cat delete

"So you're not gonna fall for the bannana in the tail pipe?" Eddie Murphy, Beverly Hills Cop, circa 1984
 
1LoudLS said:
ok lets try this again, I'll try to keep it simple so you can keep up. the myth of back pressure comes from people's misunderstanding of velocity and scavenging.

OEM exhaust systems usually have the most restriction. good companies will spend the time to make them as efficient as possible and can get them to have pretty low restriction and still quiet down the car (just look at the 2nd gen LS) the important part is to get the right sized piping, because you need to keep the exhaust pulses moving, and the faster the better. with any sized exhaust pulse, the smaller the pipe you have, the faster the exhaust pules will travel. behind each exhaust pulse is a low pressure spot that will help suck out (scavenging) the next exhaust pulse from the next cylinder in the firing order. and the faster the exhaust pulse is the stronger the scavenging will be, now there will be a point where too small of a pipe will be too much to flow the volume of the exhaust pulse, and will start to cut the gains of the faster pulse. also making it tougher to choose the right sized pipe, a smaller pipe will work great for low RPM's but will get overloaded at higher RPM's making small pipes look like they create torque but hurt horsepower. A bigger pipes will flow more volume helping a motor on the high end and helping it make more "horsepower" but the larger pipe slows down the pulses and hurts scavenging which hurts creating power at the low end and is "bad for torque". as a lot of people have a hard time understanding the true complexities of the modern combustion system, they need to make up these myths to fill in what they dont get, hence the myth of pack pressure.

this is also why it is possible for a fast car to actually have a little more power with the correct exhaust system VS. open headers, open headers have absolutely no scavenging going on, instead of having some back pressure to make torque.

this also explains why it felt like your honda back in high school lost all that torque when you through on just random straight through muffler with 3.5" piping all the way back. doesn't matter, sounded cool anyway! back to the point, back pressure had nothing to do with it.
Click to expand...
How is this any different than what I said? Yea, my statement was very simplified, but I'm comparing truck engines (gasoline V8's) to high performance engines such as the Mustang. Take the GT40. Minus the fact it uses a supercharger, the engine is more or less the 1st generation Navigator engine. Two different purposes. One needs torque because its moving a massive vehicle, and it needs to be able to tow the equiv of two more. The other is made for speed. If you open up the exhaust on a truck without having a properly tuned exhaust, you risk losing torque. In a car it may not make that much difference, but on trucks that don't rev as high (and move a lot of weight), losing that torque can hurt a lot.
 
Elemino said:
How is this any different than what I said? Yea, my statement was very simplified, but I'm comparing truck engines (gasoline V8's) to high performance engines such as the Mustang. Take the GT40. Minus the fact it uses a supercharger, the engine is more or less the 1st generation Navigator engine. Two different purposes. One needs torque because its moving a massive vehicle, and it needs to be able to tow the equiv of two more. The other is made for speed. If you open up the exhaust on a truck without having a properly tuned exhaust, you risk losing torque. In a car it may not make that much difference, but on trucks that don't rev as high (and move a lot of weight), losing that torque can hurt a lot.
Click to expand...

sure but this is not back pressure....

Back pressure is a misconception, at the very least it is the wrong way to say what is being done. If back pressure was good, then plugged up cats would make the engine run like a champ...
 
SOME back pressure on a carbeurated engine does generate more torque due to the pressure pushing the charge back up and then down through the carbeurator again. This essentially richens the charge at low RPMs which most carbs aren't too efficient at giving you a better stoichiometry. On the same engines designed around these carbs and exhaust, if you suddenly remove all back pressure and just have normal scavenging then the engine would run lean at lower RPMs and burn valves.

So, that's where the "myth" originates from.

With a self-adjusting fuel system like EFI it should be a minimal issue running lean. So with EFI having minimal back pressure is generally good.

Backpressure does quiet an exhaust system as a whole. You are also now relying on an oxygen sensor that was designed to work behind a cat to work without one. If that causes the computer to adjust the mix then you could easily get a MPG hit. Would have to ask someone who has done the delete and see what their experience was.

So, sorry for all those I offended with my statement. One of those cases where you have too many trivial things remembered and get them confused sometimes. :)

2-strokes on the other hand have to have back-pressure in order to keep you from blowing the charge right through the chamber and into the exhaust. But that's a whole different topic.
 
Funny stuff. Way back when I installed dual 3 inch exhaust on a 79 Chevy halfton, as I intended to install a 454 right after. The 350 I had it connected to lost tons of power. The reason was it lost scavenging. Install a 1 inch diameter, I'd have increased backpressure, and would have had an engine that died off at 3000RPM.

Never heard of the carb reversion thing before.

Never got around to the 454. About that time the new Silverados came out in 1999, and I just had to have one. Big mistake, it was a bigger lemon than my 93 Cougar was. The Cougar just had a bum transmission, everything on the truck EXCEPT the engine and trans broke.
 
1LoudLS said:
young bucks and science, there is a reason America is nowhere near the top of the list when compared to the rest of the world!
Click to expand...

Pfft whatever I've got stickers for aerodynamics. I have so much torquez now

:shifty:

But seriously I almost went into fluid dynamics because I love this kind of stuff (before you say anything, this is actually a fluid dynamics problem since gases act with the exact same properties as liquids unless they are heated to a plasma state... and your exhaust better not fu*king be that hot coming out of the engine or you've got a serious problem haha)
 
Stopsign002 said:
Bahahaha that first one is awesome. Its becoming painfully obvious how few people REALLY understand how cars work, or physics for that matter lol
Click to expand...

Amen! true that though, if people want to learn about something...LEARN about it. lol
 
people are stupid, and our government allows them to be stupid by putting warning labels on everything and spoon feeding them everything. If you are to :q:q:q:qing stupid to know a chainsaw can chop your leg off, you have no buisness owning it... It should never have to saw blade is sharp.... Or HOT coffee, if you order a coffee and don't say you want an iced coffee, then it is :q:q:q:qing HOT. The only things that should have warning labels are things that are not obviously dangerous, such as some chemically based items, and meds

If they would let the dumb people either learn something or just die off it would be better... Darwinism, it should exist

Problem is we won't let them.....
 
2002_lincolnLS said:
people are stupid, and our government allows them to be stupid .....
Click to expand...

Allows them? Are you kidding, they encourage it. They want the people to be stupid. Stupid people are easier to control and take advantage of.
 
kaball said:
It does exist just not in the good ol USA. The track we're on now we are practicing a form of reverse Darwinism... moving backwards instead of forwards.
Click to expand...

yeah, have you ever seen the movie, Idiocracy, they have recently moved into the documentary section of the library.... n
 
You must log in or register to reply here.

Members online

No members online now.
Total: 251 (members: 0, guests: 251)
Back
Top