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 Post subject: exhaust system design...
PostPosted: Sat May 01, 2021 3:20 pm 
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TBI Slant 6

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I have been observing different exhaust systems, to see if I can notice trends.

One aspect that I have noticed is that all truck exhaust systems I have looked at tend to have larger head pipes than the car versions.. No that is not true, the 3/4 ton trucks and larger tend to have larger diameter exhaust systems up to the converter, then they neck down. The one 1/2 ton chevy which I assume was factory, had a rather small exhaust down pipe off the manifold. Mid to late 90's Magnum engines also had smaller head pipes for emissions reasons as I understand it.

This afternoon I was working on my brother in law's Jeep Commander with a 3.7 engine. Judging by the exhaust pipe from the manifold past the converters to the Y pipe, I would have assumed it was a 5.7 Hemi. The exhaust head pipe coming off each cylinder head is 2 3/8". One side is about 32" long, the other is about 60" to where they join in front of the muffler. After the muffler I did not measure, but guessing it was about 2, maybe 2 1/4 over the rear axle and out the back.

I am starting to wonder if exhaust system volume at least on the front half of the system contributes to low end torque and has no down side..


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PostPosted: Sun May 02, 2021 5:04 am 
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Turbo Slant 6

Joined: Thu Jun 07, 2012 4:29 pm
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I can't answer your question directly, but the more important thing to remember is no one here can. Exhaust systems are probably in a dead heat with oil and spark plugs when it comes to bench-racing theories.

The average production vehicle - like it or not - has lots and lots of R&D poured into the exhaust system. Those results we can see but we don't get to know why/how they were achieved. It's usually a mix of criteria - performance, noise level, emissions, safety, cost, and fit - all coming together. Chances are the larger head pipes are there to help compensate for the fact that the exhaust will soon run into a catalytic converter that is like a big Russet potato stuffed in the tailpipe.

I just wish every male between the ages of 18-55 who buys a new 1/2 ton truck wouldn't feel the need to instantly drive the muffler shop and get his factory stainless steel, quiet exhaust system cut off with a Sawzall and replaced with a Flowmaster and soon-to-be-rusty 2" with 12" long 4" diameter tipped tailpipe.


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PostPosted: Sun May 02, 2021 6:10 am 
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TBI Slant 6

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GregCon wrote:
The average production vehicle - like it or not - has lots and lots of R&D poured into the exhaust system. Those results we can see but we don't get to know why/how they were achieved. It's usually a mix of criteria - performance, noise level, emissions, safety, cost, and fit - all coming together. Chances are the larger head pipes are there to help compensate for the fact that the exhaust will soon run into a catalytic converter that is like a big Russet potato stuffed in the tailpipe.


This is a good point I have not fully considered, and it would seem that cost is often a driving factor in what the factory puts on each vehicle.. so maybe the larger exhaust head pipe is about reducing warranty claims, rather than performance..


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PostPosted: Sun May 02, 2021 6:46 am 
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Turbo EFI
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Location: kankakee IL
Car Model: 80 volare, 78 fury 2 dr, 85 D150
You mentioned emissions...
I can't see how pipe diameter would have an effect there.
On the magnum series of engines I do know that the 1st 2-3 years of that engine being put in trucks that the manifold outlet was bigger, the pipes were bigger, than what they used from then on. Also supposedly the ones with bigger exhaust had a slightly better cam as well. Combined that gave those early magnums 5-10 hp more than the later ones, I can't say how much was because of the exhaust and how much because of the cam.
I do know that there is a point of diminishing returns as the pipe size increases. But I don't know where the line is drawn.
even up til the 80s and 90s there were a whole lot of people who had duals put on their cars with 2" pipe from front to back and these cars seemed to do fine with that.
I have a 78 Fury 2 door with a 2 barrel 318 that I had exhaust done on right after I got it in 07, and the guy at the shop insisted on using 2-1/2" from nose to tail,. on a completely stock 2 bbl engine even though I really wasn't wanting that big. (No cats either) I had asked him to put the tails a little smaller, but he didn't.
I'm thinking of having it redone for 2 reasons.
I now have a real hard time gettin the oil filter in and out, and one of the tails hits on the shock over bumps and drives me nuts. I am about to replace the sagged out leaf springs, meaning that hopefully I can also do away with the band aid air shocks I put on years ago that might help that problem. I think that honestly 2" front to back would have been plenty considering I have duals on it.
Length of pipe has alot to do with sound level, it will be quieter with tails all the way out than with turn downs ahead of the rear end.


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PostPosted: Sun May 02, 2021 2:59 pm 
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GregCon wrote:
a catalytic converter that is like a big Russet potato stuffed in the tailpipe.


That is utterly, totally false.

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Last edited by SlantSixDan on Mon May 03, 2021 7:20 pm, edited 1 time in total.

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PostPosted: Sun May 02, 2021 7:09 pm 
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Supercharged
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Location: Fircrest, WA
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GregCon wrote:
I just wish every male between the ages of 18-55 who buys a new 1/2 ton truck wouldn't feel the need to instantly drive the muffler shop and get his factory stainless steel, quiet exhaust system cut off with a Sawzall and replaced with a Flowmaster and soon-to-be-rusty 2" with 12" long 4" diameter tipped tailpipe.



Not going to happen. You live in Texas. Cutting out the catalytic converter and installing an obnoxious exhaust on your truck is a matter of pride there.


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PostPosted: Sun May 02, 2021 8:45 pm 
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Guru
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Quote:
... It's usually a mix of criteria - performance, noise level, emissions, safety, cost, and fit - all coming together...


And don't forget about sound, when it comes to new vehicle exhaust systems.
A lot of today's exhaust system's R&D work (and dollars) are spent to get the "right" sound out the tailpipe.

You now see a lot of "funny things" in a new vehicle's exhaust system and some of those things are there to get the sound that the manufacturer wants to hear.
See the link below for more on the exhaust sound engineering and design subject.
DD

https://www.tenneco.com/pdf/Leaflet_Sound_E_neu.pdf


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PostPosted: Sun May 02, 2021 10:05 pm 
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Supercharged

Joined: Thu May 12, 2005 11:50 pm
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Location: So California
Car Model: 64 Plymouth Valiant
Now they just cheat and use a sound-track...............

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PostPosted: Mon May 03, 2021 5:56 pm 
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TBI Slant 6

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Compact manifolding & small head-piping absoluely affect emissions, it reduces cold catalyst warm-up time, one of the few really incomplete systems left on modern ICEs. Plans are slowly, or finally coming around to heat-assist the cat electrically,..ie all O2 sensors, but cold weather/start is one "dirty" condition that persists.
Russet 'tater? BS. Only if failed or damaged. We're not discussing race aps.


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PostPosted: Mon May 03, 2021 6:32 pm 
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Doctor Dodge wrote:
A lot of today's exhaust system's R&D work (and dollars) are spent to get the "right" sound out the tailpipe.

You now see a lot of "funny things" in a new vehicle's exhaust system and some of those things are there to get the sound that the manufacturer wants to hear.
DD
Killer6 wrote:
Compact manifolding & small head-piping absoluely affect emissions, it reduces cold catalyst warm-up time, one of the few really incomplete systems left on modern ICEs. Plans are slowly, or finally coming around to heat-assist the cat electrically,..ie all O2 sensors, but cold weather/start is one "dirty" condition that persists.
Russet 'tater? BS. Only if failed or damaged. We're not discussing race aps.


OK, I can easily understand there is R&D work going on to get a certain sound.. And emissions is also a topic for $$ spent.. Can someone explain why a 3.7 Jeep has 2 3/8" head pipes? That flies in the face of good emissions... so it must be there for some other reason. The muffler is also huge, so I am imagining they want to keep it fairly quiet, and not obnoxious.. I want to believe that the huge head pipe is for something to do with the power curve. . . . BUT I've got no science to back up my imagination..


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PostPosted: Tue May 04, 2021 9:42 am 
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TBI Slant 6
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SlantSixDan wrote:
GregCon wrote:
a catalytic converter that is like a big Russet potato stuffed in the tailpipe.


That is utterly, totally false.


Dan, without looking for a fight, I almost completely agree that a blanket statements as the one you answer to is false. Some of the same research has been done to get sure your cat do not represent an important block for exhaust gas flow, and the topic is still under research trying to reduce it even further and to optimize them (if you google it there is a bunch of work).

... but an old and tired cat (meooooww...), from an old, oil burning engine, can become as permeable as a Russet Potato. I might say that a mistreated old dirty one can become a Russel potato, and has to be checked.

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PostPosted: Tue May 04, 2021 9:43 am 
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TBI Slant 6
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Location: Houston, TX
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Doctor Dodge wrote:
Quote:
... It's usually a mix of criteria - performance, noise level, emissions, safety, cost, and fit - all coming together...


And don't forget about sound, when it comes to new vehicle exhaust systems.
A lot of today's exhaust system's R&D work (and dollars) are spent to get the "right" sound out the tailpipe.

You now see a lot of "funny things" in a new vehicle's exhaust system and some of those things are there to get the sound that the manufacturer wants to hear.
See the link below for more on the exhaust sound engineering and design subject.
DD

https://www.tenneco.com/pdf/Leaflet_Sound_E_neu.pdf


:shock: interesting...

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PostPosted: Tue May 04, 2021 4:33 pm 
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cpslntdchrg wrote:
... but an old and tired cat (meooooww...), from an old, oil burning engine, can become as permeable as a Russet Potato. I might say that a mistreated old dirty one can become a Russel potato


That's true, and well worth keeping in mind when one is designing an exhaust system with old, tired parts. :mrgreen:

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PostPosted: Tue May 04, 2021 4:59 pm 
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TBI Slant 6
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SlantSixDan wrote:
cpslntdchrg wrote:
... but an old and tired cat (meooooww...), from an old, oil burning engine, can become as permeable as a Russet Potato. I might say that a mistreated old dirty one can become a Russel potato


That's true, and well worth keeping in mind when one is designing an exhaust system with old, tired parts. :mrgreen:


C'mon Dan!. So much for recycling... and, anyway, if you wait enough it will eventually plug! :lol: , I'm just saving you trouble looking in to the (far...far...) future, just as a bunch of the texan diesel pick up truck owners :lol:

Anyway, take care of your cats if you don't want them to become Russet potatoes (on the bright side, I guess that after the conversion, at least are easier to feed and clean, and don't scratch furniture :wink: )

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PostPosted: Tue May 04, 2021 6:10 pm 
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Turbo Slant 6

Joined: Thu Jun 07, 2012 4:29 pm
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Location: Houston
Car Model: 68 Valiant
"Dan, without looking for a fight, I almost completely agree that a blanket statements as the one you answer to is false. Some of the same research has been done to get sure your cat do not represent an important block for exhaust gas flow, and the topic is still under research trying to reduce it even further and to optimize them (if you google it there is a bunch of work)."

The problem with saying anything in today's society is there will always be someone who fails to understand what you are saying....not because they are inherently incapable, but because they are clamoring to be offended, first and foremost. Meanwhile, most people understand full well what you meant when you said it.

Without wasting too much thought....ANYTHING other than a good vacuum cleaner, perhaps, in an exhaust system will hinder flow. And don't tell me the AVERAGE cat is a high tech work of art, capable of flowing like Niagara Falls. It's not. But regardless....to take a comment literally that is clearly a tongue-in-cheek statement? To quote Dirk Diggler in Boogie Nights...."That's a YP, not a MP".

I'll offer some advice for the future...the next time someone says they love the country, don't go looking for an oak in Hillary Clinton's yard.


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