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1997 GMC Safari SLX - AWD
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Discussion Starter · #1 ·
I have a bit of a head scratcher here.

I have always wanted a tad more sound from my vehicles. So when I bought my Safari months ago, I thought about the muffler right away haha. I don't want anything TOO loud (I had a Thrush 2 chamber 17651 on my Jeep 4.0, a copy of the flowmaster 40, and it was too much haha), so I thought it would be nice to get something like a Flowmaster 50 Series. It is shorter and lighter than that stock lead weight under there, so for offroad, a bit better too!

My question is as follows.... I know that the factory muffler goes from 3inches, to a 2.5 inch tail pipe, which I assume is for back pressure (?). I have read on here that it would be stupid to run a full 3 inch exhaust, as you would lose torque.... the Jeep XJ I owned was the same concept.

So here is the headscratcher: Would you run:

1. A 3" inlet / 3"outlet muffler, with a 3" --> 2.5" step down after the muffler

OR

2
. A 3" --> 2.5" step down on the pipe in front of the muffler, and then run a 2.5" / 2.5" muffler?


I am fairly certain each one gives a different result due to muffler volume, gasses going in/out, and back pressures.... or am I nuts, and it's a case of "6 of one, half dozen of the other"?

Anyone who has done a flowmaster or similar welded performance muffler... which route did you take?

Thanks !
 

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1998 LS AWD Forest Green metallic
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I have a bit of a head scratcher here.

I have always wanted a tad more sound from my vehicles. So when I bought my Safari months ago, I thought about the muffler right away haha. I don't want anything TOO loud (I had a Thrush 2 chamber 17651 on my Jeep 4.0, a copy of the flowmaster 40, and it was too much haha), so I thought it would be nice to get something like a Flowmaster 50 Series. It is shorter and lighter than that stock lead weight under there, so for offroad, a bit better too!

My question is as follows.... I know that the factory muffler goes from 3inches, to a 2.5 inch tail pipe, which I assume is for back pressure (?). I have read on here that it would be stupid to run a full 3 inch exhaust, as you would lose torque.... the Jeep XJ I owned was the same concept.

So here is the headscratcher: Would you run:

1. A 3" inlet / 3"outlet muffler, with a 3" --> 2.5" step down after the muffler

OR

2
. A 3" --> 2.5" step down on the pipe in front of the muffler, and then run a 2.5" / 2.5" muffler?


I am fairly certain each one gives a different result due to muffler volume, gasses going in/out, and back pressures.... or am I nuts, and it's a case of "6 of one, half dozen of the other"?

Anyone who has done a flowmaster or similar welded performance muffler... which route did you take?

Thanks !
I have never experimented with mufflers like that. In fact, I have had very few vehicles other than small imports, and they were often rally cars used in competition.
If it were mine, I would buy both adapters and do a trial in both configurations.
Rod J
Issaquah, Wa
 

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I replaced the pipe to a 2.5 inch and used a dynomax muffler. Originally had a thrush forever.

For yours a reducer after the 3 inch muffler is going to be as close to a factory set up as you can get.
 

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1997 GMC Safari SLX - AWD
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Discussion Starter · #4 ·
Thanks for the input folks! Alright, so I suppose get the 3" muffler, and then drop down to the tail pipe section. The reason I was confused is that the OEM boat anchor muffler has the 3" inlet and the 2.5" outlet.... so I was curious where the restriction happens. Just on the outlet, or somewhere internally.
 

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01 Astro RWD Cargo, 97 AWD Mark III
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As long as you are rebuilding the exhaust.
Walker 50418 Y pipe/ Catalytic converter with smoother bends for better exhaust flow.
 

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2.5" through the muffler would probably generate a less droney note.

I would steer clear of Flowmasters. They're not good at muffling and they're not good at performance. A Dynomax Super Turbo would probably do a better job of both.
 

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2000 Lifted 4x4 Astro 92 V8-350 Shorty
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My question is as follows.... I know that the factory muffler goes from 3inches, to a 2.5 inch tail pipe, which I assume is for back pressure
"Back pressure" is a huge mis-conception.. and is commonly misunderstood.
It is not desirable anywhere in the system.

There is an entire world of science to exhaust flow technology. "Scavenging" and flow "velocity" are a big part of it. Look it up. Larger pipe has less scavenging and velocity than smaller pipe at lower rpms, and visa versa. Pipe sizes are "optimized" for HP, torque, and fuel economy... and bigger is not always better, at least in the wrong place. Pipe size actually tunes the sytem.

Same goes for header technology, with regard to pipe size, length, and collector size too.
Headers need to be size matched for optimum performance. A mismatch can reduce power where it's needed most.

With the Astro.. it's two pipes into one.

Most of the pulsed flow technology applies to the pipe BEFORE the muffler. Once the exhuast pulses enter and expand in the muffler..the scavenging effects mostly end. The output side is far less critical, but still needs to be adequate for volume of CFM output.

Sometimes it's also a balance of technology... and practicality.
 

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"Back pressure" is a huge mis-conception.. and is commonly misunderstood.
It is not desirable anywhere in the system.
Most of the pulsed flow technology applies to the pipe BEFORE the muffler. Once the exhuast pulses enter and expand in the muffler..the scavenging effects mostly end. The output side is far less critical, but still needs to be adequate for volume of CFM output.
I think these really sum it up.
It seems that the header/manifold size seems to matter for scavenging, then the rest really doesn't matter as long as it flows well enough with the least amount of back-pressure.
 

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"Back pressure" is a huge mis-conception.. and is commonly misunderstood.
It is not desirable anywhere in the system.
This is correct. Backpressure is always the enemy of power. Engines do not need backpressure at all.

The larger the pipe, the more flow potential. But if your engine is not using that flow potential, then the pulses are moving slower and scavenging is reduced or lost. At the same time, the larger pipes tend to produce a more bassy droney tone that can be annoying. The larger the pipe gets, the more it appears to the engine like open air, i.e. the end of the exhaust system.

What your engine wants for power is header primaries of a certain matched length and diameter flowing into a smooth collector with a tailpipe of a certain length and diameter, probably not longer than 3' behind the collector. That's it. Anything tacked on to the end is potentially robbing power. How much we let it rob is a function of how quiet we want the system to be, where we want the exhaust to exit the undercarriage, and what emissions laws require us to do.
 

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1997 GMC Safari SLX - AWD
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Discussion Starter · #10 ·
All very interesting and knowledgeable replies! Thanks folks!

One thing I did not quite get from all the tech talk ...... and forgive me if it all went over my head a bit, but a stock 4.3 in my van.... that would NOT benefit from a full 3" exhaust right? A 2.5" pipe after the muffler should suffice?

I only ask because I have a bunch of 3" mandrel bent pipe from a silverado laying about that could easily be turned into a muffler-back tail pipe for the Safari but that may be totally pointless (and may drone anyway). Cheap isn't always the answer, just because I have it on hand doesn't mean I need to use it!

And agreed on the comment about Flowmasters.... No thank you, to a welded muffler on a daily. Had one on my Jeep XJ.... holy drone.
 

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Discussion Starter · #12 ·
these are inexpensive better flowing then stock and sound decent no drone: Exhaust Muffler-Soundfx Direct Fit Muffler Walker 18274 | eBay They come in other sizes but this is for the idea just search sizes
Thanks! Appreciate it. My main idea for the muffler isn't sound, that's for sure. I am OK with a quiet/moderate system... I just want to get rid of that Trash Bin sized OEM muffler underneath haha. It's a hazard off road and just so HUGE. Just tidy up the underside of the Astro a bit
 

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One thing I did not quite get from all the tech talk ...... and forgive me if it all went over my head a bit, but a stock 4.3 in my van.... that would NOT benefit from a full 3" exhaust right? A 2.5" pipe after the muffler should suffice?
Not only would it not benefit, but you stand to lose power if any change happens. A 2.5" single system is more than enough for your van. And the smaller the tube, the less bassy drone to make you hate driving. And believe me: that drone is going to peak at whatever RPM you spend the most time at. It's the law somehow.

David Vizard's rule is 2.3CFM per open header HP. If you have a dual system, that's 2.3CFM per open header HP per bank. CFM numbers can be hard to find on mufflers, but Dynomax publishes them. A 2.5" Super Turbo definitely meets that figure for a single exhaust. Dual doesn't even need 2". You have a 190HP engine. It doesn't need that much, and it can't use any more.
 

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2000 Lifted 4x4 Astro 92 V8-350 Shorty
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Those Magnaflows are excellent mufflers....I so love the sound of the Magnaflows, though!
I'm running dual Magnaflows on my 92 350-V8... they are fantastic.
Yes they sound great too... deep powerful rumble at idle, no cruise drone whatsoever.
My 2000 V6 however, still happily running the big quiet stock muffler... no intention of changing.
 

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I'm running dual Magnaflows on my 92 350-V8... they are fantastic.
Yes they sound great too... deep powerful rumble at idle, no cruise drone whatsoever.
My 2000 V6 however, still happily running the big quiet stock muffler... no intention of changing.
I have used them on numerous VW/Audi turbo builds. On a turbo four cylinder, a single Magnaflow reduced noise to a nearly stock level (with no cat.) The turbocharger also does a lot to kill exhaust noise. On an NA engine, they would produce a nice, throaty roar with, as you said, no drone. They are sweet mufflers. If my wife understood the hobby, I would use them on my van too. But she doesn't, so I'm not.
 

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I personally would never likely put them on a V6... only V8.

I also have a couple sets of new Dynomax Ultra-Flo Stainless mufflers too, which I've never used yet. Thought of giving them a try to compare.

Ironically, I may be pulling my dual 2-1/2" Magnaflows off my 92 V8... to replace with some less flowing old-school side-pipes. Squeezing out every last HP is no longer a purpose of mine.. I have more than enough tire-spinning torque to spare. Nothing typically beats the look and sound of side-pipes on a nice rumbly V8. We'll see...
Automotive parking light Tire Wheel Vehicle Automotive lighting
 

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I don't recall who originally posted this..
but I think this is the COOLEST Astro dual system idea (true dual out the rear).
I was going to copy this idea.. but then I got lazy when my side exits worked out so well.
Wheel Tire Automotive tire Vehicle Tread
 
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