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It is common practice to have a 1/2" reduction in pipe size from the headers actual collector to the start of the exhaust system. If you continue the exhaust with the same size as the collector, the engine see that entire length before the muffler as collector length. 3" exhaust pipe would need 3 1/2" collector to be right in theory. For the street, leave it alone.
 

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Discussion Starter #22
It is common practice to have a 1/2" reduction in pipe size from the headers actual collector to the start of the exhaust system. If you continue the exhaust with the same size as the collector, the engine see that entire length before the muffler as collector length. 3" exhaust pipe would need 3 1/2" collector to be right in theory. For the street, leave it alone.
When I drop the exhaust system I'm at an 18 inch collector. So on the street the muffler is right there. That was my reasoning when I built it, to have the muffler break the pressure wave. That was my thinking 15 years ago....
Do uou think In this case 3.5 would make a difference? or only when it's open after the collector?
thx
 

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Discussion Starter #23 (Edited)
The gauge off an O2 bung IMG_7892.jpg was a great and simple idea.
Thanks again BB70 !!
I show a little over 4 pounds, right around 6400... then I run out of road ;-)
so it's choking a little. i'm thinking a 2 pound spring in a BOV (or a fabricated plate) on each side would be about right
 

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How far upstream is the 4psi?
Meaning, this is aft of the splitter, right?
How about in front of the splitter/y-pipe thingy?
The question is: How much of the 4psi is from upstream of the current measurement point.
For example, BB70's suggestion is good for verify a plugged exhaust. But, if he measures in front of the muffler, he could get a 4 psi reading. But he needs to figure out if the Cat puked its guts out or a pre-cat failed. Back in the day, it was common for a monolithic Cat to break up and deposit itself in the muffler. Misdiagnose the Cat and now you gotta sell the customer an exhaust system. Two more quick tests and you'll have the backpressure mapped out.

Last century, I had a special punch I made for my air chisel that would pierce a hole for an IR gas analyzer then plugged it with a 5/16" flanged self tapping bolt after the dyno run.
 

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Discussion Starter #25 (Edited)
No cats! I hate cats.
Each side is two mufflers back to back: a single chamber , single baffel, 3-inch muffler and then a 3-inch perforated Y-pipe muffler right behind it that has two 2.5-inch outlets.
The 4 psi was at O2 bung in the collector, ahead of the H-pipe, ahead of the mufflers.
thx!
 

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The Flowmaster/Flowdisaster mufflers are killing any flow. Replacing them with something straight through will be worth at least 50hp on a car like yours. That is the #1 thing I would change.

Replace those with some mufflers you can see straight through, and call it 'good'. As for keeping it quiet... Use the LONGEST straight through muffler you can fit under the car. You'd be amazed at just how quiet the longer absorption mufflers can be... and with zero flow degradation. You could modify your quad 2" to 2-1/4" and pick up a ton of area.

Keep it simple... else you'll end up with a stupid amount of money in an exhaust that doesn't flow very well in the end, and weighs a TON.

Good Luck
 

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I don't remember who makes them but I've seen cut outs for that were boost related. Maybe you could configure that to work somehow
 

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I want to eventually mount a little air compressor in a couple of my cars so i can operate cut outs, but they wouldnt necessaruly be cut outs per se, id route gas through more mufflers one way, and through less the other way.

For me its more to be able to shut the things up at will, quieten things down.... but ive found more muffler/more backpressure kills off bottom end wheelspin also, so its an interesting thought to ponder for street use on questionable surfaces.... doing this then opening it up at the gear shift. Not sure if rigging up some kind of controller that works off rpm is possible, probably is with aftermarket EFI and extra inputs/outputs. Anythings possible i guess if you put enough time and effort into it.

The electric cut outs seem to leak some from what ive seen. Air operated ones can probably hold tight, infact some of the air butterflies found in jap trucks here, for exhaust brakes dont seem to leak, they are air operated on/off.

I remember having a car that got 'defected' here and had to get inspected. It was a 71 so pre most emissions, i didnt pass the noise test though the first time around. Instead of redoing the zorst, i made a couple plates to sandwich between the headers and exhaust, and drilled a one inch or so hole in them, maybe it was 1.5 inches. something like that. It shut the thing up considerably, i took it back to the inspection station and it passed. WOT was crap, but lesser part throttles were stronger. Id assume it would use less fuel like this also at part throttle cruising, but i didnt test that.

It was an interesting little experiment for me back in the day in my teens, fwiw it was a warmed over 318 with heavily ported heads and a .484 cam, 600 holley, headers and a few other little things.
 

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I want to eventually mount a little air compressor in a couple of my cars so i can operate cut outs, but they wouldnt be cut outs per se, id route gas through more mufflers.

Its more to be able to shut the things up at will... but ive found more muffler/more backpressure kills off bottom end wheelspin also, so its an interesting thought to ponder for street use on questionable surfaces, doing this then opening it up at the gear shift. Not sure if rigging up some kind of controller that works off rpm is possible, probably is with aftermarket EFI and extra inputs/outputs.

The electric cut outs seem to leak some from what ive seen. Air operated ones can probably hold tight, infact some of the air butterflies found in jap trucks here, for exhaust brakes dont seem to leak, they are air operated on/off.
Why in the flying fuck would you do any retarded shit with more muffler to control wheel spin if you have any type of programmable efi?
 

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Why in the flying fuck would you do any retarded shit with more muffler to control wheel spin if you have any type of programmable efi?
I wouldnt if i had programmable efi, i just mentioned it because ive noticed that effect... and ive noticed it first on carb and distributor stuff 20+ years ago.

I said id primarily do this to quieten my junk down....for me, its about stealth when wanted... namely to avoid cop issues, but sometimes i dont want to be rattling peoples widows either especially late at night. So i know im going a little off topic from the OP, though I get the premise for those wanting to do this for other reasons.
 

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I wouldnt if i had programmable efi, i just mentioned it because ive noticed that effect... and ive noticed it first on carb and distributor stuff 20+ years ago.

I said id primarily do this to quieten my junk down....for me, its about stealth when wanted... namely to avoid cop issues, but sometimes i dont want to be rattling peoples widows either especially late at night. So i know im going a little off topic from the OP, though I get the premise for those wanting to do this for other reasons.
It's certainly a fun discussion. I just put turbos on everything....no need for mufflers...lol My daily has twins on it with dual 3" straight out back with no mufflers. Its not super loud until the wastegates open.
 

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It's certainly a fun discussion. I just put turbos on everything....no need for mufflers...lol My daily has twins on it with dual 3" straight out back with no mufflers. Its not super loud until the wastegates open.
Sure... its and added bonus with turbos. I have a turbodiesel 6.5L jap truck and its so quiet I just took the muffler off one day and run straight 3.5 pipe out of it... thats the only turbo I have right now, a mitsubishi at a lowly 10psi.

Turbo guys must be amused at how loud NA cars can be vs how slow they are. :D
 

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Sure... its and added bonus with turbos. I have a turbodiesel 6.5L jap truck and its so quiet I just took the muffler off one day and run straight 3.5 pipe out of it... thats the only turbo I have right now, a mitsubishi at a lowly 10psi.

Turbo guys must be amused at how loud NA cars can be vs how slow they are. :D
Lol!

Notice any difference in performance on the 6.5 with the straight pipe?
 
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