slant 6 will result in low end torque loss. When the headers/exhaust is reduced to 2" or 2.25", there will be an increase in low end torque relative to the 3" set up. A lot of that logic is used to size the factory exhausts (not the only reason). its not just a question of volume, it is also a question of pulses and the way they are going up and down the exhaust system. Volume must be adequate, but the timing of the pulses, and at what RPM they help or hinder exhaust scavenging must also be taken into account. The timing of the pulses varies with a large number of parameters, from cylinder volume, camshaft timing to header length and size, collector length and size, head pipe length and size, muffler size, length and type, and a few others, along with the RPM the engine is at, and all exhaust systems (as a whole) will have a specific RPM at which they are most efficient. Also, smaller diameter exhausts tend to be quieter, although mufflers can have a pretty major effect on that.

in Bruce's case, the smaller muffler increased the back pressure, and resulted in a low end torque increase, felt as a more responsive engine.

 

[Bruceynz]

I have seen these tables before, they just look crazy to me! Maybe they are to extract every last drop, I can't say as I don't see here, how many 6.4 hemi's are running a dual 2.5" system or a single 3" system?

A quick look on the net says the 6.4 hemi has a cat back 2.75" exhaust but unable to find if that is a single system or dual system.

Anyway let's see if this can be answered, hp is at high rpm's so 95% of the time when I take off from traffic lights and it changes at 2000rpm into second and into 3rd 35mph it's doing 1250rpm how much hp am I loosing? Ok answer is simple and I am being sarcastic.

Let's say engine makes 300hp at 5000rpm with dual 2.25" system and then with my single 2.5 "system will I be loosing 50hp or 5hp?

general rule is basically 1” exhaust diameter for every 100hp.
Here’s a nice little write up from Hot Rod network.
What Size Exhaust Pipes Work Best?
Your magnum would ideally have a 2.5”-3” exhaust off your headers all the way back to the tail pipe for your single pipe set up

 

[80mirada]

The 6.4 has dual exhaust. Motor Trend has tested exhaust size and mufflers on a dyno....

 

[Bruceynz]

Ok what was difference I mean if there is massive difference I will look at changing the system.

 

[BudW]

There is a lot of different topics to speak about here.
First of all:

People say the front pipes need to remain small to keep the velocity up. I call bs on that, especially on a log-manifold combo like yours.

I have no personal knowledge on this, but logic tells me this is also BS. Sense I’m no expert, you can disregard my opinion.

it should at least mimic no exhaust.

I can’t argue there – except for overall noise.


Things I do know:
Some engines like backpressure. Most engines do not. I’m sure camshafts have a lot to do with that (as AJ mentioned).

Exhaust gases shrink as they cool.
I have seen Y-pipe exits at 2½”, 2¼” exit from cat and muffler exit at 2” with little problems (for 2-bbls). Note: more about this later. I don’t know how much shrinkage there is, but logic would make me think that maybe exhaust is twice the size when it exits the engine to point it leaves the tailpipe (for a normal length car) – but I could be off base.

A single 2½” exhaust pipe will flow 509 CFM (Cubic Feet per Minute). I’m not sure what the metric equivalent is but I don’t think the equivalency it matters when you look at the numbers. Note: all CFM numbers are taken at same temperature – so keep in mind hot gases will take up more space.
A single 2½” pipe is good for roughly 232 HP (or 463 HP for duals).

A single 2¼” pipe will flow 408 CFM. ¼” smaller pipe difference (from 2½” pipe) = 20% less airflow.
A single 2¼” pipe is good for roughly 185 HP (or 371 HP for duals).

A single 3” pipe will flow 747 CFM and is good for about 339 HP (or 679 HP for duals).

Stacking mufflers (one after another) will lower CFM and increase backpressure. Splitting mufflers (one per side) is always better.
I’m not sure if it will help, but I’ve considered dual exhaust with four splitting mufflers, to get the noise down as quiet as possible. I drive long distances, and exhaust noise just wears me out.

Back to an earlier subject. Exhaust (exploding gases) makes noise. It happens and nothing that can be done, except via mufflers.

There is another “noise” phenomenon, which is created causing resonance. This is when the exhaust pipe starts to vibrate making another noise, which is usually a drone noise. Typically, this is caused by using the same size pipe from start to finish (example would be 2½” into 2½” into 2½”).
Chrysler used resonators on some cars (a secondary muffler, just before it exits the car), including many of the 426 Hemi’s and 440 6-bbl cars, to knock out that resonance noise. Tapering the exhaust (big to small) is another method to reduce the drone/vibration. Another method is to get the exhaust vibrations to self-cancel itself, but that takes a bit of math and fabrication work.

Something that @Duke5A has done (or is doing) is to add exhaust cut-outs to his existing exhaust system. This will allow for bypassing the muffler and his car will be louder. Great for “timed acceleration trials” or to intimidate other drivers. Not so great to impress the local law enforcement or the spouse.

Chrysler had a “drag race only” system for the Max Wedge cars that had a manual cut-out. You drove to the track with mufflers, get out tools to remove the caps. After racing, re-bolt caps on and drive home with mufflers. Taking this system and adding electric cut-outs would make that job easier.

For what you said, I think the last section of pipe is a bit too small. Depending on what you are wanting to do, the exhaust cut-outs might help (maybe), as well.
BudW

 

[Bruceynz]

All very interesting just watched an engine master 2.5" vs 3" dual system on big block Chev, 2.5" 601hp and the 3" made 614hp, they used full length system to try an replicate a full car system. So I dunno if you can say this, single 2.5" good for 300.5hp and 3" good for 307hp??

 

[Opticon77]

2.5" might support 300hp but they just didn't have enough engine to challenge the dual 3". Which should be good for more like 750hp.